preferred orientation of plagioclase neutron diffraction and u stage data abstract pole figures of anorthosite mylonite an 65 from san juan bautista california were determined with neutron diffraction using a 23 position sensitive detector this novel technique enables us to deconvolute the complex diffraction spectrum of this triclinic mineral into fifteen separate peaks hkl which are measured simultaneously all pole figures display strong preferred orientation with complicated triclinic distributions from which it appears that 001 poles are concentrated normal to the schistosity plane of the specimen the triclinic mineral is well suited to discuss some fundamental issues of texture representation if crystals are measured individually e g on the u stage positive and negative ends of directions can be unequivocally identified and the whole sphere rather than a hemisphere is necessary to represent the distribution of positive axes this is illustrated for a plagioclase rich amphibolite from orno huvud sweden

genesis of tectonic inversion structures seismic evidence for the development of key structures along the purbeck isle of wight disturbance the interpretation of a densely spaced and well calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the wessex basin the results help to resolve long standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important local ancillary structures with respect to the major monoclinal folds with which they are associated although the structures are entirely consistent with the effects of contractional reactivation tectonic inversion of normal faults the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns subsequent inversion geometries and resultant outcrop patterns as well as illustrating regional controls on the formation of structures the new seismic based interpretations enable a reassessment of the lulworth crumple and the ballard down fault the lulworth crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted incompetent syn rift fill in the immediate hanging wall to the purbeck fault a reactivated major normal fault the ballard down fault s origin is interpreted to result from the formation of a local late stage out of the syncline reverse fault which propagated southwards and upwards through a chalk succession as the chalk had already been rotated to form the northward dipping steep limb of the purbeck monocline at ballard down the structure cuts down stratigraphically the results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins

shear wave anisotropy of laminated lower crust beneath urach sw germany a comparison with xenoliths and with exposed lower crustal sections in order to investigate the laminated lower crust beneath urach sw germany we analysed 1 the anisotropy and reflectivity of seismic shear waves and 2 the composition of xenoliths samples we conclude that the directional dependence of shear waves of the lower crust is effectively transversely isotropic with a maximum velocity difference of 3 to 6 both seismic signature and petrological information are compared with reference profiles deduced from the exposed lower crustal sections of ivrea and calabria italy the comparison based on synthetic seismograms shows that the observed combination of shear wave anisotropy and reflectivity pattern corresponds to alternating layers containing a high amount of metapelites such as found in the calabria profile and in the val strona subsection of ivrea the main portion of the whole rock anisotropy is caused by the lattice preferred orientation of anisotropic minerals so called intrinsic anisotropy the effect of alternating rock layering so called layering anisotropy contributes only a minor portion to the whole rock anisotropy layered mafic intrusions such as found in the val sesia subsection of ivrea are nearly isotropic both seismic anisotropy and xenoliths indicate that mafic layers do not play a major role for the laminated lower crust at urach they may be restricted to a thin layer at the crust mantle boundary

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

physical weathering of marbles caused by anisotropic thermal expansion marbles as building stones as well as in their natural environments show complex weathering phenomena the most important damage scenario is based on the highly anisotropic thermal expansion coefficient of calcite i e extreme expansion parallel and contraction normal to the crystallographic c axis therefore the rock fabric and especially the lattice preferred orientation texture of calcite and or dolomite as the predominant mineral phases in marbles have a significant influence on the mechanical weathering the textures of marbles from five different locations vary from a more or less perfect prolate to moderate oblate shape of the 006 pole figure tensor accordingly the texture derived bulk thermal dilatation anisotropy covers a broad range from 0 048 to 0 680 the modelled thermal dilatations correlate with those obtained from experimental measurements the difference in magnitude is basically explained by the microcrack fabrics which was not considered in the computations all samples show a deterioration due to thermal treatment regardless of the strength of texture the directional dependence of a the total magnitude of the thermal dilatation coefficient and b of the residual strain is highest in marbles with a strong texture whereas the carrara marble with a weak texture exhibits a uniform crack formation the progressive loss of cohesion along grain boundaries due to dilatancy may serve as an example for the initial stage of physical weathering

reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within british jurassic marine mudstones the coinstone is a well known hiatus concretion level in the lower lias lower jurassic upper sinemurian of dorset southern england it has long been recognized as a layer of bored and encrusted early diagenetic clay hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones several different types of concretion of variable complexity can be distinguished of which two probably derived from slightly different stratigraphic levels have been juxtaposed by condensation at the erosion surface diagenetic and biological processes occurring before during and after exhumation on the jurassic sea floor can be recognized the relative timing of these events can be distinguished suggesting that initial concretion consolidation the first generation of septarian cracking and the precipitation of the first generation of crack lining calcite preceded exhumation these therefore probably took place at an early stage at shallow burial depths within the accumulating sediment pile the early calcite is brown uv fluorescent and inclusion rich and is similar to the first calcite generations seen in many other clay hosted septarian concretions a generally early diagnetic origin of this material is thus inferred observations on crack textures and geometries and the interactions of the post exhumation fauna of encrusters borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today the burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion even in the absence of high energy physical processes regional stratigraphic gaps may be formed as a consequence of sea level rises or falls or as a response to sediment supply reduction independent of sea level change such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

microscopic anatomy of a hot on cold shear zone insights from quartzites of the main central thrust in the alaknanda region garhwal himalaya

quantitative correlation of texture and magnetic anisotropy of compacted calcite muscovite aggregates abstract deformation of carbonate rocks leads to crystallographic preferred orientations cpos of calcite which in turn generates an anisotropy of magnetic susceptibility ams the use of the diamagnetic anisotropy which arises from the preferential orientation of calcite for texture characterization is usually hindered by 1 the weakness of the anisotropy and 2 overlapping contributions from paramagnetic and ferromagnetic minerals to the ams in this study the diamagnetic paramagnetic and ferromagnetic sub fabrics of synthetic calcite muscovite samples have been separated using high field torque measurements the cpo of calcite and muscovite has been determined by neutron diffraction and the magnetic sub fabrics are compared to the ams that was re calculated from the cpo the isolated diamagnetic sub fabric shows a good correlation with the calcite texture for calcite concentrations above 70 the paramagnetic sub fabric is shown to be a reliable texture parameter for muscovite even at a concentration of 5 the results demonstrate the capability of the ams method as a texture parameter in impure carbonate rocks

aspects of basin inversion in southern britain the wessex basin of southern england a permian to cretaceous extensional basin can be structurally divided into a set of constituent asymmetrical graben bounded by major e w trending zones of en echelon syn depositional normal faults the graben were inverted in late cretaceous and tertiary times by compressive stresses oriented roughly north south inversion structures fall into two related categories regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben fill superimposed upon and geographically delimiting the regional upwarps are roughly east west trending linear zones of en echelon inversion structures these coincide with the earlier graben bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault the linear reverse fault monocline inversion structures were a relatively inefficient method of basin shortening because upper crustal faults in the region steepen upward markedly reversal of these faults under compression resulted in a shortening discrepancy at shallow depths locally particularly in south dorset this was overcome by bucklefolding and low angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures elsewhere however the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary fill this occurred preferentially in those graben containing young poorly lithified and therefore weak sediments thus the early cretaceous depocentres of the weald and channel basins were strongly upwarped with axial uplifts of over 1000 m conversely graben having older more lithified sequences suffered little regional upwarp shortening primarily by fault reversal along the linear inversion structures the amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension

direct measurement of the quartz c axis using neutron diffraction abstract quartz c axis pole figures are widely used across structural geology and geophysics to gain insights into crustal properties yet their direct measurement using bulk diffraction techniques has hitherto been unsuccessful in this contribution we present measurements of the quartz c axis using neutron diffraction this has been achieved by combining advances in neutron diffraction instrumentation high neutron fluxes and fast data acquisition with carefully performed diffraction pattern analysis to resolve low intensity diffractions on the 003 diffraction peak hidden in the shoulder of the higher intensity 112 peak these measurements have allowed us to assess recalculated c axis pole figures i e those predicted from odf functions against measured c axis pole figures and thus assess the reliability of quartz c axis pole figures past and present we find that measured and recalculated pole figures are generally in good agreement in both strongly and weakly textured rocks and that inclusion of the 003 pole figure into quartz texture analysis routines improves odf construction

the origin and earliest state of the earth s hydrosphere the origin and earliest history of the earth s hydrosphere the inventory of excess volatiles defined by rubey in 1951 can be constrained within wide but useful limits by a consideration of empirical and theoretical evidence from astrophysics and geology models for the evolution of the solar system from the protoplanetary nebula and for the growth of the earth to its present dimensions suggest quite strongly that the hydrosphere came into being during accretion its format with h2o mostly in the oceans co2 mostly in sediments and a residual atmosphere dominated by n2 co2 and h2o was established at a very early date and has persisted without large destabilizing climatic excursions until the present day alternative accounts of early history in which the earth either loses a massive primordial atmosphere or acquires its secondary atmosphere by gradual degassing seem improbable on the basis of a series of circumstantial but cumulatively persuasive arguments the difficulty of dissipating a massive atmosphere of solar composition in reasonable times the likelihood that accretion was a highly energetic process and that it triggered early segregation of the core and the tendency of the planet to accumulate volatiles preferentially in the later stages of accretion are examples of arguments favoring an early origin for the hydrosphere several geological isotope systems which can be sampled today require early separation of the atmosphere and probably the hydrosphere as a whole these systems record radiogenic enrichment patterns in the noble gases and stable isotope fractionations which suggest an early origin of the biosphere certain geological indicators of atmospheric composition and the broadly equable character of the rock record are also consistent with a hydrosphere established in the earliest stages of history and having an initial neutral or weakly reduced composition the climatic stability of such a hydrosphere poses crucial questions especially since the early sun was less luminous we find that it is not paradoxical to have an early earth with surface temperature close to that of the present day if the major hydrospheric constituent is liquid water a near global ocean and a cloudy atmosphere provide thermal feedbacks which suffice to maintain climatic stability against the perils of the deep freeze and the runaway greenhouse large quanltities of atmospheric greenhouse gases are not required for climatic stability although co2 may well have been present in amounts larger than today s large expanses of dry land are however difficult to accommodate in the picture which we present but the evidence favors differentiation of the continents later in geological history than the early times which we consider localized glacier ice is permitted and perhaps likely and may provide a milieu favorable for chemical evolution

ultra deep subduction of yematan eclogite in the north qaidam uhp belt nw china evidence from phengite exsolution in omphacite phengite exsolution in omphacite from the yematan eclogite north qaidam uhp belt nw china is described mineralogical investigations show that the precursor omphacite in the yematan eclogite contains up to 1 16 wt k2o and 10 000 ppm h2o experimental studies document this omphacite to be formed at pressures higher than 6 gpa at 900 c the pressure temperature conditions of 3 68 gpa and 892 c for phengite exsolution in omphacite associated with garnet during the exhumation were obtained by using grt omp ph geothermobarometer we conclude that omphacitic clinopyroxene in subducted eclogites may act as a robust medium to transport h2o and potassium deep into the interior of the earth this study suggests that the yematan eclogite in north qaidam uhp metamorphic belt nw china may have been subducted and exhumed from depths of more than 200 km

early to mid cretaceous tectonics and unconformities of the wessex basin southern england sediment distribution patterns time subsidence plots seismic data and outcrop analysis of the lower cretaceous ryazanian albian of the wessex basin southern england suggest that a number of unconformable horizons exist and that there is no single late cimmerian unconformity surface a tectonic change from areally restricted deposition in the weald and channel sub basins to extensive sedimentation across the whole of southern england began in the mid aptian this change ended with transgression of basement highs in the early albian of the dorset area and later albian of the devon and london platform basin margin areas its onset can be linked to the austrian tectonic phase of europe the onset of marine deposition in the aptian is coincident with periodic transgression of the margins of the wessex basin throughout the aptian albian mid late aptian and early mid aibian transgressions were preceded by periods of erosion the aptian albian lower greensand is characterized by transgressive regressive phases suggesting a short term controlling process third order eustatic changes or intra plate mechanisms tectonic changes in the mid aptian include uplift followed by widespread subsidence this change coincides with a time of plate reorganization in the european and north atlantic areas

saturn s icy satellites thermal and structural models abstract thermal histories of the small icy saturnian satellites mimas tethys dione rhea and iapetus are constructed by assuming that they formed as homogeneous ice silicate mixtures the models include effects of radiogenic and accretional heating conductive and subsolidus convective heat transfer and lithospheric growth accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating mimas is so small that its thermal evolution is essentially purely conductive at present it is a cold nearly isothermal body any subsolidus convection or thermal activity in mimas would have been confined to a brief period in its early history and would have been due to a warm formation the four largest satellites are big enough and contain sufficient heat producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions dione and rhea have convective interiors for most of their thermal histories while tethys and iapetus have mainly conductive thermal histories with early periods of convective 0activity the thermal histories of the five satellites for the last 4 by are independent of initial conditions at present they have cold conductive interiors the model thermal histories are qualitatively consistent with the appearances of these satellites mimas has an ancient heavily cratered surface tethys and probably iapetus have both heavily cratered and more lightly cratered areas and dione and rhea have extensively modified surfaces because of their similar sizes and densities mimas and enceladus are expected to have similar surfaces and thermal histories but instead enceladus has the most modified surface of all the small icy saturnian satellites our results suggest a heat source for enceladus in addition to radiogenic and accretional heating tidal dissipation is a possibility because the water ice in these bodies does not melt resurfacing must be accomplished by the melting of a low melting temperature minor component such as ammonia hydrate

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

isotopic composition of a calcite cemented layer in the lower jurassic birdport sands southern england implications for formation of laterally extensive calcite cemented layers abstract 18opdb and 13cpdb values have been measured on 107 calcite cement samples from a laterally extensive 3 km and continuous calcite cemented layer 0 5 m thick in the coastal exposures of the lower jurassic shallow marine bridport sands in dorset southern england the samples were taken from a two dimensional grid with 10 cm horizontal and vertical spacing between samples and along individual vertical lines across the calcite cemented layer 18opdb values vary between 4 8 and 9 2 and decrease radially outwards from points with lateral spacings on the order of 0 5 1 m in the middle of the calcite cemented layer the 18opdb values therefore indicate that the calcite cemented layer was formed by merging of concretions all 13cpdb values measured are in the narrow range 2 2 to 0 5 which suggests that the dominant source of calcite cement in the layer was biogenic carbonate

ultimate strength of solids and formation of planetary cores some previous experiments and theory have suggested that the ultimate strengths of silicates are 1 2 gpa 10 20 kbar that the ultimate strength of ice is about 0 1 gpa 1 kbar and that the confining pressures of planetary interiors 100 gpa would not increase these strengths by more than a factor of two deviatoric stresses of these magnitudes are easily generated in planetary interiors by density heterogeneities of tens of kilometers radius this implies that once such heterogeneities had formed core segregation would not be further inhibited by the strength of a cold protoplanetary interior it is further suggested that core segregation may be initiated by heterogeneous impact heating well before extensive near surface melting occurs and that in the most favorable circumstance segregation could begin in a 700 km radius protoplanet or a 500 km radius icy satellite these considerations increase the likelihood that core segregation in larger planets and satellites was essentially contemporaneous with accretion

consequences of giant impacts in early mars core merging and martian dynamo evolution a giant impact is an increasingly popular explanation for the formation of the northern lowland on mars it is plausible that at the impact time both mars and the impactor were differentiated with solid silicate mantles and liquid iron cores such a large impact likely resulted in merging of the cores of both bodies a process which will have implications on the thermal state of the planet we model the evolution of the martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor s core as a single diapir the impact heating and the viscous heating induced during the core merging may affect the early thermal state of mars during several tens of million years our results show that large viscosity contrasts between the impactor s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature when the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000 the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core mantle boundary finally the direct impact heating of martian core leads to thermal stratification of the core and kills the core dynamo it takes on the order of 150 200 myr to reinitiate a strong dynamo anew the merging of the impactor s core with the martian core only delays the reinitiation of the dynamo for a very short time

seismic structural and petrological models of the subcrustal lithosphere in southern germany a quantitative revaluation anisotropy in the subcontinental lithosphere becomes increasingly important because it is observed in many seismic studies especially for p n waves typical rocks of the uppermost mantle are peridotites which predominantly exhibit a pronounced elastic anisotropy this anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation here referred to as texture of olivine to evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere specimens of the northern hessian depression germany and the balmuccia ultramafic massif northern italy have been used they comprise four olivine texture types which are characteristic for olivine textures observed worldwide the bulk rock elastic properties have been calculated using olivine and orthopyroxene textures their single crystal elastic constants at ambient pressure temperature conditions and their volume fraction clinopyroxene and spinel are assumed to be randomly distributed the effect of four different orientations of the foliation within the uppermost mantle has been evaluated since this orientation is usually unknown two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed these variations cause significant azimuthal variations of the p wave reflections coefficients at the moho primarily we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15 to 95 possibly these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface the remaining two texture types only manifest a small directional dependence when anisotropy of compressional waves is observed in seismic studies these latter types can only be of subordinate importance however all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed this ambiguity can be substantially reduced when shear waves s waves are considered the directional distribution of s wave velocities and of the s wave splitting exhibits characteristic patterns for the different olivine texture types this could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle a fundamental requirement for a more comprehensive interpretation is the availability of detailed s wave observations the maximum s wave splitting in the peridotites investigated coincides with the maximum of the faster leading s wave this may be of importance to detect s wave splitting in future seismic studies

impact origin for the martian crustal dichotomy half emptied or half filled 1 one possible mechanism for martian hemispherical dichotomy formation is excavation of the northern lowland basin in the existing crust by a single giant impact here we explore a related scenario in which the impact melt volume is sufficient to fill the excavated cavity produce a thickened crust and generate a topographic basin centered on the opposite side of the planet we investigate this mechanism by formulating a model for viscous spreading of a partially molten layer over a spherical surface we derive numerical solutions and calculate the final area of the spreading layer as a function of impactor size and shape of the layer boundary due to an initially asymmetric melt region results are compared with observations including lowland basin size shape of the dichotomy boundary age difference between crustal provinces and crustal thickness

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

geological evidence for intra jurassic faulting in the wessex basin and its margins geological observations on jurassic outcrops close to major faults in the wessex basin mendip area reveal the local presence of ammonite and brachiopod bearing sediments penetrating underlying strata toarcian and bajocian neptunian dykes and particularly sills are associated with the eypemouth fault and bajocian sills with the bride fault and mere fault in the mendip area numerous neptunian dykes of hettangian sinemurian pliensbachian and bajocian ages cross cutting carboniferous limestone are recorded typically also associated with major basement faults e g cranmore and leighton faults these periods of assumed sediment injection are taken as indicating times of displacement along the faults in question variations in facies hettangian sinemurian toarcian bajocian uppermost oxfordian kimmeridgian spatially linked to faults are documented from some areas and boreholes reveal considerable fault controlled thickness changes in hettangian sinemurian bajocian and kimmeridgian sediments the timing of jurassic faulting in the wessex basin mendip area thus polarizes into two intervals hettangian bajocian and latest oxfordian onwards correlating with the early rifting phases of the central and north atlantic respectively

can a sinking metallic diapir generate a dynamo metallic diapirs may have strongly contributed to core formations during the first million years of planetary evolutions the aim of this study is to determine whether the dynamics induced by the diapir sinking can drive a dynamo and to characterize the required conditions on the size of the diapir the mantle viscosity and the planetary latitude at which the diapir sinks we impose a classical hadamard flow solution for the motion at the interface between a spherical sinking diapir and a viscous mantle on dynamical simulations that account for rotational and inertial effects in order to model the flow within the diapir the flows are confined to a velocity layer with a thickness that decreases with increasing rotation rate these 3d flows are is then used as input for kinematic dynamo simulations to determine the critical magnetic reynolds number for dynamo onset our results demonstrate that the flow pattern inside a diapir sinking into a rotating planet can generate a magnetic field large diapirs r 10 km sinking in a mantle with a viscosity ranging from 10 9 to 10 14 pa s provide plausible conditions for a dynamo equatorial sinking diapirs are confined to a thicker velocity layer and are thus possibly more favorable for dynamo generation than polar sinking diapirs in addition equatorial sinking diapirs produce stronger saturated magnetic fields however for the range of parameters studied here estimation of the intensity of diapir driven magnetic fields suggests that they could not have contributed to the lunar or martian crustal paleomagnetic fields

tertiary structures and hydrocarbon entrapment in the weald basin of southern england abstract the weald basin of southeast england was formed by rapid subsidence associated with thermal relaxation following early mesozoic extensional block faulting the basin appears initially to have taken the form of an easterly extension of the wessex basin but became the major depocentre during the upper jurassic and lower cretaceous with associated active faulting these movements appear to have ceased prior to albian times and a full upper cretaceous cover is believed to have been deposited in a gentle downwarp which extended far beyond the confines of the weald and wessex basins major inversion of the weald basin took place in the tertiary with both gentle regional uplift which in the eastern part of the basin is estimated to have exceeded 5000 feet 1525 metres and intense local uplift along pre existing zones of weakness which led to the formation of compressional features such as tight folds and reverse faults zones of tertiary deformation appear to have been strongly influenced by underlying particularly hercynian structural trends lower jurassic source rocks reached maturity in the early cretaceous and initial migration occurred at this time often over long distances into traps closed by pre aptian faults tertiary tilting and uplift led to the breaching of many of these pre existing traps and the formation of large folded closures a second phase of hydrocarbon migration particularly of gas took place at this time with significant vertical migration along fault zones major reservoirs located to date occur in middle jurassic carbonates and upper jurassic sandstones but deep burial in the basin has caused considerable destruction of primary reservoir characteristics changes in the temperature and pressure regimes and the mobilization of fluids within the basin resulting from the tertiary uplift caused further diagenetic changes particularly in the carbonate reservoirs exploration of the weald basin remains at a very early stage with a low drilling density to date the more recent drilling has focussed on earlier structures but traps formed or modified during the tertiary movements represent important exploration objectives although general deterioration in reservoir quality towards the centre and east of the basin makes large fold closures in these areas less attractive

complete texture analysis of a deformed amphibolite comparison between neutron diffraction and u stage data abstract this paper describes a new technique of analysing complex neutron diffraction patterns allowing the quantitative texture analysis of each rock forming mineral up to trielinic lattice symmetry in polymineralic rocks the method is demonstrated using hornblende and plagioclase fabrics in alzenau amphibolite from the spessart mountains mid german crystalline rise hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements for control purposes crystallographic preferred orientations were also determined independently by individual grain measurements using a u stage from the neutron texture analyses we obtain only superposed pole figures hornblende and hornblende hornblende and plagioclase and plagioclase with plagioclase of low indexed lattice planes therefore the critical crystallographic directions were recalculated by the orientation distribution function odf which is given by texture components the optical and neutron derived pole figures are in a good agreement all pole figures display strong preferred orientation although the plagioclase texture is less pronounced the hornblende texture consists of a 001 maximum parallel to the stretching lineation with 010 showing a tendency to develop a girdle around the lineation 100 axes are distributed with a clearly split maximum representing the monoclinic angle between 100 and 001 the plagioclase texture is a rare one with 001 subparallel within the foliation and 100 oriented subparallel to the lineation 010 of plagioclase is concentrated normal to lineation within the foliation the technique outlined here is potentially the most useful one for texture analysis of polyphase rocks independent of crystal symmetry

directional dependence of p and s wave propagation and polarization in foliated rocks from the kola superdeep well evidence from laboratory measurements and calculations based on tof neutron diffraction we have measured p and s wave velocities on two amphibolite and two gneiss samples from the kola superdeep borehole as a function of pressure up to 600 mpa and temperature up to 600 degrees c the velocity measurements include compressional vp and shear wave velocities vs1 vs2 propagating in three orthogonal directions which were in general not parallel to inherent rock symmetry axes or planes the measurements are accompanied by 3d velocities calculations based on lattice preferred orientation lpo obtained by tof time of flight neutron diffraction analysis which allows the investigation of bulk volumes up to several cubic centimetres due to the high penetration depth of neutrons the lpo based numerical velocity calculations give important information on the different contribution of the various rock forming minerals to bulk elastic anisotropy and on the relations of seismic anisotropy shear wave splitting and shear wave polarization to the structural reference frame foliation and lineation comparison with measured velocities obtained for the three propagation directions that were not in accordance with the structural frame of the rocks foliation and lineation demonstrate that for shear waves propagating through anisotropic rocks the vibration directions are as important as the propagation directions the study demonstrates that proper measurement of shear wave splitting by means of two orthogonal polarized sending and receiving shear wave transducers is only possible when their propagation and polarization directions are parallel and normal to foliation and lineation respectively c 2005 elsevier b v all rights reserved

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

lattice preferred orientations and microstructures of deformed cordilleran marbles correlation of shear indicators and determination of strain path marbles from nine cordilleran deformation zones the raft river and snake ranges the ruby rincon and riverside mountains the eastern peninsular ranges mylonite zone and three sites in the death valley region were sampled for microstructural analysis of thin and ultra thin sections and pole figure measurements by x ray diffraction to interpret their deformation history microstructural observations reveal various degrees of recrystallization fabrics are predominantly symmetric but some samples display consistent asymmetries of the long grain axes schistosity and the macroscopic foliation plane defined by compositional layering these asymmetric microstructures are most pronounced in non recrystallized fabrics well developed textures are found at all sites with c axes perpendicular to the foliation and a axes in a girdle in the foliation plane the sense of shear derived from slightly asymmetric 1120 pole figures is consistent with the shear sense determined in the field and by s c structures in adjacent gneisses the magnitude of the obliquity of 1120 girdles suggests that the component of simple shear is relatively small even in samples with highly asymmetric microstructures we conclude that the obliquity of the 1120 girdles is a reliable indicator of the strain path during the last substantial increment of strain and indicates that the carbonate rocks studied deformed dominantly in pure shear this implied strain path for carbonate rocks may not necessarily reflect the regional strain path because strain may be partitioned between rocks of different compositions or between zones of predominantly pure and simple shear at all scales

stability assessment of marble statuaries of the schlossbrucke berlin germany based on rock strength measurements and ultrasonic wave velocities the degree of weathering in natural stones on buildings and sculptures has been determined for many years in numerous cases by means of ultrasonic measurements conclusions concerning the strength of the rock and the type of weathering can thus be drawn this relationship has not been established for all rock types most of the progress utilizing this method has been made in the analysis of marbles where an increasing degree of weathering shows lower ultrasonic velocities in the present study four carrara marble samples showing similar rock fabrics but with respect to weathering exhibit considerable differences are investigated porosity varies between 0 2 vol and ca 2 4 vol whereby with increasing porosity the pore radii changes as well parallel to this the ultrasonic velocities change in dry samples from about 5 5 to 1 6 km s respectively model calculations reveal that the velocity reduction is caused by cracks with an extremely small aspect ratio of about 0 005 or even less after a specific loss of strength however solution processes can become active which modify the microcracks and generate an opposite trend in the process a strong porosity increase correlates to a relatively small velocity reduction with the presence of water the v p porosity weathering relationship experiences a considerable modification parallel to the reduction of the ultrasonic velocities it was determined that the mechanical strength compressive strength flexural strength etc as well as the static young s modulus is reduced almost equally by a progressive advancement of the weathering front in one case study dealing with tensile strengths it was clearly documented how tensile cracks develop and propagate in dependence of the rock fabric the rock mechanical and ultrasonic velocity data were used for stability assessments applied to the marble statuaries from the schlossbrucke in berlin stability assessments of the sculpture group 4 reveal that some critical parts must be replaced due to safety reasons

the origin and recognition of laterally continuous carbonate cemented horizons in the upper lias sands of southern england abstract the upper lias sands of southern england contain numerous laterally extensive carbonate cemented horizons petrographical analyses of samples from outcrop sections and the marchwood no 1 borehole indicate that these horizons result from preferential cementation of bioclast rich clay poor sediments by comparison with interbedded clay rich bioclast poor sediments the alternation of the two sediment types is attributed to the effects of respectively fairweather and storm processes on a submerged marine shoal petrographical and ichnological data indicate an early distinction of the strongly and weakly cemented horizons the widespread extent of the cemented horizons as indicated by outcrop studies on the dorset coast is considered to be a direct consequence of episodic storm activity on the low relief shoal sedimentological palynological and petrophysical criteria are presented to assist in recognition of similar extensive cements in subsurface reservoir horizons that do not outcrop

evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using ebsd and fabric analyser techniques quartz crystallographic preferred orientations cpo from three distinct orthogneisses using both the electron back scatter diffraction ebsd and fabric analyser fa techniques reveal a clear trend from basal and rhomb slip for high p t conditions 670 20 degrees c 9 kbar rhomb and basal slip for medium p t 590 15 degrees c 6 kbar and a dominance of prism slip for lower p t conditions 570 degrees c 4 5 kbar the textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences locally quartz cpus are different within one thin section and in comparison to bulk orientation measurements both ebsd and the fabric analyser techniques are ideal to determine such textural heterogeneities while the ebsd is a powerful technique to determine the full cpo measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the fa is an accurate tool to determine cpus of c axis orientations in uniaxial materials in a cpo focussed study the fa is a cheap alternative to ebsd as the analysis of whole thin section can be accomplished in a very short time with minimal sample preparation with the fa it is possible to evaluate the cpus of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity

calcretes related to phreatophytic vegetation from the middle triassic otter sandstone of south west england carbonate cementation in the triassic otter sandstone of budleigh salterton sw england occurs in two distinct forms large vertical concentrically zoned cyclinders and thin subhorizontal sheets the former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain the sheets represent cementation around ancient water tables the precipitation of the rhizocretions took place at least in part during the life of the plants and 13c and 18o values support the view that evapo transpiration induced carbonate precipitation palaeosol profiles are rare in the otter sandstone reflecting the geomorphological instability of the braidplain surfaces in contrast rhizocretionary and sheet calcretes which formed several metres below the active depositional surface are abundant because they had a high preservation potential

geophysical correlation of the aptian and albian formations in the wessex basin of southern england using geophysical borehole log signatures and basic lithological information from sidewall cores and well cuttings division and correlation of the mid cretaceous sediments of the wessex basin can be achieved a sub gault early albian unconformity surface is mapped with some accuracy across the hampshire dorset area and the deposits of a previous aptian transgression are observed the distinctive log signatures aid in facies reconstructions and sequence stratigraphical analysis especially the onlap of basal gault horizons onto and across basement highs the highly variable thickness and lithological distributions characteristic of the wealden pass upward into laterally extensive facies distributions in the lower greensand and gault sequence boundaries are identified at the base of the wealden lower greensand gault within the gault cristatum subzone and base chalk

preferred orientation of phyllosilicates in phyllonites and ultramylonites abstract x ray texture goniometry operated in the transmission mode has been applied to fine grained phyllosilicate rich sc mylonites from the santa rosa mylonite zone california the whipple mountains core complex california and the austroalpine nappes of the eastern alps austria basal planes 001 phyllosilicate pole figures depart in two respects from orthorhombic symmetry a the maximum pole density deviates from the pole to the mylonitic foliation and lies along a great circle which is normal to the foliation and contains the mylonitic lineation and b the contours of equal pole density are more widely spaced on one side of the 001 maximum than on the other measured along the mylonitic lineation optical microscopy reveals s and c surfaces and sets of shear bands sb characteristic of non coaxial deformation the pole figures are used for two purposes a the first is strain estimation after the march model by imposing orthorhombic symmetry the resulting march strains are more or less the same in all californian samples this is interpreted as evidence for a steady state foliation b the second is shear sense determination it is demonstrated that consistently asymmetric preferred orientations reflect structures which are typical of non coaxial deformation it is concluded that x ray analysis of phyllosilicate rich mylonites can in addition to providing an estimate of total strain give information about deformation regime and shear sense furthermore the study documents the heterogeneous nature of deformation on macroscopic and microscopic scales and the importance of non coaxial deformation in accommodating large strains

the influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate rich rocks abstract the anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust crenulation cleavage is one of the most common microstructural fabrics found in multiply deformed phyllosilicate rich crustal rocks we calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development an initial planar foliation a moderately developed crenulation cleavage and a well developed crenulation cleavage mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method the difficulties involved with sample preparation and data acquisition of phyllosilicate rich rock samples are also discussed we compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map namely voigt and reuss averages these averages do not account for grain scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic with p wave anisotropies up to 30 9 and s wave anisotropies up to 34 2 whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic with a p wave anisotropy of 15 5 and s wave anisotropy of 10 7 progressive development of the fabric also causes the orientations of p and s wave velocity maxima and s wave polarization directions to change markedly despite the high anisotropy imparted by a planar schistosity the variety of folds and fabrics typically found in phyllosilicate rich rocks within larger scale crustal volumes will tend to mute the anisotropy possibly to the point of appearing nearly isotropic

interpreting magnetic fabrics in amphibole bearing rocks abstract if amphibole is a major constituent in a rock its magnetic fabric can be largely controlled by the crystallographic preferred orientation of amphibole this study describes the para magnetic anisotropy in two amphibolites both containing ca 70 hornblende with rather strong crystallographic preferred orientation both amphibolites display a significant and well defined anisotropy of magnetic susceptibility with the minimum susceptibility approximately normal to foliation however in one amphibolite the maximum susceptibility is parallel to the lineation whereas in the other it is not this seemingly inconsistent observation can be explained by the intrinsic susceptibility anisotropy of single crystals of hornblende and their texture in the rocks numerical models show how the principal susceptibility axes relate to macroscopic foliation and lineation for point and fiber textures this study underlines the potential of using magnetic anisotropy to obtain information about mineral fabrics in mafic rocks at the same time it highlights the necessity for taking into account single crystal properties of the mineral s responsible for the anisotropy and their crystallographic preferred orientation when interpreting magnetic fabrics

a model of metal silicate separation on growing planets abstract the thermal evolution of planets during their accretionary growth is strongly influenced by impact heating the temperature increase following a collision takes place mostly below the impact location in a volume a few times larger than that of the impactor impact heating depends essentially on the radius of the impacted planet when this radius exceeds 1000 km the metal phase melts and forms a shallow and dense pool that penetrates the deep mantle as a diapir to study the evolution of a metal diapir we propose a model of thermo chemical readjustment that we compare to numerical simulations in axisymmetric spherical geometry and with variable viscosity we show that the metallic phase sinks with a velocity of order of a stokes velocity the thermal energy released by the segregation of metal is smaller but comparable to the thermal energy buried during the impact however as the latter is distributed in a large undifferentiated volume and the former potentially liberated into a much smaller volume the diapir and its close surroundings a significant heating of the metal can occur raising its temperature excess by at most a factor of 2 or 3 when the viscosity of the hot differentiated material decreases the proportion of thermal energy transferred to the undifferentiated material increases and a protocore is formed at a temperature close to that of the impact zone

high spatial resolution models of time dependent layered mantle convection summary the degree of geochemical mixing that occurred during the mantle s early evolution is dependent on the history of convective motions in the mantle we present time dependent tnodels of mantle convection that show for certain conditions in the newly formed earth s mantle a transient phase of layered convection results during which geochemical differences between the upper and lower mantle could have been preserved the basic model assumes that the initial temperature profile for subsolidus mantle convection is strongly superadiabatic the mantle is represented by a constant viscosity boussinesq fluid in a 2 d cartesian box heated from the bottom the models have considerably greater spatial resolution 90 x 90 uniformly spaced grid points than our previous work parameters that have been vaned include the rayleigh number ra lo5 lo8 the aspect ratio 1 4 9 the buoyancy of the initial temperature profde and the maximum temperatures allowed for subsolidus convection the models support our previous assertion that there is a critical value of ra above which superadiabatic initial conditions result in a burst of convection that inverts the temperature gradient in the bulk of the mantle sufficiently to temporarily prohibit a single layer of convection instead two layers result this critical value is about ra lo6 for a mantle with the maximum amount of buoyancy for less buoyant initial conditions this critical value is somewhat higher the layering lasts on the order of lo9 yr for ra lo6 10 considering that ra for the early mantle was certainly much larger than lo6 this suggests that a transient phase of multiple layering may have prevented vigorous convection from geochemically homogenizing the early mantle however if a less buoyant initial condition prevailed or one where supersolidus convection controls the maximum temperature of the upper mantle the transient layering could not have taken place

quartz deformation across interlayered monomineralic and polymineralic rocks a comparative analysis abstract given the increased availability of ebsd and fabric analyzer facilities quartz microstructural and cpo analysis have become tools of the trade for studies investigating crustal tectonics therefore improved understanding of the complexities of quartz deformation is needed to ensure that such analyses distinguish primarily between tectonic outcomes and artefacts of the quartz deformation behaviours in different deformed rock volumes we contribute to this understanding by comparing quartz deformation between monomineralic rocks which have simple compositions and polymineralic rocks which are more complex and representative of the crust mylonite quartzites and gneisses of the main central thrust nw himalaya are used for comparison under similar deformation conditions quartz inside monomineralic rocks forms well developed cpos and dynamically recrystallized microstructures typical of grain size insensitive gsi creep mechanisms quartz inside polymineralic rocks by contrast exhibits weak cpos and fine grained microstructures commonly interpreted as grain size sensitive creep but also exhibits lattice distortions e g undulose extinction sub grain boundaries associated with gsi creep we postulate that intrinsic factors such as interphase boundaries cause these ambiguous results the lack of clear evidence for deformation mechanisms in polymineralic rocks casts doubt on the reliability of quartz microstructures and cpos for rapid diagnoses of their rheology and broader tectonic significance

texture analysis of a muscovite bearing quartzite a comparison of some currently used techniques abstract four well established techniques were applied to determine the mineral textures of a muscovite bearing quartzite 1 directional measurements on an optical u stage 2 x ray 3 neutron and 4 electron diffraction ebsd techniques 1 and 4 are of the single grain type and techniques 2 and 3 of the statistical or volume type experimental pole density diagrams were compared by means of the construction of pole figure differences difference pole figures which led to the observation that ebsd and u stage derived pole figures agree well even in detail in contrast pole figures derived from x ray and neutron diffraction are clearly different from pole figures derived from the single grain techniques visible as pronounced preferred orientation in the difference pole figures specific properties of the applied techniques may be responsible for the observed differences such as 1 missing proportionality to the grain volume in the single grain methods 2 the accessible sample volume 3 erroneous data correction or 4 statistical errors also the method of data treatment which is basically different for the single grain and statistical methods should be considered when pole figures are evaluated apart from purely economical constraints and availibility of equipment the decision on the most suitable method for a texture determination should be based on the scientific goals and specific properties of particular techniques texture measurements of the statistical type are well suited for determination of bulk textures of rocks e g as required for the calculation of anisotropic physical properties of rocks whereas single grain measurements are advantageous for the investigation of local textures and texture forming mechanisms

characterization of transpressive deformation in shear zones of the archean north caribou greenstone belt nw superior province and the relationship with regional metamorphism abstract the 2 7 3 0 ga north caribou greenstone belt ncgb host to the musselwhite bif hosted gold deposit possesses abundant shear zones on its northern margins which appear to have formed under amphibolite facies conditions protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during crustal amalgamation of the western superior province and are responsible for folding the ore hosting bif and channeling fluids the importance of shear zones in behaving as conduits for fluids during the tectonic evolution of the ncgb is not well known and their relationship with metamorphism is equivocal yet higher grade syn to post tectonic metamorphic minerals seem to correlate with loci of higher strain structural analyses support oblique transpressive collision that produced steeply dipping planar and shallowly plunging linear fabrics with dominant dextral kinematics that trend broadly parallel to the doubly arcuate shape of the belt electron backscatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize crustal conditions during transpressive deformation the dinnick lake shear zone cuts through mafic metavolcanics and at its core is an l tectonite granite composed of recrystallized quartz whole rock geochemistry shows little variation in ca na mg and k often used as indicators of hydrothermal alteration from surrounding less deformed units suggesting deformation in a dry environment microstructural analysis indicates subgrain rotation recrystallization and deformation by prism a and c slip in quartz as well as aligned hornblende that suggest deformation temperatures above 500 c quartz in mafic rocks along the margins of the shear zone also exhibits a basal a slip component indicating a slight decrease in strain or temperature although the ncgb exhibits some first order evidence of vertical tectonism dome and keel geometries the dominant strain record within shear zones is that of horizontal oblique transpressive displacement this is in agreement with other greenstone belts in the western superior province where vertical tectonism and horizontal tectonism were coeval