the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

wellbore stability analysis and prediction of minimum mud weight for few wells in krishna godavari basin india abstract analysis of wellbore instability contains evaluation of rock mechanical properties and the state of in situ stresses in this analysis the only convenient factor is the mud weight i e the fluid density of the drilling fluid if the mud weight is greater than the predicted the mud will enter into the formation causing tensile failure fracture stress conversely a lower mud weight can result in shear failure collapse stress of rock which is known as borehole breakout here we present three types of failure criteria mohr coulomb mogi coulomb and modified lade rock mechanical rocks such as poisson s ratio uniaxial compressive strength cohesion have been computed from compressional and shear wave velocities internal friction angle is obtained from gamma ray log the failure criteria have been applied to two wells located at krishna godavari k g onshore and three wells at offshore to obtain minimum mud weight required to sustain its stability the caliper log for above mentioned wells have experienced complete to moderate failure considered as a lower bound of onset of failure at selected depth intervals it is observed that mohr coulomb failure criteria overestimates the predicted mud weight for the safe drilling on the contrary mogi coulomb failure criteria is closer to the expected result

evolution of mica fish in mylonitic rocks abstract mineral fish are lozenge shaped porphyroclasts single crystals in a finer grained matrix which occur in ductile shear zones and which are commonly used as shear sense indicators mineral fish of biotite tourmaline k feldspar garnet hypersthene and quartz occur in mylonites but most common are white mica fish these mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations the principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation concomitant selective grain size reduction occurs by recrystallisation cataclastic separation pressure solution and diffusional mass transfer microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds all the presented fish shaped porphyroclasts or mineral fish have a specific shape preferred orientation with their long axis at a small antithetic angle with respect to the foliation they represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands

2 d rotation behavior of a rigid ellipse in confined viscous simple shear numerical experiments using fem abstract the rotation behavior of rigid elliptical inclusions adherent to the viscous matrix in simple shear flow is investigated using a 2 d finite element numerical model several simulations were performed using different ratios s between shear zone width and inclusion s least principal axis a computational strategy was devised to calculate pressure and viscous forces exerted on the inclusion and deduce its angular velocity for large s values results agree remarkably well with theoretical predictions while for small s values results deviate significantly from theory but are in agreement with previous analogue experiments the numerical model provided detailed and coherent information about the physical parameters involved in the process e g pressure strain rate and vorticity distributions within the model

india national gas hydrate program expedition 02 operational and technical summary abstract the india national gas hydrate program is being steered by the government of india s ministry of petroleum and natural gas mopng with participation of directorate general of hydrocarbons dgh oil and natural gas corporation limited ongc and the national oil companies and research institutes of india the india national gas hydrate program expedition 01 nghp 01 established the presence of gas hydrate in the krishna godavari kg and mahanadi basins and in the offshore area of the andaman sea basin however the gas hydrates discovered during nghp 01 were mainly distributed as fracture filling material in fine grained clay rich sediments the india national gas hydrate program expedition 02 nghp 02 was carried out with an objective to discover gas hydrate in sand rich sediment along the eastern offshore margin of india ongc planned and executed nghp 02 on the behalf of the mopng nghp 02 started on march 3 2015 and was completed on july 28 2015 total 147 days using the japanese scientific drilling vessel chikyu d v chikyu during nghp 02 42 holes at 25 sites were drilled cored and or surveyed with downhole logging tools these sites were located in four areas along the eastern margin of india and formally named area a mahanadi basin three sites area b northern part of the kg basin twelve sites area c central part of the kg basin six sites and area e southern part to the kg basin four sites all 25 sites established during nghp 02 were first drilled and logged with logging while drilling lwd tools and an additional 17 holes were then drilled and or cored with conventional coring tools hpcs escs or pressure coring tools pctb wireline logging was conducted in 10 holes and formation tests using a dual packer modular formation dynamics tester mdt tool were carried out in two holes the onboard science team used the laboratory facilities on the d v chikyu to examine and analyse the physical properties geochemistry and sedimentology of all the cores collected during the expedition core samples were also analysed in additional post expedition shore based studies conducted in numerous domestic and international gas hydrate research laboratories the nghp 02 sediment cores were archived at the national gas hydrate core repository in mumbai india which is associated with the ongc gas hydrate research and technology centre ghrtc the necessary data for characterizing the occurrence of gas hydrate such as interstitial water chlorinities core derived gas chemistry physical and sedimentological properties thermal images of the recovered cores pressure core and downhole measured logging data lwd and or conventional wireline log data were obtained from most of the drill sites established during nghp 02 almost all the drill sites yielded evidence for the occurrence of gas hydrate however the inferred in situ concentration of gas hydrate varied substantially from site to site for the most part the interpretation of downhole logging data core thermal images interstitial water analyses and pressure core images from the sites established during nghp 02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained sand rich sediments this paper presents the operational and technical summary of nghp 02

mantled porphyroclast gauges we investigate the behavior of the isolated mantled porphyroclast in a shear zone the method employed is a finite element model three distinct phases clast mantle and matrix are present the rheologies are power law with exponents ranging from 1 to 5 and the far field boundary condition is simple shear the effective viscosity of the mantle is assumed to be less than those of the clast and the matrix we show for which parameter sets mantled porphyroclasts reach super horizontal stabilization with respect to the shear plane and sense clasts in natural mylonites frequently exhibit similar orientations which are interpreted as stable inclinations the systematic examination of the matrix mantle clast system allows for the construction of attractor maps that can be directly used as gauges for i the effective viscosity contrast between matrix and mantle ii the production rate of mantle material around the clast as a function of the bulk shear strain and iii for the total shear strain the necessary data required to use the attractor maps are simple geometrical parameters that can be measured in the field i e clast aspect ratio clast inclination versus the shear plane mantle thickness and mantle and clast area this new method successfully reproduces the characteristics of natural porphyroclasts and is in good agreement with data from natural shear zones q 2005 elsevier ltd all rights reserved

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

synthesis of deep multichannel seismic and high resolution sparker data implications for the geological environment of the krishna godavari offshore eastern continental margin of india abstract proven hydrocarbon occurrence in the krishna godavari kg basin a part of the eastern continental margin of india initiated intense exploration activities to discern the potential hydrocarbon bearing structures payoff zones recent discovery of gas hydrates in its offshore triggered detailed exploration measures adopting advanced seismic data acquisition techniques supported by deep drilling to understand its genesis interpretation of multichannel seismic mcs sections highlighted several distinct horizons from lower cretaceous to recent in the sediment overburden and the nature and configuration of the underlying crust the sections also depicted the presence of growth faults shale bulging toe thrust faulting delta progradation active sliding slumping buried channels pinchout structures and reflection free zones indicating the different types of sedimentary processes varied geological environments and neotectonic activity in the kg offshore high resolution sparker hrs data revealed subsidence gas fluid expulsion pathways occasional diapiric intrusions and growth faulting in the upper slope and toe thrust faulting upthrust and diapiric intrusions in the lower slope the mid slope is characteried by a well stratified e w trending basin bounded by upthrust and diapirism the entire region is associated with gas escape features blanking zones columnar type gas vents fault controlled mud shale diapirs large dimension gas saturated zones and pockmarks the growth faults shale bulging and toe thrusting seen in the mcs data since oligocene time could be linked to shale diapirism upthrust gas fluid expulsion subsidence and mini basin formation observed on the shallow hrs records synthesis of these datasets suggests that the distribution of gas hydrate in the shallow sediments may be controlled by the deeper structural and tectonic configuration therefore knowledge about shallow sedimentary column and its connectivity with deeper strata through faults fractures and other structural features and the geological architecture are significantly important to evaluate the genesis of gas hydrates

the development of sheath folds in viscously stratified materials in simple shear conditions an analogue approach sheath folds are highly non cylindrical folds occurring in a variety of geological settings and have been studied using different approaches with the present work we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions the sheath folds develop from an initial non cylindrical deflection located on the highly viscous layer the analogue experiments investigated the influence of 1 variations in the viscosity ratio between the high viscosity layer and the matrix hvl m 2 variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer af thvl and 3 progressive simple shear the results show that increases in hvl m will produce progressively less elongated sheath folds while increases in af thvl will result in more elongated sheath folds we present regime diagrams with hvl m and af thvl for different shear strains illustrating under which conditions sheath folds form in case the original deflection amplitude and layer thickness as well as can be retrieved for sheath folds in nature then their geometry can be used to quantify the effective hvl m

numerical modelling of clast rotation during soft sediment deformation a case study in miocene delta deposits a numerical model for a rotated clast in a sedimentary matrix is presented quantifying the deformation in associated soft sediment deformation structures all the structures occur in a southwards prograding deltaic sequence within the miocene ingering formation deposited at the northern margin of the fohnsdorf basin eastern alps austria debris flow and pelitic strata contain boudins pinch and swell structures ptygmatic folds rotated top to s reverse faults and rigid clasts developed under different stress conditions within the same layers the deformation around a 24 10 cm trapezoid shaped rigid clast resembling the clast geometry in metamorphic rocks has been modelled using a 2d finite element modelling software under the chosen initial and boundary conditions the rotational behaviour of the clast mainly depends on the proportions of pure and simple shear best fitting results were attained with a dominantly pure shear deformation 65 85 with stretching parallel and shortening normal to the bedding in this specific model set up the initial sedimentary thickness is reduced by 30 explained by stretching due to sediment creeping and compaction the high amount of pure shear deformation proposed is compatible with the observed layer parallel boudinage and pinch and swell structures rotated faults and ptygmatic folds were caused by the minor component of bedding parallel simple shear

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

late quaternary environmental and sea level changes from kolleru lake se india inferences from mineral magnetic geochemical and textural analyses abstract the present study employs mineral magnetic geochemical and textural parameters to elucidate the climate and sea level change signatures from kolleru lake sediments representing the inter deltaic part of the krishna godavari sedimentary basin kg basin for this purpose four cores were obtained which included significant peat the conventional radiocarbon dating suggested that the peat formation took place during 9 ka 6 ka with an average sedimentation rate of 0 31 cm y thermomagnetic analysis identifies titanomagnetite magnetite and hematite as the dominant magnetic mineralogy of the kg basin sediments the variation in magnetic mineralogy is related to paleoclimatic and paleoenvironmental processes the transitions between titano magnetite and hematite coincide with peat layers of the cores as indicated by sirm saturation isothermal remanent magnetization magnetic susceptibility and s ratio defined by irm 0 3t sirm values the various mineral magnetic parameters and s ratio along with major elemental concentrations of ti al fe and zr show signatures of an arid event before 10 ka between 9 ka 8 ka periods and during the last 5 5 ka the overall climate of warm and humid type was inferred from 10 ka to 9 ka which correlates with early holocene optima and 6 ka to 5 5 ka of middle holocene the presence of evaporite crystals in the lowermost part of the cores corroborated by mineral magnetic and geochemical proxies indicates sub aerially weathered facies of the arid late pleistocene similarly the peat layers rich in mangrove pollen indicate sea level changes between 10 ka and 6 ka the sub surface position of the pleistocene weathered layer and the holocene peat layer suggests tectonic subsidence of the area which is situated over the basement graben known as gudiwada sub basin

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

combining impedance inversion and seismic similarity for robust gas hydrate concentration assessments a case study from the krishna godavari basin east coast of india abstract gas hydrate saturations were calculated based on archie s relation and rock physics modeling utilizing log measurements of electrical resistivity and p wave velocity through the gas hydrate stability zone ghsz at two sites in the krishna godavari kg basin off the east coast of india acoustic impedance inversion was then performed around the well sites for regional extrapolation of the borehole data well log based gas hydrate concentration estimates and core data are in general agreement with the seismic impedance inversion results at the individual well sites however the correlation with seismic data and thus the confidence in the extrapolation decreases with distance from the well site to address the general problem of unknown regional confidence limits in the extrapolation and aid in regional gas hydrate assessment analyses a new approach is introduced by calculating the running sum of the seismic similarity attribute across the gas hydrate stability zone the running sum of the similarity attribute can be used locally on a 2d seismic line or 3d seismic volume for defining the limit of well data extrapolation around a given well site by normalizing the running sum of the similarity attribute from all available 2d seismic data in the kg basin a regional map was generated yielding effective confidence limits for extrapolation of well log data such maps of regional confidence limits can be used strategically in basin wide gas hydrate assessments as they provide a measure of probability to find a given gas hydrate concentration and may also offer a guide for defining a minimum regional spacing between well sites to address the overall structural complexity of the basin which is reflected in the similarity of the seismic data

rotating micro structures in antarctic cold basal ice implications for glacier flow and its interpretation structural analyses were conducted in the basal zone of an antarctic glacier the studied basal ice sequence was retrieved from a 20 m long subglacial tunnel dug at the margin of the glacier and is at the temperature of 17 c for the first time rotating clast systems embedded within debris rich ice were thin sectioned using specially designed cutting techniques the observed structures reflect the occurrence of pervasive shearing at the base of the glacier and can be used as shear sense indicators in addition some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice it is showed here that cautious analysis of deformation structures present in debris bearing ice may bring insights not only into the flow dynamics of the embedding matrix but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets

influence of total organic carbon deposition on the inventory of gas hydrate in the indian continental margins abstract total organic carbon toc content of marine sediments represents residual carbon originally derived from terrestrial and marine sources which has survived seafloor and shallow subseafloor diagenesis ultimately its preservation below the sulfate reduction zone in marine sediments drives methanogenesis within the gas hydrate stability zone ghsz methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate in this paper we examine the inventory and sources of toc in sediments collected from four regions within the ghsz along the indian continental margins the recovered sediments vary in age from oligocene to recent mean toc abundance is greatest in the krishna godavari k g basin and decreases progressively to the mahanadi basin andaman wedge and kerala konkan k k basin this decrease in toc is matched by a progressive increase in biogenic caco3 and increasing distance from terrestrial sources of organic matter and lithogenic materials organic carbon sources inferred from c n and 13ctoc range from terrestrial k g basin to mixed marine and terrestrial mahanadi basin to marine dominant andaman wedge and k k basin in the k g basin variation in the bulk 13ctoc is consistent with changes in c3 and c4 vegetation driven by monsoon variability on glacial interglacial timescales whereas in the mahanadi basin a shift in the 13ctoc likely reflects the onset of c4 plant deposition in the late miocene a large shift the 13ctoc in the k k basin is consistent with a change from c3 to c4 dominated plants during the middle miocene we observe a close relationship between toc content and gas hydrate saturation but consider the role of sedimentation rates on the preservation of toc in the zone of methanogenesis and advective flow of methane from depth although toc contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data seismic borehole log pressure core and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions although toc content may be a first order indicator for gas hydrate potential the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur

numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks abstract the rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field experimental and numerical modelling studies often centred on the question whether objects rotate or not in non coaxial deformation with numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations originally shape and orientation were investigated while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century due to improved numerical techniques anisotropic rheology has become the focus of most recent studies indicating that it is a primary factor in the rotation behaviour of objects we present new simulations investigating the role of anisotropy on different scales relative to the object and show how this influences the rotation rate as well as the inclusion patterns in case of syntectonically growing porphyroblasts these simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects the variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field

rotation of rigid elliptical cylinders in viscous simple shear flow analogue experiments we used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness and for very small ratios s between shear zone width z and the shortest principal axis e2 of the ellipse that constitutes the base of the elliptical cylinder 3 5 s z e2 1 2 former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and or a very large ratio between shear zone thickness and inclusion size s 1 and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow except for inclusions with infinite axial ratio our results depart greatly from the analytical predictions experiments were carried out at constant shear zone thickness but variable s and degree of coupling at the inclusion matrix interface the results show that 1 confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness even for synthetic rotation 2 the deviation in the angular velocity and or in the sense of rotation increases as s approaches 1 and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane and 3 a slipping inclusion matrix interface greatly enhances deviation from theoretical predictions with antithetic rotations to as much as 20

an outline of shear sense analysis in high grade rocks ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved however high grade shear zones are common and dominate in many precambrian terrains high grade shear zones should be analysed in a different way from low grade zones the plane on which shear sense markers should be observed the vorticity profile plane is more difficult to find than in low grade shear zones the most reliable shear sense markers in high grade shear zones are shear bands mineral fish mantled porphyroclasts sigmoids and asymmetric boudins 2006 published by elsevier b v on behalf of international association for gondwana research

comparing the cross polar cap potentials measured by superdarn and amie during saturation intervals 1 assimilative mapping of ionospheric electrodynamics amie and super dual auroral radar network superdarn are two commonly used techniques to measure cross polar cap potential pc in this study i compare pc inferred from amie with that inferred from superdarn during saturation intervals measurements from defense meteorological satellite program dmsp and polar cap pc index are also used to help estimate true pc seven saturation events in 2000 were selected for each event i ran the amie software driven by magnetometer data only by magnetometer data and superdarn data and by superdarn data only then i compared pc obtained from amie with that from superdarn i found that 1 pc inferred from superdarn is significantly lower than that inferred from other techniques including amie when pc is large e g greater than 150 kv 2 pc inferred from amie with superdarn input is much higher than pc inferred from superdarn 3 pc inferred from amie with magnetometer input tends to give the same results as that inferred from amie with magnetometer and superdarn input 4 pc inferred from amie with magnetometer input is in general in agreement with pcinferred from amie with superdarn input it is suggested that reason for the low values of superdarn derived pc during saturation intervals is that the superdarn algorithm to compute pc from measurements is not applicable to such intervals

rigid inclusions rotate in geologic materials as shown by torsion experiments jeffery showed 86 years ago that a sphere rotates synthetically with applied simple shear at a rate equal to half the shear strain rate however it can be argued that rocks do not behave like viscous fluids hence jeffery s model does not apply in order to test this argument we took a two phase synthetic aggregate rock made of 70 halite and 30 muscovite as anisotropic ductile matrix with embedded rigid bodies with contrasting shapes circular cylinder sphere and cube the sphere approximates the shape of a garnet and the cube the shape of pyrite crystals both so common in nature when subject to torsion at 250 mpa and 100 c the matrix deformed homogeneously and all rigid bodies rotated as theoretically predicted for a viscous fluid matrix therefore we conclude that rocks deforming in a ductile fashion behave macroscopically like a continuum hence fluid mechanics can be used to study rigid inclusion rotation behaviour

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

a critique of vorticity analysis using rigid clasts abstract estimating the kinematic vorticity numbers from rock fabrics presents many problems in this paper we use numerical modeling to investigate the reliability of those widely used vorticity analysis methods using rigid clasts we use very simple flows steady state homogeneous and having monoclinic symmetry to represent plane straining zones and transpressional zones and assume that the rotation of rigid clasts perfectly obeys the theory of jeffery 1922 proceedings of the royal society of london a102 161 179 these are assumptions made in current vorticity analysis using rigid clasts even with these simple assumptions our modeling shows that the current methods have intrinsic uncertainties so large that it is pointless to use the estimated vorticity numbers to constrain shear zone boundary conditions and kinematics it is perfectly consistent with numerical modeling results if the currently reported vorticity numbers estimated from rigid clasts in the range of 0 50 0 85 are all interpreted as being from natural shear zones with close to simple shearing flows the large uncertainties arise because the motion of rigid clasts is intrinsically a three dimensional problem

observed saturation of the ionospheric polar cap potential during the 31 march 2001 storm 1 theoretical arguments and mhd simulations have suggested that the potential drop across the polar cap ionosphere approaches a constant value when the imf is very large and southward this idea has been difficult to test because the conditions producing this effect are extremely rare during the 31 march 2001 storm the imf was directed southward for over 6 hours with a magnitude of 36 nt to 20 nt while the dmsp f13 satellite crossed the polar region obtaining a good measure of the true polar cap potential drop the observed potentials are compared with predictions from a theoretical model of the saturation process hill et al 1976 siscoe et al 2002 clear evidence of a non linear response consistent with the model predictions for height integrated pedersen conductivities in the range of 5 10 s are shown

2 d shape preferred orientations of rigid particles in transtensional viscous flow ghosh and ramberg tectonophysics 34 1976 1 70 studied the two dimensional 2 d rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression sr 0 at low medium shear strains we now extend this theoretical study to transtension sr 0 by deriving a new analytical solution to non interacting populations of rigid particles and to high shear strains we used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock our results show that 1 shape preferred orientations spos developed in viscous simple shear flow are transient and cyclical except for particles with aspect ratio or material lines which tend to the shear direction 2 stable spos can develop in transpression they dip towards the shear sense but when defining an s c structure with the shear foliation this composite fabric is always antithetic and thus indicates the wrong sense of shear 3 stable spos can also develop in transtension they dip opposite to the shear sense but when defining an s c structure with the shear foliation this composite fabric is always synthetic and thus indicates the correct sense of shear 4 spos produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations which must be demonstrated a priori

microstructure and mechanical properties of halite coarse muscovite synthetic aggregates deformed in torsion abstract we investigated the behavior of a synthetic two phase aggregate composed of 80 halite 20 coarse muscovite under torsion deformation at 100 200 and 300 c a confining pressure of 250 mpa and a constant strain rate of 3e 4 s 1 at all temperatures the two phase aggregate deformed homogeneously at the sample scale the strength of the aggregate and muscovite deformation depended on temperature strain rate and initial orientation of muscovite greatest dimension relative to the shear plane with muscovite initially parallel to the shear plane halite flowed plastically and long muscovite grains were not strong enough to behave as a rigid inclusion rotating in a ductile matrix instead high aspect ratio grains deformed by folding which was visibly accomplished by slip along mica 001 cleavage and by great flattening of halite in the core of the folds when initially normal to the shear plane high aspect ratio muscovite grains passively rotated towards the shear plane in both cases small and low aspect ratio muscovite grains behaved mostly as mica fish with mica grains tilted opposite to shear sense similarly to natural mylonites porphyroclast systems and rolling structures were also common in the microstructure a strong foliation made of halite ribbons and aligned muscovite flakes rapidly developed but did not make the composite aggregate weaker when compared with single phase halite comparison with synthetic aggregates of single phase halite shows that the two phase aggregate was much stronger than single phase halite in all cases comparison with a synthetic aggregate with calcite porphyroclasts shows that the strength of the aggregate with muscovite at 100 c was lower below a strain of ca 2 6 and higher beyond this value and that the aggregate with calcite was stronger at 200 c strain rate stepping tests 1e 5 s 1 2e 3 s 1 indicate that the two phase aggregate behaved as power law viscous with stress exponents of ca 12 and 10 at 100 and 200 c respectively the mechanical data obtained in this study represent the actual rheological behavior of the aggregates because we used soft polymer jackets

are isolated stable rigid clasts in shear zones equivalent to voids abstract particles in shear enclose important information about a rock s past and can potentially be used to decipher the kinematic history and mechanical behavior of a certain outcrop or region isolated rigid clasts in shear zones often exhibit systematic inclinations with respect to the shear plane at small angles tending towards the instantaneous stretching direction of the shear zone this shape preferred orientation cannot be easily explained by any of the analytical theories used in geology it was recently recognized that a weak mantle surrounding the clast or a slipping clast matrix interface might be responsible for the development of the observed inclinations physical considerations lead us to conjecture that such mantled rigid clasts can be effectively treated as voids that are not allowed to change their shape the resulting equivalent void conjecture agrees well with numerical and field data and has the following important geological implications i clasts in shear zones can have stable positions in simple shear without the requirement of an additional pure shear component ii the stable orientation can be approached either syn or antithetically hence the clast can rotate against the applied shear sense iii the strain needed to develop a strong shape preferred orientation is small 1 and therefore evaluations based on other theories may overestimate strain by orders of magnitude iv the reconstruction of far field shear flow conditions and kinematic vorticity analysis must be modified to incorporate these new findings

composition and origin of authigenic carbonates in the krishna godavari and mahanadi basins eastern continental margin of india abstract the mineralogical and stable isotopic composition of authigenic carbonates from the krishna godavari kg and mahanadi basin provide a deeper insight into the processes inducing carbonate formation in the sediments of the eastern continental margin of india in the bay of bengal authigenic carbonate cements micro nodules bioturbation casts and tubes from 12 core locations drilled during the indian national gas hydrate program nghp expedition 01 were investigated for this study three main processes responsible for authigenic carbonate precipitation are identified organoclastic sulfate reduction anaerobic oxidation of methane and methanogenesis evidence of vigorous methane seepage is indicated in carbonates recovered at sites 7 10 12 kg basin and 19 mahanadi basin these methane derived carbonates display typical paragenetic carbonate mineralogies aragonite high mg calcite with 15 mol mgco 3 ca rich dolomite two separate horizons of methane derived carbonates are correlated between 4 drill holes up to 16 km apart the upper horizon has been dated with 14 c 40 100 51 600 a bp 1950 and clearly indicates that methane seepage has been much more vigorous in the past possibly due to the effect of lower sea level on the gas hydrate stability zone across the margin

sampling disturbance in hydrate bearing sediment pressure cores nghp 01 expedition krishna godavari basin example sampling natural sediments causes unavoidable disturbance as recovered sediments experience changes in stress and strain during drilling core recovery transportation handling and early stages of testing in hydrate bearing sediments the potential for sampling disturbance may be aggravated since pressure and temperature changes can lead to hydrate dissociation and gas exsolution pressure core technology attempts to recover and characterize hydrate bearing sediments while preserving them under in situ pressure and temperature conditions which is an essential requirement to assess the mechanical physical chemical and biological properties of natural hydrate bearing sediments previous studies on near surface sampling effects are extended in this study to evaluate additional sampling disturbances relevant to hydrate bearing sediments 1 hydrate dissociation due to mechanical extension 2 negative pore pressure generation during unloading mandelecryer effect 3 secondary hydrate formation 4 changes in hydrate mass as a function of changes in pressure and temperature within the stability field 5 hydrate anomalous preservation and its benefits for pressure core handling and testing and 6 relaxation aging following sampling results provide valuable insight to sampler design coring and operation procedures high pressure chamber design and pressure core testing techniques

seismic imaging of a fractured gas hydrate system in the krishna godavari basin offshore india gas hydrate was discovered in the krishna godavari kg basin during the india national gas hydrate program nghp expedition 1 at site nghp 01 10 within a fractured clay dominated sedimentary system logging while drilling lwd coring and wire line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect three dimensional 3d seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures a system of two fault sets was identified part of a typical passive margin tectonic setting the lwd derived fracture network at hole nghp 01 10a is to some extent seen in the seismic data and was mapped using seismic coherency attributes the fractured system around site nghp 01 10 extends over a triangular shaped area of 2 5 km2 defined using seismic attributes of the seafloor reflection as well as seismic sweetness at the base of the gas hydrate occurrence zone the triangular shaped area is also showing a polygonal nearly hexagonal fault pattern distinct from other more rectangular fault patterns observed in the study area the occurrence of gas hydrate at site nghp 01 10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source the triangular shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits additionally the fault associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone it is proposed that there are additional locations in the kg basin with possible gas hydrate accumulation of similar tectonic conditions and one such location was identified from the 3d seismic data 6 km nw of site nghp 01 10

gas hydrate distribution and carbon sequestration through coupled microbial methanogenesis and silicate weathering in the krishna godavari basin offshore india abstract the national gas hydrate program expedition 01 cored ten sites in the krishna godavari basin located on the southeastern margin of india a comprehensive suite of pore water solute concentrations and isotope ratios were analyzed to investigate the distribution and concentration of gas hydrate along the margin in situ diagenetic and metabolic reactions and fluid migration and flow pathways gas hydrate was present at all of the sites cored and in situ microbial methanogenesis leads to estimates of depth integrated average gas hydrate saturations that are typically

deltaic land subsidence and sea level fluctuations along the east coast of india since 8 ka a palynological study the consequences of climate change and land subsidence are often related to sea level rise fall mangrove extinction and migration were assessed through palynological studies in two sedimentary cores in order to address climate relative sea level rsl and its relation to geomorphology since 8420 cal yr bp from kanuru 500 cm deep and since 5850 cal yr bp from machilipatnam 118 cm deep krishna delta india four units nine phases of sediment depositional environment were identified on the basis of marine terrestrial palynomorphs in unit 1 the marine palynomorphs indicate palaeoshoreline in kanuru from 8420 to 8300 cal yr bp which is now 4 5 km from the present day shoreline but after a short span a retreat in sea level took place between 8300 and 7040 cal yr bp about three to four short term intermittent rise fall in rsl were recorded in unit 2 between 7040 and 3980 cal yr bp in kanuru and between 5225 and 3240 cal yr bp in machilipatnam during this period the diversity of mangroves was

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

investigation of a rare event where the polar ionospheric reverse convection potential does not saturate during a period of extreme northward imf solar wind driving

biases in three dimensional vorticity analysis using porphyroclast system limits and application to natural examples a description of the systematic errors associated with the measurement of the vorticity number from poryhroclasts in natural systems is presented and discussed we show that strong biases and systematic errors could derive both from some erroneous physical i e no slip across clast matrix boundary homogeneity within the matrix as well as geometrical assumptions i e the radius ratio and angular measurements carried out in two dimensions on outcrop surfaces and thin section by comparing natural datasets of porphyroclast shape preferred orientation spo with different theoretical curves plots we suggest that at least one of the jeffery physical assumptions can be tested when applying vorticity techniques the comparison of different possible sources of systematic errors indicates that for medium to low vorticity numbers wm 0 8 vorticity data are strongly biased and that a minimum systematic error of 0 2 should be taken into account finally we use data from natural shear zones from the southern variscan belt in sardinia to test and discuss the starting assumptions of the jeffery model one of the main challenges of modern structural geology is to try to obtain information about the rheology and kinematics of flow occurring in the deepest crust and upper mantle several microstructure fabrics e g fabric asymmetry associated with mylonites represent potentially important sources of information to obtain kinematic data as well as for estimation of deformation parameters see passchier trouw 2005 for a review in particular porphyroclast systems have been shown to be a sensitive indicator of flow parameters and can be used to constrain the relative ratio of combined pure and simple shear i e the vorticity of the flow several theoretical and practical studies indicate that syn kinematic recrystallization of mantles sor d shaped tails around porphyroclasts and shape preferred orientation spo of porphyroclasts may store information on the amount of shear strain with the aim of developing a vorticity gauge theoretical and experimental models for study of the rotation and behaviour of elliptical porphyroclasts in a fluid have been proposed and developed the first attempt began with the pioneering works of jeffery 1922 and eshelby 1957 that found solutions for rigid objects immersed in a viscous and linear elastics fluid respectively bretherton 1962 ghosh ramberg 1976 and bilby kolbuszewski 1977 further adapted the basic jeffery analytical solution to material science and structural geology subsequent investigations used the previous theoretical calculation to develop efficient strain and vorticity gauge techniques capable of quantifying the flow parameters within shear zones passchier 1987 wallis 1992 simpson de paor 1993 holcombe little 2001 three main analytical techniques passchier 1987 wallis 1995 holcombe little 2001 are commonly employed to characterize flow within shear zones using rigid porphyroclats jessup et al 2007 all these techniques use the measurement of the axial ratio of porphyroclast r the shape factor fig 1 and the angle of long axis porphyroclast with respect to foliation or stretching direction h fig 1 and the distribution of the cross plotted curves to define the mean vorticity number wm these curves define a threshold number rc below which the porphyroclasts continuously rotate and above which they record stable sink position however as many authors passchier 1987 marques coelho 2001 have already noted the jeffery model assumes 1 rigid particles immersed in a non confined viscous flow 2 a perfect coupling on matrix particle interface and 3 no interference between particles during deformation use of the jeffery model implies that these physical assumptions are met and or need to be at least preliminarily tested although techniques to estimate the vorticity of flow have become routinely used on sheared rocks in a variety of tectonic settings e g klepeis et al 1999 xypolias doutsos 2000 xypolias koukouvelas 2001 bailey eyster 2003 law et al 2004 carosi et al 2006 2007 iacopini et al 2008 sarkarinejad et al 2008 2010 frassi et al 2009 larson godin 2009 relatively few detailed analysis of error sources have been proposed and a critical discussion from prior d j rutter e h tatham d j eds deformation mechanisms rheology and tectonics microstructures mechanics and anisotropy geological society london special publications 360 301 318 doi 10 1144 sp360 17 the geological society of london 2011 publishing disclaimer www geolsoc org uk pub ethics at university of aberdeen on november 19 2012 http sp lyellcollection org downloaded from

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

storm time ionosphere and plasmasphere structuring sami3 rcm simulation of the 31 march 2001 geomagnetic storm we present the first self consistent modeling study of the ionosphere plasmasphere system response to a geomagnetic storm we use the coupled sami3 rice convention model rcm of the global ionosphere and inner magnetosphere with self consistent electrodynamics to simulate the 31 march 2001 magnetic storm we find that the penetration electric fields associated with the magnetic storm lead to a storm time enhanced density sed in the low to middle latitude ionosphere and that the separation in latitude of the appleton anomaly peaks increases the sed exhibits magnetic conjugacy occurring in both the northern and southern hemispheres moreover mapping the boundary of the sed into the equatorial plane coincides with the development of a plume like structure in the plasmasphere these preliminary results are consistent with observations

simulation of the polar cap potential during periods with northward interplanetary magnetic field 1 in this paper we examine the response of the ionospheric cross polar cap potential to steady purely northward interplanetary magnetic field imf using the lyon fedder mobarry global magnetohydrodynamic simulation of the earth s magnetosphere the simulation produces the typical high latitude reversed cell convection that is associated with northward imf along with a two cell convection pattern at lower latitude that we interpret as being driven by the viscous interaction the behavior of the potential can be divided into two basic regions the viscous dominated region and the reconnection dominated region the viscous dominated region is characterized by decreasing viscous potential with increasing northward imf the reconnection dominated region may be further subdivided into a linear region where reconnection potential increases with increasing magnitude of northward imf and the saturation region where the value of the reconnection potential is relatively insensitive to the magnitude of the northward imf the saturation of the cross polar cap potential for northward imf has recently been documented using observations and is here established as a feature of a global mhd simulation as well the region at which the response of the potential transitions from the linear region to the saturation region is also the region in parameter space at which the magnetosheath transitions from being dominated by the plasma pressure to being dominated by the magnetic energy density this result is supportive of the recent magnetosheath force balance model for the modulation of the reconnection potential within that framework and including our current understanding of the viscous potential we present a conceptual model for understanding the full variation of the polar cap potential for northward imf including the simulated dependencies of the potential on solar wind speed and ionospheric conductivity

controls on the geometry of tails around rigid circular inclusions insights from analogue modelling in simple shear abstract we used analogue modelling to investigate the factors controlling tail geometry in porphyroclast systems results show that 1 inclusions can develop in both slipping and non slipping modes but inclusions only form in the latter 2 inclusions develop when the mantle production rate is constant and the mantle is transected by the separatrix inclusions form when the mantle is initially outside the separatrix and later comes inside this line 3 in the slipping mode the wedge shaped tail of inclusions always has a straight external surface parallel to the shear plane whereas in the non slipping mode the external surface is curved inwards external embayments 4 together with earlier theoretical results and inclusions always show stair stepping of tails when embedded in a viscous matrix under homogeneous simple shear deformation 5 maximum stair stepping occurs in the slipping mode and is at least equal to the inclusion diameter if our models bear significant similarity to nature then i the straight or curved character of inclusions could mean that they had respectively a slipping or non slipping interface with the surrounding recrystallized matrix and ii inclusions may result from shear deformation under retrogressive metamorphic conditions in thrust systems

development of highly non cylindrical folds around rigid ellipsoidal inclusions in bulk simple shear regimes natural examples and experimental modelling abstract the three dimensional development of strongly non cylindrical folds around competent ellipsoidal inclusions in bulk simple shear regimes is studied in experiments and natural examples examples from the continental allochthonous terrane of the braganca nappe complex ne portugal show rim folds and sheath folds associated with different parts of rigid ellipsoidal boudins experimental work has been carried out with models made from analogue materials silicone putty and plasticine and deformed in a simple shear machine we have considered three different models to simulate natural examples and the results show that fold morphology depends on the shape of the inclusion and position around the inclusion although the bulk deformation regime is layer parallel homogeneous simple shear we can distinguish local deformation regimes responsible for the folding associated with different parts of the rigid body flow and strain patterns must therefore be complex around inclusions or populations of inclusions prolate to oblate strain ellipsoids can be expected in different positions close to the rigid inclusion in our experiments the competent inclusions do not rotate synthetically with the applied bulk simple shear e g clockwise rotation of inclusion in dextral shear instead they back rotate early in the shearing history and keep this position throughout the experiment this is the result of the size relationship between the inclusion and the finite width of the shear zone the fold pattern around rigid inclusions may be used as a shear sense criterion

mineral lineation produced by 3 d rotation of rigid inclusions in confined viscous simple shear abstract the solid state flow of rocks commonly produces a parallel arrangement of elongate minerals with their longest axes coincident with the direction of flow a mineral lineation however this does not conform to jeffery s theory of the rotation of rigid ellipsoidal inclusions reis in viscous simple shear because rigid inclusions rotate continuously with applied shear in 2 dimensional 2 d flow the rei s greatest axis e1 is already in the shear direction therefore the problem is to find mechanisms that can prevent the rotation of the rei about one axis the vorticity axis in 3 d flow the problem is to find a mechanism that can make e1 rotate towards the shear direction and so generate a mineral lineation by rigid rotation about two axes 3 d analogue and numerical modelling was used to test the effects of confinement on rei rotation and for narrow channels shear zone thickness over inclusion s least axis wr 1 can rotate backwards from an initial orientation w e1 parallel to the shear plane in great contrast to jeffery s model 3 back rotation is limited because inclusions reach a stable equilibrium orientation 4 most importantly and in contrast to jeffery s model and to the 2 d simulations in 3 d the confined rei gradually rotated about an axis orthogonal to the shear plane towards an orientation with e1 parallel to the shear direction thus producing a lineation parallel to the shear direction the modelling results lead to the conclusion that confined simple shear can be responsible for the mineral alignment lineation observed in ductile shear zones

riedel shear control on the development of pennant veins field example and analogue modelling abstract the wall rocks of a crustal scale sinistral ductile shear zone in namibia the purros mylonite zone contain two types of asymmetric quartz veins bedding surfaces contain sigmoidal quartz veins with limited thickness along their symmetry axes that can be classified as tension gashes a second type of veins consists of a striated central fault vein separating pennant type quartz filled terminations the tips of these pennant veins have a different orientation to those of the tension gashes analogue experiments were carried out using a sheet of silicone powder suspended on a slab of poly dimethyl siloxane pdms both deformed in simple shear these experiments produced open fractures very similar to the pennant veins that form by intersection of r and r riedel shear fractures these fractures rotate and slip during progressive deformation opening pennant shaped gaps we interpret the natural pennant veins to form by the same mechanism of r and r shear fracture initiation and subsequent rotation and opening since this mechanism differs from that of previously described vein types such as wing cracks tension gashes and swordtail or fishmouth termination veins which mainly open as tension veins we consider pennant veins as in new independent class of asymmetric mineral filled veins

the not so simple effects of boundary conditions on models of simple shear analog modeling of geological processes such as folding instabilities or the behavior of inclusions in a matrix often employs a linear simple shear rig in theory a homogeneous plane strain flow is prescribed at the boundaries of such deformation rigs but in practice the resulting internal deformation of the analog material commonly paraffin wax or silicone putties often deviates strongly from the intended homogeneous strain field this can easily lead to misinterpretation of such analog experiments we present a numerical finite element approach to quantify the influence of imperfect simple shear boundary conditions on the internal deformation of a homogeneous viscous analog material the results demonstrate that imperfect boundary conditions in the vorticity normal plane can cause the heterogeneous strain observed in some analog experiments however in other experiments the analog material lies on top of a weak lubricating material or is sandwiched between two such materials these layers lead to a viscous drag force acting on the analog material resulting in imperfect simple shear boundary conditions in the third dimension for this experimental configuration the numerical results show that the lubricating layers are responsible for the heterogeneous strain observed in analog models the resulting errors in internal strain can be as high as 100 and these difficult to avoid boundary effects must be considered when interpreting analog simple shear experiments

porphyroblast rotation during crenulation cleavage development an example from the aureole of the mooselookmeguntic pluton maine usa in the low pressure high temperature metamorphic rocks of western maine usa staurolite porphyroblasts grew at c 400 ma very late during the regional orogenesis these porphyroblasts which preserve straight inclusion trails with small thin section scale variation in pitch were subsequently involved in the strain and metamorphic aureole of the c 370 ma mooselookmeguntic pluton the aureole shows a progressive fabric intensity gradient from effectively zero emplacement related deformation at the outer edge of the aureole 2900 m map distance from the pluton margin to the development of a pervasive emplacement related foliation adjacent to the pluton the development of this pervasive foliation spanned all stages of crenulation cleavage development which are preserved at different distances from the pluton the spread of inclusion trail pitches in the staurolite porphyroblasts as measured in two dimensional 2 d thin sections increases nonlinearly from 16 to 75 with increasing strain in the aureole these data provide clear evidence for rotation of the staurolite porphyroblasts relative to one another and to the developing crenulation cleavage the data spread is qualitatively modelled for both pure and simple shear and both solutions match the data reasonably well the spread of inclusion trail orientations 40 75 in the moderately to highly strained rocks is similar to the spread reported in several previous studies we consider it likely that the sample scale spread in these previous studies is also the result of porphyroblast rotation relative to one another however the average inclusion trail orientation for a single sample may in at least some instances reflect the original orientation of the overgrown foliation

tectonostratigraphy and depositional patterns in krishna offshore basin bay of bengal the krishna godavari basin along the east coast of india covers the deltaic and interdeltaic areas of the krishna and godavari rivers and extends into the offshore figure 1 this basin is situated along the shoulder of a rifted passive continental margin which developed during the separation of india from antarctica in the late jurassic this rifting episode is superimposed on an earlier northwest southeast gondwana graben trend such as the pranhita godavari graben figure 2a which extends from onshore into the krishna godavari basin this is an earlier prerift depositional episode onto precambrian crystalline basement which commenced in the early permian and continued through the middle jurassic the late jurassic rift episode caused mechanical extension along the eastern margin leading to the formation of northeast southwest horst and graben features based on bouguer gravity data murty and ramakrishna 1980 recognized different tectonic elements figure 2b such as krishna graben bapatla horst west godavari subbasin including the median kaza kaikalur horst tanuku horst and east godavari subbasin including narasapur yanam high