microstructures and c axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites abstract quartzites sheared in low metamorphic grade shear zones that extend along the contact with migmatitic gneissic complexes in the quadrilatere ferifero region southeastern brazil show quartz microstructures indicating deformation by dissolution dissolution occurred adjacent to the foliation planes preferentially in quartz porphyroclasts oriented with their 0001 planes parallel to the foliation dissolution worked as an efficient mechanism to remove porphyroclast edges smoothing foliations and enabling strain accommodation both dissolution and crystal plasticity were competitive crystallographically controlled processes operating in different domains of the evolving microstructure dissolution occurred along the foliation planes while isovolume crystal plasticity and accompanying recrystallization operated essentially in the quartzose domains between folia phyllonites formed in the most strained domains of the shear zones fine grained quartz grains in phyllonites show uncommon c axis fabrics c axis oriented at low angles to the stretching lineation which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution domainal c axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer strengthening c axis maxima at low angles with the stretching lineation at the expense of crystal plastic processes during progressive phyllonitization

structural geology and regional tectonic significance of the ramgarh thrust himalayan fold thrust belt of nepal 1 the ramgarh thrust is one of the major fault systems of the himalayan thrust belt in nepal and northern india the ramgarh thrust sheet is 0 2 2 0 km thick and can be traced along strike the entire length of the himalaya in nepal the fault generally places the oldest paleoproterozoic rocks in the lesser himalayan series upon younger lesser himalayan rocks or lower miocene foreland basin deposits regional balanced cross sections suggest that the ramgarh thrust had at least 120 km of initial south vergent displacement subsequently the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying lesser himalayan rocks ramgarh hanging wall strata are greenschist grade phyllite quartzite and augen gneiss all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top to the south sense of shear structural fabrics in the ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet regional and local mapping of the ramgarh thrust in nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat except where local lateral ramps complicate its geometry similarly the structurally overlying main central thrust always places a hanging wall flat in greater himalayan series rocks upon the regionally flat ramgarh thrust sheet these geometric relationships preclude kinematic and thermal models that elevate greater himalayan and lower lesser himalayan rocks along high angle thrust ramps in the vicinity of the present traces of the ramgarh and main central thrust faults instead the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the main central thrust the present steep dips of the ramgarh and main central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the lesser himalayan zone the ramgarh thrust sheet overlaps a bed length of at least 100 km in lower miocene foreland basin deposits indicating that a significant amount of displacement on the thrust must have occurred after 15 ma growth of the lesser himalayan duplex and additional slip on the frontal part of the ramgarh thrust occurred from 12 to 5 ma the presence of a major greenschist grade metasedimentary thrust sheet composed of lesser himalayan rocks directly below the main central thrust suggests that the famous inverted metamorphism in this region is a result of structural inversion similarly the concept of a broad zone of intense shear strain related exclusively to emplacement of the main central thrust sheet is probably invalid in nepal

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

quartz recrystallization regimes c axis texture transitions and fluid inclusion reequilibration in a prograde greenschist to amphibolite facies mylonite zone ribeira shear zone se brazil abstract the crystal plastic behavior of quartz mylonites from the ribeira shear zone se brazil a major strike slip structure that was active during a prograde metamorphic phase related to the neoproterozoic brasiliano pan african orogeny was investigated using a multi method approach geothermobarometry results indicate deformational conditions ranging from 300 to 630 c and 500 700 mpa a strong correlation between mapped metamorphic zones and a dominance of different dynamic recrystallization mechanisms of quartz occurs within the mylonite zone bulging recrystallization blg dominates within the chlorite zone between 300 and 410 c subgrain rotation recrystallization sgr operates within the biotite zone from 410 to 520 c and grain boundary migration recrystallization gbm dominates in the garnet zone above 520 c the development of quartz c axis textures is mainly governed by temperature and dynamic recrystallization mechanisms textures from blg zone mylonites are characterized by maxima around z sgr zone mylonites display single girdles or asymmetric type i crossed girdles and gbm zone mylonites comprise maxima around y and intermediate between x and z the scarcity or absence of water bearing fluid inclusions in quartz mylonites from the sgr and gbm zones which are dominated by carbonic inclusions suggests water deficient conditions whereas blg zone mylonites are dominated by water bearing inclusions this evidence indicates that water was available in the protoliths but has been eliminated with increasing deformation and deformation temperature no effect of the water content variation on the quartz microstructural and recrystallized grain size evolution was detected and little influence on c axis texture development was observed most of the fluid inclusion densities were reequilibrated during the shear zone exhumation history recording a decompression in the range of 300 500 mpa while microstructural reequilibration effects related to the prograde metamorphism are largely preserved fluid inclusion microstructures and densities from two sgr zone samples preserved evidence for a near isothermal compression within the interior of the ribeira shear zone during the prograde metamorphism

a p t t d discontinuity in east central nepal implications for the evolution of the himalayan mid crust abstract rock specimens collected from lower middle and upper portions of the exhumed himalayan mid crust in the upper tama kosi region of east central nepal yield pressure temperature time deformation p t t d paths that demonstrate the presence of a structural metamorphic and geochronologic discontinuity a p t t d path from below the discontinuity defines a loading path with deformation at staurolite grade metamorphic conditions occurring at 10 8 ma p t t d paths from above the discontinuity in contrast record an earlier and protracted metamorphic history that includes decompressional heating monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be 21 ma with decompression related garnet breakdown initiating at c 19 ma and likely continuing until c 15 ma these contrasting p t t d histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen these results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen

low latitude ionospheric d region dependence on solar zenith angle phase and amplitude measurements of vlf radio signals on a short nearly all sea path between two hawaiian islands are used to find the height and sharpness of the lower edge of the daytime tropical d region as a function of solar zenith angle sza the path used was from u s navy transmitter npm 21 4 khz on oahu to keauhou 306 km away on the west coast of the big island of hawaii where ionospheric sensitivity was high due to the destructive interference between the ionospherically reflected wave and the ground wave particularly around the middle of the day the height and sharpness are thus found to vary from h 69 3 0 3km and 0 49 0 02 km 1 for sza 10 at midday to h 80 km and 0 30 km 1 as the sza approached 70 90 near dawn and dusk for this tropical path additional values for the variations of h and with solar zenith angle are also found from vlf phase and amplitude observations on other similar paths the short path nwc to karratha in nw australia and the long paths nwc to kyoto in japan and nau puerto rico to st john s canada significant differences in the sza variations of h and were found between low and middle latitudes resulting from the latitudinally varying interplay between lyman and galactic cosmic rays in forming the lower d region both latitude ranges showed 0 30 km 1 during sunrise sunset conditions 1

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

effects of st patrick s day geomagnetic storm of march 2015 and of june 2015 on low equatorial d region ionosphere d region effects of the 17 19 march 2015 a st patrick s day super geomagnetic storm dst 223 nt using a navigational transmitter very low frequency vlf signal nwc 19 8 khz recorded at a low latitude indian station allahabad geomag lat 16 45 n have been analyzed and compared with similar strength of the 22 25 june 2015 storm dst 204 nt during the march storm nwc signal amplitude decreased on 17 march main phase of the storm and recovered on 27 march which is 1 day after the recovery of the storm whereas for the june storm vlf amplitude decreased for 2 days only during its recovery phase the decrease in the amplitude was pronounced during evening terminator for both the storms the modeling of vlf signal anomaly on 17 march and on 25 june using long wave propagation capability code shows an increase in the d region reference height h by 2 6 km and 2 5 km for march and june storms respectively the d region electron density ne determined using storm time h and sharpness factor shows a decrease in the ne during the main phase followed by a slow recovery during the recovery phase of the march storm whereas june 2015 storm showed a decrease in the ne only on 25 and 26 june morlet wavelet analysis of the amplitude for both the storms shows a presence of strong wave like signatures suggesting propagation of atmospheric gravity waves traveling ionosphere disturbances to the low latitude d region due to the joule heating at high latitudes

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

heterogeneous brittle ductile deformation at shallow crustal levels under high thermal conditions the case of a synkinematic contact aureole in the inner northern apennines southeastern elba island italy abstract we present an example of interaction between magmatism and tectonics at shallow crustal levels in the late miocene the metamorphic units of the eastern elba island northern apennines were intruded at very shallow crustal levels by a large pluton 60 km 2 with the development of an hectometre sized contact aureole defined by growth of low pressure high temperature mineral assemblages p max max 650 c structural data show that the contact aureole is associated with a km sized antiform of the foliation and by several metre to decametre thick high strain domains consisting of strongly foliated rocks containing synkinematic ht lp mineral assemblages and ductile shear zones of variable thickness these shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration associated with the thermal peak of contact metamorphism to subgrain rotation and bulging recrystallization the latter mostly associated with the cataclastic overprint these transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation deformation accompanied the development and cooling of the contact aureole which recorded the switch from high temperature ductile to low temperature brittle conditions the geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks

heterogeneous sub simple deformation in the gol e gohar shear zone zagros sw iran insights from microstructural and crystal fabric analyses petrofabrics and microstructural characteristics of the quartz rich mylonites were investigated in the nw se trending gol e gohar shear zone with length of ca 25 km within the zagros transpression zone sw iran the semi quantitative vorticity analysis using quartz c axis fabric rxz method and rigid porphyroclast geometry rigid grain net method reveals that mean kinematic vorticity number wm increases towards the baghat thrust fault displaying the occurrence of spatial and temporal variations of flow during progressive deformation the wm values show pure shear dominated flow progressively was changed to a simple shear dominated flow toward to the late stages of ductile deformation implying that the accelerating flow path was accompanied with progressive cooling and strain localization at the lower parts of the zone the measured vertical thinning dip parallel elongation suggests that thrust emplacement and metamorphic rock extrusion likely occurred by a combination of simple shear with a pure shear component of deformation i e general shear the estimated deformation temperatures based on the quartz c axis fabric patterns and their opening angles highlight greenschist to amphibolite facies conditions

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

deformation temperatures and flow vorticities near the base of the greater himalayan series sutlej valley and shimla klippe nw india abstract we report new deformation temperature and flow vorticity data from the base of the greater himalayan series ghs exposed in the sutlej valley and shimla klippe of nw india we focus on three groups of transects across the hanging wall of the main central thrust mct in order of relative foreland hinterland positions they are the shimla klippe western and eastern sutlej transects deformation temperatures indicated by quartz c axis fabric opening angles increase both from foreland to hinterland at a given structural distance above the mct and up structural section from the mct within individual transects deformation temperatures in the immediate hanging wall to the mct are estimated at 510 535 535 550 and 610 c on the shimla western sutlej and eastern sutlej transects respectively the steepest inferred field gradients in deformation temperatures are recorded adjacent to the mct and progressively decrease up structural section following a power law relationship comparison with temperature estimates based on multi mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions vorticity analyses indicate that shearing along the base of the ghs occurred under sub simple shear conditions wm values of 0 9 1 0 with a minor component of pure shear

middle crustal ductile deformation patterns in southern tibet insights from vorticity studies in mabja dome kinematic kinematic vorticity wm and deformation temperature analyses were performed to test the hypothesis that mid crustal rocks exposed in mabja dome southern tibet were penetratively deformed within a southward flowing mid crustal channel during the late eocene early oligocene to early miocene outcrop and thin section kinematic indicators show a downward transition from mixed top n and top s shear in chloritoid and garnet zone rocks through dominantly top s shear in garnet and kyanite zone rocks to solely top s shear in staurolite zone and deeper rocks along mineral elongation lineation parallel transects wm in schists and orthogneisses decreases with structural depth from w0 80 w40 pure shear to w0 55 w63 pure shear deformation temperature increases from w450 ci n

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

the famatina complex nw argentina back docking of an island arc or terrane accretion early palaeozoic geodynamics at the western gondwana margin

significance of transport parallel strain variations in part of the raft river shear zone raft river mountains utah usa this paper examines part of the raft river shear zone rrsz in northwestern utah that exhibits an extreme transport parallel increase in strain intensity coupled with a transition from flattening to constrictional strain detailed geologic mapping and finite strain quartz c axis fabric and kinematic vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport parallel elongation accommodated by a stretching fault at the base of the shear zone the domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks this domain is characterized by strain in the constrictional field caused by transport perpendicular flow into the area of high transport parallel elongation where rocks cut by the stretching fault are rheologically strong the rrsz locally records flattening strain and lower strain intensities limiting the amount of stretching needed at the base of the shear zone in any one direction and or recording transport perpendicular flow into adjacent highly extended domains the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone normal shortening and transport parallel elongation and these observations provide an example of deformation partitioning in a crustal scale structure driven by local rheological transitions

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

on the origin of orogens in order to understand how orogens work a quantitative approach demonstrating proof of concept is essential our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of working numerical models that are consistent with both the underlying physics and first order geological constraints we present a simple conceptual temperature magnitude t m framework for orogenesis in terms of the progression from small cold to large hot orogens and we use forward numerical models to test hypotheses corresponding to specific stages along the t m spectrum small cold orogens are analyzed using crustal scale singularity s point models in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics the transition from oceanic subduction to continental collision and the subsequent evolution of large hot orogens has been investigated using both crustal and upper mantle scale models the latter including dynamic subduction of suborogenic mantle lithosphere large hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot weak lower crustal infrastructure beneath plateaus tectonic processes are dominated by ductile flow of weak crust in response to differential pressure while plateau flanks form external thrust sense wedges we discuss four topical issues in orogenic tectonics including the response of the suborogenic mantle lithosphere to convergence the interaction of climate and tectonics the current debate concerning wedge versus channel flow models to explain the himalayan tibetan system and the interpretation of metamorphic architecture in terms of orogenic processes we conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins accompanied by shortening thickening and heating of deformed crust

leucogranite intruding the south tibetan detachment in western nepal implications for exhumation models in the himalayas the most popular models regarding the exhumation of the greater himalayan sequence ghs such as extrusion channel flow critical taper and wedge extrusion require prolonged activity of the two bounding shear zones and faults the main central thrust mct and the south tibetan detachment std we present the crystallization age of an undeformed leucogranite that intrudes both the ghs and the tethyan himalaya sequence ths zircon and monazite u pb ages in the leucogranite give ages between 23 and 25 ma constraining at that time the end of shearing along the std our results limit the contemporaneous activity of the mct and std to a short period of time 1 2 ma and thus argue against exhumation models requiring prolonged contemporaneous activity of the mct and std

response of the low latitude d region ionosphere to extreme space weather event of 14 16 december 2006 the response of the d region low latitude ionosphere has been examined for extreme space weather event of 14 16 december 2006 associated with a x1 5 solar flare and an intense geomagnetic storm dst 146 nt using vlf signals from northwest cape australia nwc 19 8 khz and lualualei hawaii callsign npm 21 4 khz transmitters monitored at suva geographic coordinates 18 10 s 178 40 e fiji modeling of flare associated amplitude and phase enhancements of nwc 3 6 db 223 and npm 5 db 153 using long wave propagation capability code shows reduction in the d region reflection height h by 11 1 km and 9 4 km and enhancement in ionization gradients described by increases in the exponential sharpness factor by 0 122 and 0 126 km 1 for the nwc and npm paths respectively during the storm the daytime signal strengths of the nwc and npm signals were reduced by 3 2 db on 15 and 16 december for about 46 h and recovered by 17 december modeling for the nwc path shows that storm time values of h and were reduced by 1 2 km and 0 06 km 1 respectively morlet wavelet analysis of signal amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases the reduction in vlf signal strength is due to increased signal attenuation and absorption by the earth ionosphere waveguide due to storm induced d region ionization changes and hence changes in d region parameters the long duration of the storm effect results from the slow diffusion of changed composition ionization at d region altitudes compared with higher altitudes in the ionosphere

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

kinematics of rock flow and fabric development associated with shear deformation within the zagros transpression zone iran this paper presents quantitative data on the finite strain quartz crystal fabric geometry of flow and deformation temperatures in deformed quartzite samples to characterize the ductile deformation along the thrust sheets constituting the sanandaj sirjan metamorphic belt within the zagros mountains of iran the results of this study emphasize the heterogeneous nature of deformation in this belt showing a spatial variation in strain magnitude and in degree of non coaxiality a dominant top to the se sense of shear is indicated by the asymmetry of microstructures and quartz c axis fabrics quartz c axis opening angles suggest deformation temperatures range between 435 50 c and 510 50 c which yield greenschist to amphibolite facies conditions during the ductile deformation mean kinematic vorticity number w m measured in the quartzite samples ranges between 0 6 and 0 9 with an average of 0 76 which indicates that extrusion of the metamorphic rocks of the region was facilitated by a significant component of pure shear strain traced towards the basal thrust of the zagros thrust system from northeast to southwest the quartz grain fabrics change from asymmetric cross girdle fabrics in the internal part of the deformation zone to an asymmetric single girdle fabric at distances close to the basal thrust this variation may depend on the structural depth and on the geometry of the ductile deformation zone the observed increase in strain and vorticity within the study area is comparable with patterns recorded within metamorphic rock extrusions within other orogens in the world

midcrustal discontinuities and the assembly of the himalayan midcrust detailed quartz lattice preferred orientation lpo data define two structural discontinuities in the exhumed high grade metamorphic core of the himalaya exposed in the upper tama kosi region of east central nepal the structures are marked by abrupt breaks in a general trend of up structural section increasing quartz lpo defined deformation temperatures deformation associated with the upper structural discontinuity which occurs within sillimanite grade rocks is postpeak metamorphism in both the hanging wall and the footwall new geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 ma indistinguishable from previously published ages for the footwall movement across this structure represents early miocene strain localization and thickening in the himalayan midcrust movement across the lower discontinuity which occurs between staurolite and kyanite grade rocks appears to be synmetamorphic with material in its footwall at approximately 10 ma but postpeak metamorphism for material in its hanging wall this movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core the recognition of two thrust sense discontinuities in the exhumed himalayan core in the tama kosi region is consistent with other similar structures recognized along the himalaya the widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large hot orogens

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

some new aspects of kinematic vorticity analysis in naturally deformed quartzites abstract two vorticity methods have been proposed to estimate kinematic vorticity number in naturally deformed quartzites these combine the degree of quartz c axis fabric asymmetry either with the strain ratio in the xz plane of finite strain r xz or with the angle between the oblique grain shape fabric and the main foliation here it is suggested that the incorporation of r xz data with angle values can also serve as an additional method of vorticity analysis the use of a new nomogram that provides a rapid means of evaluating the consistency of vorticity estimates obtained by the three methods is also discussed a comparison of vorticity estimates from low grade test samples using all methods yields that three methods may record different parts of deformation history this hypothesis was then used to discuss the spatial and temporal variation of kinematic vorticity in a crustal scale shear zone this natural example showed that the new method is able to record late stage changes in the flow regime which are slightly or not detectable by previous methods also it is noteworthy that vertical vorticity profiles based on results of rigid porphyroclast analysis tend to average profiles obtained by the estimates of quartz based methods

tectonometamorphic discontinuities within the greater himalayan sequence in western nepal central himalaya insights on the exhumation of crystalline rocks abstract the core of the greater himalayan sequence in the mugu karnali area western nepal is affected by a thick shear zone with development of nearly 4 km of mylonites mangri shear zone it is a contractional shear zone showing a top to the sw and wsw sense of shear the shear zone developed during the decompression in the sillimanite stability field of rocks that previously underwent relatively high pressure metamorphism deformed under the kyanite stability field p t conditions indicate that the footwall experienced higher pressure 1 0 0 9 gpa than the hanging wall 0 7 gpa and similar temperatures 675 700 c u pb in situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 ma samples from the lower part of the greater himalayan sequence underwent similar ductile shearing at 17 13 ma these ages and the associated p t t paths revealed that peak metamorphic conditions were reached 5 7 ma later in the footwall of the shear zone with respect to the hanging wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself mangri shear zone with the other recently documented tectonic and metamorphic discontinuities within the greater himalayan sequence point to the occurrence of a regional tectonic feature the high himalayan discontinuity running for more than 500 km along the strike of the central himalayas it was responsible of the exhumation of the upper part of the greater himalayan sequence starting from 28 ma well before the activation of the main central thrust and the south tibetan detachment our data point out that exhumation of the greater himalayan sequence was partitioned in space and time and different slices were exhumed in different times starting from the older in the upper part to the younger in the lower one

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

refining the structural framework of the khimti khola region east central nepal himalaya using quartz textures and c axis fabrics abstract new quartz texture and c axis fabric data from across the paleoproterozoic ulleri phaplu melung orthogneiss in the khimti khola region of east central nepal provide new constraints on the internal structural framework of the himalaya that help shed light on the convergence accommodation processes active in the upper portion of the crust during orogenesis these data outline a strain history that varies across the unit deformation near the base of the unit occurred at 605 50 c with evidence of significant static recrystallization and recovery preserved in quartz whereas deformation near the top of the unit occurred at 540 50 c with quartz characterized by dynamic recrystallization mechanisms the strength of the quartz c axis fabrics follows a similar spatial pattern with those from near the top of the unit recording stronger fabrics than those measured from lower in the unit together these data are interpreted to indicate strain localization possibly at progressively lower temperature near the top of the ulleri phaplu melung orthogneiss this interpretation is consistent with cooling ages that indicate the upper boundary of the unit coincides with an out of sequence shear zone this study not only provides a structural characterization of the shear zone helping to refine the kinematic framework of this portion of the himalaya but also confirms the utility of fabric strength analysis in deciphering strain localization within pervasively deformed rocks

kinematics and vorticity of flow associated with post collisional oblique transpression in the variscan inner zone of northern sardinia italy abstract quartz c axis fabrics vorticity of flow and deformation temperatures were analyzed in samples collected in the central sector of the variscan inner zone in sardinia italy in order to constrain the kinematics of exhumation of medium and high grade metamorphic rocks during d2 post collisional deformation we investigated two shear belts with opposing shear senses these shear belts are developed parallel to the trend of the inner zone which marks the boundary between two metamorphic complexes our data suggest that the sinistral shear belt initiated during the early stage of d2 deformation in a simple shear dominated regime whereas the dextral shear belt initiated later in a pure shear dominated regime we document a general non coaxial flow regime with contemporaneous pure and simple shear components geometrical relationships reveal the operation of oblique extrusion during a compressive regime where the coaxial component acted sub horizontal and orthogonal to the trend of the axial zone changes in the kinematics vorticity of flow and strain geometry that occurred during d2 deformation may be related to post collisional orogen parallel displacement caught up in the variscan orogeny

quartz c axis fabric development associated with shear deformation along an extensional detachment shear zone chapedony metamorphic core complex central east iranian microcontinent abstract lattice preferred orientations lpos of quartz were used to establish differences in deformation geometry finite strain and temperature across an extensional detachment shear zone within the chapedony metamorphic core complex in the central east iranian microcontinent along the northern flank of gondwana quartz c axis data show a continuous evolution across the core complex from asymmetric type i crossed girdles at the southwest margin to broken asymmetric type i crossed girdle and single girdle with a large concentration of axes plotted in the center of the stereoplot at the central parts of the core complex and small circle girdle pattern at the northeast margin these variations in quartz c axis patterns imply change in strain geometry during deformation from plane strain to general flattening and pure flattening integrating analyses of quartz c axis opening angles quartz c axis patterns and recrystalization regimes of quartz and feldspar suggests deformation temperatures range between less than 400 c and 650 c which yield greenschist to amphibolite facies conditions mean kinematic vorticity number wm measured in the mylonite samples ranges between 0 67 and 0 71 which indicates that exhumation of the metamorphic rocks of the cmcc was facilitated by a significant component of pure shear strain within a general shear regime

kinematics and vorticity in kangmar dome southern tibet testing midcrustal channel flow models for the himalaya 1 kinematic kinematic vorticity wm and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the kangmar gneiss dome southern tibet record ductile deformation patterns of a frozen segment of a southward flowing midcrustal channel microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top north and top south shear in garnet zone rocks to dominantly top north shear in staurolite kyanite zone and deeper rocks kinematic vorticity values indicate an increase in pure shear component with depth from 48 pure shear in chloritoid zone rocks through 62 in garnet zone to staurolite kyanite zone rocks to 68 pure shear in an orthogneiss the deepest exposed rocks deformation temperatures inferred from grain scale microstructures and quartz lattice preferred orientations increase from 300 c 400 c in chloritoid zone rocks to 600 c in the deepest exposed rocks these temperatures are equivalent to temperatures derived from garnet biotite thermobarometry indicating that wm was recorded during peak metamorphism this ductile deformation zone was cut by the brittle southern tibetan detachment system stds that juxtaposed metasedimentary rocks upon the orthogneiss on the basis of these relations midcrustal rocks in the core of kangmar dome record 1 general shear vertical thinning and n s horizontal extension with a component of top north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the stds 2 an increase in pure shear with structural depth a consequence of an increase in lithostatic load and 3 displacement of the high temperature shear zone by the brittle stds our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

dominant simple shear deformation during peak metamorphism for the lower portion of the greater himalayan sequence in west nepal new implications for hybrid channel flow type mechanisms in the dolpo region abstract i conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid crustal rocks exposed in the dolpo region west nepal my results indicate that the main central thrust is located 5 km structurally below the previous mapped locations deformation temperature increasing up structural section from 450 c to 650 c and overlap with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the ghs occurred at close to peak metamorphic conditions i interpreted the telescoping and the inversion of the paleo isotherms at the base of the ghs as produced mainly by a sub simple shearing wm 0 88 1 pervasively distributed through the lower portion of the ghs my results are consistent with hybrid channel flow type models where the boundary between lower and upper portions of the ghs broadly corresponding to the tectonometamorphic discontinuity recently documented in west nepal represents the limit between buried material affected by dominant simple shearing and exhumed material affected by a general flow dominates by pure shearing this interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow and critical wedge type processes at different structural depth

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

thermal structure and tectonic evolution of the scandian orogenic wedge scottish caledonides integrating geothermometry deformation temperatures and conceptual kinematic thermal models in the caledonides of northwest scotland two independent geothermometers fe mg exchange and quartz c axis fabric opening angle are used to characterize the thermal structure of the lower part of the scandian 435 420 ma orogenic wedge within the moine ben hope and naver sgurr beag thrust sheets traced from west foreland to east hinterland fe mg exchange thermometry yields peak or near peak temperatures ranging from 484 50 c to 524 50 c in the immediate hangingwall of the moine thrust to 601 50 c in the immediate hangingwall of the ben hope thrust to 630 50 c in the naver thrust sheet preserved metamorphic facies and textural relationships are consistent with thermometric estimates deformation temperatures calculated from quartz c axis fabric opening angles across two similar orogen perpendicular transects also yield systematic increases glen golly ben klibreck 520 630 c ullapool contin 465 632 c traced towards the naver and sgurr beag thrusts in addition deformation temperatures show a pronounced increase along the leading edge of the moine thrust sheet moving south towards the assynt window which is interpreted to reflect deeper exhumation of the thrust plane above the assynt footwall imbricate stack because temperatures calculated from metamorphic assemblages are within error of the quartz fabric derived deformation temperatures that are of demonstrably scandian age the metamorphic sequence between the moine and naver sgurr beag thrusts is interpreted to have developed during the scandian orogeny integration of our results with previous 2d thermal mechanical studies allows development of new conceptual thermal kinematic models of scandian orogenesis that may be broadly applicable to other collisional systems furthermore it highlights the critical nature of coupling between orogen kinematic and thermal evolution

deformation mechanisms and inverted thermal profile in the north almora thrust mylonite zone kumaon lesser himalaya india abstract the thermal structure of a 2 2 km thick mylonite zone from the kumaon lesser himalaya india is interpreted based on deformation mechanisms and microstructures the mylonite zone lies along the north almora thrust nat a s dipping thrust that marks the northern flank of a large synformal crystalline klippe almora klippe approximately 17 km north of the town of almora the mylonite zone evolved during the upper eocene emplacement of the almora klippe part of a larger munsiari thrust sheet following the collision of india and eurasia in lower middle eocene mylonitic rocks near the top of the zone show dynamicrecrystallization of both feldspar and quartz indicating deformation at amphibolite grade 500 600 c near the center of themylonite zone the feldspars are fractured and quartz is dynamically recrystallized though to a lesser degree than near the top of the zone this evidence combined with the occurrence of secondary epidote chlorite biotite and calcite in these rocks suggests that mylonitization occurred at greenschist grade 500 400 c near the base of the mylonite zone biotite is present feldspars are fractured and altered to sericite and quartz shows limited plastic deformation in the form of undulose extinction and subgrain formation indicating that temperatures of 400 c or less were reached during deformation deformation mechanisms and metamorphic reactions in the mylonite zone rocks thus point to a decrease in the temperature of deformation from 500 300 c near the top of the thrust zone to about 400 c near its base suggesting an inverted thermal profile we interpret this inverted thermal profile in the mylonite zone to have resulted from gradual cooling of the zone by heat transfer to a cold footwall at the time of thrusting

relationships between displacement and distortion in orogens linking the himalayan foreland and hinterland in central nepal greater himalayan sequence rocks exposed in the manaslu himal chuli himalaya can be separated into distinct upper and lower parts deformation recorded in both parts occurred at temperatures ranging between 450 c and 640 c and is characterized by almost equal coaxial and noncoaxial components across the upper greater himalayan sequence peak metamorphic temperatures are essentially isothermal whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars km in the lower greater himalayan sequence however both metamorphic pressure and temperature decrease with structural depth the abnormal pressure gradient in the upper greater himalayan sequence is attributed to 50 vertical thinning during southward displacement while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the main central thrust shear zone and the progressive incorporation of more rock into the greater himalayan sequence deformation in the upper portion of the greater himalayan sequence was characterized by extending flow i e extension in the direction of flow whereas deformation in its lower portion was characterized by compressing flow i e compression in the direction of flow extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands while compressing flow is emblematic of displacement and distortion in orogenic foreland regions the transition between the upper and lower parts of the greater himalayan sequence therefore represents a fundamental transition between hinterland style deformation involving processes such as lateral midcrustal flow and foreland style deformation involving critical taper thrust fold wedge development

folding and shear deformation in quartzites inferred from crystallographic preferred orientation and shape fabrics abstract folding and simple shear deformation frequently occur together on all scales the kinematic information from crystallographic preferred orientation cpo and microstructural information from shape fabrics are used to investigate the relationships of simple shear and folding in small scale folds three fold samples were analysed sample 1 with the fold axis perpendicular sample 2 oblique and sample 3 essentially parallel to the shear direction all folds have formed by buckling in a simple shear deformation regime the principal kinematic directions for each sample lie in a single plane normal to the fold axial plane and at variable angles to the fold axis the cpos allow a distinction to be identified between coaxial and simple shear deformation components in different parts of each fold the shear senses determined by cpos and shape fabrics indicate that the shear deformation and the buckling of the layers occurred approximately simultaneously in all samples cpo analysis of the flexural slip components of the folding suggests that the fold axes have not rotated substantially towards the extension direction since their initiation the variable orientations of the fold axes are explained by variable original orientations of the anisotropy with respect to the shear direction

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

long duration geomagnetic storm effects on the d region of the ionosphere some case studies using vlf signal the present work investigates the effects of long duration geomagnetic storms on vlf signal during ionospheric sunrise time commonly known as d layer preparation time dlpt depth the vlf signal at 19 8 khz transmitted from northwest cape australia and received at a low latitude station tripura india is used for the present analysis the data for the analysis are selected from november 2008 to october 2011 in the active period of the geomagnetic storms the average dlpt depth is found to have a negative correlation coefficient of 0 91 with geomagnetic ap index it is also found that with each 10 unit increase of ap index the dlpt depth the day and night asymmetry level changes by 1 25 db the results are supported with modeled international reference ionosphere iri electron density data and dlpt depth at 71 km height for the three positions namely receiver position signal hop position and the transmitter position along the total great circle path it is found that the receiver position electron density is the main controlling factor for dlpt depth the correlation between iri electron density and dlpt depth increases from 0 13 at transmitter position to 0 33 at the first hop position to 0 46 at the receiver position respectively the percentage change of post storm electron density at 71 km height is found to increase by more than 100 at the receiver position the results are discussed on the basis of the electron density changes over the signal propagation path mainly caused by the geomagnetic storms

nighttime ionospheric d region equatorial and nonequatorial 1 nighttime ionospheric d region parameters are found to be generally well modeled by the traditional h and as used by wait and by the u s navy in their earth ionosphere vlf radio waveguide programs new comparisons with nonequatorial mainly all sea vlf path observations reported over several decades are shown to be consistent with the previously determined height h 85 0 km and sharpness 0 63 km 1 these paths include npm hawaii to washington d c omega hawaii and nlk seattle to japan nwc n w australia to madagascar and nba panama to colorado in marked contrast transequatorial path observations even when nearly all sea are found to be often not well modeled for example for omega japan and jji japan to dunedin new zealand the observed amplitudes are markedly lower than those which would be expected from h 85 0 km and 0 63 km 1 or any other realistic values of h and other transequatorial observations compared with modeling include nwc to japan omega hawaii to dunedin and npm hawaii to dunedin it is suggested that the effects of irregularities in the equatorial electrojet may extend down into the nighttime d region and so account for the observed equatorial vlf perturbations through scattering or mode conversion

kinematics of the dadeldhura klippe shear zones w nepal implications for the foreland evolution of the himalayan metamorphic core mapping combined with structural analyses in the foreland edge of the metamorphic core of the himalayas in sw nepal highlights the existence of two north dipping shear zones with opposite sense of shear here the metamorphic core is mainly affected by non coaxial top to the south sense of shear at temperatures between 450 c and 550 c that switch to a top to the north sense of shear at the top of the metamorphic core we regionally correlate this upper shear zone with the south tibetan detachment system ar dating on white mica indicates that both shear zones operated between 23 ma and 17 ma restoration of the folded south tibetan detachment in far western nepal yields a minimum dip slip distance of 190 km compatible with predictions made by models of extrusion of a weak mid crustal channel our results support an orogenic model in which channel flow in the hinterland coexisted with thrust wedge mechanics in the foreland

unusual transition in quartzite dislocation creep regimes and crystal slip systems in the aureole of the eureka valley joshua flat beer creek pluton california a case for anhydrous conditions created by decarbonation reactions abstract microstructures and quartz c axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the eureka valley joshua flat beer creek composite pluton temperatures of deformation are estimated to be 740 50 c based on a modified c axis opening angle thermometer of kruhl j metamorph geol 16 1998 142 in quartzite layers located closest 140 m to the pluton wall rock contact flattened detrital grains are plastically deformed and partially recrystallized the dominant recrystallization process is subgrain rotation dislocation creep regime 2 of hirth and tullis j struct geol 14 1992 145 although grain boundary migration dislocation creep regime 3 is also evident complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact and coincides with recrystallization taking place dominantly through grain boundary migration regime 3 within the quartzites strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains the c axis fabrics indicate that a slip operated within the quartzites closest to the pluton wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism c slip the spatial inversion in microstructures and slip systems apparent high temperature deformation and recrystallization further from the pluton contact and apparent low temperature deformation and recrystallization closer to the pluton contact coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units marble and calc silicate assemblages dominate close to the pluton wall rock contact whereas mixed quartzite and pelite assemblages are dominant further from the contact we suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc silicate assemblages in the inner aureole quartzites may have produced high x co 2 water absent fluids during deformation the presence of high x co 2 fluid is inferred from the prograde assemblage of quartz calcite and not wollastonite diopside k feldspar in the inner aureole quartzites we suggest that it was these dry conditions that suppressed prism c slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization which would normally be associated with lower deformation temperatures in contrast the prograde assemblage in the pelite dominated outer part of the aureole is biotite k feldspar these wet pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism c slip and regime 3 recrystallization because other variables could also have caused the spatial inversion of c axis fabrics and recrystallization mechanisms we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz our conclusions are based on a small number of samples and therefore the unusual development of crystal fabrics and microstructures in the aureole to the ejb pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks

deformation temperatures vorticity of flow and strain in the moine thrust zone and moine nappe reassessing the tectonic evolution of the scandian foreland hinterland transition zone abstract examination of deformation temperature 3 d strain and flow vorticity wm in mylonites from the assynt loch more region of the moine thrust zone mtz allows quantitative kinematic and thermal characterization of shearing at the base of the scandian 435 425 ma orogenic wedge quartz microstructures and c axis fabric opening angles from mylonites in the immediate hangingwall and footwall to the moine thrust suggest that deformation temperatures are highest in the eastern part of the assynt region including mylonites close to alkaline intrusive complexes and decrease along strike both to the north stack of glencoul loch more and to the south knockan quartz c and a axis fabrics together with limited 3 d strain data indicate that deformation in both the footwall and hangingwall mylonites dominantly involved plane strain to general flattening although domains of more constrictional flow are identified adjacent to thrust transport parallel lineaments in the overlying moine nappe rigid grain analyses indicate a remarkably constant flow vorticity for tens of kilometers along orogenic strike 40 60 pure shear in both the hangingwall and footwall mylonites integration with previously reported strain and vorticity estimates from the assynt region indicates a 50 75 sub vertical shortening perpendicular to gently dipping foliation that must have been accommodated by either volume loss or extrusion of material toward the synorogenic topographic surface extrusion implies a causal link between upper and lower crustal processes with significant implications for the kinematic and geometric evolution of the scandian wedge

telescoping of isotherms beneath the south tibetan detachment system mount everest massif abstract petrologic and microstructural crystal fabric data indicate that isotherms recorded in greater himalayan series ghs schists and gneisses in the footwall to the south tibetan detachment system stds have undergone extreme telescoping during penetrative flow associated with southward extrusion of the ghs in the rongbuk valley to the north of mount everest we have made three vertical sampling traverses from the stds down into the ghs and estimated temperatures associated with penetrative deformation using the opening angles of quartz c axis fabrics measured on dynamically recrystallized grains from north to south the deformation temperature data indicate apparent thermal field gradients of 369 385 and 420 c per km for our three traverses traced over a maximum vertical sampling distance of 0 5 km adopting a differential flow path model simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment parallel transport magnitudes of 25 170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the stds depending on assumed original geothermal gradient dip of detachment etc these particle transport estimates are similar to those previously calculated from barometry data of ghs rocks in the everest region and are compatible with channel flow models for extrusion and exhumation of the ghs

a structural transect in the lower dolpo insights on the tectonic evolution of western nepal abstract we present the results of a structural transect in lower dolpo cross cutting the upper part of the lesser himalaya lh the higher himalayan crystallines hhc and the lower part of the tibetan sedimentary sequence tss the mct zone affects the upper part of the lh as well as the lower part of the hhc and shows a later brittle reactivation mean vorticity in the mct points to non coaxial deformation these data together with available kinematic data along the belt on the south tibetan detachment system stds and in the core of the hhc point to increasing simple shear toward the tectonic boundaries a top to the sw high temperature shear zone toijem shear zone is recognized in the middle part of the hhc at the boundary between units 1 and 2 it developed during the earlier stages of exhumation of the hhc enhancing the decompression of the hanging wall and the emplacement of leucogranite dykes and sills its development could be explained by a change in the velocity profile during the extrusion of the hhc triggered by first order changes in rock types of the tectonic unit the stds is marked by a wide zone of high strain and by a metamorphic jump from amphibolite facies in the carbonate rocks of the upper part of the hhc to greenschist facies marbles in the lower part of the tss the development of a pervasive foliation towards the bottom of the tss indicates increasing strain related to top down to the ne tectonic transport a low p metamorphic event marked by the growth of post d1 biotite porphyroblasts at the base of the tss is related to the conductive heating from the underlying hhc

analysis of deformation paths in shear zones the geometry of ductile shear zones can be used to solve problems of regional tectonics if the deformation path of material in the zones is sufficiently understood flow in many shear zones may have deviated from simple shear and consequently data on the final deformation state such as finite strain and volume change are insufficient for reconstruction of the deformation path even if flow parameters were constant during progressive deformation additional information on the flow vorticity number is also needed and can be obtained from fabric elements such as sets of folded boudinaged veins rotated porphyroblasts and blocked rigid objects mohr circle constructions are presented as a tool to calculate deformation parameters from fabric data to represent the deformation path graphically and to reconstruct flow parameters from the shape of this path if the vorticity number or the volume change rate changed during progressive deformation the deformation path can be partly reconstructed using sets of fabric elements which register mean and final values of these parameters

quartz c axis fabrics in deformed conglomerates some support for a skeletal approach to fabric analysis abstract preferred orientation patterns of c axes in deformed quartzite rocks are conveniently characterized by 1 their skeletal outline and 2 by the way this skeleton is populated by c axes it has been recognized that the fabric skeleton principally reflects the kinematics of the flow during at least the later part of the deformation history whilst the distribution of c axes over the fabric skeleton is less reliable in this respect because the c axes orientation distribution may be dominated by the effect of an uneven and non random distribution of c axes prior to deformation little evidence has been presented however to confirm this interpretation quartz c axis fabrics in quartzite pebble conglomerates from southern spain show variable and incomplete patterns in individual pebbles which however all have a similar fabric skeleton and link to form more complete patterns when c axes from several pebbles are plotted together in view of the common deformation history of the pebbles within a given sample the variable and incomplete fabrics in single pebbles can only be explained by the effect of initial non randomness of c axes prior to deformation it follows that the c axis orientation distribution in this case bears little relationship with the inferred kinematics of the flow whilst the fabric skeleton shows a consistent relationship with the kinematic framework

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

perturbations of midlatitude subionospheric vlf signals associated with lower ionospheric disturbances during major geomagnetic storms 1 we examine the effects on the midlatitude ionospheric d region of the 7 april 2000 storm and the halloween storm of late october 2003 by means of the associated perturbations of several subionospheric vlf signals propagating in both the northern and southern hemispheres we use vlf nighttime data from the holographic array for ionospheric lightning research hail located in the united states l 2 3 as well as data from palmer station antarctica l 2 4 on 7 april 2000 a 5 db depression in vlf amplitudes is observed at multiple hail stations with a depression onset that occurs later for vlf signal paths at lower latitudes on both 7 april 2000 and 31 october 2003 fluctuations in the amplitude of the vlf signals are first observed in the premidnight sector and persist through the end of the data recording period dawn the frequency content of the fluctuations is predominantly in the 0 01 to 0 02 hz range but extends up to 0 03 hz increases in the energetic electron flux in the loss cone as measured by the noaa poes satellites are observed on both 7 april 2000 and 31 october 2003 we suggest that both the signal depressions and subsequent fluctuations are associated with variations in the precipitation flux of energetic electrons onto the upper atmosphere auroral activity patterns based on data from the noaa poes satellites show that the equatorward edge of the auroral oval expanded equatorward to lower l shells l 3 during both geomagnetic storms using the auoral activity patterns and multiple vlf lf signal paths we provide evidence that the fluctuations and the signal depression coincide with the equatorward edge of the auroral oval extending over the perturbed vlf lf great circle paths quantitative modeling of subionospheric vlf wave propagation incorporating energetic electron flux measurements and the associated altitude profiles of secondary ionization yields results consistent with the variations in the vlf signal amplitude observed

a model for the development of a domainal quartz c axis fabric in a coarse grained gneiss abstract quartz c axis fabrics in high grade coarse grained layered gneisses reveal a distinctive pattern with three perpendicular maxima which can be interpreted in terms of syntectonic recrystallization and growth mapping of the distribution of c axes within a single thin section demonstrates that the fabric is domainal fabric domains are on the scale of millimetres and are associated with individual quartz rich patches no single domain exhibits the overall fabric of the complete thin section within each domain weighting of the c axis fabric by the area of each quartz grain demonstrates that the largest grains have their c axes preferentially at the maxima a model is presented to account for the development of this fabric grains which can keep their stored strain energy to a minimum by the use of a single glide system oriented approximately parallel to the plane and direction of maximum resolved shear stress grow at the expense of other grains the largest most successful grains deform by glide on only one of either the basal a or the prism a systems depending on the crystallographic orientation of the grain each domain develops its own glide direction by co operation of the glide systems of its constituent grains the successful grains in a domain are those which can accommodate that direction the bulk strain of the rock is achieved by the addition of the strains distinct to each domain