separation of high frequency pn phases and mantle refracted p phases at distances between 6 and 18 in the western pacific by ocean bottom seismograph array observations of high frequency hf pn phases by an oceanbottom seismometer obs array were made at distances between 6 and 18 in the north western pacific at this distance range mantle refracted p phases arrive earlier than hf pn phases these two phases are successfully separated for near surface earthquakes by applying appropriate filtering since the dominant frequencies of hf pn phases are higher than 6 hz and the frequencies of mantle refracted p phases are rather low about 3 hz the apparent velocities of hf pn phases in this distance range are estimated to be from 8 3 to 8 1 km sec or even lower while those of mantle refracted p waves are from 8 4 to 8 6 km sec the reported value of 8 33 km sec for hf pn phases appears to be an average of both hf pn and mantle refracted p phases therefore these observations suggest that a proposed additional hf pn wave guide for the higher velocity of 8 33 km sec is questionable

pure shear and simple shear calcite textures comparison of experimental theoretical and natural data abstract the pattern of lattice preferred orientation texture in deformed rocks is an expression of the strain path and the acting deformation mechanisms a first indication about the strain path is given by the symmetry of pole figures coaxial deformation produces orthorhombic pole figures while non coaxial deformation yields monoclinic or triclinic pole figures more quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models for this comparison a representation in the sensitive three dimensional orientation distribution space is extremely important and efforts are made to explain this concept we have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the taylor model we were able to simulate the observed changes with strain history coaxial vs non coaxial and the profound texture transition which occurs with increasing temperature two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results a marble from the santa rosa mylonite zone in southern california displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear a limestone from the tanque verde detachment fault in arizona has a monoclinic fabric from which we can interpret that 60 of the deformation occurred by simple shear

vorticity analysis in calcite tectonites an example from the attico cycladic massif attica greece abstract although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited we use new micro structural petrofabric and vorticity data to analyse the kinematics of flow in outcrop scale calcite mylonite zones these zones are genetically related to a crustal scale ne directed ductile thrust basal thrust that emplaced the blueschist over the basal unit during the exhumation of the attico cycladic massif calcite microstructures reveal that the last stage of deformation occurred at temperatures 200 300 c achieved by mild heating which is possibly related with the reburial of the basal thrust s footwall vorticity analyses were based on the degree of asymmetry of calcite c axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis both methodological approaches provide consistent estimates with a simple shear component between 55 and 82 w n 0 76 0 96 the use of the stress axis 1 orientation recorded by twin c axis pairs to quantify vorticity generally gives significantly lower simple shear component comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation related deformation in the nappe pile was steady state

channel bed steps along nahal yael negev desert israel abstract the height and spacing of channel bed steps formed in boulder and bedrock in the ephermeral channels of the nahal yael watershed show a strong correlation with the channel slope the distance between steps is inversely proportional to slope at slopes of 0 to 20 but then remains fairly constant at higher slopes step height increases proportionally with channel slope these bed steps are similar to those described on perennial channels elsewhere where step formation has been attributed to the effects of high flows which submerge the clasts forming the steps however the ephemeral flows in the tributary channels of the nahal yael system are not sufficient to submerge the clasts forming the boulder steps using rouse s criteria channel bed steps are creating maximum flow resistance at slopes of 12 to 50 along the channels of nahal yael using davies criteria bed steps are creating maximum flow resistance at slopes of 3 to 10 the bed steps also relate to sediment transport nahal yael has a low sediment yield by world standards and the increase in flow resistance due to step formation may serve to increase sediment movement both the boulder and bedrock steps at nahal yael are hypothesized to reflect the scales of flow turbulence along the channels we were unable to evaluate the role of flow volume or boundary roughness in controlling these scales but channel slope appears to exert some control on the scale of turbulence based on the correlation between slope and bed steps the hypotheses developed from field evidence at nahal yael should be testable in flume simulations

shear zones developed in an experimentally deformed quartz mylonite specimens of an experimentally deformed quartz mylonite show the development of shear zones these shear zones are of two types type a shear zones are discrete and within the specimen whereas type b shear zones occur where the specimen has been sheared past the corner of the piston type a shear zones are comprised of elongate recrystallized grains with long axes oriented 30 40 to the shear zone boundary type b shear zones consist of strongly flattened and recrystallized ribbon grains augen occur where the deformation history is coaxial around the piston corner where the deformation history is non coaxial only strongly flattened grains occur the c axis fabrics which develop in the shear zones comprise girdles or maxima approximately normal to the foliation defined by the grain elongation shape fabric an asymmetry with respect to the shape fabric is present in many of the fabrics this asymmetry results from either using a quartz mylonite with a strong preferred orientation as the starting material or as a result of the non coaxial deformation where it can be demonstrated that the asymmetry results from the non coaxial deformation the asymmetry is a leading edge with respect to the sense of shear the results of this study suggest that care must be taken in interpreting asymmetric quartz c axis fabrics in natural shear zones particularly if it is suspected that the protolith to the shear zone had an initially strong preferred orientation type a shear zones originate as brittle zones extending from the specimen corners which become recrystallized as pressure and temperature are increased these shear zones show initial non coaxial deformation dominantly simple shearing followed by later flattening parallel to the shortening direction type b shear zones form when the piston punches into the barreling specimen the strain history of these shear zones is characterized by initial flattening up to 30 followed by shearing past the piston corner

sub moho seismic profile in the mariana basin ocean bottom seismograph long range explosion experiment abstract ocean bottom seismograph obs long range explosion experiments were carried out in the mariana basin in 1973 and 1976 seven large shots 8 5 1 5 ton as well as several tens of small shots were fired the maximum range of observation was about 1900 km as many as 25 obs stations were deployed in an array of about 800 km it is found that the sub moho p wave velocity structure is of stratified nature being composed of alternating high and low velocity layers high velocity layers with apparent velocities of 8 1 8 2 8 4 8 6 and 8 7 km s are identified low velocity layers sandwiched between the high velocity layers of 8 4 8 6 and 8 7 km s are very prominent the sub moho high velocity lid with an apparent velocity of 8 4 km s is very thin thinning of this lid thickening of the low velocity layer and the presence under it of another high velocity layer 8 6 km s appear to characterize the uppermost mantle structure beneath the mariana basin

the geomorphic significance of step pools in mountain streams abstract this paper develops a conceptual model to define the changing role and significance of step pools as energy dissipators in steep mountain channels although energy dissipation is a significant function of step pools this role changes over time with variations in discharge steps are effective in reducing stream energy at low flows but their effectiveness diminishes with increasing stage accordingly the manner in which energy dissipation occurs also varies with increasing flows spill resistance gives way to a dominance in form and grain resistance the conceptual model for the changing role of step pools is illustrated with data from hydraulic analysis and modeling of step pools in the santa monica mountains of california the model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics the model offers a new articulation for the geomorphic significance of step pools in mountain streams and it serves as a useful template for a more complete understanding of step pools over a longer time scale

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

texture development of calcite by deformation and dynamic recrystallization at 1000 k during torsion experiments of marble to large strains abstract torsion deformation experiments were performed on solid cylinders of carrara marble at high temperature 1000 k and constant twist rate about 4 10 4 rad s 1 to large twist angles between 80 and 840 these conditions correspond to simple shear deformation at constant shear strain rate 3 10 4 s 1 and variable amounts of shear strain of 1 2 5 and 11 at the outer mantle of the sample cylinders lattice preferred orientations lpo were measured on polished thin sections using automated electron backscatter diffraction ebsd to analyze the microtextures in orientation imaging micrographs oim shear deformation produced first a microstructure of elongated grains with shape and orientation close to the finite strain ellipse of the imposed shear strain 1 and 2 a deformation texture developed with monoclinic sample symmetry and an oblique c axis distribution relative to the shear plane with increasing shear the marble recrystallized by subgrain rotation and nucleation of small grains starting along grain boundaries which ultimately replaced the whole deformation microstructure a steady state recrystallization microstructure evolved with nearly equant grains of about 10 m in size 5 and 11 the lpo changed completely from the monoclinic deformation texture into a distribution with a sharp single orientation component with r 10 1 4 parallel to the shear plane and a 1 2 1 0 parallel to the shear direction this recrystallization texture has orthorhombic symmetry with respect to shear plane and shear direction during the transition the applied flow stress decreased simple shear deformation of calcite was modeled with the self consistent polycrystal plasticity theory it included a model for dynamic recrystallization based on a balance between growth and nucleation model results agree with experimental data if slip on r 10 1 4 a 1 2 1 0 is introduced as a potential slip system for calcite in that case the steady state recrystallization texture has an easy slip preferred orientation the hypothesis remains speculative until this slip system is actually confirmed by direct observation

strength characteristics of the r f and c slip systems in calcite abstract numerical modelling of crystallographic preferred orientation or texture development in plastically deformed materials requires specification of all potential slip and twinning systems and their relative strengths or critical resolved shear stresses crrs it is widely believed that the most important intracrystalline deformation systems in calcite are twinning on e 1 018 40 4 1 slip on r 10 1 4 2 021 and slip on f 1 012 2 2 01 the superscripted signs indicating slip sense it is these systems which have been used in modelling texture development in calcite polycrystals to date however modelling has been limited by insufficient and partly conflicting data on the relative strengths of the various slip and twinning systems in calcite deformation experiments on calcite single crystals performed by us in recent years taken together provide new data for a more complete and consistent quantification of the strength properties of the calcite glide systems in our experiments single crystals of calcite were compressed in both the 4041 and 2243 orientations at temperatures and constant strain rates in the range 300 800 c and 3 10 4 3 10 8 s 1 respectively the stress strain curves obtained mostly show two yield points slip line analysis shows that these yield points are related to the onset of macroscopically observable slip first on an r 2 021 system and then on an f 10 1 1 or c a system the latter two systems dominate at higher temperature 600 c at strain rate 3 10 5 s 1 and have not been taken into account in any texture modelling so far with increasing absolute temperature tabs calculated crss values c for the r f and c systems decrease in a manner which can be described by empirical power law functions of the type c a tabs b where b r 2 021 and f 10 1 1 systems the yield stresses for r 2 021 and f 10 1 1 slip were found to be rather insensitive to strain rate with conventional power law stress exponents 12 comparing the present results with previous data we conclude that two regimes of slip system activity exist namely a low temperature regime involving e twinning slip on r 2 021 and on f 2 2 01 positive direction not reported and a high temperature regime with r 2 021 f 10 1 1 and c a slip this slip system transition occurs at t 400 c at strain rate 10 4 10 5 s 1 and is expected to be associated with one or more texture transitions the results point to a clear need for renewed texture modelling of calcite polycrystals

the periodic nature of step pool mountain streams rhythmic sequences of steps and pools in steep mountain streams have captured the attention of numerous workers but whether periodicities exist in the occurrence of step pools has not been investigated in a comprehensive way and the implications for such periodicities have not been explored spectral analysis was applied to bed elevation data from streams in the santa monica mountains of southern california to investigate periodicity in the occurrence of step pools results indicate significant step pool periodicities confirmed by independent field measurements that vary from 0 43 channel widths to 2 40 channel widths spectral analysis of channels affected by external influences tends to indicate multiple significant peaks that reflect higher variance in step pool spacing underlying periodicities are nevertheless de tected suggesting that external factors obscure but do not destroy the effects of more general internal mechanisms periodicity in the occurrence of step pools suggests that step pools are bedforms that represent a fundamental mutual adjustment between flow channel morphology and energy expenditure a rhythmic channel thalweg supports the notion that step pools are analogous to meandering in the vertical dimension and implies that because step pool channels are not able to adjust energy expenditure in the plane dimension they instead adjust boundary roughness in the vertical dimension a continuum of rhythmic gravel bedforms that displays the results of such adjustments and that shows how step pools merge into pools and riffles in the stream channel system is discussed and illustrated with empirical data from the santa monica mountains although many differences exist between steep mountain streams and broad alluvial rivers the findings of this study suggest that generalized principles of uniform energy expendi ture may also apply to rhythmic step pool streams these findings are potentially valuable in the management and design of steep channels

combining natural microstructures with composite flow laws an improved approach for the extrapolation of lab data to nature so far rheological predictions that are based on experimentally derived flow laws appear inconsistent with microstructural observations in natural calcite mylonites because grain size sensitive gss diffusion creep is predicted while microstructural criteria suggest grain size insensitive gsi dislocation creep as the dominant deformation mechanism a way out of this discrepancy between experiment and nature can be found by combining full grain size distributions with composite gsscgsi flow laws we applied this approach to natural carbonate mylonites from the helvetic alps switzerland for steady state microstructures our calculations indicate an increasing gss component with increasing temperature at geologically constrained strain rates of 10 k10 10 k11 s k1 the modeling results are consistent with microstructural observations of the natural mylonites in these rocks dynamically steady state microfabrics show a systematic change in grain size grain size distribution and grain aspect ratio with temperature while crystallographic preferred orientations remain of similar strength when single value mean grain sizes are used rather than grain size distributions rheological modeling fails to give reasonable results further paleostresses estimated from conventional recrystallized grain size piezometers were found to be unrealistically high but were reduced to 80 100 mpa when results of the composite modeling were used hence combining composite flow laws with microstructural data from natural mylonites forms a promising approach for a better extrapolation of experimental data to nature and therefore provides new insight into changes in rheology during large scale geodynamic processes q 2005 elsevier ltd all rights reserved

the p wave structure of the lithosphere asthenosphere in the western pacific a comparison of ocean versus continent abstract in this paper we present the lithosphere asthenosphere p wave velocity structures in the western pacific as determined by ocean bottom seismometer array observations to illustrate the contrasts and similarities between continental and oceanic regions these structures are compared with representative velocity structures beneath continental shield regions and the tectonically active regions the subcrustal lithosphere in the western northwest pacific basin possesses a layered p wave structure apparent surface velocities are 8 0 8 4 and 8 6 km s and extends to a depth of about 149 km the lithosphere is underlain by a low velocity layer which appears to be the major transition zone from the lithosphere to asthenosphere this velocity structure is similar to that of the continental shield regions in the east mariana basin the uppermost mantle layer below the moho is about 30 km thick with a thin high velocity lid 8 3 km s at its base these layers form the subcrustal lithosphere below which several alternating low and high velocity layers together form a thick asthenosphere the upper mantle structure beneath the east mariana basin seems to show an intermediate feature between continental shield regions and tectonically active regions the differences between the upper mantle structure of the continent and ocean could be regarded as aspects of an evolution of upper mantle the western northwest pacific basin could be regarded as being almost matured while the east mariana basin could be regarded as being rejuvenated

calcite textures examples from nappes with strain path partitioning abstract strain path partitioning partitioning of deformation into components of pure and simple shear in two alpine nappes is qualitatively indicated by a variation in orientation of foliation microkinematic criteria quartz textures and a comparison with strain path partitioning in theoretical nappe models the degree of non coaxial deformation relative to the total deformation is quantified by using the asymmetry of textures in calcite tectonites the texture interpretation is based on both polycrystal plasticity models and experimental data calcite tectonites in the graz paleozoic thrust sheet austria and the morcles nappe switzerland were deformed in a strain path with both pure and simple shear components texture asymmetry supports the strain path derived from geological observations as an exception a high strain ultramylonite deformed nearly exclusively in pure shear it is suggested that this represents a softening instability since deformation in pure shear is energetically more favorable than in simple shear the incompatibilities caused by heterogeneous pure shear are accommodated by detachments and heterogeneous simple shear adjacent to the pure shear zone

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

bed stability variations after check dam construction in torrential channels south east spain the effects of check dams on the bed stability of torrential channels have been analysed in several tributary basins of the segura and guadalentin rivers south east spain in order to illustrate the large variability in channel bed forms and bed sediment sizes along the stream 52 reaches of 150 m in length were surveyed this variability is due to the behaviour of check dams which depends on bedrock control bed slope channel roughness lateral sediment input and a highly variable sediment transport capacity though the purpose of check dams is to diminish the boundary shear stress reducing the longitudinal slope and to stabilize the channel bed downstream they reduce the volume of channel stored material favouring local scour processes and upstream they can destabilize the sidewalls the results enable us to evaluate the impact of every check dam on the bed morphology distinguishing the structures installed in limy marl areas e g catchment of the carcavo rambla cieza and in schist and slate terrains e g catchment of the torrecilla rambla close to lorca in the first type bedrock and moderately thick granular beds predominate downstream from the check dams so that the length of bedrock reaches and increase of roughness due to scour processes are the best indicators to verify its geomorphological effectiveness on the other hand the metamorphic areas drained by ramblas and gullies produce great quantities of gravel that are retained by check dams creating more uniform and permeable beds where the balance between sedimentation and scouring and the ratio c84 0 rbs appear to be the parameters most frequently adopted to estimate the bed stability analysis of slope adjustments and the application of other indices to estimate the bed substrate stability lrbs sri and the structural influence of the dams sibs corroborate the differences in bed stability found in the corrected reaches in each catchment copyright 2007 john wiley sons ltd

the influence of crystallographic orientation and grain boundary migration on microstructural and textural evolution in an s c mylonite abstract the analysis of the subgrain and dislocation sub structure present in grains in different crystallographic orientations within a domainal s c mylonite is reported the study reveals a range in subgrain size and development which is related to the crystal orientation of the host grain in the mylonite subgrains in grains with easy slip orientations contain large subgrains while grains in more complex slip orientations contain small subgrains the dislocation densities preserved in the mylonite are extremely variable even within individual subgrains and may be the result of a late stress pulse the analysis indicates that grain boundary migration was an important process in the microstructural evolution initiating the selective removal of some orientations and thus influencing the final crystallographic preferred orientation present in different domains the data collected is also used to assess the potential stability and preservation of microstructures during the non steady state deformation expected at the end of a deformation event

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

vorticity determination distribution partitioning and the heterogeneity and non steadiness of natural deformations abstract an investigation on vorticity in general heterogeneous and non steady flows is presented the bulk vorticity of a flow is solely determined by the imposed boundary conditions and it exerts no constraints on local flow the distribution of bulk vorticity in different rheological domains and the partitioning of the distributed vorticity into shear induced vorticity and spin are both time dependent this indicates that heterogeneous flows are inevitably non steady in a quantitative example it is demonstrated that even though the imposed boundary movement is a constant simple shear local flow can be non steady spinning and non coaxial super simple shear flow with internal kinematic vorticity number greater than i occurs but the finite strain does not necessarily pulsate the geological implications of this investigation are discussed the significance of heterogeneity and non steadiness in natural deformations are emphasized it is concluded that vorticity distribution and partitioning is the link between different scale structures the internal kinematic vorticity number is not a criterion of finite strain oscillation

antidunes on steep slopes 1 when rates of subcritical flow are increased on gentle slopes the bed successively produces ripples dunes and flat beds antidunes defined here as all bed undulations for which the surface gravity waves are in phase with the bed profile appear only in high flow rates and may be found in some extreme natural flow events inversely on steep slopes flume experiment s 1 approximately flows are supercritical and antidunes are observed to appear just after the beginning of sediment motion and to disappear for high flow rates using data from new experiments we seek to improve the prediction of antidune geometry on steep slopes an equation for antidune wavelength was deduced from dimensional analysis and fitted to new experimental data the equation was successfully evaluated using a data set extended to 167 values with data from the literature obtained on both steep and gentle slopes this equation gives results very similar to the usual analytical equation from kennedy when tested on the results from gentle slope experiments but proved to be better adapted for antidune wavelength on steep slopes

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

mechanisms of dynamic recrystallization in naturally deformed dolomite inferred from ebsp analyses abstract in order to investigate the mechanisms of dynamic recrystallization in dolomite single grain orientation measurements on a mantled porphyroclast have been carried out by means of electron back scattered patterns ebsps the local orientations in the grain fabric were analysed to establish meaningful orientation families in addition to a twinned grain the porphyroclast consists of two domains each with a specific orientation pattern both domains are polygonized by subgrain rotation which gives great and small circle dispersion patterns of the crystal orientation data in the pole figures however even the simple and well defined dispersion patterns for which one can assume a single slip system are not consistent with any of the theoretical dispersion patterns of the known slip systems additional mechanisms like rigid body rotation nucleation and or combinations of different slip systems are the likely explanation no definitive orientation relationship has been found between the porphyroclast and the mantle the lack of such a relationship together with the shape of the grain boundary point to grain boundary migration the texture of the mantle agrees with the global texture of the sample indicating that mechanisms of crystal plasticity were active during flow around the clast the results demonstrate the potential of ebsp data for the deduction of active mechanisms but a proper interpretation requires a better theoretical foundation this could be achieved by modelling dispersion patterns for different mechanisms and different combinations and by including orientation correlations

channel units in small high gradient streams on vancouver island british columbia descriptions and a classification are presented of channel units in very high gradient first order mountain channels bedrock cascades boulder cascades rapids chutes riffles glides and pools had distinct mean bed gradients of 0 49 0 45 0 20 0 12 0 09 0 06 and 0 03 respectively multiple pairwise comparisons showed that these gradients are significantly different except bedrock cascades were not different from boulder cascades and glides were not different from chutes riffles or pools channel width and entrenchment do not discriminate channel units in any consistent way but characteristic substrates occur constituent substrates of bedrock and boulder cascades are nominally apparent while chutes also are bedrock bound mixtures of boulders and cobbles constitute rapids while mixtures of cobbles and gravel constitute riffles glides and pools generally are composed of boulders cobbles and gravel in various proportions log steps sometimes delimit units particularly pools but wood is not a dominant element in these channels gradients of channel units in these small first order dominantly nonalluvial channels are significantly higher than correspondingly identified units in larger channels farther down the drainage system the reason for this distinction is supposed to be related to channel scale hence potential stream power

thermo kinematic evolution of the annapurna dhaulagiri himalaya central nepal the composite orogenic system the himalayan orogen represents a composite orogenic system in which channel flow wedge extrusion and thrust stacking operate in separate orogenic domains with distinct rheologies and crustal positions we analyze 104 samples from the metamorphic core greater himalayan sequence ghs and bounding units of the annapurna dhaulagiri himalaya central nepal optical microscopy and electron backscatter diffraction ebsd analyses provide a record of deformation microstructures and an indication of active crystal slip systems strain geometries and deformation temperatures these data combined with existing thermobarometry and geochronology data are used to construct detailed deformation temperature profiles for the ghs the profiles define a three stage thermokinematic evolution from midcrustal channel flow stage 1 7008c to 550 6508c to rigid wedge extrusion stage 2 400 6008c and duplexing stage 3 280 4008c these tectonic processes are not mutually exclusive but are confined to separate rheologically distinct orogenic domains that form the modular components of a composite orogenic system these orogenic domains may be active at the same time at different depths positions within the orogen the thermokinematic evolution of the annapurna dhaulagiri himalaya describes the migration of the ghs through these orogenic domains and reflects the spatial and temporal variability in rheological boundary conditions that govern orogenic systems

uppermantle anisotropy and the oceanic lithosphere rayleigh wave and love wave dispersion data for oceanic paths cannot be satisfied by an isotropic uppermantle and it is incorrect to invert these two datasets independently for separate isotropic structures available phase and group velocity data are inverted for oceanic structure as a function of age taking into account anelastic dispersion sphericity and anisotropy the resulting models are quite different from previous results which ignore the above effects and the extra parameters involved in proper anisotropic inversion the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age and an anisotropic low velocity zone with an age dependent anisotropy the lid or seismic lithosphere reaches a maximum thickness of 50 km which is about one half the thickness obtained in previous isotropic or pseudo isotropic inversions the seismic lithosphere therefore may be comparable in thickness to the elastic or flexural lithosphere raising the possibility that both are controlled by effects other than temperature stress and time such as mineralogy crystal orientation or partial melting the velocities in the low velocity zone are higher than inferred by previous surface wave studies

natural calcite c axis fabrics an alternate interpretation microstructures and crystallographic fabrics in natural calcite mylonites from newfoundland canada are discussed and compared with other natural and experimental examples in the newfoundland rocks porphyroclasts deformed mainly by twinning and intracrystalline slip and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration t 350 c porphyroclast c axis fabrics consist of a broad point maximum oriented normal to the shear plane recrystallized grains in the ultramylonites have strong c axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane experimental simple shear calcite c axis fabrics have been divided by previous workers into low lt and high temperature ht fabrics twinning and intracrystalline r slip have been identified as the dominant deformation mechanisms in low temperature experiments whereas intracrystalline r and f slip dominate in high temperature experiments lt fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane ht fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane the transition between lt and ht fabrics occurs at temperatures lower than 400 c in fine grained limestones and lower than 700 c in marbles at laboratory strain rates natural calcite c axis fabrics in agreement with study almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane because their fabrics are similar to experimental lt fabrics they have been interpreted as lt fabrics by comparison with experimental and computer simulated lt fabrics the obliquity of their c axis point maximum with respect to the shear plane normal has been used as a shear sense indicator and as a measure of the degree of non coaxiality of the deformation two lines of evidence mitigate against calcite c axis fabrics as shear sense and strain path indicators in mylonite zones first the obliquity of the c axis point maximum in natural examples is not consistent for a given sense of shear secondly mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c axis fabrics of most calcite mylonites therefore natural calcite mylonite c axis fabrics do not generally correlate with either lt or ht experimental fabrics instead natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures 800 850 c and underwent pervasive dynamic recrystallization by grain boundary migration consequently c axis fabrics of calcite mylonites cannot be used as a measure of the degree of non coaxiality of the deformation nor can they be used as shear sense indicators

dimensions of standing waves at steps in mountain rivers 1 oscillations of the water surface are common features of flows around critical conditions fr 1 and of both natural and structure induced undular hydraulic jumps the paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers the purpose of the research is to analyze what parameters affect the phenomenon to provide semiempirical formulations for wave dimensions and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive the dimensionless wavelength and wave amplitude were correlated to the froude number at the sill the first wavelength appears linearly related to the froude parameter whereas the amplitude of the first wave shows a less defined behavior possibly indicating that a maximum value occurs for sill froude numbers around 1 6 1 7 the corresponding wave steepness i e wave amplitude length results between 0 15 0 16 and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum probably related to hydrodynamic conditions just before wave breaking the transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams and the hydraulic and morphological implications of this finding are also discussed with particular emphasis on step stability and step wavelength

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

strain dependent variation of microstructure and texture in naturally deformed carrara marble this study investigates the microstructure and texture variations across a mm scale shear zone in carrara marble of the alpi apuane italy the microstructures have been investigated for grain size texture and shape fabrics textures have been measured with computer integrated polarization microscopy cip and electron back scattered diffraction ebsd separating porphyroclast and recrystallized grains the deformation which post dates an earlier deformation phase and subsequent annealing is strongly localized the microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation the bulk shear strain rate is estimated to be about 10 11 sec 1 at deformation temperatures of approximately 325 c 30 c the protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone core mantlestructures are produced by dominant rotation recrystallization accompanied by some grain boundary migration the microstructural transition from protomylonite to mylonite coincides with a change in texture with increasing strain the single c axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation maximum normal to the shear plane which is consistent with dominant basal hai and r h 2201i slip the microstructural and textural variations depend on the proportion of recrystallized grains as dynamic recrystallization progresses with finite strain the texture development is finite strain dependent the comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization 2006 elsevier ltd all rights reserved

deep structure of the east european platform according to seismic data abstract the paper presents a review and analysis of new seismic data related to the structure of the mantle beneath the east european platform analysis of observations of long range profiles revealed pronounced differences in the structure of the lower lithosphere beneath the russian plate and the north caspian coastal depression the highest p velocities found at depths around 100 km are in the range 8 4 8 5 km s 1 deep structure of the baltic shield is different from the structures of both these regions no evidence of azimuthal anisotropy in the upper mantle was found a distribution of p velocity in the upper mantle and in the transition zone consistent with accurate travel time data was determined the model involves several zones of small and large positive velocity gradients in the upper mantle rapid increases of velocity near 400 and 640 km depths and an almost constant positive velocity gradient between the 400 and 640 km discontinuities the depth of the 640 km discontinuity was determined from observations of waves converted from p to sv in the mantle

new u pb zircon age constraints on the timing of deformation in north central newfoundland and implications for early paleozoic appalachian orogenesis u pb zircon ages of five felsic intrusions in eastern notre dame bay bracket the absolute ages of a number of key units and events in the newfoundland appalachians the age of 507 3 2 ma obtained for the twillingate pluton confirms an age obtained by earlier workers it strengthens evidence of widespread cambrian arc volcanism in newfoundland and places a lower limit on the initiation of deformation in central newfoundland the loon bay suite which includes the loon bay batholith and a swarm of quartz and feldsparphyric tonalite dikes has an age of 408 2 ma this age places a late silurian upper limit on the timing of thrust fauting and three generations of folding in the new world island area of notre dame bay this age also constrains the timing of sinistral ductile movement on the reach fault and of initiation of high angle brittle faulting in the region an age of 422 2 ma for a composite mafic and felsic dike on the port albert peninsula places an upper limit on the age of the botwood group in that area this age also supports the correlation between the botwood group and mixed felsic and mafic igneous rocks of the springdale group in western notre dame bay placed in context with the timing of other units and events in the newfoundland appalachians our ages show that the formation and deformation of tectonic terranes making up the orogen did not take place in an orderly sequential fashion instead at any one time more than one tectonic element was being generated subducted or deformed although the locus of strain varied within the orogen deformation occurred continuously from the late cambrian epoch until the end of the paleozoic era

on the stability of step pool mountain streams although step pools are generally considered to be stable bedforms stability is not absolute but depends on size scale and perspective hydraulic analysis of the stability of step pool sequences in the santa monica mountains california indicates that they are active channel features that are generally restructured within 5 to 100 years the degree of mobility depends on step particle size steps are stable within small temporal and spatial scales where they function as independent variables that dissipate stream energy and regulate channel hydraulics but stability decreases at larger scales where step pools become dependent variables that respond to discharge and sediment load hence the role of step pools changes from energy dissipation to one of channel adjustment these results underscore the need to consider larger spatial and temporal scales in order to reveal the complete function and significance of step pools in mountain fluvial systems

microfabrics of calcite ultramylonites as records of coaxial and non coaxial deformation kinematics examples from the rocher de l yret shear zone western alps microfabrics were analysed in calcite mylonites from the rim of the pelvoux massif western alps france wnw directed emplacement of the internal penninic units onto the dauphinois domain produced intense deformation of an eocene age nummulitic limestone under lower anchizone metamorphic conditions slightly below 300 c two types of microfabrics developed primarily by dislocation creep accompanied by dynamic recrystallisation in the absence of twinning coaxial kinematics are inferred for samples exhibiting grain shape fabrics and textures with orthorhombic symmetry their texture crystallographic preferred orientation cpo is characterised by two c axis maxima symmetrically oriented at 15 from the normal to the macroscopic foliation non coaxial deformation is evident in samples with monoclinic shape fabrics and textures characterised by a single oblique c axis maximum tilted with the sense of shear by about 15 from the analysis of suitably oriented slip systems for the main texture components under given kinematics it is inferred that the orthorhombic textures which developed in coaxial kinematics favour activity of b10 11n and b02 21n slip along the f and r planes respectively with minor contributions of basalban slip in contrast the monoclinic textures which developed during simple shear are most suited for duplex ban slip along the basal plane the transition between the dominating slip systems for the orthorhombic and monoclinic microfabrics is interpreted to be due to the effects of dynamic recrystallisation upon texture development since oblique c axis maxima documented in the literature are most often rotated not with but against the shear sense calcite textures alone should not be used as unequivocal shear sense indicators but need to be complemented by microstructural criteria such as shape preferred orientations grain size estimates and amount of twinning

channel reach morphology in mountain drainage basins a classification of channel reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types colluvial bedrock and five alluvial channel types cascade step pool plane bed pool rime and dune ripple coupling reach level channel processes with the spatial arrangement of reach morphologies their links to hillslope processes and external forcing by confinement ripar ian vegetation and woody debris defines a process based framework within which to assess channel condition and response potential in mountain drainage basins field investigations demonstrate character istic slope grain size shear stress and roughness ranges for different reach types observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad justed to the relative magnitudes of sediment supply and transport ca pacity steep alluvial channels cascade and step pool have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply whereas low gradient alluvial channels pool rime and dune ripple have lower transport capacity to supply ra tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge general differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source transport and response segments the spatial distribution of which provides a watershed level conceptual model linking reach morphology and channel processes these two scales of channel network classification define a framework within which to in vestigate spatial and temporal patterns of channel response in moun tain drainage basins

anisotropy of the mantle inferred from observations of p to s converted waves summary seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km in this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range the idea of the method is to detect and analyse the sh component of the waves converted from p to s in the mantle the procedure of record processing includes frequency filtering axis rotation transformation of the record to a standard form stacking the standardized sh component records of many seismic events and the harmonic analysis of amplitude as a function of the direction of wave propagation when applied to the long period records of norsar the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizontal axis of symmetry our preferred model postulates anisotropy of 1 per cent in a layer 50 km thick at the base of the upper mantle

geophysical constraints on geodynamic processes at convergent margins a global perspective abstract convergent margins being the boundaries between colliding lithospheric plates form the most disastrous areas in the world due to intensive strong seismicity and volcanism we review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure seismicity and geometry of subducting slab we present global maps of free air and bouguer gravity anomalies heat flow seismicity seismic vs anomalies in the upper mantle and plate convergence rate as well as 20 profiles across different convergent margins a global analysis of these data for three types of convergent margins formed by ocean ocean ocean continent and continent continent collisions allows us to recognize the following patterns 1 plate convergence rate depends on the type of convergent margins and it is significantly larger when at least one of the plates is oceanic however the oldest oceanic plate in the pacific ocean has the smallest convergence rate 2 the presence of an oceanic plate is in general required for generation of high magnitude m 8 0 earthquakes and for generating intermediate and deep seismicity along the convergent margins when oceanic slabs subduct beneath a continent a gap in the seismogenic zone exists at depths between ca 250 km and 500 km given that the seismogenic zone terminates at ca 200 km depth in case of continent continent collision we propose oceanic origin of subducting slabs beneath the zagros the pamir and the vrancea zone 3 dip angle of the subducting slab in continent ocean collision does not correlate neither with the age of subducting oceanic slab nor with the convergence rate for ocean ocean subduction clear trends are recognized steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate with slab rotation towards a near vertical dip angle at depths below ca 500 km at very high convergence rate 4 local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs however near isostatic equilibrium may exist in broad zones of distributed deformation such as tibet 5 no systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation magmatic activity crustal faulting horizontal heat transfer and also due to low number of heat flow measurements across many margins 6 low upper mantle vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels

optical microstructure of partly recrystallized calcite in some naturally deformed marbles abstract deformed marbles from the mt lofty ranges south australia have microstructures characterized by deformed relics of calcite interspersed with much finer grained recrystallized calcite and with minor quartz and diopside the deformed calcite relics contain numerous subgrains and show abundant evidence of strain induced migration of twin and grain boundaries new calcite grains have nucleated and grown preferentially along twins especially at twin intersections at grain and kink like boundaries and apparently randomly inside deformed relics optical microstructural evidence suggests that dynamic syntectonic recrystallization has taken place along with some static recrystallization probable mechanisms for the initiation of recrystallization include bulge nucleation and subgrain nucleation

a field investigation of the influence of sediment supply on step pool morphology and stability abstract the effect of the sediment supply on step pool morphology and stability was studied through the investigation of 42 step pool reaches eight in the vosges and 34 in the alps france step pool geometry step wavelength l and height h was measured for each reach and the data set split into two distinct groups comprising 25 connected to a sediment source step pool reaches and 17 disconnected step pool reaches the geometry analysis showed differences between the two groups with the shape factors k l hr and the h hr ratios hr is the residual pool depth being lower for disconnected step pools this was interpreted to result from progressively changing geometry over time with disconnected step pool geometry defining the probable asymptotic characteristic associated with this long term process the measured step pool dimensions were in good agreement with equations based on maximum flow resistance abrahams et al 1995 energy of a falling jet comiti et al 2005 and channel spanning deposition chartrand et al 2011 however the average shape factor for the comiti et al equation was k 17 instead of 8 as the authors had proposed the data set was extended to 255 values with data from the literature which confirmed this average value for k field evidence moss cover and stone imbrication indicate that connected step pools are mobile whereas disconnected step pools are stable this was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are 20 50 years for connected step pools and even less when considering scouring effects and greater than 50 years for disconnected step pools this difference in step mobility was ascribed to the fact that with sediment supply the channel can freely adjust its local slope allowing a relatively efficient transfer downstream of large boulders by scouring effects

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

development of preferred orientation in plane strain deformed limestone experiment and theory carbonate rocks deform preferentially by twin gliding on e 01 18 and slip on r 10 14 and f 02 21 in polycrystalline aggregates strong textures develop we report on experimentally produced textures in triaxial plane strain geometry with orthorhombic symmetry at 200 c and 400 c pole figure of the experimentally deformed specimens are compared quantitatively with theoretical simulations based on the taylor theory using both slip and mechanical twinning as mechanisms agreement at low and high temperature is satisfactory and documents that models developed for f c c metals can be applied to low symmetry minerals provided that deformation mechanisms are known and that mechanical twinning is properly accounted for comparison with experimental results indicates that strain was nearly homogeneous at the conditions considered and the same may apply to many geological textures three texture types are described which are differentiated mainly by the relative importance of e twinning

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

anisotropic models of the upper mantle long period rayleigh wave and love wave dispersion data particularly for oceanic areas have not been simultaneously satisfied by an isotropic structure in this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy anelastic dispersion sphericity and gravity we assume that the surface wave data represents an azimuthal average of actual velocities thus we can treat the mantle as transversely isotropic the resulting models for average earth average ocean and oceanic regions divided according to the age of the ocean floor are quite different from previous results which ignore the above effects the models show a low velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies the correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive for most of the earth sh sv in the vicinity of the low velocity zone neat the east pacific rise however sv sh at depth consistent with ascending flow anisotropy is as important as temperature in causing radial and lateral variations in velocity the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age this layer defines the lid or seismic lithosphere in the pacific the lid thickens with age to a maximum thickness of 50 km this thickness is comparable to the thickness of the elastic lithosphere the lid thickness is thinner than derived using isotropic or pseudo isotropic procedures a new model for average earth is obtained which includes a thin lid this model extends the fit of a prem type model to shorter period surface waves

the evolution of a colluvial hollow to a fluvial channel with periodic steps following two transformational disturbances a wildfire and a historic flood abstract the transition of a colluvial hollow to a fluvial channel with discrete steps was observed after two landscape scale disturbances the first disturbance a high severity wildfire changed the catchment hydrology to favor overland flow which incised a colluvial hollow creating a channel in the same location this incised channel became armored with cobbles and boulders following repeated post wildfire overland flow events three years after the fire a record rainstorm produced regional flooding and generated sufficient fluvial erosion and sorting to produce a fluvial channel with periodically spaced steps an analysis of the step spacing shows that after the flood newly formed steps retained a similar spacing to the topographic roughness spacing in the original colluvial hollow prior to channelization this suggests that despite a distinct change in channel form roughness and bedform morphology the endogenous roughness periodicity was conserved variations in sediment erodibility helped to create the emergent steps as the largest particles d84 remained immobile becoming step features and downstream soil was easily winnowed away

bed morphology and generation of step pool channels flume experiments have been carried out to study the formation processes and the bed morphology of step pool channels from the experiments different step types and step configurations could be distinguished depending on the stream power these step types can be seen as an image of the generation mechanisms of step pool systems these results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution in contrast to a variety of other studies antidunes did not contribute to the generation of the step structures however the data of the presented study fits well into the region of antidune formation proposed by kennedy for sand bed rivers this observation points out that step pool field data located in the kennedy region do not inevitably prove that antidunes played a role in step development it is rather proposed that in kennedy s region of antidune formation there exist hydraulic conditions where the flow resistance is maximized it is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height independently of whether these are antidunes in sand and gravel bed rivers or step pool units in boulder bed streams the considerations of the kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self stabilization by modifying the step pool structure towards a geometry that provides maximum flow resistance and maximum bed stability copyright 2008 john wiley sons ltd

measurements and modelling of possible mantle constituents from a long line seismic refraction experiment in the west philippine basin summary a long line seismic refraction experiment extending to a range of 500 km was conducted in the west philippine sea the refraction data corrected for crustal delays are used to estimate the velocity structure in the upper lithosphere the seismic structure is converted to velocity versus pressure using a simple overburden formula west philippine sea velocity pressure data are compared with possible mantle constituents compositional models indicate that the velocity structure is between eclogite and eclogite dunite type phases listed in christensen 1974 before correcting for temperature however correcting the velocity data for the possible mantle constituents to the steady state temperature estimated for the basin indicates that the west philippine sea upper mantle has much higher seismic gradients than those predicted for any of the single phase components

origin of step pool systems in mountain streams mountain streams very often present a step pool aspect antidune formation alone dispersion and sorting theory or velocity reversal as advocated for the origin of riffle pool sequences do not explain how steps and pools form experiments have been conducted to clarify their origin it was found that the step pool aspect appears after high flows had deformed the bed during the presented tests a self armoring process occurred which resulted in a stabilization of the bed when coarsening of the top layer was little classic antidunes resulted an important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance the substantial increase in resistance was an essential factor in bed stabilization

step pool formation models and associated step spacing step pool bedforms develop under conditions of high flow rates and high sediment influx and transport rates step setting events are capable of mobilizing and rearranging the large step forming grain sizes thus observations of the processes of step formation are limited to flume experiments the results of laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps are presented over 350 step formations were observed and documented creating a large data set for detailing and evaluating step formation processes existing step formation models focus on step development from antidunes particle clusters transverse ribs or depositional berms or from a hydraulic regime that includes cascade flow and hydraulic jumps these models were developed using field measurements of artificial grade control structures or stable step pool systems during low flow conditions or degradational flume experiments where flows elucidated the step pool bedform in the channel profile but did not create equilibrium transport of the large step forming clasts this research evaluates the pre existing formation models and proposes three new models the rough bed exhumation and dune models the rough bed model was dominant throughout the experiments followed by the exhumation model and then the dune model a single model was not responsible for all step formations the rough bed exhumation and dune models all influenced step development and every step sequence was created through a mixture of formation processes this research generated a large data set where each step spacing is paired with the corresponding step formation model making it possible to identify a distinct exclusion zone length for each model thus given knowledge of the step formation model the minimum distance to the next step downstream can be predicted from the final spacing of a step sequence it may be possible to deduce the likely formation mechanisms active during a step resetting event copyright 2007 john wiley sons ltd

evolution of a calcite marble shear zone complex on thassos island greece microstructural and textural fabrics and their kinematic significance abstract the deformation history of a monophase calcite marble shear zone complex on thassos island northern greece is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system shear zone boundary szb strain localization within the massive marble complex is linked to decreasing p t conditions during the exhumation process of the metamorphic core complex solvus thermometry indicates that temperatures of 300 350 c prevailed during part of the shear zone deformation history the coarse grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation a component of heterogeneous non coaxial deformation is first recorded within the adjacent protomylonite enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone where high strain was accommodated by non coaxial flow this study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations single c axis maximum perpendicular to the szb by different dominant deformation mechanisms separated a axis pole figures a and a axis show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry

the morphologic structure of step pools in mountain streams abstract step pool sequences in streams in the santa monica mountains of southern california impart a characteristic morphologic structure that varies with slope but despite the strong association with slope that suggests a direct process control analysis of 464 step pools in 13 study reaches reveals the relationship to reflect correlation but not necessarily causality the direct control on step height is particle size whereas discharge is the probable causal process determining step wavelength because both particle size and discharge generally change with channel slope downstream a slope variation with step wavelength and height is the apparent indirect geometric result a conceptual model of process form linkages is proposed based on these results whereby the step pool morphology is produced by positive relationships between wavelength and discharge and between height and particle size the model offers new insights for explaining the step pool morphology and it is potentially useful in channeling future efforts toward the appropriate scales and variables

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

on the origin of step pool sequences in mountain streams this paper reports a field test of the prevailing antidune theory to explain the mechanisms of step pool formation in steep mountain streams the theory is supported by laboratory data but it has not received extensive field testing that would enable wide applicability to natural conditions data from the santa monica mountains of california are used to test the antidune model where step wavelength and reconstructed hydraulic conditions are compared with the formation domain predicted by experimental and theoretical studies results show general agreement between field and laboratory data step wavelength and hydraulic requirements are consistent with those of the antidune process of formation but results also suggest that true antidunes may be difficult to achieve in the steep headwaters where channels may be dominated by large roughness elements these findings strengthen the link between theory and observation in clarifying how step pools form they contribute to the basis for elucidating why step pools form in mountain streams

high temperature texture in naturally deformed carrara marble from the alpi apuane italy abstract from deformation experiments and numerical modelling a large type variety of crystallographic preferred orientations textures are known for calcite in contrast naturally deformed samples usually show the low temperature lt texture type with only minor texture variations the high temperature ht texture type is rarely described and mostly not very well defined based on neutron diffraction measurements and a quantitative texture analysis by means of the iterative series expansion method and the texture component model this study gives evidence for the ht texture type in a deformed marble from the alpi apuane in italy the microstructure of the sample shows elongated grains with long short axis ratios of up to 10 1 the long axes of the grains are oriented parallel to the general direction of transport indicating prolate strain but no shear sense from the texture a shear sense can be deduced that cannot be fully brought in line with the regional deformation and thermal history the results indicate a larger texture variety of naturally deformed calcite rocks than generally assumed this should stimulate further systematic texture studies for a better understanding of the texture forming mechanisms and the closely connected understanding of the kinematic significance of textures for the analysis of regional deformation histories

characteristic dimensions of the step pool bed configuration an experimental study 1 the dominant dimension of the step pool bed form is its spacing which is determined by the locations at which large step forming grains are deposited we conducted laboratory experiments involving active transport of widely sorted sediment including large grains whose deposition readily formed steps direct observation of step development indicates that deposition of step forming grains was predominantly associated with bed roughness impeding transport or with earlier flow and transport conditions not contemporaneous with step development neither mechanism is likely to occur at regular locations on the bed measured step spacing was found to follow a poisson distribution supporting the conclusion that steps do not develop in preferred or regular locations this contrasts with previous arguments that regular step spacing develops in response to deposition forced at regular locations beneath water surface waves we observe neither the regular distribution nor the associated depositional mechanism although the observed step distribution is well defined with a strong mode that could be interpreted as indicating regular spacing

geometric scaling of step pools in mountain streams observations and implications understanding the geometric structuring of alluvial step pools is an enduring problem in mountain stream geomorphology many previous studies propose that the key control parameter governing the step pool form is mean bed slope as explored through several examples however we find that slope control formulations are inconsistently demonstrated for broad data sets on this basis we reexamine the bed form architecture of step pools with a new descriptive parameter termed the aspect ratio active stream width divided by step drop height we test with data collected from three different mountain settings for step pools observed over mean bed slopes ranging from 2 to 22 we find that step wavelength and height are well described by when these dimensions are normalized by the step drop height furthermore we observe that varies inversely with mean bed slope context for our findings is provided by recent field and laboratory work which highlights that the cross stream component to the overall flow structure in step pools and scour pools is significant this is important because it expands upon and relates our work to formative models which are based on jet scour and the associated three dimensional flow structure of step pools our work has practical implications for the design of step pool channels because existing approaches are based in large part on structuring artificial systems according to a narrowly defined geometry we observe however that under certain conditions this approach may yield constructed step pools that are unstable as an alternative approach we propose that the geometric relationships developed here can be applied to identify artificial step pool geometries that will be more fully reflective of natural analogs and thus possibly more stable

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

experimental investigations of the evolution of step pools in rills with heterogeneous soils in yuanmou dry hot valley sw china abstract as a common type of microtopography in rill channels the formation and dynamics of step pools are prerequisites for the occurrence of rills however the evolution of step pools in a rill sprs is still unclear scouring experiments in field plots were conducted to explore the dynamic evolution of sprs and the effect of steps on pools under conditions of strong runoff and soil like materials aggregated with gravels in yuanmou dry hot valley sw china using three dimensional laser scanning spatial analysis of a gis geographic information system and statistical analysis 11 parameters were used to characterize step pool morphology the results showed that sprs formed after several scourings at particular locations and some of them disappeared in the late stage a diverse range of sprs formed with geometrical parameters initially increasing then stabilizing and finally decreasing during scouring pool shape transformed from a deep v shape to a shallow v shape and then to a u shape the evolution of a typical pool included initial active stable and fading stages there were 59 oval shaped pools 88 06 of the total number of pools in plane form and 37 deep pools 55 22 in cross section and significant differences in the plane form and profile morphology of a pool presented themselves during its evolution sprs developed more easily in dry red soil like materials compared with vertisol like materials the geometrical parameters of pools correlated with step size and pools became deeper with increasing step height and longer with increasing step height and decreasing step slope the spatial heterogeneity of soil like materials as well as hydraulic conditions led to complex and nonlinear spr evolution it is helpful to further understand the mechanism of rill incision and evolution

seismic anisotropy the state of the art ii summary the theory causes observations and possible applications of seismic anisotropy in the earth have developed considerably since the previous state of the art paper was published in 1977 the behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle the top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress field the upper part of the crust is frequently anisotropic probably due to cracks differentially aligned by the non lithostatic stresses the possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities

vorticity analysis in the zagros orogen shiraz area iran quantitative vorticity analyses in orogenic belts are essential for studying the kinematics of deformation and can be performed using a range of methods the combination of microstructural analysis for vorticity with other methods creates a more rigorous analysis in order to determine the degree of non coaxiality and spatial pattern of vorticity during deformation in the zagros orogenic belt a study area containing the boundary of the zagros folded belt and the zagros fold and thrust belt is selected the study area is situated in the shiraz region of e zagros in iran the kinematic vorticity analysis is carried out using 4 methods based on 1 the degree of asymmetry of the calcite c axis fabric 2 the assumption that the orientation of the long axes of calcite within an oblique stylolite foliation delineates the direction of the instantaneous stretching axis 3 the assumption that the tension gash tips determine the direction of the instantaneous stretching axis and 4 stylolite teeth determine the direction of the instantaneous stretching axis c axis data from calcite give a kinematic vorticity number between 0 68 and 0 83 and the orientation of the long axes of calcite grains yields a range between 0 5 and 0 84 stylolites provide a kinematic vorticity number between 0 5 and 0 79 and tension gashes provide a kinematic vorticity number between 0 56 and 0 81 this range of vorticity numbers confirms the contributions of both simple 33 59 and pure shear 41 67 twining of calcite also reveals that the last stage of deformation occurred at a temperature of 170 200 c spatial analysis reveals an increase in the simple shear component from the sw of the zagros folded belt to the ne of the zagros fold and thrust belt

flow regimes bed morphology and flow resistance in self formed step pool channels received 3 july 2008 revised 28 december 2008 accepted 13 january 2009 published 25 april 2009 1 we used a mobile bed flume with scaled grain size distribution channel geometry and flow to examine morphology and hydraulics of stepped channels we hypothesized that 1 step geometry and flow resistance differs significantly as a function of the range of grain sizes present 2 a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways and 3 the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions results support each of these hypotheses and help to illuminate the complexity of v q relationships in stepped channels in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step forming events therefore a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels nonetheless models based on dimensionless velocity and unit discharge appear more robust compared to those based on the darcy weisbach friction factor

formative conditions and sedimentary structures of sandy 3d antidunes an application of the gravel step pool model to fine grained sand in an experimental flume antidunes and their sedimentary structures can be useful in reconstructing paleo hydraulic conditions especially for large discharge events however three dimensional 3d antidunes in sand sized sediments have not yet been studied extensively as compared to either two dimensional 2d antidunes or antidunes in gravel sized sediments in this study we estimated formative conditions of gravel step pool morphologies and applied them to the formation of 3d antidunes over a sand bed formative conditions are expressed in terms of a relationship between the water discharge per unit width and the bed slope flume experiments demonstrated that 3d mound like antidune configurations and their associated internal sedimentary structures could be preserved internal sedimentary structures were characterized by shallow lens like structures whose bases were erosional although gently dipping concave upward lamination was dominant convex upward lamination was occasionally observed the dimensions of lenticular lamina sets can be used to estimate antidune geometry thus if 3d antidunes can be interpreted in the stratigraphic record it is possible to estimate the paleo hydraulic parameters such as water discharge and bed slope more precisely than previously copyright 2010 john wiley sons ltd

calcite texture development in experimentally induced ductile shear zones specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear a combination of simple and pure shear and in simple shear temperatures were 400 c and 500 c confining pressure was 100 mpa in the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2 3 mm wide shear zone shear displacement is documented by marker lines and circles shear strain varies between 0 84 and 1 56 strain is recorded by flattening of individual grains defining a foliation normal to the axis of principal finite shortening e1 no twinning is observed on a macroscopic scale

a tem study of microstructures associated with solution cleavage in limestone abstract microstructures associated with clay filled seams in the scaglia rossa limestone from the umbria marche apennines italy were characterized with transmission electron microscopy tem in order to study one of the mechanisms that presently falls under the heading of pressure solution only seams that formed after lithification often called solution of spaced cleavage were examined although some observations supported suggestions that much of the clay in the solution cleavage seam could be authigenic this report focuses primarily on the limestone microstructures no evidence of plastic deformation could be observed around the seams with an optical microscope however the tem resolved dislocations concentrated within limited areas of single grains comparisons with experimentally deformed calcite microstructures and with computer simulations of tem dislocation contrast allowed further interpretation of the character and density of dislocations dislocation concentrations were frequently associated with voids and pits the pits and voids were usually elongate in a calcite dislocation slip plane and were interpreted as dissolution features these and other observations suggested that selective dissolution of localized dislocation tangles was responsible for solution cleavage formation similar microstructures associated with the stress corrosion cracking have been interpreted elsewhere as evidence of plastic strain enhanced heterogeneous dissolution further interpretations indicated that a solution cleavage surface grows laterally like a fracture tip

high frequency seismic attenuation of oceanic p and s waves in the western pacific analyses of oceanic p and s data from an earthquake in the kuril islands which occurred along the trend of a 1500 km long ocean bottom hydrophone array deployment near wake island have yielded constraints on high frequency seismic attenuation in the western pacific spatial attenuation and q were determined for group velocities between 8 2 and 3 0 km s and for frequencies between 2 5 and 22 5 hz there is greater attenuation of oceanic p than oceanic s at all frequencies suggesting either the presence of significant bulk attenuation in the lithosphere or that oceanic p propagates as a leaky mode the apparent attenuation of oceanic p and s is distance dependent with lower attenuation beyond 30 the apparent q of oceanic p increases with frequency as q 0 7 from about 300 at 2 5 hz to 1500 at 17 5 hz the apparent q of oceanic s increases with frequency as q 1 1 from about 400 at 2 5 hz to 3000 at 22 5 hz

simulation of fabric development in recrystallizing aggregates ii example model runs abstract this study investigates the role of the coupling of dynamic recrystallization and lattice rotations in fabric development a new two dimensional computer simulation of polycrystalline deformation is used which combines homogeneous straining internal lattice rotations and dynamic recrystallization processes five example runs of the simulation are described here which compare the progressive development of fabrics resulting from different recrystallization regimes and different straining geometries it is found that these fabrics can evolve significantly with progressive strain from one strong fabric pattern to another the effect of recrystallization is not only to create point maxima concentrations of c axes but also to modify and create girdle distributions correlations are made between the fabrics generated by this model and previously reported quartz and ice fabrics

mechanics microstructure and ams evolution of a synthetic porphyritic calcite aggregate deformed in torsion in order to investigate the mechanical microstructural and ams evolution of porphyritic mylonites we made a synthetic aggregate composed of 70 fine calcite 3 mm in length defined a conspicuous

the role of veining and dissolution in the evolution of fine grained mylonites the mcconnell thrust alberta microstructural observations of the mcconnell thrust fault rocks demonstrate the importance of vein formation and calcite dissolution in the evolution of the mcconnell thrust limestone mylonites the protolith to the mylonite is a micritic limestone the mylonite contains numerous discontinuous bedding parallel calcite veins that range from relatively undeformed except for twinning and undulose extinction to highly sheared the highly sheared veins show evidence for dynamic recrystallization and grain size reduction along grain and twin boundaries that can lead to complete recrystallization to a grain size of approximately 1 3 m the bulk of the calcite matrix within the mcconnell thrust mylonite is interpreted to have formed by dynamical recrystallization of veins the emplacement of relatively coarse grained veins promoted the activation of dislocation creep within the fault rocks of the thrust in the absence of the increase in grain size afforded by vein emplacement it is anticipated that the fine grained protolith would have deformed predominantly by solution transfer with grain boundary sliding and localized brittle shearing the layer parallel calcite veins are found in close proximity to sheared clay seams which are interpreted to represent the insoluble residue left behind after dissolution of calcite these clay seams may have contributed to the build up of high pore fluid pressures which if intermittently reaching lithostatic conditions during displacement would result in transient brittle failure and thrust surface parallel vein emplacement in an otherwise ductilely deforming rock

microstructure and fabric transitions in calcite tectonites from the sierra alhamilla spain a suite of marble specimens from the sierra alhamilla spain deformed to large strains under natural conditions at about 300 c shows distinct variations in microstructure and fabrics it can be demonstrated that the development of crystallographic preferred orientations and grain shape fabrics are strongly dependent on recrystallized grain size this is interpreted to reflect the relative importance of various deformational mechanisms superplasticity seems to have a sharp upper grain size limit at 10 15 smm within the power law creep regime deformation in fine grained aggregates is probably dominated by diffusional in coarser ones by dislocation mechanisms the transition is a gradual one and may span a grain size range of 30 sm

a lithospheric seismic refraction profile in the western north atlantic ocean summary a study of the seismic velocity structure of the subcrustal oceanic lithosphere was conducted in the western north atlantic north of the lesser antilles where the age of the lithosphere is 80 95 ma a 1000km long north south array of 18 ocean bottom seismographs obs spaced at 60 km intervals recorded arrivals from explosions at ranges up to 800km and arrivals from earthquakes in the northern caribbean region at ranges up to 1400 km analysis of the travel time and amplitude data from the explosions provided the velocity structure down to about 65 km below sea level the earthquake data were intended to extend the travel times to ranges over 1100 km and thus sample the deeper part of the lithosphere however the geometry and velocity structure of the downdipping slab beneath the puerto rico trench deflected the ray paths between the caribbean hypocentres and the obs array such that no rays travelling through the lower lithosphere were detected by the array only rays through the upper lithosphere reached the obs array the results show that the subcrustal oceanic lithosphere consists of two main regions the uppermost region starts just below the m discontinuity and exhibits velocities of 8 2 8 4 km s 1 this region is characterized by three alternating zones of low 8 2 km s 1 high 8 4 km s 1 and low 8 2 km s 1 velocity the second region lies below 53 km and exhibits high velocities of 8 5 8 6 km s 1 a sharp boundary separates these two regions suggesting that a major change in composition or phase occurs there the velocity structure within the upper region shows broad agreement with petrological models for the generation of oceanic lithosphere starting from a pyrolite composition the alternating zones of low and high velocity correspond to zones depleted of and enriched in the basaltic fraction of the primary mantle material respectively this is a consequence of the processes of migration and differentiation of the upwelling asthenosphere near the mid ocean ridge axis however none of the petrological models based on a pyrolite or peridotite initial composition can easily explain the high velocities of over 8 5 km s 1 observed for the lower region of the seismic velocity model we speculate that this region may consist instead primarily of eclogite which is acceted from the asthenosphere as the lithosphere cools and ages the results do not permit definition of the lower boundary of the lithosphere which probably lies below the 65 km maximum depth resolved in this study s wave observations were less reliable than the p wave data but generally support the p wave velocity model no evidence was found for azimuthal anisotropy within the upper region of the subcrustal lithosphere however the existence in the western north atlantic of a weakly anisotropic or even isotropic lower lithosphere with a significant component of eclogite requires verification by future experiments

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd