deformation at an archean subprovince boundary northern minnesota structural analysis in the northern margin of the quetico subprovince part of the archean superior province of the canadian shield in minnesota reveals that the main deformation involved polyphase folding f1 recumbent and nappe like and f2 upright east west trending and tight to isoclinal during regional ductile transpression and amphibolite facies metamorphism a younger deformation developed during the latter stages of regional transpression resulted in the generation of localized ultraphyllonites along the steeply dipping rainy lake seine river fault rlsrf the major fault separating the quetico subprovince from the rainy lake wrench zone a wedge shaped block between the quetico and wabigoon subprovinces the transpression involved north south shortening and east west dextral shear the presence of shear zones in amphibolite facies wall rock south of the fault and in lower grade ultraphyllonites within the rlsrf suggests that localization of shear occurred by work and or reaction softe

charriage crustal anatexie et decrochements ductiles dans les maures orientales var france au cours de l orogenese varisque la cinematique et la signification tectonique des deformations enregistrees par les gneiss catazonaux des maures orientales socle varisque de provence france sont precisees les evenements successifs peuvent etre correles avec les evolutions metamorphiques et magmatiques l histoire orogenique des maures orientales apparait composee des evenements suivants 1 metamorphisme a haute pression uniquement conserve dans des reliques d eclogite et de quartzite a silicates calciques 2 metamorphisme catazonal accompagne d anatexie contemporain d un transport subhorizontal vers le nne d echelle crustale cet episode est responsable de la dilaceration des unites lithologiques de leur laminage et de leur mylonitisation 3 intrusion d une premiere generation de granitoides anatectiques 4 deuxieme episode tectonique dans des conditions epi a mesozonales responsable du deplacement vers le nord des maures orientales par rapport aux maures occidentales le long d un decrochement senestre de 4 km d epaisseur faille de ramatuelle plan de la tour dans lequel les granitoides d anatexie precoces sont mylonitises en dehors de la zone de faille cet episode est marque par des plis ouverts droits ou deverses vers l ouest orientes n s parallelement a la direction de transport 5 reactivation cataclastique moderee de la faille de ramatuelle plan de la tour avec un mouvement dextre localement accompagne dans la partie est de la region par des chevauchements mineurs vers le sud 6 intrusion d une deuxieme generation de granites anatectiques vers 320 ma

unexpected characteristics of the 150 km echoes observed over gadanki and their implications recent discovery of two distinct types of 150 km echoes namely type a and type b and subsequent progress in the large scale kinetic simulation of photoelectron induced plasma waves have begun a new era in resolving the five decades long 150 km echoing riddle in this paper we present hitherto unrevealed three important and unexpected findings on the two distinct types of 150 km echoes based on gadanki radar observations our observations show unexpected predominance of type a echoes strong seasonal dependence of both type a and type b echoes and a surprising connection of the type b echoes to the unusually deep solar minimum of 2008 2009 we discuss how these results provide important new clues in tethering the competing processes involved in the daytime 150 km echoes and have significance in the recently proposed photoelectron induced plasma fluctuations as a potential mechanism for the 150 km echoes

kinematics and geochronology of multistage ductile deformation along the eastern alxa block nw china new constraints on the relationship between the north china plate and the alxa block abstract the ne trending bayanwula shan lang shan is an important tectonic belt lying between the north china plate ncp to the east and the alxa block to the west an understanding of its nature and the timing of deformation are essential to understand the relationship between the ncp and the alxa block two phases of ductile deformation have been observed in this belt large scale top to the west ductile thrusting characterized the early deformation in the bayanwula shan lang shan nearly east west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed different types of sheath and oblique folds with east west trending fold hinges are also developed in the region the shear strain of this ductile thrust is up to 17 the ductile deformation may have resulted from the top to the west thrusting of the northern part of the ncp over the alxa block and may have occurred ca 351 ma biotite 40 ar 39 ar age later ductile deformation was expressed as ne trending sinistral shear along the entire bayanwula shan lang shan and likely occurred ca 250 ma biotite and muscovite 40 ar 39 ar ages this shear may have resulted from the collision between the yangtze and north china plates to the south during the triassic combined with recently obtained detrital zircon u pb ages for the area the ductile deformation events in the eastern alxa block indicate that the block may not have been part of the ncp at least before the end of the devonian both blocks were located in the paleo asian ocean during the paleozoic and collided or amalgamated with each other at the end of the devonian

the migif hafafit gneissic complex of the egyptian eastern desert fold interference patterns involving multiply deformed sheath folds abstract the wadi hafafit complex whc is an arcuate belt of orthogneisses migmatites and other high grade metamorphic rocks which marks the boundary between the central eastern and the south eastern deserts of egypt in the whc gneissic meta gabbro outlines macroscopic fold interference patterns characterized by elliptical to irregular culminations cored by gneissic meta tonalite to meta trondhjemite the five main culminations of the whc have previously been labeled a most northerly b c d and e most southerly a detailed structural investigation of b c d and e reveals that these structures are a result of the interference of four macroscopic fold phases the first three of which may represent a single deformation event the first folding involved sheath like fold nappes which were transported to the n or nw assisted by translation on gently dipping mylonite zones the regional gneissosity and mineral extension lineations formed during this folding event the fold nappes were deformed by mainly open upright small macroscopic and mesocopic folds with approximately ne trending hinges as a probable continuation of the latter folding the sheaths were buckled into large macroscopic folds and monoclines with the same ne trends the fourth macroscopic folding resulted from shortening along the ne sw direction producing mainly nw se trending upright gently plunging folds gravitative uplift is disputed as a component of the deformation history of the whc the peculiarities of the fold interference pattern result from the interesting behaviour of sheath folds during their refolding

relationship between non cylindrical fold geometry and the shear direction in monoclinic and triclinic shear zones we investigate the relationship between non cylindrical fold geometry and the shear direction of the hosting high strain zone by numerical modeling and show that apical axes of non cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component the common practice of using well developed sheath folds as indicators for the shear direction is not reliable and the use of immature non cylindrical or sheath folds appears more reliable hinge lines of mature sheath folds approach parallelism with the fabric attractor to which all material lines rotate which can have variable angles with respect to the shear direction in thinning zones the fabric attractor is the direction of the maximum principal strain rate of the pure shear component and is parallel to the shear zone boundary in thickening zones it lies somewhere in the quadrant between the direction of the maximum principal strain rate of the pure shear component perpendicular to the shear zone boundary and the simple shear direction and is generally oblique to the shear zone boundary the exact location depends on the flow geometry of the shear zone 2007 elsevier ltd all rights reserved

regional flow perturbation folding within an exhumation channel a case study from the cycladic blueschists abstract kilometre scale cylindrical folds and associated parasitic folds that trend at small angles to the transport lineation are analysed along a 100 km long transport normal segment of the cycladic blueschists in an attempt to reconstruct the 3d structural architecture within an exhumation channel reversals in the polarity of both fold vergence and the hinge lineation obliquity occur in a flow normal direction defining transport parallel culmination and depression surfaces that root downwards onto an underlying detachment fold patterns generated around culmination and depression surfaces support models of flow perturbation folding where folds initiate at small angles or sub parallel to transport in response to wrench dominated differential shearing successive culmination and depression surfaces are separated from one another by along strike distances of 20 km although atypical fold geometries developed in the flanks of major culmination and depressions follow their own patterns revealing that smaller perturbations occur within the larger scheme major culminations are interpreted to reflect regions of surging flow marked by increased velocity during exhumation whilst the opposite is true for depressions this behaviour implies that on a regional scale differential shear varies laterally in an irregular sinusoidal manner defining areas of relative high and relative low displacement within the exhumation channel

ductile and brittle deformations in the northern snake range nevada abstract the northern snake range east central nevada is one of the metamorphic core complexes of the sevier hinterland within the range a major decollement separates an upper plate composed of brittlely deformed paleozoic sedimentary rocks mostly carbonates from a lower plate composed of metamorphic upper precambrian lower cambrian rocks intruded by gneissic granites a study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that the northern snake range decollement has been a zone of intense non coaxial e vergent shear and transport in a n115 e direction outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains of at least 20 the interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition but might result from two distinct phases of deformation ductile deformation in and below the decollement could have occurred before brittle deformation in the upper plate brittle faulting in the upper plate related to basin and range extension reactivated the upper surface of the ductile shear zone high topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent oligo miocene basins

kilometre scale sheath folds in the western betics south of spain the three dimensional exposures of the sierra blanca marbles and neighbouring rocks in the western betic cordillera of the south of spain show tight to isoclinal f3 folds re folded by a larger sheath like structure the relationships between both structures are as follows 1 the mean axial surface for sierra blanca sheath fold can be defined as the great circle of best fit through the s axes of sub cylindrical f3 folding domains 2 the asymmetric f3 folds indicate a single mainly eastward normal sense of shear 3 the mean attitude of the axial plane crenulation foliation sc is also sub parallel to the mean axial surface for sierra blanca sheath fold the regional context of the sierra blanca sheath fold is finally discussed and a model of a heterogeneous high strain extensional zone proposed

evolution of the eastern waddington thrust belt and its relation to the mid cretaceous coast mountains arc western british columbia the eastern waddington thrust belt cuts the eastern margin of the coast mountains mid cretaceous magmatic arc thrust faults carried triassic rocks of the intermontane superterrane early cretaceous volcanic and clastic rocks and volcanic and plutonic rocks of the active arc to the northeast outward from the core of the arc minimum shortening across the thrust belt is estimated as 50 or 40 km synkinematic through postkinematic metamorphism produced an inverted metamorphic gradient with the structurally higher magmatic arc as the likely heat source radiometric thermochronologic and structural data indicate that the thrust belt was active at 84 ma and probably at 87 ma and suggest that the peak of postkinematic metamorphism occurred about 82 84 ma the thrust belt was intruded by postkinematic plutons in latest cretaceous and early tertiary time 68 ma and 58 ma the eastern waddington thrust belt is coeval with or slightly younger than a system of west directed thrusts in the western and southern parts of the coast mountains arc the prominence of these structures suggests that synmagmatic contraction played a major role in development of the arc the distribution and character of syn and postthrusting metamorphism indicate that this contraction rather than localized loading by magma produced the metamorphism

tertiary deformation and metamorphism se of tibet the folded tiger leap decollement of nw yunnan china the yulong haba xue shan range in the northwestern part of yunnan china is a large n s antiform that folds the paleozoic series of the yangzi platform the upper yangzi river jinsha jiang has cut a 3500 m deep valley hu tiao gorge across this antiform thus exposing folded bedding parallel ductile shear zones decollements with transport toward the ssw in the present geographical coordinates the large finite shear strain implies tens of kilometers of transport pointing to the regional significance of these decollements rb sr radiometric dating of phlogopites that crystallized in marbles within the foliation planes yields the age of the metamorphic and deformation event 35 9 0 3 2 ma the age derives from an internal rb sr isochron made on different size fractions of the same mineral which provides a novel demonstration of the feasibility of such plots transport on the decollement and related shortening occurred prior to or at the onset of extrusion of indochina along the ailao shan red river shear zone 80 km west of the yulong shan the 39ar 40ar age spectra of k feldspar from the core of the yulong shan suggest uplift by antiformal folding around 17 ma as indochina s extrusion came to an end we infer that other large scale cenozoic decollements such as that exhumed in the yulong shan underlie some of the vast folded areas that surround the eastern himalayan syntaxis transport on such decollements first toward the south and then toward the east and folding above them might have occurred during two principal shortening phases whose ages bracket indochina s escape toward the se

structural analysis of sheath folds with horizontal x axes northeast canada abstract early proterozoic supracrustal rocks occur below a thick nappe of archaean basement gneiss in the melville peninsula where sheath folds are exposed in a wide zone of middle proterozoic dynamothermal metamorphism outcrop patterns of truncated isoclinal sheath folds resemble cylindrical folds except in relatively small areas around the paraboloidal caps bulk extension axes are parallel to strike in the belt as shown by isoclinal sheath folds with horizontal central axes x axes as well as similarly aligned mullion structure and rotated scapolite prisms extension axes converge from northeast to southwest in the apparent flow direction

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

thrusting in the lower crust evidence from the oygarden islands kemp land east antarctica layered orthogneisses of the oygarden islands preserve evidence for four high grade deformation events d 1 to d 4 archaean d 1 and d 2 structures are only patchily preserved due to extensive recrystallization during d 3 and d 4 which represent effects of the c 1000 ma rayner structural episode ductile thrusting at middle to lower crustal levels occurred during d 3 which is separated into two mutually cross cutting phases based on structural geometry the two phases represent changes in finite strain that developed during progressive deformation east directed transport during d 3a developed subhorizontal thrusts that contain co axial east trending f 3a folds and l 3a lineations buckling as a consequence of constriction in thrust duplexes developed upright f 3b folds coaxial to f 3a folds and steeply south dipping d 3b shear zones garnet clinopyroxene and garnet orthopyroxene bearing assemblages in mafic lithologies and garnet sillimanite bearing assemblages in pelitic lithologies reflect d 3 conditions of p 9 kbar and t 800 850 c the well exposed d 3 duplex structures indicate that shortening of the lower crust may be accommodated by extensive strain partitioning to develop contemporary kilometre scale thrust stacking and ductile flow

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

a provisional structural thickness map of the otago schist new zealand metamorphic zones terranes and lithologic associations provide a subdivision of the lithologically monotonous otago schist on a 10 to 50 kilometer scale in this study form line maps of schist foliation have been used to construct a provisional structural thickness map of the otago schist that allows a more detailed 1 5 km scale subdivision of the entire schist the map shows the macroscopic thickness of schist measured orthogonal to dominant foliation relative to the datum of the caples rakaia terrane boundary the structural thickness map provides a new spatial reference frame for the otago schist that depicts the differing levels of exhumation of penetratively deformed metamorphic rocks the structural thickness parameter shows a good correlation with metamorphic grade white mica grain size and k ar and ar ar ages thus justifying the simplifying assumptions behind the map s construction the map reveals several hitherto little highlighted features including along and across strike correlations interference patterns between mesozoic and cenozoic macroscopic folds net displacement across known and inferred major faults and relative structural position of mesothermal gold deposits

structural analysis of sheath folds in the sylacauga marble group talladega slate belt southern appalachians remnant blocks of marble from the moretti harrah dimension stone quarry provide excellent exposure of meter scale sheath folds tubular structures with elliptical cross sections 4 ryz 5 are the most common expression of the folds the tubes are elongate subparallel to stretching lineation and are defined by centimeter scale layers of schist eccentrically nested elliptical patterns and opposing asymmetry of folds s and z are consistent with the sheath fold interpretation sheath folds are locally numerous in the moretti harrah quarry but are not widely distributed in the sylacauga marble group reconnaissance in neighboring quarries provided no additional observations the presence of sheath folds in part of the talladega slate belt indicates a local history of plastic non coaxial deformation such a history of deformation is substantiated by petrographic study of an extracted hinge from the moretti harrah quarry the sheath folds are modeled as due to passive amplification of initial structures during simple shear using both analytic geometry and graphic simulation as indicated by these models relatively large shear strains y 9 and longitudinal initial structures are required the shear strain presumably relates to nw directed displacement of overlying crystalline rocks during late paleozoic orogeny

descending ion layer property in the gadanki radar observations of 150 km echoes and its implication to the echoing phenomenon 1 gadanki radar observations of the daytime 150 km echoes displaying descending features resembling the descending ion layer behavior are presented the descending pattern is intriguing since it indicates the possible role of density gradient associated with the ion layer in generating the irregularities responsible for 150 km radar echoes not envisioned before unusually strong snr and narrow spectral features of these echoes clearly indicate the role of sharp density gradients in the echoing process given the fact that gadanki is located at magnetically low latitude it is argued that these descending echoing layers are produced by interchange instability on the gradient of daytime descending ion layer formed by meridional wind shear associated with tidal gravity waves quite similar to that observed during nighttime the observations reported here are first of its kind illustrating the role of density gradient layers and are important in furthering our understanding on the puzzling 150 km echoing phenomenon

scale invariant sheath folds in salt sediments and shear zones abstract sheath folds are developed in a broad spectrum of geological environments in which material flow occurs including gravity driven surficial deformation in ignimbrites unconsolidated sediments and salt together with deeper level ductile shear zones in metamorphic rocks this study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales elliptical closures defining eye folds represent y z cross sections through highly curvilinear sheath folds our analysis of the published literature coupled with field observations reveals remarkably similar ellipticities r yz for sheath folds in metamorphic shear zones r yz 4 23 salt r yz 4 29 sediment slumps r yz 4 34 glaciotectonites r yz 4 48 and ignimbrites r yz 4 34 nested eye folds across this range of materials n 1800 reveal distinct and consistent differences in ellipticity from the outer r yz to the inner most r y z elliptical rings of individual sheath folds the variation in ratios from outer to inner rings r r yz r y z in gravity driven surficial flows typically displays a relative increase in ellipticity to define cats eye folds r r yz that are remarkably constant r 2 0 99 across 9 orders of magnitude sheath y axes range from 0 1 mm to 75 km our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain with r also reflecting the rheological significance of layering associated with original buckle fold mechanisms the scaling pattern of sheath folds reflects the length scales of the precursor buckle folds and width of deformation zones across a broad range of materials and environments with continued deformation the layering marking the original folds may become increasingly passive to define sheath folds these empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials strain rates and scales encompassing a variety of deformation settings

the maggia nappe an extruding sheath fold basement nappe in the lepontine gneiss dome of the central alps the lepontine gneiss dome represents a unique region of the central alps where oligocene miocene amphibolite facies grade rocks and fold nappes of the deepest tectonic level of the alpine orogenic belt are exposed in a tectonic window the cenozoic structures of the maggia nappe reveals a giant tens of kilometre scale tubular fold structure that cross cut through the surrounding lower penninic nappes from its root situated in the southern steep belt of the alps near bellinzona the mesozoic sedimentary cover of the maggia nappe is typical for the helvetic stratigraphic domain the age of formation of the lower penninic fold nappes by ductile detachment of the upper european crust during its underthrusting below the higher penninic and austroalpine orogenic lid and adriatic indenter was estimated between 40 and 30 ma maximal pressures of 8 9 kbars and temperatures of 600 700 c were attended during and after the nappe emplacement some 30 22 ma ago the maggia and surrounding nappes are crosscut by the isograds of the barrovian regional metamorphism

high resolution observations of 150 km echoes at jicamarca high resolution soundings of equatorial upper e region echoes at jicamarca exhibit gravity wave modulated signal intensity distributions and doppler velocities corresponding to vertical plasma drifts a gravity wave wind driven interchange instability is suggested as a possible generation mechanism of the field aligned plasma irregularities responsible for the echoes the echoes may be utilized in gravity wave studies and in zonal electric field measurements by radars with modest sensitivities

alteration associated with gold deposition at the getchell carlin type gold deposit north central nevada wall rock alteration at the getchell underground deposit was examined to determine the effects of au bear ing fluids on host lithologies and the relationship between k bearing alteration minerals and au deposition the major minor and trace element geochemistry of highly altered and mineralized to unmineralized rocks from the getchell deposit was quantified for more than 50 samples collected along 13 transects through calcareous siltstone and carbonaceous limestone and along one transect through a rhyodacite dike each transect in sedimentary rocks was collected along a single homogeneous bed that could be followed from high grade ore to moderately altered rock or waste rock analyses were obtained for 39 elements 10 oxides and loss on ignition using multiple techniques petrographic studies were integrated with geochemistry and x ray diffraction and electron microbeam analyses to identify ore and alteration minerals and to correlate mineralogy with geochemical fluxes geochemical analyses indicate that ti al zr and th behave as immobile elements immobility isocon diagrams show that significant hg sb se te tl and cs are typically added to the wall rocks with au fe is im mobile in some transects added to wall rocks in others and removed from rhyodacite host rocks minor s and w are usually added to the rock whereas minor cu and mo are variably added or removed sio 2 appears to be either added during alteration or remain constant elements that are consistently removed during alteration include ca mn sr and sc and minor mg ba and k na was significantly removed from the rhyodacite au correlates positively with ag hg sb se si te and tl and negatively with ca mn and sc element fluxes correspond with observations of mineral abundances and show that sulfidation and decar bonitization are spatially associated with gold mineralization au hg sb tl cu and possibly te and w were incorporated in trace element rich pyrite as host rock fe was sulfidized fe was added to mineralized rocks in some transects indicating that pyritization occurred in addition to sulfidation cs is a component of galkhaite a trace mineral that is relatively common in some parts of the getchell system decarbonitization removed as much as 95 wt percent calcite from mineralized rocks in some transects and is responsible for loss of ca mn sr and sc although sio 2 as and s are significantly added to carlin type systems they do not appear as major added components in the isocon diagrams this is because the halos for these components are much larger than the areas examined by the sample transects x ray diffraction analyses identified minerals in ore and waste samples and quantified clay minerals including illite and montmorillonite kaolinite was not identified or is present in low abundances in both mineralized and unmineralized rocks montmorillonite does not correlate with gold and textures are consistent with pore filling deposition of montmorillonite following au deposition illite abundance exhibits a positive correlation with au r 2 0 80 intergrowths of illite with ore stage pyrite support a genetic relationship between the two minerals and indicate that illite formed as part of the au related hydrothermal event mineral abundances and textures suggest that illite formed by alteration of k feldspar during au deposition results show that dating of k bearing phases in sedimentary rocks other than illite would provide ages unrelated to gold deposition

structural evolution in thrust belts and relative plate motion the upper pennine piemont zone of the internal alps southwest switzerland and northwest italy the earliest ll extension lineation of the polyphase deformation sequence in the combin and zermatt saas fee units of the piemont zone of the upper pennine nappes trends roughly nw se and the geochronological and stratigraphical constraints indicate that the nappe transport and the associated stretching lineation occurred between about 100 and 70 ma this accords in a general way with the relative motion of the african plate to the european plate during the late cretaceous as determined from reconstructions of the opening of the atlantic ocean and suggests that late cretaceous thrusting in the swiss alps was driven fairly directly by the africa europe plate motion

high grade reworking of central australian granulites part 1 structural evolution abstract four distinct deformational events d2 d5 accompanying a granulite facies metamorphic cycle m2 m5 are shown to structurally overprint rework pre existing granulites m1 of the northeast strangways range in the central arunta block the first metamorphic cycle m1 at 1800 ma involved an anticlockwise p t path peaking at 850 950 c at 8 9 kbar m1 metamorphism involved widespread partial melting that produced stromitic migmatites and map scale concordant granitic gneisses isobaric cooling after the peak and accompanying hydration gave rise to a wide variety of coronitic and symplectitic reaction textures that enclose and replace m1 mineral parageneses no kinematic structuring is associated with m1 and the period encompassing the thermal peak and immediately subsequent are thought to have been absent of deviatoric stress the second metamorphic cycle m2 m5 possibly at 1400 1500 ma involved a clockwise p t path with a maximum p of 9 10 kbar and a thermal peak of approximately 800 c this metamorphic cycle accompanied the major ductile deformations recognized d2 d5 and has been labelled the proterozoic reworking the proterozoic reworking has been divided into two thermo barometric and structurally distinct periods d2 d3 involved regionally inclined ene wsw non coaxial shear of high bulk shear strains giving rise to an intense often mylonitic pervasive fabric s2 l2 and isoclinal and sheath folds on all scales f2 f3 s2 l2 is the first recognized kinematic fabric and does enclose and partially recrystallize m1 mineral parageneses including late m1 metamorphic reaction textures d2 d3 deformation was due to crustal shortening and gave rise to crustal over thickening loading accompanying prograde metamorphism m2 d4 d5 involved upright open and asymmetrical folding f4 and e w trending shear zone development s5 within a regionally extensive system of inclined oblique sinistral transpression both d2 d3 and d4 d5 episodes occurred under the same approximately e w directed compressive stress and are considered sequential episodes in the one tectonothermal cycle f4 folding accompanied significant melt formation and the peak of metamorphism of the m2 m5 metamorphic cycle s5 shear zones accompanied 3 4 kbar of decompression with cooling presumably during uplift in isostatic response to crustal over thickening in d2 d3 progression from inclined non coaxial shear to transpression occurred in response to an increase in the relative buoyancy forces limiting crustal thickening as is typical of many compressional orogens however the proterozoic reworking is atypical of simple linear or arcuate mountain belts because late stage shearing s5 is aligned sub parallel not at a high angle to the transport vector of the earlier ductile deformations d2 d3 consequently this region is modelled in terms of crustal shortening directed along the length of an e w trending orogen confined to the north and south by relatively stable crustal blocks when e w shortening could no longer be accommodated by crustal thickening strain was partitioned by sinistral transpression between the north and south bounding crustal blocks

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

the motloutse complex and the zimbabwe craton limpopo belt transition in botswana abstract the archaean rocks of northeastern botswana are divided into named lithostratigraphic complexes each a different assemblage of stratiform lithologies granitoids and gneisses and each with differences in metamorphic grade structural style and metallic mineralisation the motloutse complex lies between four other complexes it includes granitoids and orthogneisses typical of the zimbabwe craton to the east detached segments of the quartzite carbonate pelite association of the anomalous matsitama schist belt to the west and migmatites similar to those of the limpopo belt to the south but is itself a distinctive lithological assemblage it shows a higher grade of metamorphism than the adjacent cratonic terranes but lacks the granulites of the limpopo belt it is delineated by relatively abrupt changes in structural style and fabric orientation of which most if not all coincide with shear zones or faults it is partly bounded against the limpopo belt by the magogaphate shear zone but also flanks the western boundary of the limpopo belt central zone west of 27 35 e the motloutse complex was formed in the late archaean between 2 7 ga and 2 5 ga ago in a zone of plutonism which separated a crustal fragment bearing the depositional basin of the matsitama shelf type sequences from the developing greenstone belts to the east during regional deformation elements of the matsitama belt and associated paragneisses were thrust from the southwest over the active magmatic zone and were then infolded with and underplated by migmatites orthogneisses and granitoids of the developing motloutse complex the terranes which were thus accreted to the southwestern margin of the zimbabwe craton during this episode of deformation and crustal thickening d1 d3 were then cut across by dextral movement on the magogaphate shear zone during d4 so juxtaposing the limpopo belt with the motloutse complex the area of the motloutse complex was previously regarded as a transitional boundary between the craton and the mobile belt but is here seen instead as an intracratonic accretionary assemblage on which the ene wsw limpopo trend was partly superimposed during d4 shearing delineation of lithostratigraphic complexes elsewhere in the zimbabwe craton and the limpopo belt may be possible and can be expected to clarify the pattern to which these provinces were assembled during the archaean prior to d4 deformation

kinematic and temporal relationships between parallel fold hinge lines and stretching lineations a microstructural and crystallographic preferred orientation approach folds with hinge lines subparallel to stretching lineation are common features in shear zones and represent extreme cases of fold modification however their microstructures and in particular their kinematic significance remain poorly understood here we describe the meso and micro scale structures and crystallographic preferred orientation cpo of quartz and muscovite from a metric scale shear modified fold from plattjen saas fee western internal alps the studied fold is of class 2 and the samples collected around the fold have essentially the same microstructures quartz c axes plotted in the tectonic coordinate system exhibit an apparent reverse obliquity on opposing limbs muscovite fabrics are controlled essentially by the orientation of 001 planes if the cpo data are plotted in a common reference frame e g either fold or geographical all samples exhibit essentially the same patterns for both minerals in particular relative to the fold axis and axial surface the reversal of quartz cpo obliquity on opposing limbs is no longer evident the small obliquity 15 between the intersection lineation of the two planes of 001 of muscovite and the fold axis results in the apparent reversal of shear sense on opposite limbs when viewing the cpo related to the macroscopic fabric the homogeneity of the microstructures and cpos of quartz and muscovite across the fold support the interpretation that the fold developed during simultaneous activation of heterogeneous and homogeneous simple shear with the former responsible for fold development and the latter responsible for fold rotation and the overwhelming of the heterogeneous deformation these processes led to the observed parallelism between fold hinge line and local stretching lineation

3d visualization of sheath folds in ancient roman marble wall coverings from ephesos turkey abstract archaeological excavations and restoration of a palatial roman housing complex in ephesos turkey yielded 40 wall decorating plates of folded mylonitic marble cipollino verde derived from the internal hellenides near karystos greece cipollino verde was commonly used for decoration purposes in roman buildings the plates were serial sectioned from a single quarried block of 1 25 m3 and provided a research opportunity for detailed reconstruction of the 3d geometry of meterscale folds in mylonitized marble a gocad model is used to visualize the internal fold structures of the marble comprising curtain folds and multilayered sheath folds the sheath folds are unusual in that they have their intermediate axis normal to the parent layering this agrees with regional tectonic studies which suggest that cipollino verde structures formed by local constrictional non coaxial flow sheath fold cross section geometry exposed on the surface of a plate or outcrop is found to be independent of the intersection angle of the fold structure with the studied plane consequently a single surface cannot be used as an indicator of the three dimensional geometry of transected sheath folds

ionospheric variability over indian low latitude linked with the 2009 sudden stratospheric warming in this paper we analyze radar observations of e b drift and plasma irregularities ionosonde observations of e and f layer parameters including spread f and magnetic field observations made from indian low latitudes linked with the 2009 sudden stratospheric warming ssw event e b drift variations presented here are the first of their kind from the indian sector as far as the effect of ssw is concerned difference of magnetic fields observed from the equator and low latitude h and e b drift show linear relation and both show remarkably large positive values in the morning and negative values in the afternoon exhibiting semidiurnal behavior remarkable changing patterns in the critical frequency of f2 layer fof2 and f3 layer fof3 were observed after the occurrence of ssw large variations with quasi 16 day periodicity were observed in h fof2 and fof3 both semidiurnal and quasi 16 day wave modulation observed after the 2009 ssw event are consistent with those reported earlier we also noted quasi 6 day variations in h and fof2 soon after the ssw commencement not much reported before during the counterelectrojet events linked with the ssw event while equatorial es esq disappeared as expected there were no blanketing es esb a finding not reported and discussed earlier esb was also not formed at the off equatorial location indicating the absence of required vertical wind shear but e region plasma irregularities were observed by the ionosonde and radar with a close relationship between the two weak f region irregularities were observed in the postmidnight hours and case studies suggest the possible role of ssw related background electric field in the manifestation of postmidnight f region irregularities

sheath fold formation around slip surfaces terra nova 24 417 421 2012 abstract sheath folds are associated with shear zones and have been used as high strain indicators we present results from a three dimensional analytical study showing that the flow perturbation around a planar weakness acting as a slip surface in simple shear leads to the formation of sheath folds at low shear strain closed traces of marker layers are exhibited in cross sections cut perpendicular to the shear direction in the vicinity of the slip surface tip in cross sections cut parallel to the shear direction and normal to the shear plane flanking folds are observed our model captures the first order observations of natural sheath folds such as diversity of shapes and multiple eye structures natural equivalents of the planar weaknesses that we model include fractures veins and rheologically weak layers flow perturbation around these zones of weaknesses is proposed as an alternative mechanism of sheath fold formation which emphasizes the coeval operation of brittle and ductile processes in shear zones

high temperature deformation in the basal shear zone of an eclogite bearing fold nappe sveconorwegian orogen sweden ductile shear zones associated with emplacement of high pressure nappes are key features to resolve exhumation mechanisms the eastern segment of the sveconorwegian orogen hosts an eclogite bearing fold nappe whose basal shear zone shows structures associated with emplacement of the eclogite bearing nappe and decompression under high to intermediate pressure granulite and upper amphibolite fades conditions based on detailed structural mapping of a 4 km well exposed section of the basal shear zone we describe two major phases of deformation an early deformation stage d1 formed a penetrative gneissic foliation and tectonic layering including isoclinal folds f1 the sequence was subsequently affected by up to km scale tight south vergent folds f2 with sheared out limbs at the outcrop scale asymmetric f2 folds are commonly s vergent but symmetric folds with different degrees of tightness are also present melt was present at all stages of deformation and the structural relations demonstrate mutual feedback between melt localization and fold formation f2 folds have shallowly e plunging fold axis parallel to a stretching lineation defined by high grade mineral aggregates both constitute prominent structures of the basal shear zone f2 folds are associated with an axial planar fabric s2 defined by upper amphibolite and locally granulite facies mineral assemblages d2 shear structures are associated with top to the east kinematic indicators throughout the section the d1 episode was responsible for emplacement of the eclogite nappe into its present structural position the subsequently developed lineation parallel folds are interpreted to form by general shear where the structures reflect the regional e directed flow of the entire eclogite nappe c 2015 elsevier b v all rights reserved less

on a summer maximum in the occurrence frequency of 150 km f1 radar echoes over pohnpei 1 using three and a half years of 50 mhz radar data collected from pohnpei federated states of micronesia 6 96 n 158 19 e geographic 0 3 magnetic dip latitude we show for the first time that the occurrence frequency of 150 km or f1 echoes has a broad but conspicuous maximum during northern summer in this longitude sector with minimal activity in other months given a seasonal similarity to sporadic e es we suggest that an es layer instability cosgrove and tsunoda 2002 generates a polarization electric field which maps along geomagnetic field lines to the f1 region there forms thin plasma sheets that provide gradients to excite 3 m scale plasma waves via an interchange process

rotation of fold hinge lines associated with simple shear during southerly directed thrusting ouachita mountains southeastern oklahoma abstract the broken bow uplift in southeastern oklahoma contains the oldest rocks exposed in ouachita orogenic belts and has experienced tour phases of deformation first generation folds are tight overturned sverging and have a well developed slaty cleavage second generation folds are coaxial with first generation structures are open to tight are inclined and reveal s vergence northerly dipping faults truncate second generation fold limbs third phase structures including recumbent folds and pencil structures are interpreted to be contemporaneous with s directed thrusting the hinges of earlier folds documented in the area least affected by thrusting are horizontal and trend e w the hinges of folds of the same generation within fault zones plunge 30 50 toward the north or northwest thus it is concluded that the earlier folds have been passively rotated during s direeted thrusting as have the contemporaneous folds and pencil structures a simple shear model associated with s directed thrusting is proposed to explain these geometrical relations the rotated fold hinges provide quantifiable shear strain gauges which can be used to quantify shear strain in the broken bow uplift as well as in other orogenic belts a model for shear strain calculation is developed using the general geometric relationship between fold hinge lines the shear direction and the shear plane

quiet variability of equatorial e b drifts during a sudden stratospheric warming event 1 we present strong evidence that during the january 2008 minor sudden stratospheric warming ssw event the equatorial vertical e b drifts exhibit a unique and distinctive daytime pattern we do not think one event causes the other however both events might be related through the global effects of planetary waves the drifts were measured by the jicamarca incoherent scatter radar located under the magnetic equator we have observed an anomalous temporal variation of the vertical e b drifts during the minor ssw event showing a semidiurnal variation with very large amplitudes lasting for several days large differences in the e b drifts were observed during a period of large increase of temperature and a large decrease of mean zonal wind in the high latitude stratosphere 60 90 n this high correlation is an unexpected finding which might shed new light on sources and mechanisms of quiet time ionospheric variability

when shearing is a cause of folding abstract numerous publications on shearing as a cause of folding do not refute the idea of buckling being the main process responsible for the formation of fold structures deformations of other types may either produce conditions for layer parallel shortening needed for buckling or modify fold shapes but do not generate new fold structures on any scale the origin of similar folds commonly considered to be a result of inhomogeneous shearing across layering may be explained by buckling of a multilayer where layers have uniform favourable rheological properties tightening and amplification of folds near fault surfaces is not necessarily a result of shearing caused by friction at fault surfaces on the contrary in these cases faulting is a result of strain localization during folding recumbent isoclinal folds so common in metamorphic rocks originate mainly by buckling of subvertical planar structures intrafolial folds are usually very flattened relics of former folds the idea of sheath folds being produced mostly by shearing is not confirmed by the available geological data sheath folds that were not produced by shearing are rather common more likely they are formed by constriction or flattening of folds with curved hinges the latter are a result either of fold interference or of the constriction in the layering plane

kilometre scale sheath fold at mattmark and implications for transport direction in the alps folds were the first tectonic features of rocks to be studied and as such are taken as a reference for the interpretation of other structures intuitively fold axes were assumed to be subperpendicular to the direction of transport nevertheless many studies of metamorphic belts showing intense ductile deformation describe folds with axes close to the transport direction in particular sheath folds1 9 are highly curvilinear folds elongated parallel to the direction of transport we discuss here a spectacular large sheath fold situated in a crustal shear zone in the monte rosa nappe swiss alps our analysis implies that both the geometry and significance of folds in the internal alps must be re examined we further demonstrate the existence of crustal ductile shear zones in an area of the alps where previously emphasis was generally placed on fold nappes the movements on these shear zones is often tens of kilometres

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

can crustal extension be distinguished from thrusting in the internal parts of mountain belts a case history of the entrelor shear zone western alps abstract criteria for the distinguishing between structures that accommodate crustal scale extension or crustal scale shortening may be applied to late orogenic shear zones developed in the internal parts of mountain belts this is done for a large shear zone that has been mapped in the nw italian alps around the internal basement massif of gran paradiso the structure involved is termed the entrelor shear zone it carries greenschist facies european continental rocks the brianconnais grand st bernard unit onto an old subduction complex of oceanic piemonte and continental gran paradiso material that preserve eclogitic assemblages evidence for the map scale continuity of the entrelor shear zone is presented together with kinematic data to show how folds stretching lineations and shear criteria relate to a regional episode of ese directed shearing however folds initiated systematically oblique to the shear direction on a ne sw axis with continued shearing the fold population remains skewed e w of the population of stretching lineations the shear zone may be traced for 70 km around the dome of the gran paradiso massif and displays a minimum displacement of 20 km this entrelor structure shows variable offset of metamorphic grade however for much of its length greenschist facies rocks are carried in its hanging wall onto eclogites in the footwall tracing these units regionally shows that this apparent extensional offset of metamorphic facies is the result of restacking of an originally inverted metamorphic sequence i e hp piemonte on lp brianconnais this contractional behavior is consistent with the profile of the shear zone relative to the modern surface the entrelor shear climbs up from generally buried in the west to generally eroded in the east the conclusion that the entrelor shear zone is probably a thrust rather than an extensional structure relies on linking outcrop data into a semi regional context false interpretations may result from examining only small parts of the structure however although these map and structural criteria are consistent with contractional kinematics associated with crustal shortening confirmation is required through linked petrological and isotopic studies the entrelor shear zone is thus a good illustration of the need for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of mountain belts

mechanisms for folding of high grade rocks in extensional tectonic settings abstract this review of structures developed in extensional high grade terrains combined with results of centrifuge analogue modelling illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust thickened orogen several extensional fold mechanisms such as folding within detachment shear zones are similar to those in contractional settings the metamorphic p t t path and not fold style or mode of formation is therefore required to determine the tectonic setting in which some folds developed other mechanisms such as rollover above and folding between listric normal shear zones and folding due to isostatic adjustments during crustal thinning are unique to extensional tectonic settings several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation it is shown that isoclinal recumbent folds refolded by open upright folds may develop during regional extension in the deep crust folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment or from enhanced back rotation of layers between normal shear zones during back rotation folding layers rotated into the shortening field undergo further buckle folding and all may rotate towards orthogonality to the maximum shortening direction this mechanism explains the presence of many transposed folds folds with axial planar pegmatites and folds with opposite vergence in extensional terrains examples of folds in high grade rocks interpreted as forming during regional extension included in this paper are from the grenville province of canada norwegian caledonides albany mobile belt and leeuwin complex of western australia ruby mountains in the basin and range province of nevada the at sund area of greenland the napier complex of enderby land in east antarctica and the kigluaik mountains in western alaska

ductile shear deformation of quartzite in an alpine crustal thrust ambin massif abstract in this paper we show that fabric analysis gives good results when the sampling is done in an homogeneous material in a well known tectonic environment a quartzitic layer situated in the autochtonous basement of the ambin massif below the nappe des schistes lustres french italian alps has been choosen because the first ductile deformation phase is clearly defined by the microstructures and because the metamorphic history is well known the quartz c axis diagrams are characterized by two asymmetric crossed girdles and the one containing the maxima is inclined westwards we propose that these fabrics result from plastic deformation of the quartzite occurring with the basal plane as a dominant gliding plane the fabrics define a shear sense towards the west the angle between the basal planes and the foliation which varies from 15 to 40 is at least qualitatively related to the macroscopic strain our study confirms the existence of a regional large crustal thrust which has occurred during the obduction of oceanic material schistes lustres on the european continent

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

the overprint of the alpine tectono metamorphic evolution on the hercynian orogen an example from the apuane alps northern apennines italy abstract the apuane alps represent a tectonic window in the nappe structure of the northern apennines in its lowermost part the apuan metamorphic complex i e autochthonous auct and massa unit reveals a polyphase structural pattern as the result of two main alpine events of tertiary age the first event is responsible for intense isoclinal folding and the development of a pervasive schistosity in greenschist facies the second is connected with the subsequent late stage uplift the pre triassic rocks in the core of the main isoclinal anticlines were severely tectonized completely transposing a pre existing foliation and a greenschist pre alpine synkinematic metamorphism the traces of which are obvious and widespread hercynian structures of megascopic mesoscopic and microscopic size are outlined here

radar observations of daytime 150 km echoes from outside the equatorial electrojet belt over gadanki 1 we present systematic observations of daytime 150 km echoes from gadanki 13 5 n 79 2 e 6 3 dip latitude the echoing region is confined to 142 162 km altitude and show forenoon descent and afternoon ascent displaying a necklace shape quite similar to that of the equatorial 150 km echoes signal to noise ratio velocity and spectral width are as high as 13 db above noise 5 to 35 m s 1 and 2 to15 m s 1 respectively the observed necklace shape is first of its kind from gadanki and the present observations have almost all characteristics of equatorial daytime 150 km echoes the gadanki radar observations hence provide first experimental evidence on the occurrence of daytime 150 km echoes outside the equatorial electrojet belt

further investigations on 150 km echoing riddle using simultaneous observations of 150 km and e region echoes from off electrojet location gadanki 1 in this paper we present a detailed study of the off electrojet daytime 150 km echoes based on simultaneous observations of daytime 150 km and e region echoes made on a few days per month during july 2005 to august 2006 using the gadanki radar daytime 150 km echoes are found to occur in all seasons over gadanki although echo occurrence is found to be maximum in equinoxes and minimum in summer no clear seasonal preference over gadanki has been observed in contrast e region echo occurrence made simultaneously shows a strong seasonal dependence with remarkably low occurrence in winter as compared to other seasons interestingly the 150 km echoes show a distinctly different morphology in winter as compared to other seasons echoes are observed in two distinct height regions in winter as compared to one broad region in other seasons also the strongest echo and the lowest altitude of echo occurrence are found to be associated with winter further a significant share of doppler velocities is found to be downward in winter as compared to predominantly upward velocities observed in other seasons no seasonal dependence in the spectral width of 150 km echoes however has been observed seasonally averaged doppler velocities are quite similar to those of the f region drifts reported from jicamarca again seasonally averaged drifts are found to be downward in winter afternoon as compared to upward velocities observed in other seasons notwithstanding the seasonal differences observed in the 150 km echoes echo occurrence is found to have a common feature of strong forenoon afternoon asymmetry with forenoon occurrence being greater these observations are discussed in the light of present understanding on the daytime 150 km echoing phenomenon

from plate tectonics to the design of the dul hasti hydroelectric project in kashmir india abstract the dul hasti hydroelectric scheme which consists of a 65 m high gravity dam a 10 6 kin long headrace tunnel and an underground powerhouse is being built on the chenab river a tributary of the indus river within the himalayan range of kashmir india this paper shows how the geological knowledge at the time of signature of the contracts between the national hydroelectric power corporation and a french consortium in september 1989 was radically modified through successive on site investigations geological field surveys and seismotectonic analyses carried out at various scales the actual geological tectonic geotechnical and hydrogeological conditions discovered in the last three years active faulting deep infilled graben sheath folds general mylonitisation of the rockmass confined aquifers with very high heads of water widespread opening along joints filled with silty material highly susceptible to untamping and piping under even low heads of water have led the designer to modify the tunnel layout the tunnel design and the construction methods in particular the assessment of tunnelling conditions requires modifying the tunnel boring machine and installing additional equipment for frequent advance investigations ahead of the tunnel face and special treatments of zones of low mechanical properties an identified joint filled with silty material under a high head of water will be crosscut by a drilling a complete ring of pressure relief holes crossing the jointaround the tunnel b a special grouting treatment under very high pressure including cement grouts and silica gels or in some cases resins and c increasing the final lining of the tunnel using a high strength concrete to bear the full ground water pressure on crossing rock formations with reduced mechanical properties a long and numerous rock bolts or full length adhesive anchors will have to be implemented near the tunnel head to prevent further softening of the rock during radial plastic deformation and b the external diameter of the tunnel will have to be increased to maintain the final internal diameter and the minimum foreseen concrete lining thickness the sequence of investigations performed since 1960 highlights the fact that the acquisition of such an extremely complex geological and tectonic database is a long difficult and ongoing process moreover it shows that a reliable estimation of geological tunnelling conditions geology hydrogeology old and active tectonics and geotechnics depends on an understanding of the present and past geodynamics of a given region at different scales from plate tectonics to fault scarps and microtectonics

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

vergence and facing patterns in large scale sheath folds the careful geometric analysis of minor structural detail elucidates the relationships and evolution of associated large scale curvilinear hinge geometries developed during wnw directed caledonian thrusting exposed in neoproterozoic moine psammites of the moine nappe reversals in the polarity of structural facing associated with minor folding mark the position of major sheath folds which parallel transport upwardly convex sheaths closing in the direction of thrust transport cored by older gneissose basement inliers are termed culminations whilst those opening in the transport direction and cored by moine psammites are termed depressions sheath folds are bisected by transport parallel and foliation normal culmination depression surfaces which separate not only the reversals in facing but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domains relative to the transport direction the sense of obliquity of minor z and s folds is thus dependent on position with respect to the surfaces of culmination and depression and not the fold axial surfaces surfaces of culmination and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome culmination on antiform saddle depression on antiform inverted saddle culmination on synform and basin depression on synform configurations the curvilinear hinges of minor folds may also be asymmetrical about the transport direction and within the plane of the regional foliation to define patterns of fold hinge line vergence classical concepts of fold limb vergence may thus relate to larger antiformal and synformal hinges whilst the fold hinge line vergence defines major curvilinear hinges associated with culminations and depressions major sheath folds may therefore be interpreted in terms of both minor fold hinge line and limb vergence coupled with fold axis obliquity and reversals in the polarity of structural facing the ability to recognise consistent and reliable structural relationships between facing and hinge obliquity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude 1999 elsevier science ltd all rights reserved

equatorial 150 km irregularities observed at pohnpei first observations of meter scale equatorial 150 km plasma irregularities outside the american longitude sector are reported the observations conducted at pohnpei western pacific with a wind profiling st radar demonstrate the feasibility of making ionospheric electric field measurements with low cost instruments and also reveal a potentially important anisotropy in the angular distribution of 150 km vhf radar echoes

sheath folds as discriminators of bulk strain type abstract nested elliptical closures defining eye folds represent classic y z cross sections through highly curvilinear sheath folds generated during intense ductile deformation in metamorphic rocks systematic analysis of 1425 such eye folds based on our own field observations and examples from the published literature reveals distinct and consistent differences in ellipticites measured from the outer r yz to the inner most r y z elliptical rings of individual sheaths the variation in overall aspect ratios from outer to inner rings is defined as r where r r yz r y z and may display a relative increase or decrease in ellipticity to define cats eye r bulls eye r 1 fold patterns respectively layer thicknesses may also be measured along the y axis parallel to the axial surface t y and at 90 to this along the z axis t z to define the ratio of t yz sheath folds generated during broadly simple shear deformation k 1 display mean r yz 4 61 t yz 3 31 and cats eye folds r 0 69 sheath folds generated during general shear also display cats eye folds with identical mean r 0 69 values but greater thickness variations and elliptical ratios t yz 4 35 r yz 5 76 thus within both simple and general shear dominated deformations the overall variation in layer thickness t yz and ellipticity of eye folds r yz increases with increasing deformation and a greater component of pure shear whilst the r value remains constant and reflects original fold patterns sheath folds formed during constrictional k 1 deformation display markedly lower aspect ratios r yz 2 42 and thickness variations t yz 2 94 together with distinctive bulls eye folds r 1 23 these empirical relationships suggest fundamental and universal constraints on curvilinear fold generation across this broad spectrum of deformation types and allow sheath folds to act as both effective 95 consistent and robust discriminators of bulk strain type

tectonic evolution of the yaounde segment of the neoproterozoic central african orogenic belt in southern cameroon the deformation history of the neoproterozoic central african orogenic belt in southern cameroon is well recorded in the low to high grade rocks outcropping in the area around yaounde the fabrics in these rocks are con sistent with two main ductile deformation events d1 and d2 d 1 predated emplacement of calc alkaline dioritic bodies and caused the formation of nappes that resulted in high pressure granulite metamorphism of soft sediments a strong overprinting of these nappes during d2 symmetric extension probably associated with large scale foliation boudinage and or gneissic doming and intense magmatic underplating gave rise to regional flat lying fabrics the latter were further buckled by d3 and d4 folding phases defining a vertical constriction occurring with a major east west to nw se shortening direction the corresponding f3 and f4 folds trend north south to ne sw and east west to nw se respectively and represent the main regional strain patterns based on the east west to nw se maximum shortening orientation indicated by f3 folds it is proposed that the nappe stacking phase d 1 occurred in the same direction the deformation history in the area can thus be described as corresponding principally to alternating east west to nw se contractions and north south to ne sw orogenic parallel extensions at the regional scale this could be due to the transaharan east west collisional system

on the linkage of daytime 150 km echoes and abnormal intermediate layer traces over sanya 1 the daytime 150 km echoes which are associated with the upper e region field aligned irregularities have been observed around the equatorial electrojet region but never before at magnetic latitudes near the northern anomaly crest region we present first results of daytime 150 km echo over sanya china a station located far away from the dip equator in the northern hemisphere a layer of weak radar echoes with spectral width less than 10 m s 1 was seen in the height range from 145 km at 12 15 lt to 152 km at 13 45 lt on 21 july 2010 the interesting aspect is that the rare observation of daytime 150 km echoes with the small sanya vhf radar was preceded by the occurrence of an unusual intermediate layer which is identified as abnormal traces at the upper e region in corresponding ionograms the abnormal intermediate layer associated with possible gravity wave activity that implicates the presence of upper e region density gradients could make a significant contribution to the growth of irregularities responsible for the rarely detected daytime 150 km echoes over sanya

relationships between distributed and localized shear in the tectonic evolution of a caledonian fold and thrust zone northwest ireland broad zones of distributed shear operating through mid crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete large magnitude displacements the relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation pre existing structural anisotropy strain rate and migration of fluids rigorous structural analysis of lower amphibolite facies dalradian metasediments in northwestern ireland demonstrates that an early d 1 discrete ductile detachment was subsequently reactivated during distributed non coaxial d 2 deformation operating in a broad zone through the structural pile regional shear was directed towards the southeast and resulted in the generation and translation of kilometre scale isoclinal recumbent sheath folds which close and face towards the transport direction the d 1 detachment is clearly folded around the hinges of these major folds whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear shear criteria along the detachment indicate a southeast directed translation of the major folds in sympathy with regional shear a broad zone of d 3 translation operating through the nappe pile resulted in coaxial refolding of large scale f 2 folds by the d 3 ballybofey nappe producing a complex fold interference pattern non coaxial d 3 deformation resulted in continued reactivation of local decollements together with the initiation of east southeast directed oblique thrusts and partial dismemberment of d 2 folds detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed

on the sheet like nature of 150 km f1 radar echoes 1 the so called 150 km or f1 echoes are shown to arise from field aligned irregularities fai that are embedded in a sheet like region using a 49 92 mhz radar on christmas island which has an antenna beam directed toward geographic east at a zenith angle of 29 5 high resolution doppler velocity variations within the radar scattering volume are used to infer that the sheet like region could have both tilt and curvature this finding is used to infer that plasma density n must be enhanced within the sheet like region and that fai could be generated by an interchange instability driven by a vertical electric field or a zonal neutral wind that acts on the horizontal component of the gradient in n how tilted sheet like structures could form in the f1 region is discussed

a computer model of sheath nappes formed during crustal shear in the western gneiss region central norwegian caledonides abstract the eastern western gneiss region of central norway is part of the deepest exposed norwegian caledonides where basement gneisses and an overlying thrust nappe sequence have been folded into large fold nappes structural analysis of a fold nappe within the central part of the district the grovudal area suggests that it has a strongly sheath like form and that other fold nappes of the western gneiss region may also have sheath like forms the structural history within the grovudal area is dominated by intense east directed subhorizontal shear in an overthrust sense followed by asymmetric refolding with an easterly vergence a computer generated kinematic model was developed to test whether the regional interference patterns could be explained by sheath fold development during this type of deformation the computer model shows that the major regional interference patterns could have been formed by such a kinematic history but does not rule out other possibile histories the proposed kinematic history is however compatible with the regional tectonic history of the main caledonian nappe pile suggesting that the complex nappe interference patterns typical of the region were formed in a kinematically simple but intense ductile deformation associated with caledonian continental imbrication

quartz fabric and strain partitioning in sheath folds an example from the voltri group western alps italy abstract a mesoscopic sheath fold defined by an almost pure quartzitic level in quartz mica schists of the voltri group southeastern border of the western alps was sampled and studied in detail microstructural analysis was carried out on thin sections from different structural domains in the quartzitic layer with the emphasis to analyse quartz shape fabrics and the crystallographic preferred orientation of the c axes of this mineral quartz was dynamically recrystallized but its degree of preferred orientation is not as high as it would be expected in a high shear strain regime required for the development of sheath folds this may suggest strain partitioning between the quartzitic level and the micaceous matrix crystallographic preferred orientation of c axes and grain shape fabric asymmetry indicate the same sense of shear on both fold limbs in the xz plane this is in agreement with a model of sheath fold development by the passive rotation of material lines clustering of the c axis poles shows a configuration indicative of a different strain state in different domains of the sheath fold and suggests an evolution of the fold characterized by combined simple shear and coaxial progressive deformation

experimental study of sheath fold development around a weak inclusion in a mechanically layered matrix article i nfo we present quantitative laboratory models investigating the mechanics of sheath fold formation around a weak inclusion in simple shear sheath folds are intriguing highly non cylindrical structures that stimulated extensive field and experimental studies leading to an ongoing debate concerning their formation and evolution through a parametric study we test the influence of a mechanically layered matrix on the devel opment of sheath folds using silicone models our models show how 1 the viscosity ratio between the layers in the matrix and 2 the layer thickness control the shapes of the resulting folds and their visibility all experiments with a weak inclusion resulted in strong deformation of the layers an increase in viscosity ratio however leads to less evolved sheath folds which are shorter and show a larger opening and dip angle in contrast to former studies we show that a mechanically layered matrix does not hinder the forma tion of sheath folds the visibility of the sheath folds in our models strongly depends on the aspect ratio be tween the inclusion height and the layer thickness we observed sheath folds for a ratio larger than 7 5 the experiments reproduced the first order features of natural sheath folds our results challenge previously pub lished studies where sheath folds were considered as purely passive structures we demonstrate that sheath folds readily develop around slip surfaces suggesting that this might be a relevant formation mechanism in nature

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

geometrical similarity in successively developed folds and sheath folds in the basement rocks of the northwestern indian shield an intensely deformed gneiss migmatite terrane and a relatively undeformed granulite granitoid terrane constitute the bulk of precambrian basement in the northwestern indian shield this article traces the structural evolution in the gneiss migmatite terrane where traditional methods of structural analysis are difficult to apply and shows how successively developed folds can assume identical geometry and orientation at an advanced stage of progressive ductile shearing the gneiss migmatite terrane exemplifies a regional scale ductile shear zone that preserves the history of polyphase folding and sheath folding geometrical similarity between individual domain scale sheath folds and mesoscopic regional scale folds implies that sheath folding is common at all scales in the gneiss migmatite terrane as the mylonite foliation that traces successive folds is curviplanar the successively initiated hinge lines were curvilinear from their inception in the shear zone at the advanced stage of ductile shearing the hinge line curvatures were accentuated due to their rotation towards subvertically directed maximum stretching x and variably oriented fold axial planes were brought into approximate parallelism with the upright principal plane xy of the bulk strain ellipsoid eventually all the folds irrespective of their relative order of development became strongly non cylindrical extremely tight isoclinal and approximately co planar with respect to each other it is due to the above geometrical modifications during ductile shearing that folds irrespective of their order of development now appear identical with respect to isoclinal geometry axial plane orientation and hinge line curvilinearity evidence from the fold orientations the deformed lineation patterns and the sheath fold geometry suggest that the shearing occurred in a general shear type of bulk strain and nnw sse directed subhorizontal compression resulted in subvertically directed stretching in the gneiss migmatite terrane

intense non coaxial shear and the development of mega scale sheath folds in the arunta block central australia abstract the strangways orogenic belt of 1800 ma granulites in the central arunta block was completely reworked at granulite facies grades during the middle proterozoic 1400 1500 ma the bulk strain regime of the initial episodes of this reworking is presented ductile reworking involved non coaxial deformation on a regional scale and produced map scale sheath folds high bulk shear strains are inferred on microscopic and macroscopic scales and the spatial variation in bulk shear strain throughout the region is discussed the high bulk shear strains were accommodated by east over west ductile ramping and fold repetition resulting in easterly inclined structures crustal shortening and crustal over thickening

daytime 150 km echoes observed with the equatorial atmosphere radar in indonesia first results 1 results of the daytime 150 km echoes from 10 36 s magnetic latitude observed using the equatorial atmosphere radar ear which are first of its kind from magnetic southern hemisphere are presented the echoing region of 145 165 km shows forenoon descent and afternoon ascent with signal intensity modulation with time resembling a necklace and the doppler spectra are narrow which are quite similar to those reported earlier from other locations westward and upward southward irregularity drifts are found to be consistent with daytime background electric fields providing a tool to investigate low latitude daytime electrodynamics detectability of these echoes suggests the absence of strong latitudinal dependence in contrast to the earlier belief suggesting a possibility of their detection at mid latitudes also the echoes show zonal anisotropy that is opposite to that observed over pohnpei the implications of these observations are discussed in the light of present understanding of 150 km echoing phenomenon

kinematic records of subduction and exhumation in the ile de groix blueschists hercynian belt western france deciphering between deformations related to subduction and exhumation in hp metamorphic rocks represents a challenge for the understanding of the dynamic processes involved and more particularly the exhumation mechanisms an analysis of shear criteria in the ile de groix blueschists in the hercynian belt of southern brittany western france is carried out in terms of relative chronology and in relation to lithology mainly micaschists and glaucophane bearing metabasites structural mapping shows that foliations are flat lying bearing stretching lineations oriented in a n150 e 30 direction associated shear criteria are dominantly top to northwest and locally in the southeast part of the island both top to northwest and southeast both shearing deformations lead to intense finite strains as demonstrated by the occurrence of sheath folds and the general orientation of fold axes parallel to the stretching lineation detailed mapping in this area showing opposite senses of shear demonstrates that only top to southeast shear is observed in rocks containing well shaped lawsonite pseudomorphs in rocks affected by top to northwest shear the lawsonite pseudomorph shape is destroyed by intense stretching the top to southeast shear is coeval with prograde hp metamorphism and precedes a top to northwest shear coeval with a retrograde metamorphic path in the frame of the hercynian belt of southern brittany the southeast directed shear is attributed to thrusting related to a northward dipping subduction and the northwest directed shear to exhumation controlled by a northward dipping extensional detachment

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

3d geometry and kinematic evolution of the wadi mayh sheath fold oman using detailed mapping from high resolution photography abstract the wadi mayh sheath fold in north eastern oman is one of the largest and best exposed sheath folds known and presents a unique opportunity to better understand this somewhat enigmatic style of deformation we undertook high resolution photographic surveying along wadi mayh to document the sheath fold in 61 georeferenced panoramic photomerges here we present ten such images that provide a structural interpretation of the sheath fold and surrounding structure we resolve this structure in a simplified three dimensional model and in two orthogonal cross sections and propose a kinematic evolution to explain the geometry the wadi mayh sheath fold is the most prominent example within what we suggest is a composite sequence of sheath folds which is itself enclosed within a ssw closing recumbent syncline at the base of the major saih hatat nappe sheath folding is accommodated within permian saiq formation limestones showing carpholite assemblages 6 8 kbar 275 375 c a major discontinuity separates this sequence from enveloping older rock units the sequence formed during progressive top to north ductile shearing as the overlying nappe migrated northwards with respect to the underthrusting hulw unit this process occurred during ssw directed exhumation of partially subducted continental crust in ne oman approximately 15 ma after obduction of the oman ophiolite initiated

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

geology of the northern part of the nanga parbat massif northern pakistan and its implications for himalayan tectonics the northern outcrop termination of indian continental crust lies at the nanga parbat massif from where this contribution presents new field data the tectonic contact with the structurally overlying kohistan island arc is concordant and ductile is associated with interleaving of nanga parbat and kohistan lithologies and may be correlated with the main mantle thrust found elsewhere in the nw himalayas this ductile shear zone is locally overprinted by cataclastic faults associated with exhumation of the massif but overall the northern outcrop termination of the massif is controlled by erosion through a gently northward plunging antiformal structure this folds both the main mantle thrust and the underlying concordant 9layered unit9 of the nanga parbat basement thus there is no indication that the massif acted as a promontory to the indian continent during collision nor that it is a structure entirely bound by neotectonic faults ductile shear fabrics associated with the 9main mantle thrust9 are cross cut by leucogranite sheets and pegmatites these may represent the stockwork to a significant crustal melt granite body described here in the northern massif this jutial granite shows many geochemical characteristics in common with similar bodies in the high himalayas which are consistent with anatexis of the buried indian continental basement rocks however the granite is enriched in heat producing elements particularly th 22 ppm and shows extremely high 87 sr 86 sr ratios 0 88 the high concentrations of radiogenic sr are also an attribute of a suite of hitherto enigmatic leucogranitic pegmatites that laces the nanga parbat massif suggesting that these may represent a stockwork to a largely buried body of which the jutial granite is a small exposure the enrichment in heat producing elements within the granite may reflect similarly high heat production in the source indian continental crust requiring in turn a fundamental re examination of the thermal evolution of this crust during himalayan metamorphism and exhumation

late permian early triassic orogeny in japan piling up of nappes transverse lineation and continental subduction of the honshu block abstract field surveys in the oga atetsu and yamaguchi areas of southwest japan have been conducted in order to precise the structure of the permian orogen a stack of nappes is recognized comprising from top to bottom 1 the oga nappe which is considered to be a seamount complex 2 hp sangun metamorphics 3 the permian yakuno ophiolite and 4 the permian detrital maizuru group which is interpreted as the sedimentary cover of a continental block called here the honshu block outcropping as the older granite this stack of nappes is overthrust by the paleozoic hida basement consisting of ht gneisses granites and late carboniferous shallow water sediments microtectonic analysis of the sangun schists shows that the subhorizontal schistosity bearing a submeridian lineation was formed during the synmetamorphic phase asymmetric pressure shadows shear bands and sigmoidal minerals show that the synmetamorphic deformation corresponds to a ductile shear from north to south the permian early triassic orogeny is interpreted as the result of a collision between the hida gneiss or south china block and the honshu block the intervening oceanic area gave rise to southward directed nappes the permian orogenic belt extends at least from taiwan to central japan

the geometry and topology of natural sheath folds a new tool for structural analysis abstract curvilinear sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the x y axial surface and the x z medial culmination depression surface which bisects the fold nose however 10 000 geometric analyses of minor folds and fabrics formed during ductile thrusting in the caledonides of northern scotland reveals that major dome and basin sheath folds can display distinct and predictable asymmetries across both axial and medial surfaces the strain is typically heterogeneous so that structural fabrics and younging evidence are preserved within sheath folds at varying stages of development this allows an analysis of the evolution of such structures from tongue folds to more extreme tubular forms geometric relationships between measured orientations of fold hinges axial planes extension lineations and foliations are compared on fabric topology plots ftps which provide an effective tool for monitoring planar and linear fabric rotations with increasing progressive non coaxial deformation they consistently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces and on crossing medial surfaces from short to long hinge line segments axial and medial surfaces effectively therefore divide major sheath folds into quadrants with different amounts senses and combinations of planar and linear fabric rotation within each domain such heterogeneous deformation implies that models of intense non coaxial deformation uniformly affecting pre existing folds may overestimate bulk displacement and shear strain variable fold hinge line rotation about medial surfaces also provides an effective mechanism for the closure of major sheaths which may otherwise project for unfeasible distances in the x direction bedding cleavage intersections are developed at greater angles to the transport direction than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing in cross section asymmetric tear drop eyes indicate the sense of fold hinge line vergence whilst fold limb and hinge line asymmetry may be combined on 3 d vergence ranking and rotation grids which allow location and relative strain states of minor structures to be accurately predicted within the overall sheath framework systematic fabric analysis on ftps may be applied to the investigation of ductile deformation across a broad range of scenarios

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions