the rigid grain net rgn an alternative method for estimating mean kinematic vorticity number wm the use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number wm is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks the most common methods broadly grouped into those that use tailed and tailless porphyroclasts have been applied to many different tectonic settings however attempts have not been made to unify the various methods here we propose the rigid grain net rgn as an alternative graphical method for estimating wm the rgn contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution phd method we use the rgn to unify the most commonly used wm plots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix test samples from the south tibetan detachment tibet yield indistinguishable results when the rgn is compared with existing methods because of its ease of use ability for comparing natural data sets to theoretical curves potential to standardize future investigations and ability to limit ambiguity in estimating wm the rgn makes an important new contribution that advances the current methods for quantifying flow in shear zones 2006 elsevier ltd all rights reserved

the tectonic significance of u th pb ages of monazite inclusions in garnet from the himalaya of central nepal abstract textural observations chemical zoning and 196 in situ la icp ms 232 th 208 pb and 238 u 206 pb dates of monazite inclusions in greater himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite cenozoic prograde and retrograde monazite are commonly intergrown and both of these components are frequently intergrown with lower paleozoic monazite because zones in most inclusions are too small to date and because some inclusions show little zoning of y and th chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of th pb dates comparison of 238 u 206 pb and 232 th 208 pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions most cenozoic monazite crystallized between 42 29 and 22 12 ma and these age ranges may record prograde and retrograde metamorphism of greater himalayan rocks respectively dates younger than 12 ma are discordant without exception the

the south tibetan detachment system in the rongbuk valley everest region deformation features and geological implications abstract structural and microstructural investigations into the south tibetan detachment system in the rongbuk valley southern tibet pointed to the presence of a complex extensional setting with the development of two major fault zones with movement in a top to ne direction the lower one is an at least 1000 m wide ductile shear zone that brings into contact the overlying north col formation subject to low grade metamorphism and the high grade micaschist and gneiss of the higher himalayan crystallines characterized by polymetamorphism from barrovian to low pressure high temperature conditions the upper low angle fault zone is characterized by a metric thick level of cataclasites and brings into contact the slightly metamorphosed ordovician limestones and the north col formation the lower ductile shear zone is thought to accommodate at least 5 km of vertical displacement whereas the upper low angle normal fault zone is thought to accommodate at least 1600 m of vertical displacement the lower ductile shear zone is thus to be considered the main extensional feature of the stds in the rongbuk valley

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

effect of finite strain on clast based vorticity gauges abstract clast based vorticity gauges utilize orientations of grains assumed to have behaved as isolated rigid particles suspended in a flowing viscous matrix a fundamental assumption behind use of the method is that sufficient strain has accumulated for high aspect ratio grains to rotate into positions approaching their stable sink orientation and that clasts below a critical aspect ratio may be observed in any orientation relative to the flow plane we constructed a numerical model to explore the effect of variable finite strain on development of the orientation distribution of a large population of rigid clasts embedded in a viscous medium for end member pure and simple shear and for several distinct general shear flows our model predicts the technique will tend to produce vorticity overestimates for lower vorticity flows for a wide range of finite strain the model also indicates that clast populations in moderate to high vortical flows tend to develop shape preferred orientations that closely resemble those expected for flows of lower vorticity we conclude that clast based methods are not effective for extracting detailed kinematic information from a mylonite deformed in a flow with arbitrary boundary conditions in fact it appears that most general shear flows continued long enough to develop moderate high finite strains will tend to produce a clast orientation distribution that will yield a visual estimate of the critical aspect ratio that suggests approximately equal contributions of pure and simple shear components

rupture characteristics of the deep bolivian earthquake of 9 june 1994 and the mechanism of deep focus earthquakes the m w 8 3 deep 636 kilometers bolivian earthquake of 9 june 1994 was the largest deep focus earthquake ever recorded seismic data from permanent stations plus portable instruments in south america show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers rupture proceeded at 1 to 3 kilometers per second along the down dip azimuth of the slab and penetrated through more than a third of the slab thickness this extent is more than three times that expected for a metastable wedge of olivine at the core of the slab and thus appears to be incompatible with an origin by transformational faulting these large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the earth s surface

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

out of sequence deformation and expansion of the himalayan orogenic wedge insight from the changgo culmination south central tibet 1 the changgo culmination one of the north himalayan domes in south central tibet consists of a multiphase granite core surrounded by a deformed metasedimentary carapace the granitic core records general non coaxial shear with a top to the south sense shear component the contact between the core and the carapace is a shear zone characterized by general non coaxial shear with a top to the north shear sense interpreted to be the northern continuation of the south tibetan detachment system stds the shear zone contains lenses of leucogranite dated at 35 4 ma this is interpreted to reflect eocene crustal thickening coeval with the earliest shortening event recorded in the carapace the main phase of the changgo granite crystallized at 23 5 ma while undeformed aplite dikes the youngest phase observed in the granite were intruded at 22 1 ma aplite dikes crosscut the main deformation fabric within the changgo granite therefore that deformation and associated south directed shearing must have ended between 23 5 ma and 22 1 ma the dikes are strained within the stds indicating that final displacement along the stds must post date 22 ma yet be older than 18 4 ma the cooling age of muscovite in the shear zone it is proposed that the exhumation of the changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge subsequently deformation in the wedge migrated toward the foreland expanding the orogenic wedge laterally and moving the locus of displacement from the main central thrust structurally downward to the main boundary thrust

determining vorticity axes from grain scale dispersion of crystallographic orientations kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones the appropriate plane in which to interpret shear sense indicators is the vorticity normal surface which is often inferred using fabric foliation lineation orientation strain modeling however suggests that such fabrics can be unreliable for determining a kinematic framework we demonstrate the application of a new quantitative method crystallographic vorticity axis cva analysis that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale we apply cva analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries in all cases the aggregate of calculated grain scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis this new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information and without assumptions about the kinematics of deformation

mechanical and chemical compaction in fine grained shallow water limestones abstract artificial compaction of in situ cores of modern sediments from differing sedimentary environments resulted in 1 reduction of sediment thickness by 50 percent and more 2 reduction of initial porosities of 65 to 75 percent to 35 to 45 percent 3 creation of megascopic textures almost identical to many ancient lime mud and wackestones 4 creation of organic wispy stylolite like layers that in nature probably would initiate dissolution and stylolite development given burial and time 5 chemical compaction evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and sem evidence of solutional interfitting of 1 to 4 m size aragonitic carbonate grains 6 obliteration of pellets and birdseye or fenestral voids in those sediments here early cementation was lacking pellets were preserved in slightly cemented bahama bank muds compacted to half the original thickness 7 obliteration of identifiable marine grasses and vertical root tube voids 8 mashing of sediment filled circular burrows to produce ellipsoidal structures significant mechanical compaction resulted from pressures simulating less than 1 000 ft 305 m of burial increasing loads to an equivalent of more than 10 000 ft 3 400 m did not significantly increase compaction or reduce sediment core length chemical compaction pressure dissolution was detected only in sediment cores compacted to pressures greater than 10 000 ft 3 400 m of burial these short term experiments suggest that chemical compaction would begin at much shallower depths given geologic time compaction experiments that caused chemical compaction lend support to the well established hypothesis that cement required to produce a low porosity low permeability fine grained limestone is derived internally dissolution ion diffusion and reprecipitation are considered the m st likely processes for creating significant thicknesses of dense limestones in the geologic record continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids possibly including hydrocarbons

source characteristics of large deep earthquakes constraint on the faulting mechanism at great depths 1 source parameters for eight large deep earthquakes are inferred from broadband data of the global seismograph network we completed an unprecedented data set composed of 14 largest deep earthquakes mw ranging from 7 0 to 8 3 from six different subduction zones studied with identical source parameter inversion methods the study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate events in cold slabs show high aftershock activity high rupture velocity and high seismic efficiency in contrast earthquakes in warm slabs generally have low aftershock productivity low rupture velocity and low seismic efficiency however the 23 june 1991 argentina event has source properties that are rather characteristic for earthquakes in cold slabs and that differ from those of other large south american events we suggest that two types of deep earthquakes occur in south america and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes in this model shear failure occurs through a thermal runaway mechanism within a shear zone at great depth and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production the 1991 argentina event located in a pocket of deep seismicity within the slab core may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones whereas the other known south american large deep events such as 1963 peru 1970 colombia and 1994 bolivia are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab

systematic changes in the incoming plate structure at the kuril trench 1 recent seismic structural studies in trench outer rise regions have shown that vp within the incoming oceanic plate systematically decreases toward the trench probably owing to bending and fracturing of the plate to understand the mechanisms acting to reduce vp vs is critical because the vp vs ratio is a sensitive indicator of lithology porosity and the presence of fluid in the outer rise region of the kuril trench we conducted an extensive seismic refraction and reflection survey that revealed systematic changes in vp vs and vp vs our results suggest that water content within the incoming oceanic plate increases toward the trench accompanied by the development of bending related fractures at the top of the oceanic crust consistent with the seawater percolation our results support the idea that plate bending and fracturing during the bending in the outer rise of the trench play an important role in the water cycle of subduction zones

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

fault plane orientations of intermediate depth earthquakes in the middle america trench 1 intermediate depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones the orientation of presubduction faults is particularly well known offshore of middle america where seismic reflection profiles show outer rise faults dipping toward the trench and extending 20 km into the lithosphere if water is transported along these faults and incorporated into hydrous minerals the faults may be reactivated later when the minerals dehydrate in this case the fault plane orientations should be the same in the outer rise and at depth after accounting for the angle of subduction to test this hypothesis we analyze the directivity of 54 large mw 5 7 earthquakes between 35 and 220 km depth in the middle america trench for 12 of these earthquakes the directivity vector allows us to identify the fault plane of the focal mechanism between 35 and 85 km depth we observe both subhorizontal and subvertical fault planes the subvertical fault planes are consistent with the reactivation of outer rise faults whereas the subhorizontal fault planes suggest the formation of new faults deeper than 85 km we only observe subhorizontal faults indicating that the outer rise faults are no longer being reactivated the similarity with previous results from the colder tonga kermadec subduction zone suggests that the mechanism generating these earthquakes and controlling fault plane orientations depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate depth seismicity exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes

thermometry of quartz mylonites importance of dynamic recrystallization on ti in quartz reequilibration quartz mylonites from the tonale fault zone in the alps northern italy have been investigated by the ti in quartz geothermometer titaniq in order to test its applicability to measure deformation temperatures the eastern part of the tonale fault zone was contact metamorphosed by the synkinematic intrusion of the adamello pluton forming an 800 m wide mylonitic shear zone with a synkinematic temperature gradient from 280 c at the frictional viscous transition to 700 c at the pluton contact as derived from metamorphic mineral assemblages deformation microstructures from quartz mylonite samples systematically collected across the mylonitic shear zone display the entire range of dynamic recrystallization in quartz which comprise bulging recrystallization blg subgrain rotation recrystallization sgr and grain boundary migration recrystallization gbm titaniq geothermometry yields the near peak deformation temperature for quartz mylonites deformed at metamorphic temperatures above 540 c in the zone of gbm however for mylonites formed under lower temperatures in the zones of sgr and blg the preexisting ti concentrations were not reset it is suggested that this is due to the sluggish ti volume diffusion rates below 500 c and the short duration of contact metamorphism and deformation even in the higher temperature samples the reequilibration of ti in quartz content was achieved by grain boundary migration rather than by volume diffusion hence our results show that gbm is crucial for the reequilibration of ti in quartz while quartz mylonites deformed by either blg or sgr which predominate in natural shear zones at greenschist facies metamorphic conditions most likely yield inherited temperatures

global large deep focus earthquakes source process and cascading failure of shear instability as a unified physical mechanism abstract we apply a multiple source inversion method to systematically study the source processes of 25 large deep focus depth 400 km earthquakes with mw 7 0 from 1994 to 2012 based on waveform modeling of p pp sh and ssh wave data the earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub events 1 category one with non planar distribution and variable focal mechanisms of sub events represented by the 1994 mw 8 2 bolivia earthquake and the 2013 mw 8 3 okhotsk earthquake 2 category two with planar distribution but focal mechanisms inconsistent with the plane including eighteen earthquakes and 3 category three with planar distribution and focal mechanisms consistent with the plane including six earthquakes we discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture transformational faulting and shear thermal instability we suggest that the inferred source processes of large deep focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre existing weak zones with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub events controlled by orientations of the pre existing weak zones the proposed mechanism can also explain the observed great variability of focal mechanisms the presence of large values of clvd compensated linear vector dipole and the super shear rupture of deep focus earthquakes in the previous studies in addition our studies suggest existence of relationships of seismic moment source duration 3 and moment source dimension 3 in large deep focus earthquakes

a cascading failure during the 24 may 2013 great okhotsk deep earthquake on 24 may 2013 the largest ever recorded deep earthquake occurred beneath sea of okhotsk a multiple point source inversion procedure is applied to constrain source process of this earthquake based on waveform modeling of both direct p and sh waves and near surface reflected pp and ssh waves our results indicate that the earthquake consists of six major subevents separated in space and time encompassing a horizontal dimension of 64 4 km along n160 e and a downward depth extension of 35 4 km the geographic distribution and focal mechanisms of the inferred subevents and foreshock aftershock locations do not fit into plane rupture we suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region with the perturbation of stress generated by a shear instability triggering another

the structural evolution of the qomolangma formation mount everest nepal abstract re examination of specimens collected across the qomolangma formation exposed in the summit pyramid on mount everest confirms a complex multi deformational history two pre oligocene deformational events are recorded within the calcite dominated rocks from the main summit 8848 m the first event is characterized by an rxz of 1 3 1 4 in detrital quartz grains which increases to 1 6 after the second event the finite strain recorded in calcite in the same rocks is higher with an rxz of 2 7 2 8 the deformation in the main summit appears to have been accommodated dominantly through grain boundary sliding in calcite in a slightly constrictive environment with a vorticity of 0 81 deformation in the south summit 8750 m contrasts with that of the top and is instead interpreted to reflect miocene movement along the qomolangma detachment with no earlier deformation preserved u pb geochronology on synkinematic rutile indicates that movement on the structure occurred at 16 3 5 0 ma rocks within the qomolangma formation in the summit pyramid of mount everest appear to record the basic history of the himalaya with early deformation likely related to crustal thickening and later more spatially restricted deformation reflecting the miocene extrusion of the metamorphic core

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

metamorphic history of the south tibetan detachment system mt everest region revealed by rscm thermometry and phase equilibria modelling this study combines microstructural observations with raman spectroscopy on carbonaceous material rscm phase equilibria modelling and u pb dating of titanite to delineate the metamorphic history of a well exposed section through the south tibetan detachment system stds in the dzakaa chu valley of southern tibet in the hanging wall of the stds undeformed tibetan sedimentary series rocks consistently record peak metamorphic temperatures of 340 c temperatures increase down section reaching 650 c at the base of the shear zone defining an apparent metamorphic field gradient of 310 c km 1 across the entire structure u th pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 ma deformation within this 1 km thick zone of distributed top down to the northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo isotherms along the upper margin of the greater himalayan series ghs we interpret the preservation of such a high metamorphic gradient to be the result of a progressive up section migration in the locus of deformation within the zone this segment of the stds provides a detailed thermal and kinematic record of the exhumation of footwall ghs rocks from beneath the southern margin of the tibetan plateau

ti resetting in quartz during dynamic recrystallization mechanisms and significance the ubiquity of quartz in continental crust and the involvement of sio 2 in multiple metamorphic processes such as reactions fluid flux and solution transfer processes makes quartz an obvious choice for reconstructing prograde metamorphic conditions in various rock types recent studies have shown the usefulness of analyzing ti distribution in quartz to constrain pressure temperature relative time deformation p t t d in metamorphosed tectonites new high precision single crystal x ray diffraction volume constraints on ti doped and chemically pure quartz provide further evidence for substitution of ti 4 for si 4 in the tetrahedral site in quartz with resultant lattice strain on the structure recent applications of the ti in quartz thermobarometer to dynamically recrystallized quartz have identified recrystallized subgrains that contain lower ti concentrations ti than their host porphyroclasts in addition ti are lower than expected for the temperatures of recrystallization atomistic simulations that estimate energetic perturbations resulting from ti incorporation into the quartz lattice indicate that significant increases in strain energy occur only at very high ti the strain energy increase is negligible for ti typical of quartz grown under mid crustal conditions this suggests that lattice strain rarely provides an appreciable driving force for ti loss from quartz instead it appears that subgrain boundaries and dislocation arrays migrating through recrystallizing quartz crystals can promote localized re equilibration thermodynamically regulated by the composition of the intergranular medium typically undersaturated in ti it therefore appears that analyses from dynamically recrystallized quartz cannot be meaningfully interpreted until methods are developed that can account quantitatively for the reduction of ti resulting from crystal plastic flow

rongbuk re visited geochronology of leucogranites in the footwall of the south tibetan detachment system everest region southern tibet abstract the hermit s gorge area adjacent to the rongbuk glacier on the north side of mount everest is a critical location to establish the timing and duration of movement along the ductile strand of the south tibetan detachment system stds a low angle north dipping normal fault that bounds the upper part of the greater himalayan sequence monazite from four leucogranite samples in the hermit s gorge that bracket the timing of ductile fabric development has been dated using both u th pb id tims and la mc icpms results suggest that the earliest 16 4 ma leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc silicates whereas structurally higher sills and dykes that post date fabric development are slightly younger all within uncertainty of one another at 15 6 to 15 4 ma field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the stds as being on going at 16 4 ma but had ceased prior to 15 6 ma while brittle faulting along the stds is younger than 15 4 ma combined with data from the everest massif and surrounding region ages of granite crystallization and ductile shearing propagated up structural section and northward with time

the dynamic rupture process of the 1993 kushiro oki earthquake we have constructed a dynamic rapture model of the 1993 kushiro oki earthquake mw 7 6 that is consistent with the observed seismograms this earthquake occurred beneath the city of kushiro japan at a depth of about 100 km and the aftershocks are limited to a horizontal plane between a double seismic zone in the subducting slab excellent near field strong motion records are available in our analysis a kinematic inversion is followed by a dynamic crack calculation and this pair of operations is repeated until convergence to the data is obtained two different conditions concerning the dynamic stress drop were considered in the dynamic calculation and a dynamic model was constructed for each condition the difference in these conditions has little effect on the results the final dynamic model shows that this earthquake occurred with a localized high stress drop of about 40 mpa in the subducting slab which indicates that the region between the double seismic zone is strong enough to sustain such a high stress one of the two high strength regions in the final model lies near the upper plane of the double seismic zone and represents a barrier due to a substantial change of stress state in the slab in the region where the initial breaks occurred 5 s before the main rupture both the strength and the stress drop are smaller than in the adjacent region this implies that before the main rupture the accumulated stress had already been released in the area of the initial breaks the aftershocks tend to occur near the high strength area in some of them the p wave first motions are opposite to those of the main shock

measurement of differential rupture duration as constraints on the source finiteness of deep focus earthquakes 2 synthetic tests to estimate errors in rupture vectors and their effect on fault plane identification 1 the directivity method of warren and silver 2006 has been used to distinguish the fault planes of deep earthquakes in the tonga and middle america subduction zones these studies identified exclusively subhorizontal fault planes between 100 and 300 km depth raising the question of whether the observations represent new constraints on the physical mechanism of the earthquakes or a bias in the methodology here the strengths weaknesses and biases of the method are investigated through the comprehensive analysis of 120 synthetic earthquakes with varying depth rupture vector orientation rupture complexity signal to noise ratio and station distribution these synthetic tests which allow the evaluation of the effect of each of the varied parameters on the rupture vector determination show that fault planes can be identified for a wide variety of conditions the method underestimates rupture velocities by 76 94 but this does not affect the determination of the orientation of the rupture vector the most important parameter for determining the orientation of the rupture vector and thereby allowing identification of the fault plane is the distribution of stations recording the earthquake better coverage around the earthquakes results in tighter constraints on the rupture vector the broader distribution of recording stations for deeper earthquakes results in tighter constraints on the rupture vectors for deeper earthquakes and therefore more ease in identifying the fault plane however the difference in resolution does not systematically prohibit the identification of certain orientations of fault planes at shallower depths and suggests that previous observations of exclusively subhorizontal fault planes between 100 and 300 km depth are not the result of systematic bias in the methodology instead the observed dominance of subhorizontal fault planes provides important constraints on the physical mechanism of intermediate depth earthquakes the observed subhorizontal fault orientation is inconsistent with the reactivation of the dominant trenchward dipping outer rise normal faults

compaction and decompaction algorithms for sedimentary carbonates abstract the mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration backstripping analyses and quantitative models for simulation of carbonate stratigraphy deep water chalks and calcareous oozes are not treated here empirical porosity depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity depth curves for shallow water carbonate muds and sands the curves are exponential in form this review indicates that mud rich shallow marine carbonates behave differently than sand rich grain supported sediments with regard to reduction in porosity and thickness quantitative analysis of compaction behavior in carbonate sediments reveals that barring early cementation carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths e g 50 thickness reduction with about 150 200 m overburden however primary porosities of carbonate muds approximately 70 80 are reduced only to about 42 under that overburden thus the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity this indicates that interparticle cementation with or without chemical compaction is required to transform muddy carbonate sediments into lithified rock in the absence of early lithification carbonate sands compact more slowly than muds primary interparticle porosities of about 40 45 can be maintained in the first 200 300 m of burial with little thickness reduction

the 2013 okhotsk deep focus earthquake rupture beyond the metastable olivine wedge and thermally controlled rise time near the edge of a slab the 2013 m8 3 okhotsk earthquake involves two primary mechanisms of deep focus earthquake rupture mineral phase transformation of olivine to spinel and thermal shear instability backprojection imaging of broadband seismograms recorded by the north american and european networks indicates bilateral rupture toward ne and sse the rupture paths of the ne segment and other regional m7 earthquakes are confined in narrow regions along the slab contours consistent with the phase transformation mechanism however the sse rupture propagates a long distance across the slab and aftershocks are distributed across a 60 km wide zone beyond the plausible thickness of the metastable olivine wedge favoring thermal shear weakening while the ne rupture is only visible at high frequencies the sse rupture is consistently observed across a broad frequency range this frequency dependent rupture mode can be explained by lateral variations of rise time controlled by thermal thinning of the slab near its northern end

tibetan garnet records early eocene initiation of thickening in the himalaya tectonic reconstructions of the himalayan orogeny depend on the age at which crustal thickening commenced to investigate this age we analyzed garnet from middle crustal rocks exposed in the north himalayan mabja and kangmar gneiss domes of tibet using lu hf geochronology garnet yielded lu hf ages of 54 52 ma in mabja and 51 49 ma in kangmar samples on the basis of microstructural and major element and rare earth element zoning observations the lu hf ages are interpreted as recording garnet growth during contractional deformation in the middle crust at 54 3 0 6 ma followed by variable recrystallization during subsequent high temperature ductile extension the new lu hf ages are the first to confirm that crustal thickening and contraction in the tibetan himalaya was broadly synchronous with the early eocene collision between greater india and the eurasian plate

application of inventory data for estimating characteristics of and regional climate change effects on mountain glaciers a pilot study with the european alps a parameterization scheme using simple algorithms for unmeasured glaciers is being applied to glacier inventory data to estimate the basic glaciological characteristics of the inventoried ice bodies and simulate potential climate change effects on mountain glaciers for past and potential climate scenarios glacier changes for assumed mass balance changes are calculated as step functions between steady state conditions for time intervals that approximately correspond to the characteristic dynamic response time a few decades of the glaciers in order to test the procedure a pilot study was carried out in the european alps where detailed glacier inventories had been compiled around the mid 1970s total glacier volume in the alps is estimated at about 130 km3 for the mid 1970s strongly negative mass balances are likely to have caused a loss of about 10 20 of this total volume during the decade 1980 90 backward calculation of glacier length changes using a mean annual mass balance of 0 25m w e a 1 since the end of the little ice age around 1850 ad gives considerable scatter but satisfactory overall results as compared with long term observations the total loss of alpine surface ice mass since 1850 can be estimated at about half the original value an acceleration of this development with annual mass losses of around 1 m a 1 or more as anticipated from ipcc scenario a for the coming century could eliminate major parts of the presently existing alpine ice volume within decades

glacier and permafrost signals of 20th century warming currently available evidence of 20th century warming from glaciers and permafrost is briefly reviewed the signals are clear and strong warming of polar firn and permafrost and mass losses of glaciers at lower latitudes were most striking towards the middle of the century the easily observable length reduction of mountain glaciers confirms the global character of the evolution a probably intermittent reversal of the trend was observed in places after about 1950

frictional melting during the rupture of the 1994 bolivian earthquake the source parameters of the 1994 bolivian earthquake magnitude m w 8 3 suggest that the maximum seismic efficiency was 0 036 and the minimum frictional stress was 550 bars thus the source process was dissipative which is consistent with the observed slow rupture speed only 20 of the locals wave velocity the amount of nonradiated energy produced during the bolivian rupture was comparable to or larger than the thermal energy of the 1980 mount st helens eruption and was sufficient to have melted a layer as thick as 31 centimeters once rupture was initiated melting could occur which reduces friction and promotes fault slip

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

enrichment of co2 in fluid inclusions in quartz by removal of h2o during crystal plastic deformation abstract ductile strain induced leakage of h 2 o from mixed co 2 h 2 o fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure co 2 fluid inclusions in metamorphic rocks the h 2 o needed for hydrolytic weakening of quartz under stress may be provided by the h 2 o in fluid inclusions with dislocation creep as dislocations nucleate on the walls of fluid inclusions or intersect them h 2 o can be transported with the dislocations from the inclusion to the grain boundaries the process should continue so long as there is stress on the host quartz and the inclusion contains h 2 o the h 2 o taken to the grain boundaries would due to its wetting properties be wicked out along the grain boundaries if the h 2 o is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal plastic flow then a residual inclusion bearing components other than water should remain a mixed co 2 h 2 o inclusion would as a consequence of the process become a pure co 2 fluid inclusion during dynamic recrystallization the co 2 would collect at grain boundary triple junctions and following grain boundary migration would become incorporated into the recrystallized quartz as fluid inclusions this mechanism of generating pure co 2 inclusions could result in co 2 fluid inclusions having densities appropriate for the pressure temperature conditions during deformation using estimates of temperature of deformation the total confining pressure during deformation could be determined from the density of the co 2 fluid in these inclusions

signals of 20th century warming from the glaciers in the central italian alps the lombard alps in the central sector of the italian alps are one of the most intensively glacierized regions on the southern side of the chain about 113 km2 of ice cover all of the glaciers have been retreating since the beginning of the 20th century but the trend is not uniform since the 1950s there has been a drop in the percentage of retreat ing snouts and an increase in stationary and advancing snouts after 1985 the glaciers in the lombard alps began a new recession phase that is still going on this pattern is con sistent with temperature variations in lombardy in the 20th century according to data from sondrio station four alternating warming and cooling phases can be distinguished in 20th century warming the lombard glaciers well recorded the last three phases showing evident signs of the 1955 88 cold phase with a response time of about 20 years and may be considered good indicators of 20th century temperature trends

the effect of strain heterogeneity on graphical strain analysis methods abstract the most frequently used graphical strain analysis methods have been derived based on the assumption of homogeneous strain for the sample size investigated in view of the heterogeneity of strain generally displayed by naturally deformed rocks on all scales it is important to determine what effect varying degrees of strain heterogeneity have on these methods an oolitic limestone was deformed at various strain rates and temperatures in order to achieve the desired variation by altering the relative contributions of intercrystalline and intracrystalline deformation mechanisms coefficients of determination for the best fit line to plots of the long vs short axes of the deformed ooids were obtained and compared to the coefficient from a simulated homogeneous deformation of the same limestone the coefficient for the homogeneously strained sample was 0 80 where as the experimentally deformed samples had coefficients ranging from 0 17 to 0 73 these samples were subsequently analyzed by the graphical strain analysis methods proposed by dunnet 1969 and elliott 1970 the analyses showed that the typical patterns associated with the use of these methods may be the result of strain heterogeneity specifically the scatter of points with no strong central concentration typically seen with dunnet s r f plots and the variable contour envelope shapes of elliot s shape factor grid method can be accounted for in this manner these patterns had previously been attributed solely to variations in initial shape and fabric of the strain markers strain analysis using the r f method is influenced less by strain heterogeneity than the shape factor grid method the reduction in fluctuation of long axis orientation of strain markers that occurs during constant volume deformation allows reasonable average values of the strain to be obtained even when the degree of strain heterogeneity is high the shape factor grid method is more critically affected because the identification of the initial fabric of the markers from a contour envelope derived from the deformed data is a necessary part of the strain analysis the contour envelope shape for the initial fabric of the markers will be distorted if the subsequent strain undergone by the markers is not homogeneous

stratigraphic correlation of cambrian ordovician deposits along the himalaya implications for the age and nature of rocks in the mount everest region the depositional age and stratigraphic correlations of metamorphosed and variably deformed rocks of mount everest are poorly known because of limited recovery of diagnostic fossils detailed study of cambrian and ordovician strata from along the length of the himalaya has produced a coherent stratigraphy that stretches from northern india to tibet our work also demonstrates that the north col formation rocks everest series between the qomolangma and lhotse detachments of the south tibetan detachment system still locally preserve sedimentary textures and primary stratigraphy that match those within cambrian strata similar to 1100 km to the west in northern india this demonstrates a coherency of depositional systems and stratigraphic architecture for cambrian deposits along much of the himalaya tethyan margin it also allows for the first time identification of precise depositional ages of several units in the everest region in particular the yellow band carbonate and directly underlying siliciclastic strata which are both shown to be late middle cambrian in age detrital zircon data presented herein for a sample from these siliciclastic strata contain a similar age spectrum to those from middle cambrian strata in northern india as well as grains as young as ca 526 ma both of which support the depositional age and continuity of depositional systems along the length of the himalaya highly fractured rocks of the ordovician lower chiatsun group in the hanging wall of the south tibetan detachment system in nyalam 75 km to the west of everest correlate with ordovician strata of the mount qomolangma formation on mount everest our correlations indicate that the base of the summit pyramid of everest the foot of the third step is composed of a 60 m thick white weathering thrombolite bed the top of this ancient microbial deposit crops out only 70 m below the summit of mount everest

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

the effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites abstract this study investigates the grain scale mechanisms that facilitated strain localization in the lower crust within the east athabasca mylonite triangle of northern saskatchewan canada there two distinct lithotectonic units the upper deck and northwestern domain are juxtaposed across a km scale shear zone that has been previously interpreted to record thrust and or normal sense displacement our study reports new quartz c axis opening angle ti in biotite and ti in quartz geothermometry and amphibole plagioclase geothermobarometry within syn kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies 725 c and 0 8 gpa while sharing similar conditions the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn kinematic hydrated mineral phases detailed microstructural and crystallographic preferred orientation cpo analyses reveals that specimens poor in hydrated minerals 10 pressure solution and re precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz cpo these results indicate a strong mineralogical and fluid control on deformation and therefore which strain softening process es may dominate in lower crustal shear zones

geologic and u pb geochronologic evidence for early paleozoic tectonism in the dadeldhura thrust sheet far west nepal himalaya abstract the dadeldhura thrust sheet inm western nepal consists of proterozoic lower paleozoic sedimentary and plutonic rocks and their metamorphic equivalents that rest structurally on proterozoic strata of the lesser himalayan sequence although regional metamorphism and ductile deformation were widespread during tertiary thrust emplacement relicts of early paleozoic tectonism are preserved locally new field and geochronologic studies together with the findings of previous workers indicate that this early paleozoic tectonism included 1 regional metamorphism to at least garnet grade 2 regional folding of a thick metamorphic sequence into a broad east west trending syncline 3 outcrop scale folding of metasedimentary rocks 4 emplacement of cambro ordovician granitic bodies during and after the metamorphism and deformation 5 uplift and erosion of the metamorphic sequence with garnet grade rocks locally exposed at the surface and 6 derivation of ordovician conglomeratic sandstones from the early paleozoic orogen similar records of metamorphism deformation and uplift erosion have been found in other regions of the himalaya indicating that rocks of the dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system future tectonic models of himalayan orogenesis must accommodate this early paleozoic event

fault plane orientations of deep earthquakes in the izu bonin marianas subduction zone 1 we make use of the effects of rupture directivity on waveform shape to analyze the rupture processes of 58 large deep earthquakes in the izu bonin marianas subduction zone for more than half of the analyzed earthquakes we determine a best fitting unilateral rupture direction using teleseismic data for 20 of these earthquakes the constraints on the rupture direction allow the fault plane to be identified where the subducting slab dips at a moderate angle near horizontal fault planes dominate at all studied depths 50 600 km within more steeply dipping slabs fault planes tend to dip toward the south and west rotated into the plane of the slab the poles of the definitively identified faults form a single tight cluster pointing up and toward the surface of the slab identified ruptures have a tendency to propagate away from the top surface of the slab between 100 and 300 km depth but appear to be randomly oriented at greater depths the occurrence of predominantly near horizontal faults at intermediate depths agrees with previous observations in several other subduction zones however at depths 300 km the results from the izu bonin marianas system differ from those previously obtained for tonga kermadec where both horizontal and vertical faults were identified we consider various physical mechanisms to explain our observations and conclude that while pre existing slab structures may be reactivated if they are favorably oriented the observed asymmetry in deep fault orientations may instead result from external forces acting on the slab and resulting changes in slab morphology

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

the 1993 kushiro oki japan earthquake a high stress drop event in a subducting slab a spatial slip distribution of the 1993 kushiro oki earthquake mw 7 6 is undoubtedly constrained using near field data and the detailed aftershock distribution this earthquake has peculiar features in that the slip on the fault plane is restricted within an extremely small area about 40 km 20 km between the upper and lower planes of the double seismic zone in the subducting slab beneath hokkaido the total seismic moment averaged slip and stress drop in this area are 3 3 1020 nm 5 5 m and 4 2 107 pa respectively this earthquake indicates that the region of the slab between the planes of the double seismic zone beneath hokkaido is strong enough to sustain a stress of about 4 107 pa the slip motion of this event is consistent with retrograde slab migration along the southern kuril trench and may represent a process by which retrograde motion of subducting slabs could occur

onset of mid crustal extensional flow in southern tibet evidence from u pb zircon ages new ion microprobe u pb dates from zircon in deformed orthogneiss and migmatite and an undeformed granite in mabja dome are the first to constrain the timing of peak metamorphism and onset and duration of mid crustal ductile extension in southern tibet at 35 0 0 8 ma and 12 19 million years the structural metamorphic and intrusive his tories in mid crustal rocks exposed in these north himalayan gneiss domes are similar to those recorded in the greater himalayan sequence suggesting that middle crust was continuous from beneath southern tibet southward to the high himalaya strain compatibility indicates that 35 ma ductile extension in mid crustal rocks of southern tibet was accommodated to the south at shallow crustal levels via normal slip along the southern tibetan detachment system the oldest age estimate for slip along this normal fault zone and extrusion of its footwall if gravitational collapse is an additional important process driving extension then southernmost tibet may have been at or near maximum elevation by late eocene early oligocene time

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

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

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

why calcite can be stronger than quartz 1 in the neves area eastern alps calcite forms asymmetric centimeter scale single crystal porphyroclasts in quartz mylonites developed during hydrous amphibolite facies metamorphism at 550 c under these conditions coarse calcite was clearly stronger than the surrounding polycrystalline dynamically recrystallized quartz matrix experimental results indicate that coarse calcite is less strain rate sensitive than wet quartzite consistent with an inversion in strength on extrapolation to natural strain rates for this to occur wet quartzite must be weak flowing at differential stress of 10 mpa the lack of high temperature twins showing bulging or recrystallization in calcite clasts is consistent with such low stresses during shear zone development under near peak metamorphic conditions the maximum effective viscosity ratio of coarse calcite to quartzite for these conditions is probably not large 10 however numerical modeling shows that ratios of around 2 are sufficient to maintain near rigid calcite clast behavior for power law rheology with stress exponents appropriate to quartz n 3 4 and coarse calcite n 6 the inversion in relative strength reflects the difference in influence of water on the crystal plastic flow of calcite and quartz water has a dramatic effect for quartz but little or no effect for calcite quartz rich rocks under hydrous amphibolite facies conditions in the middle to lower crust are therefore relatively weak in fact weaker than coarse calcite and flow at much lower stresses than dry quartz rich rocks at similar crustal levels

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

the double seismic zone in kuril kamchatka the tale of two overlapping single zones the classic double seismic zone in the central portion of the kuril kamchatka arc seems to turn into a regular wadati benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc respectively to delineate this lateral changeover we studied 33 large to moderate sized mb 5 1 earthquakes that occurred at depths shallower than 200 km between 1963 and 1991 these events took place in two regions where the double seismic zone terminates and single seismic zones emerge precise earthquake source parameters including focal depths and mechanisms are determined from a careful analysis of a mixture of analog and digital p and sh waveforms recorded at teleseismic distances our results indicate that the double seismic zone is confined between approximately 46 and 53 n northeast of 53 n intermediate depth earthquakes 70 200 km show only downdip compressional focal mechanisms furthermore the wadati benioff zone there seems to be a continuation of the upper compressional layer of the double seismic zone thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower extensional sheet of the double seismic zone similarly southwest of 46 n intermediate depth earthquakes show only downdip extensional mechanisms the single seismic zone is unlikely to consist of two layers of extensional seismicity in fact nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain in other words these extensional events did not occur in the same seismogenic structure as the upper compressional layer of the double seismic zone however we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower extensional layer of the double seismic zone lateral variation from a single zone under extension in the southwest to a double seismic zone and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone on the other hand the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250 450 km is under extension in the southwestern end of the arc whereas the rest of the arc is under compression vanishing of the lower extensional layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending a similar reasoning applies to the southwestern termination of the double seismic zone

initiation of crustal shortening in the himalaya new monazite u th pb petrochronological data from the annapurna region of central nepal outline a protracted thermal history spanning 30 ma from the early eocene c 48 ma to the early miocene c 18 ma rare earth element data collected concomitant with the isotopic analyses are consistent with prograde metamorphism and crustal thickening between 48 and 30 ma and anatexis between 28 and 18 ma the timing of metamorphism recorded in these rocks is consistent with records of crustal shortening derived from ultrahigh pressure rocks in the western himalaya and exhumed metamorphic rocks in southern tibet although previous records of early shortening metamorphism related to the initial collision of india with asia are spatially associated with the northern margin of the indian plate the ages presented in this study extend that early record south into the main himalayan range these new data provide important geological constraints on this early poorly documented history

time scale for adjustment of glaciers to changes in mass balance the length of time tm over which a glacier responds to a prior change in climate is investigated with reference to the linearized theory of kinematic waves and to results from numerical models we show the following tm may in general be estimated by a volume time scale describing the time required for a step change in mass balance to supply the volume difference between the initial and final steady states the factor f in the classical estimate of m l u where i is glacier length and u is terminus velocity has a simple geometrical interpretation ft is the ratio of thickness change averaged over the full length i to the change at the terminus although both u and f relate to dynamic processes local to the terminus zone the ratio f u and therefore tm are insensitive to details of the terminus dynamics in contrast to conclusions derived from some simplified kinematic wave models a more robust estimate of tm independent of terminus dynamics is given by tm h b where h is a thickness scale for the glacier and b is the mass balance rate negative at the terminus we suggest that tm for mountain glaciers can be substantially less than the 1o2 103 years commonly considered to be theoretically expected

supershear rupture in a mw 6 7 aftershock of the 2013 sea of okhotsk earthquake earthquake rupture speeds exceeding the shear wave velocity have been reported for several shallow strike slip events whether supershear rupture also can occur in deep earthquakes is unclear because of their enigmatic faulting mechanism using empirical green s functions in both regional and teleseismic waveforms we observed supershear rupture during the 2013 moment magnitude m w 6 7 deep earthquake beneath the sea of okhotsk an aftershock of the large deep earthquake mw 8 3 the m w 6 7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second suggesting efficient seismic energy generation comparing it to the highly dissipative 1994 m w 8 3 bolivia earthquake the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes

shear instability in a viscoelastic material as the cause of deep focus earthquakes the mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs we assume subducting slabs deform such that the spatially averaged strain rate becomes time independent furthermore we assume that rheology depends on temperature in the manner of an activation process using this model we investigate quantitatively the conditions under which shear instability takes place i e the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs first we study instability in a material whose rheological properties are spatially homogeneous shear instability takes place in a homogeneous material when exceeds a certain critical level e c which depends strongly on the stress in the slabs but does not depend on the detail of the creep law the value of the activation energy next we study instability in a material that contains shear zones similar to those often observed by field geologists i e a thin layer made of material with lower vicosity than the surrounding region we find that shear zones trigger shear instability even for the case of provided that g the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones exceeds 1 and that f the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones becomes lower than 1 applying this condition to subducting slabs with shear zones we find that shear zones trigger shear instability even when the average strain rate is lower than by an order of magnitude from these results and from an estimate of in subducting slabs we conclude that shear instability is a mechanism likely to induce deep focus earthquakes

measurement of differential rupture durations as constraints on the source finiteness of deep focus earthquakes 1 the physical mechanism of deep earthquakes can be constrained by identifying their fault planes resolving the fault plane ambiguity is a classical problem in seismology and we present a method to distinguish the fault plane using observations of source finiteness source finiteness is observable on seismograms at different azimuths and distances for single event ruptures as variations in the apparent rupture duration and for complex ruptures as differences in the traveltime delay between subevents for each earthquake the rupture duration or traveltime delay will be shortest in the direction of rupture propagation and longest in the opposite direction rather than measuring the actual rupture duration at each station we use a cross correlation technique that includes a stretching factor to measure the differential rupture duration between each pair of stations these differential measurements then allow us to identify the rupture direction rupture velocity and fault plane for each earthquake first we test the method on two synthetic earthquakes which represent earthquakes composed of one and two events the method works well for both examples although attenuation can bias the determined rupture direction for the single event case next we apply this method to p waves from broadband seismograms from four intermediate and deep focus earthquakes composed of two subevents the 23 january 1997 bolivian earthquake mw 7 1 276 km depth the 27 october 1994 earthquake south of the fiji islands mw 6 7 549 km depth the 21 july 1994 japan sea earthquake mw 7 3 471 km depth and the 11 november 1998 fiji islands earthquake mw 6 3 149 km depth each focal mechanism contains a subvertical and a subhorizontal nodal plane for three of the events our analysis shows that rupture propagated subhorizontally and we identify the subhorizontal nodal plane as the fault plane for the smallest event the rupture azimuth but not the rupture dip is well constrained and we cannot conclusively identify the fault plane rupture velocities vary from 0 18 to 0 63 of the local p wave velocity

determination of fault friction from reactivation of abyssal hill faults in subduction zones abyssal hill faults are reactivated on the outer slope of trenches when they strike within 25 from trench parallel otherwise new faults form parallel to the trench we use the observed transition angle 25 and a three dimensional failure analysis to determine the coeffi cient of friction on reactivated abyssal hill faults the stress state in the outer slope is modeled as the superposition of the overburden stress and bending induced plane strain deformation if new trench parallel faults fail according to a linear failure relationship with no cohesion then determination of the coeffi cient of sliding friction s on reactivated faults is independent of the absolute value of the principal stresses and depends only on the poisson ratio of the crust and the slope of the failure law for new faults f we fi nd that reactivated faults dipping at 45 are 30 weaker than surrounding crust e g s 0 6 for f 0 85 these results suggest that the variation in the strength of oceanic crust due to the seafl oor spreading fabric is small and that the coeffi cient of sliding friction on oceanic faults is consistent with that observed in the laboratory

aftershock distribution of the october 4 1994 mw8 3 kurile islands earthquake determined by a local seismic network in hokkaido japan on october 4 1994 an earthquake with magnitude mw8 3 occurred in the western part of kurile islands at 43 42 n 146 81 e and 33 km in depth the hypocenter parameters were determined by hokkaido university in japan aftershocks following this remarkable event were located using data from a local seismic network operated by hokkaido university we found that most of the aftershocks occurred 1 on the fault plane of the mainshock 2 in the subducting plate around the fault plane of the mainshock and 3 in the focal area of the largest aftershock which occurred on october 9 with mw7 3 both 2 and 3 were not active immediately after the mainshock considering the time sequence of the aftershock activity we identified one of the nodal planes of the harvard quick cmt solutions as the fault plane of the mainshock the strike is almost parallel to the trench axis and the dip angle is near vertical it is obvious that this event is different from a low angle thrust type interplate earthquake the distribution of aftershocks strongly suggests that it is an intraplate event