the dav and periadriatic fault systems in the eastern alps south of the tauern window alpine deformation of austroalpine units south of the tauern window is dominated by two kinematic regimes prior to intrusion of the main periadriatic plutons at 30 ma the shear sense was sinistral in the current orientation with a minor north side up component sinistral shearing locally overprints contact metamorphic porphyroblasts and early periadriatic dykes direct rb sr dating of microsampled synkinematic muscovite gave ages in the range 33 30 ma whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40ar 39ar age of 46 ma the transition from sinistral to dextral transpressive kinematics related to the periadriatic fault occurred rapidly between solidification of the earlier dykes and of the main plutons subsequent brittle ductile to brittle faults are compatible with n s to nnw sse shortening and orogen parallel extension antithetic riedel shears are distinguished from the previous sinistral fabric by their fine grained quartz microstructures with local pseudotachylyte formation one such pseudotachylyte from speikboden gave a 40ar 39ar age of 20 ma consistent with pseudotachylyte ages related to the periadriatic fault the magnitude of dextral offset on the periadriatic fault cannot be directly estimated however the jump in zircon and apatite fission track ages establishes that the relative vertical displacement was 4 5 km since 24 ma and that movement continued until at least 13 ma

lithologic and glacially conditioned controls on regional debris flow sediment dynamics debris flow is an efficient process of sediment transfer from slope base to piedmont depositional fans in mountain drainage basins to advance understanding of debris flow sediment dynamics at the regional scale we analyze a historical 1998 2009 database of debris flows from 77 basins of alto adige province northeastern italy by combining information on event volumetric deposition high resolution digital topography and quaternary sediment mapping we are able to link debris flow sediment flux to morphometry lithologic variability and sediment availability we show that basin wide specific sediment yield ssy scales as an inverse power function of basin area this function is strongly controlled by the way rock type and abundance of quaternary deposits affect the rate of downstream sediment recruitment when sediment flux associated with each debris flow event is subsumed across discrete spatial increments of the entire region a complex sedimentary signature in the area ssy space is apparent that is ssy increases downstream up to areas as large as 1 km2 and starts to decline beyond this scale regardless of sediment availability we propose that this area ssy relation is characteristic of debris flow dominated settings

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

shear zone patterns and strain distribution at the scale of a penninic nappe the suretta nappe eastern swiss alps abstract shear zone patterns within the suretta nappe of the swiss alps are studied in order to understand finite strain and bulk tectonic displacements the suretta nappe is chosen for two main reasons the availability of late variscan granites deformed during alpine tectonics only and because of continuous outcrop from bottom to top of the nappe this alpine nappe recorded two major ductile deformation phases which developed under low grade metamorphic conditions the analysis of shear zone patterns in the roffna granite reveals that the first deformation is well developed in the entire nappe the second deformation however is more localized in the lower and upper parts the bulk asymmetry of the shear zone patterns confirms the kinematics recorded by other penninic nappes in this area first a stacking of the nappe towards the nw followed by the top to the e shearing during post thickening extensional tectonics the shear zone patterns and strain distribution allow us to deduce kinematics at the nappe scale

dissolution precipitation creep of k feldspar in mid crustal granite mylonites abstract the deformation of k feldspar within lower amphibolite facies granite mylonites from the gran paradiso nappe north western alps italy is primarily accomplished by dissolution replacement and precipitation processes with little or no evidence for dislocation glide or creep new elongate grains of k feldspar precipitate in dilational domains within and around k feldspar porphyroclasts as a part of the process of myrmekite formation that progressively consumes the porphyroclasts the new grains grow in epitaxial continuity with the parent porphyroclasts which hinders the development of a bulk crystallographic preferred orientation cpo in the aggregates of new grains when large amounts of relict k feldspar are still present in mylonites the magmatic k feldspar is eventually transformed into 100 300 m thick nearly monomineralic fine grained 20 50 m in size aggregates of new grains showing an oblique shape fabric and a cpo the observed cpo is not consistent with the activity of any slip system in k feldspar instead it is interpreted to result from dissolution precipitation creep with a slow reaction rate parallel to 010 and 001 crystallographic axes and a fast reaction rate parallel the 100 axis consistent with such a dissolution precipitation mechanism boundaries of new k feldspar grains are highly corroded when oriented approximately parallel to the extensional instantaneous stretching axis whereas boundaries approximately orthogonal to the same stretching axis show well developed crystal facets the cpo developed is weak which suggests that the anisotropy in the dissolution growth rate of k feldspar is also weak

how is strain localized in a meta granitoid mid crustal basement section spatial distribution of deformation in the central aar massif switzerland abstract this study investigates strain distribution in granitoid rocks formerly in the middle crust in the central aar massif switzerland and places the deformation behavior in the tectonic framework of the alpine orogeny strain is heterogeneously distributed in terms of strain partitioning forming several hundreds of closely spaced shear zones sz 80 sz km with sz thicknesses the majority of the steep shear zones preferentially accommodated upward motion by the southern block leading to an increase in peak metamorphic conditions from 250 c in the north to 450 c in the south of the aar massif the shear zones initiated at about 18 20 km depths during a stage of crustal thickening handegg phase subsequent deformation reactivated some shear zones with a gradual transition from reverse dip slip over oblique slip to strike slip shear zones under local transpressional conditions oberaar phase

the relationship between stress changes and strong earthquakes since 1900 around the bayan har block in northern tibet china abstract a 3d viscoelastic finite element model of the qinghai tibet plateau including the bayan har block is established in this article this model takes into consideration differences in the regional geological structure the main active fault zone the active tectonic block hereafter referred to as a block and boundary fault zone the irregular topography and layered lithospheric structure and the seismic velocity structure of the crust and mantle moreover the present tectonic background stress field in the research area is reconstructed using the constraints of the observed values for the rate of crustal horizontal movement and the direction of maximum principal compressive stress on these bases the historical strong earthquake sequences with ms 7 that have occurred around the bayan har block since 1900 ad are simulated based on the coulomb failure stress and equivalent stress data the relationships between stress evolution and strong earthquakes the interactions between strong earthquakes are investigated in addition we have also explored the changes in stress on the longmenshan fault zone before and after the 2008 wenchuan earthquake as well as the relationships between stress evolution and regional seismicity the results show that the occurrence of strong earthquakes around the bayan har block may be related to the increase in the total stress in the source area the influence of long term tectonic stress loading on strong earthquakes cannot be ignored the 2008 wenchuan earthquake induced an increase in stress in the southern section of the longmenshan fault zone indicating that the 2008 wenchuan earthquake has an obvious promoting effect on the 2013 lushan earthquake the seismicity of the bayan har block and its adjacent areas may be affected by the regional total stress state

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

control of the geometry of precursor brittle structures on the type of ductile shear zone in the adamello tonalites southern alps italy abstract amphibolite facies ductile shear zones developed during the immediate post intrusive cooling history of the adamello tonalites southern alps italy shear zones include i thin a few mm s thick fault like shear zones that accommodate shear strain values up to several 100 s the dominant type ii mylonitic horizons dm s thick in sharp contact with the undeformed wall rock iii continuous shear zones with sigmoidal shaped s or composite s c foliations a transition between the different types occurs along strike over short distances discontinuous shear zones are also localized on pegmatite dykes and quartz veins and both i and ii are discontinuously outlined by quartz layers detailed surface mapping reveals that most fault like shear zones are arranged en echelon mainly forming contractional steps markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips differential displacement is mainly accommodated at contractional steps by the development of foliated domains geochemical analyses of major and trace elements show that there is no compositional change along strain gradients the overall features are consistent with nucleation of shear zones on former sets of en echelon joints in many cases intruded by pegmatite dykes or filled with quartz reactivation of joints produced strongly localized shear zones whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing these jogs were progressively involved in the accommodation of shear displacement and overall shear zone development

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

shear zones as a new type of palaeostress indicator this paper proposes that shear zones act as palaeostress indicators in much the same way as brittle faults several hundred shear zones from both the limbs of the variscan langland mumblcs anticline south wales are treated using the graphical and numerical methods of fault slip analysis estimated palaeostress directions derived from shear zones are characterized by greater scatter than where brittle faults are used as stress indicators an important reason for this is the fact that on some shear zones the direction of maximum shear stress slip lineation is not perpendicular to the line of s c intersection as a consequence the use of slip lineations produces better solutions than cases where the slip direction is inferred as a line perpendicular to the s c intersection the filtering of data to reduce the scatter in the results produces insignificant improvement and is considered unnecessary

geothermobarometry in albite garnet orthogneisses a case study from the gran paradiso nappe western alps abstract the aim of this paper is to estimate syntectonic p t conditions within albite and garnet bearing orthogneisses these rocks are generally characterized by the assemblage quartz albite biotite phengite cafe garnet epidote titanite garnet contains up to 55 mole per cent of grossular k feldspar is a relict magmatic phase p t conditions are estimated using several independent methods first it is shown that exchange reactions based on the fe mg partitioning between garnet and biotite or garnet and phengite cannot be used to estimate temperatures in these rocks due to the high grossular content of garnet second maximum and minimum pressures are constrained respectively by the occurrence of albite instead of jadeite quartz and by the assemblage phengite biotite quartz third phase equilibria in albite and garnet bearing metagranites are modelled in the system k 2 o cao feo al 2 o 3 sio 2 h 2 o equilibrium curves are calculated for the observed phase compositions uncertainties in p t estimates mainly result from the choice of appropriate non ideal solution models for the garnet an application is developed for granites from the gran paradiso nappe western alps these granites show an heterogeneous deformation of alpine age expressed by mylonitic shear zones cutting across weakly deformed domains estimated p t conditions for the synkinematic assemblages are 10 16 kbar at 550 50 c

nucleation and growth of myrmekite during ductile shear deformation in metagranites myrmekite is extensively developed along strain gradients of continuous lower amphibolite facies shear zones in metagranites of the gran paradiso unit western alps to evaluate the role of stress strain energy and fluid phase in the formation of myrmekite we studied a sample suite consisting of weakly deformed porphyric granites wdgs foliated granites fgs representative of intermediate strains and mylonitic granites mgs in the protolith most k feldspar is microcline with different sets of perthite lamellae and fractures in the wdgs abundant quartz oligoclase myrmekite developed inside k feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction in the wdgs stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between k feldspar and albite in the fgs and mgs k feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine grained recrystallized myrmekite and perthite aggregates this was accompanied by a more pervasive fluid influx into the reaction surfaces and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone in the mgs myrmekite forms complete rims along the outer boundary of the small k feldspar porphyroclasts which are almost completely free of internal reaction interfaces therefore we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress mass balance calculations indicate that in al si conservative conditions myrmekite growth was associated with a volume loss of 8 5 this resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite

lithological and seasonal control on rainfall thresholds for the possible initiation of landslides in central italy we compiled a catalogue of 442 rainfall events with landslides in the abruzzo marche and umbria regions central italy between february 2002 and august 2010 for each event we determined the duration d in hours and the cumulated total event rainfall e in mm using rainfall measurements obtained from a dense network of 150 rain gauges landslides were mapped as single points using google earth and were attributed a level of mapping accuracy p in four classes p1 1 km2 p10 10 km2 p100 100 km2 p300 300 km2 using this information we determined cumulated event rainfall rainfall duration ed thresholds which are entirely equivalent to rainfall intensity duration id thresholds and avoid unnecessary conversions to determine the ed thresholds we modified a statistical method first proposed to determine id thresholds adopting a bootstrapping statistical technique we expanded the method to determine the uncertainty associated with the parameters that define the power law threshold model and the sensitivity of the thresholds to the number of d e data points in the empirical data set we determined ed thresholds for 1 and 5 exceedance probability levels i for the entire study area and for the three individual regions in the study area abruzzo marche and umbria ii for three main lithological domains in the study area post orogenic sediments flysch deposits carbonate rocks and iii for two seasonal periods may september october april comparison of the ed thresholds indicates that the cumulated amount of rainfall necessary to trigger landslides in flysch deposits is larger than in post orogenic sediments and carbonate rocks however the ed thresholds for post orogenic sediments and carbonate rocks were statistically indistinguishable in the study area also the seasonal thresholds were statistically different for rainfall durations shorter than 12 h and longer than 100 h but were indistinguishable in the range 12 d 100 h the results obtained are directly applicable to landslide forecasting based on empirical rainfall thresholds and have implications for landslide hazard and risk assessment as well as for landslide erosion and landscape evolution studies both in the study area and in the neighboring regions

brittle ductile brittle deformation during cooling of tonalite adamello southern italian alps abstract late to post magmatic deformation in slightly diachronous contiguous intrusions of the north western adamello batholith southern alps italy is recorded as from oldest to youngest i joints ii solid state ductile shear zones iii faults associated with epidote k feldspar veins and iv zeolite veins and faults structures ii to iv are localized on the pervasive precursory network of joints i which developed during the earliest stages of pluton cooling high temperature 500 c ductile overprinting of joints produced lineations defined by aligned biotite and hornblende on the joint surfaces and highly localized mylonites the main phase of faulting producing cataclasites and pseudotachylytes occurred at 250 c and was associated with extensive fluid infiltration cataclasites and pseudotachylytes are clustered along different e w striking dextral strike slip fault zones correlated with the activity of the tonale fault a major tectonic structure that bounds the adamello batholith to the north ductile deformation and cataclastic veining episodes occurred at p 0 25 0 3 gpa during rapid cooling of the batholith to the ambient temperatures 250 c that preceded the exhumation of the batholith timing of the sequence of deformation can be constrained by 39 ar 40 ar ages of 30 ma on pseudotachylytes and various existing mineral ages in the whole composite adamello batholith multiple magma pulses were intruded over the time span 42 30 ma and each intrusive body shows the same ductile to brittle structural sequence localized on the early joint sets this deformation sequence of the adamello might be typical of intrusions undergoing cooling at depths close to the brittle ductile transition

complex flow in lowest crustal anastomosing mylonites strain gradients in a kohistan gabbro northern pakistan 1 strain localization in near paleo moho metagabbros of the kohistan arc northern pakistan produced anastomosing shear zones the two dimensional 2 d analysis of strain gradients in planes parallel to the general flow direction reveals simple shear strain 7 combined with about 50 surface loss the pargasite garnet assemblage of the mylonite has a density 2 8 higher than the protolith which is insufficient to account for the measured surface loss furthermore major and trace element compositions of the gabbro and the mylonite indicate isochemical deformation average vp and acoustic impedance measured at room temperature and up to 300 mpa increase from gabbro through the gradient zone to mylonite they are consistent with density measurements the three dimensional analysis of shape preferred orientation and lattice preferred orientation of sheared and synkinematic minerals indicates that the 2 d surface loss reflects sideways and lengthwise material transfer sideways and lengthwise material transfers take place in widening and lengthening mylonites respectively the anastomosing shear zone pattern impels this complexity of the regional flow we conclude that shear zones with a thinning component are likely representative of deep crustal flow in other tectonic environments

shear behavior of greenschist along foliation plane considering anisotropy the shear behavior of rock mass along its structural plane i e joint bedding plane and foliation plane plays a key role in determining rock strength which is highly anisotropic the shear behavior along the foliation plane is rarely addressed from engineering geology point of view in the effect of anisotropy of on the shear behavior along foliation plane not to mention the effect of anisotropy in this study the direct shear behavior of greenschist along the foliation plane is investigated using both intact and sheared greenschist samples identical concrete replicas for the sheared greenschist samples are prepared using 3d scanning and printing in this study and sheared in four directions i e 0 90 180 and 270 under four normal stresses left sigma text n sigma text c 0 1 0 2 0 3 0 4 right the intact greenschist samples show significantly higher shear strength compared to the sheared samples due to the bonding strength along foliation plane the shear behavior of greenschist along foliation plane is anisotropic due to the 3d roughness of foliation plane and the anisotropic behavior diminishes as the normal stress increases moreover the replicas of sheared greenschist samples shows same shear strength as the sheared greenschist samples thus suggesting that the concrete is suitable for simulating greenschist shear behavior including the peak shear strength residual shear strength and stiffness which also hints that the shear behavior of the sheared greenschist is dramatically controlled by surface roughness of the joint rather than rack by comparing the results of the shear greenschist with concrete replicas

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

progressive eclogitization under fluid present conditions of pre alpine mafic granulites in the austroalpine mt emilius klippe italian western alps metabasites from the austroalpine mt emilius klippe preserve different stages of the evolution from pre alpine granulite to alpine eclogite facies assemblages the pre alpine protolith includes layered garnet i clinopyroxene i plagioclase hornblende granulites and gabbros the eclogitic paragenesis comprises garnet ii clinopyroxene ii epidote ca amphibole chlorite eclogitic deformation is composite and involves two main episodes d1a d1b d1a produces a layer parallel foliation and incomplete transformation of granulites and is concluded by an episode of brittle deformation leading to the formation of eclogitic veins d1b gives rise to widespread s l tectonites and mylonites the eclogitic veins and surrounding haloes enriched in fluids during veining control the nucleation of d1b shear zones deformation is accomplished through ductile flow of reaction weakened plagioclase and hornblende sites and cataclasis of garnet i and clinopyroxene i both meso and microscale observations indicate that fluids play a primary role during eclogitization in both enhancing deformation and the kinetics of metamorphic reactions fluids influence plastic flow rates in matrix aggregates and promote fracturing of hard minerals the extent of metamorphic re equilibration is closely related to the density of the fluid pathways

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

brittle precursors of plastic deformation in a granite an example from the mont blanc massif helvetic western alps abstract in the mont blanc helvetic massif granites record mesoscale alpine structures which include joints veins cataclastic to mylonitic shear zones and foliated granites a detailed structural analysis indicates that brittle deformation predates plastic strain joints never pass through and veins are offset by cataclastic shear zones and mylonites the mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions plastic deformation is first localized in the brittle discontinuities and the fine grained matrix of cataclasites then it involves the granite within brittle shear zones and this is initially accomplished mainly by flow of reaction softened aggregates of sericite widely replacing the strain supporting magmatic plagioclase the brittle to plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity and these characteristics are derived from reactivation of and focusing along pre existing brittle discontinuities in addition mylonites may inherit high angles of intersection and may contain granite porphyroclasts these features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics

volume strain strain type and flow path in a narrow shear zone this study explores the state of finite strain and changes in the mean kinematic vorticity number grain size whole rock chemistry and mineralogy across an upper amphibolite facies shear zone in a metadiorite northern malawi east central africa p t conditions during shear zone formation and deformation were approximately 700 750 c and 5 7 kbar and are slightly less than p t conditions for the regional peak of metamorphism the major rock forming minerals plagioclase hornblende biotite and quartz were deformed by crystal plastic processes accompanied by except for hornblende dynamic recrystallization the modal abundance of all four major rock forming minerals shows no systematic change from the country rock into and across the shear zone indicating that shear zone development was not associated with retrograde mineral reactions the grain size of the major rock forming minerals decreases within the shear zone plagioclase and hornblende which occur as porphyroblasts outside the shear zone exhibit a bimodal grain size distribution within the shear zone quartz has a unimodal grain size distribution in the shear zone major and trace element chemistry does not change systematically across the shear zone implying no volume change in the mylonite matrix strain data for plagioclase and hornblende by the fry method and fabric strain as deduced from rf analysis of plagioclase and quartz grains demonstrate a slightly constrictional strain type k 1 5 across the shear zone the quantitative finite strain data for the different residual minerals as obtained by unlike methods show no systematic variation but recrystallized plagioclase grains record higher strain than the residual grains the mean kinematic vorticity number changes from approximately 0 3 outside to approximately 0 8 within the shear zone indicating that the bulk deformation path deviated from progressive simple shear the estimates for finite strain and the degree of noncoaxiality account for approximately 50 of thinning normal to the shear zone

quartz fluid inclusion abundance and off fault damage in a deeply exhumed strike slip seismogenic fault abstract off fault damage zones comprise highly fractured rocks surrounding the dynamic slip surface of faults these damage zones modify fault zone rheology and rupture dynamics by changing the bulk elastic properties and modulating fluid flow damage zones in the brittle upper crust reaching widths 100 m are commonly characterized by measuring fracture density but identifying the extent of off fault damage in deeply exhumed faults is challenging due to post and inter seismic viscous plastic deformation we measured fluid inclusion abundance in quartz deformed at 400 500 c from an ancient seismogenic strike slip fault shear zone to evaluate whether it can be a proxy for damage zone width in contrast to upper crustal fault zones displaying a high low trend of healed microfracture density the shear zone has a low high low trend of fluid inclusion abundance from the core toward the host rock we find that pattern explicable through removal of fluid inclusions by recrystallization after co seismic deformation allowing us to use fluid inclusion abundance to measure damage zone extent which is 80 m the low high abundance region from the shear zone core our findings indicate that extensive co seismic damage zones may extend from earth s surface to the base of the seismogenic zone well within the frictional to viscous transition

crustal thickness map of the chinese mainland from teleseismic receiver functions abstract we present a new crustal thickness map of the chinese mainland and adjacent areas based on a compilation of previously published data more than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the chinese mainland the combined dataset provided good coverage especially over eastern and central china and our analysis revealed crustal thickness variations of the chinese mainland with considerably more detail than in previous models our crustal thickness map shows a large variation from a thickness of less than 30 km in the mesozoic basin of eastern china to over 80 km in the tibetan plateau the thickest crust was found in central tibet while the thinnest continental crust was observed beneath eastern china normal to slightly thick crust 38 51 km was found for the stable precambrian basins and orogenic belts comparison between our model and other models reveals that the models share similar large scale and smaller scale features over large portions of eastern china where the data coverage is good however large differences up to 25 km in crustal thickness occur in areas with complex crustal structure or poor data coverage we also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness

preservation of sub microscopic structural relicts in micas from the gran paradiso massif western alps implications for 40ar 39ar geochronology abstract permian igneous biotite and white mica that were re heated at 500 550 c in the middle late eocene were investigated by laser step heating and in situ 40ar 39ar techniques in conjunction with scanning electron microscopy sem electron microprobe emp and transmission electron microscopy tem to assess the influence of short lived metamorphism on k ar systematics large intra and inter grain age variations ranging from the triassic to the late eocene are primarily related to the extent of metamorphic re equilibration brown biotite porphyroclasts from weakly re equilibrated samples are characterized by core to rim zoning in major element composition mineral structure and 40ar 39ar ratios titanium concentration and fe mg fe ratios decrease from crystal cores towards the rims which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas compositional changes are coupled with structural modifications towards the crystal edges where the disordered igneous 1m 2m1 stacks are replaced topotactically by highly ordered metamorphic 1m polytypes apparent ages up to 66 ma were determined in crystal cores by in situ laser probe analyses while ages as young as 45 ma are typical of crystal rims step heating experiments of these zoned biotites yielded a discordant saddle shaped age spectrum with a concordant central segment yielding an error weighted mean age of 44 9 0 3 ma biotite porphyroclasts from a more re equilibrated specimen gave markedly different results with uniform major elements compositional profiles homogeneous argon distribution throughout the crystals and a flat age spectrum at 36 5 0 3 ma tem investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1m polytypes during alpine metamorphism white mica yielded comparable results with spot ages up to 218 ma in domains characterized by high na na k ratios and low si and ti content representing the best preserved igneous relicts significantly younger apparent ages down to 44 39 ma are typical of domains affected by partial metamorphic re equilibration at high pressure conditions as indicated by increasing si and ti contents or by exhumation related exsolution of amoeboidal quartz these results indicate that different intra grain and inter grain ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal chemical domains core to rim age gradients in biotite result from inward directed recrystallization from the crystal edges eventually leading to flat age profiles as metamorphic replacement of pre existing porphyroclasts runs to completion this study calls for caution when estimating thermal histories from apparent age gradients or from age vs grain size correlations unless the presence of pre metamorphic mineral relicts can be ruled out

quartz microstructures and crystallographic preferred orientation which shear sense do they indicate abstract crystallographic preferred orientation cpo and shape fabrics of dynamically recrystallized quartz are currently used as shear sense indicators we show that the cpo and shape fabric in a polyphase rock do not necessarily indicate the shear sense at the global scale of the shear zone but rather at the local scale of the deforming and recrystallizing quartz aggregates in lower amphibolite facies shear zones in the gran paradiso metagranodiorite magmatic quartz grains have recrystallized dynamically by subgrain rotation and grain boundary migration and deform inside a very fine grained feldspar mica matrix the quartz cpo has a peripheral c axis maximum inclined synthetically with the local shear sense the orientation of the surface fabric is related to the orientation of the c axis maximum and the microscopic shear sense in the quartz aggregate the geometry of the surface fabric ranges from monoclinic to symmetric depending on the relative contribution of grain boundary migration it is inferred that flow partitioning between the quartz aggregates and the matrix controls the local kinematics cpos are only reliable shear sense indicators in polyphase rocks if the most highly strained parts are analyzed where spin of the aggregates with respect to the shear zone boundary has ceased

quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep abstract in the gran paradiso metagranodiorite western alps small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation cpo and a monoclinic surface orientation distribution function odf the polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep in the ultramylonite the quartz cpo is randomized and the surface odf becomes orthorhombic the transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates concomitant with the precipitation of k feldspar biotite between quartz grains in quartz reduction from the dynamically recrystallized grain size in the aggregates 110 m to the size of the dispersed grains in the ultramylonite 25 m occurs through the following processes k feldspar precipitates at opening sites along grain boundaries strain incompatibility pinning the grain size in quartz aggregates coalescence of k feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates solution precipitation reduces the size of the dispersed grains to less than subgrain size 45 m

peristerite compositions in quartzofeldspathic schists franz josef fox glacier area new zealand plagioclase compositions vary from an0 1 2 5 to an32 with increasing grade in chlorite zone to oligoclase zone quartzofeldspathic schists franz josef fox glacier area southern alps new zealand this change is interrupted by the peristerite composition gap in rocks transitional between greenschist and amphibolite facies grade oligoclase an20 24 and albite an0 1 0 5 are found in biotite zone schists below the garnet isograd with increasing grade the plagioclase compositions outline the peristerite gap which is asymmetric and narrows to compositions of an12 and an6 near the top of the garnet zone in any one sample oligoclase is the stable mineral in mica rich layers above the garnet isograd whereas albite and oligoclase exist in apparent textural equilibrium in adjacent quartz plagioclase layers the initial appearance of oligoclase in both layers results from the breakdown of epidote and possibly sphene carbonate is restricted to the quartz plagioclase rich layers and probably accounts for the more sodic composition of oligoclase in these layers the formation of more ca rich albite and more na rich oligoclase near the upper limit of the garnet zone coincides with the disappearance of carbonate and closure of the peristerite gap garnet appears to have only a localized effect on ca enrichment of plagioclase in mica rich layers within the garnet zone the na content of white mica increases sympathetically with increasing ca content of oligoclase and metamorphic grade comparison of the peristerite gap in the franz josef fox glacier schists and schists of the same bulk composition in the haast river area 80 km to the s indicates that oligoclase appears and epidote disappears at lower temperatures and that the composition gap between coexisting albite and oligoclase is narrower in the franz josef fox glacier area it is suggested that a higher thermal gradient 38 40 c km and variations in si al ordering during growth of the plagioclases between the two areas may account for these differences in the alpine schists the peristerite gap exists over a temperature and pressure interval of about 370 515 c and 5 5 7 kbar 550 700 mpa ph2o

networking of shear zones at the brittle to viscous transition cap de creus ne spain a crustal scale shear zone network at the fossil brittle to viscous transition exposed at cap de creus ne spain evolved by coeval fracturing and viscous mylonitic overprinting of an existing foliation initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips shear zones interconnected along and across their shearing planes separating rhomb shaped lozenges of less deformed rock lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above in this way deformation became homogeneous on the scale of initial fracturing metre to decametre scale in contrast the shear zone network represents localisation of strain on a decametre length scale the strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain as fracturing evolved to mylonitic shearing and as the shear zones coalesced to form a through going network subparallel to the shearing plane crustal strength must therefore be considered as strain and scale dependent

distribution of landslides in the upper tiber river basin central italy abstract a comprehensive analysis of the distribution of landslides was completed in the upper tiber river basin central italy landslide occurrence was ascertained through a large scale inventory obtained through air photo interpretation api and local field surveys lithological and bedding information was obtained through photo geological and field mapping aided by the review of geological maps and reports the probability density of landslide areas was found comparable to statistics obtained by investigators in other areas of the world adoption of empirical relationships to link landslide area and volume allowed estimating the total volume of landslide material in the 4098 km2 catchment in excess of 5 9 109 m3 statistics of terrain elevation and slope in stable areas were found different from those in landslide areas in particular modal terrain slope in landslide areas was less steep 11 9 than that in areas where landslides were not mapped 13 3 analysis of the landslide and slope areas revealed that a relationship exists between the size of the slope and the size of the most common landslide analysis of the spatial relationship between landslides and the geological setting revealed that slope failures are most abundant where soft and weak rocks crop out and where bedding is chaotic or disorganized where rocks are regularly bedded landslide abundance is largest in dip slopes these new findings are important to understand the long term evolution of the upper tiber river basin and may prove useful in the assessment of landslide hazard and risk

an upper mantle s wave velocity model for east asia from rayleigh wave tomography abstract we present a new shear velocity model of the upper mantle beneath the east asia region derived by inverting rayleigh wave group velocity measurements between 10 and 145 s combined with previously published rayleigh wave phase velocity measurements between 150 and 250 s rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2d dispersion maps for 10 145 s periods the group velocity maps benefit from the inclusion of new data recorded by the china national seismic network and surrounding global stations the increase in available data has resulted in enhanced resolution compared with previously published group velocity maps the horizontal resolution across the region is about 3 for the periods used in this study the new shear wave velocity models indicate varying velocity structure beneath eastern china which yields estimates of a lithosphere asthenosphere boundary depth from around 160 km beneath the yangtze block to approximately 140 km beneath the western part of the north china craton ncc up to depths of 70 100 km beneath the eastern ncc northeast china and the cathaysia block the models reveal the subduction of two opposite facing continental plates under the southern and northern margin of tibet an obvious low velocity anomaly appears in the top 200 km of the upper mantle beneath northern tibet which is inconsistent with the presence of subducted asian or indian mantle lithosphere beneath northern tibet the cenozoic volcanism fields in the mongolian plateau are characterized by an obvious upper mantle negative anomaly but no signature of deep seated plume was observed

ultramylonite bands derived from cataclasite and pseudotachylyte in granites northeast japan abstract small scale ultramylonite and cataclasite bands millimeters to tens of centimeters thick are developed in granitic rocks west of the hatagawa fault zone hfz in the abukuma belt northeast japan they occur as single or paired bands with sharp planar boundaries trending nne ssw and often form networks and conjugate sets the very small s c angle and the high displacement thickness ratio of the bands suggest that the shear strain is high the ultramylonite bands are commonly associated with cataclasite bands and mineral veins and rarely with pseudotachylyte some cataclasite bands contain mylonitized layers in which quartz fragments are strongly deformed and dynamically recrystallized on the other hand some ultramylonites are fractured producing fragments that have rotated during later cataclasis the major element content of the ultramylonite bands is similar to that of the surrounding granitic rocks strongly suggesting that the ultramylonite bands have formed through in situ deformation of the granitic protolith without significant mass transfer mineralogy and microstructures of some ultramylonites suggest the strong possibility that they are derived from pseudotachylyte the ultramylonite bands are interpreted as forming in the 10 15 km deep cataclastic plastic transition zone under greenschist facies conditions where co seismic fracturing and aseismic plastic flow have alternated

microstructures in feldspars from a major crustal thrust zone the grenville front ontario canada abstract samples collected across the nw grenville front in ontario canada have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade the differential displacement across this 5 km thick section of the grenville front tectonic zone is at least 10 km at upper to mid amphibolite conditions both plagioclase and potassium feldspars behave plastically grainsize reduction occurs through grain boundary migration and or subgrain rotation and produces a core and mantle structure at middle to lower amphibolite grade grain size reduction is accomplished through grain boundary migration at lower amphibolite to upper greenschist grade grain size reduction occurs by the nucleation and growth of new grains with a different composition the abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine grained matrix in ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction myrmekite is well developed on the high stress boundaries in k feldspar at upper to mid greenshist conditions feldspars deform mainly by slip on synthetic shear fractures deformation twins undulatory extinction deformation bands and kink bands occur in plagioclase and perthite flames and myrmekite occur principally on high stress boundaries in k feldspar grain size reduction in feldspar is accomplished through cataclasis at middle to lower greenschist facies deformation in feldspars is dominated by antithetic fractures the transition from the dominance of synthetic to antithetic fractures is gradational below lower greenschist grade rocks deform by pervasive cataclasis of all minerals

nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions abstract in the neves area the pre alpine intrusive protolith of the zentralgneise unit tauern window eastern alps is well preserved in a kilometric scale low strain domain without pervasive alpine deformation it is compositionally heterogeneous consisting predominantly of granodiorites with lesser leucocratic granites and different generations of lamprophyres and aplites the intrusive rocks are crosscut by fractures that were locally infiltrated by fluids and surrounded by alteration haloes incipient alpine amphibolite facies ductile deformation is strongly localized on these precursor fractures and on lithological planar heterogeneities resulting in the development of several different types of shear zones fractures without alteration haloes initially accommodate slip entirely on the fracture itself with increasing deformation a foliation is progressively developed in the adjacent host rock eventually producing a single heterogeneous ductile shear zone with the typical sigmoidal foliation pattern strong layers aplite dykes and bleached alteration haloes developed to either side of precursor fractures localize shear on their boundaries to produce characteristic paired shear zones shearing is more evenly distributed within weak layers lamprophyres and quartz veins with a marked discontinuity in shear strain against the adjacent little deformed granodiorite shear zone development was accompanied by the formation of new fractures and quartz rich veins in the host rock which in turn also localized shear magmatic contacts fractures and quartz veins are mostly steeply dipping and effectively span the complete range of strike orientations the kinematics of the overprinting strike slip shear zones was determined by the orientation of the initial discontinuities relative to the local principal compressive stress axis 1 here oriented ca 345 discontinuities of almost all orientations show shear reactivation even in the case of very low resolved shear stress indicating an overall viscous response of the system without a specific yield stress the geometry and kinematics of the shear zone network suggest that the overall deformation in low strain domains was close to coaxial during deformation along the shear zone network compatibility was maintained by fracturing with the development of new quartz veins and by distributed ductile deformation of the host rock especially within contractional domains of the shear network and at shear zone intersections deformation never propagates into the undeformed homogeneous granodiorite as discrete ductile shear zones but is limited by the original extent of the precursor discontinuities shear zone development in intact rock is always preceded by fracturing which localizes subsequent shear reactivation

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

temperature constraints on microfabric patterns in quartzofeldsphatic mylonites ribeira belt se brazil abstract based on samples from the major high temperature tres rios alem paraiba padua transpressive shear zone in the ribeira orogenic belt brazil we discuss the applicability of titaniq geothermometry to constrain peak temperature conditions during high temperature mylonitization of quartzofeldspatic rocks and explore the microfabrics formed at these conditions we discuss various aspects of the titaniq method and conclude that deformation occurred at temperatures ranging from 612 to 740 20 c in the studied segment of the shear zone this high temperature deformation resulted in relatively large grain size quartz ribbons and abundant intracrystalline deformation however the cpo fabrics are weak and microstructures suggest that quartz deformation was accommodated by dynamic recrystallization involving grain boundary migration with subsequent grain growth and later some subgrain rotation during exhumation we relate the weak fabrics to diffusion processes during or immediately after dynamic recrystallization and dislocation creep and to the effect of competing slip systems during deformation in terms of rheology evidence for dauphine twinning in our samples suggest strain softening during mylonitization and we suggest that such twinning may add to the rheologically weak nature of quartzo feldspatic portions of hot middle to lower crust

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

on folding and shear zone development a mesoscale structural study on the transition between two different tectonic styles abstract an example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis folds and shear zones were formed under retrogressive conditions after the peak of the regional hercynian metamorphism and both affect a previous regional schistosity a gradual change in structural style on a regional scale is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones geometrical analysis reveals that the bulk structural pattern is compatible with a non coaxial strain history structural variation reflects a gradual change in mechanical behaviour of schists which show a marked variation in crystallinity at the time of deformation

structure and metamorphism of the gran paradiso massif western alps italy the pressure temperature time trajectory and structural history of high pressure rocks presently exposed in the gran paradiso massif provide constraints on the processes that caused their thermal evolution and exhumation high pressure metamorphism of the rocks is found to have culminated at temperatures around 525 c and pressures of 12 to 14 kbar after high pressure metamorphism the rocks cooled during initial decompression while undergoing top to the west shear on chlorite bearing shear bands and larger scale shear zones biotite bearing shear bands and larger shear zones related to top to the east deformation affected the gran paradiso massif during reheating to temperatures of around 550 c at 6 to 7 kbar further exhumation occurred at relatively high temperatures a potentially viable explanation of the observed stage of reheating before final cooling and exhumation is breakoff of a subducting slab in the upper mantle allowing advective heat transfer to the base of the crust electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0357 6

high temperature fracturing and ductile deformation during cooling of a pluton the lake edison granodiorite sierra nevada batholith california abstract in the bear creek area of the sierra nevada batholith california the high temperature postmagmatic deformation structures of the lake edison granodiorite include steeply dipping orthogneiss foliations joints and ductile shear zones that nucleated on joints and leucocratic dykes exploitation of segmented joints resulted in sharply bounded thin shear zones and in large slip gradients near the shear zone tips causing the deformation of the host rock at contractional domains the orthogneiss foliation intensifies towards the contact with the younger mono creek granite and locally defines the dextral rosy finch shear zone rfsz a major kilometre wide zone crosscutting the pluton contacts joints predominantly strike at n70 90 e over most of the lake edison pluton and are exploited as sinistral shear zones both within and outside the rfsz in a narrow 250 m thick zone at the contact with the younger mono creek granite within the rfsz the lake edison granodiorite includes different sets of dextral and sinistral shear zones joints the latter corresponding to the set that dominates over the rest of the lake edison pluton these shear zones joints potentially fit with a composite y r r shear fracture pattern associated with the rfsz or with a pattern consisting of y r shear fractures and rotated t mode i extensional fractures the mineral assemblage of shear zones and the microstructure and texture of quartz mylonites indicate that ductile deformation occurred above 500 c joints and ductile shearing alternated and developed coevally the existing kinematic models do not fully capture the structural complexity of the area or the spatial distribution of the deformation and magmatic structures future models should account more completely for the character of ductile and brittle deformation as these plutons were emplaced and cooled

the ultimate fate of a synmagmatic shear zone interplay between rupturing and ductile flow in a cooling granite pluton abstract the neoarchean cundimurra pluton yilgarn craton western australia was emplaced incrementally along the transpressional cundimurra shear zone during syndeformational cooling discrete networks of cataclasites and ultramylonites developed in the narrowest segment of the shear zone showing the same kinematics as the earlier synmagmatic structures lithological boundaries between aplite pegmatite veins and host granitic gneiss show more intense pre cataclasite fabrics than homogeneous material and these boundaries later became the preferred sites of shear rupture and cataclasite nucleation transient ductile instabilities established along lithological boundaries culminated in shear rupture at relatively high temperature 500 600 c here tensile fractures at high angles from the fault plane formed asymmetrically on one side of the fault indicating development during seismic rupture establishing the oldest documented earthquake on earth tourmaline veins were emplaced during brittle shearing but fluid pressure probably played a minor role in brittle failure as cataclasites are in places tourmaline free subsequent ductile deformation localized in the rheologically weak tourmaline rich aggregates forming ultramylonites that deformed by grain size sensitive creep the shape and width of the pluton shear zone and the regime of strain partitioning induced by melt present deformation and established during pluton emplacement played a key role in controlling the local distribution of brittle and then ductile subsolidus structures

numeric modeling of the strain accumulation and release of the 2008 wenchuan sichuan china earthquake abstract as one of the most devastating earthquakes the 2008 wenchuan earthquake occurred on imbricate high angle listric reverse faults the faults dip 70 above 15 km depth become 30 to 40 below 15 km depth and presumably root into a subhorizontal brittle ductile transition zone below about 22 2 km depth in this paper we use the viscoelastic finite element method to simulate coseismic deformation and stress change and recurrence of major earthquakes associated with the high angle listric reverse fault our modeling shows that changes of coseismic equivalent stress mainly occur in the vicinity of the seismogenic fault especially the hanging wall of the fault in the depth range above 12 km modeled coseismic slip distribution and modeled average recurrent interval corroborate with geological geodetic and seismological observations we modeled 250 000 yr slip history of the fault slip the result shows that minor slips which obey slip predictable model take place on 30 40 dipping fault plane at depth below 15 km when the isotropic stress accumulation on the gentle fault reaches the critical level the entire gently dipping fault ruptures to form a large slip the large slips on the gently dipping fault comply with a time predictable model of recurrences these larger slips on the gently dipping fault occur simultaneously with slips on the steeply dipping fault and might have triggered slips on the steeply dipping fault to form great events such as the 2008 wenchuan earthquake

local shear zone pattern and bulk deformation in the gran paradiso metagranite nw italian alps the gran paradiso nappe of the northwestern alps mostly consists of augen gneisses derived from the alpine deformation of permian granitoids the regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top to west sense of shear the granitoid protolith is preserved in the kilometre scale low strain domain of the piantonetto valley and mainly consists of a porphyritic metagranite including joints leucocratic dykes and biotite rich schlieren in this low strain domain the alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite the shear zones mostly dip towards s se in a shallow shear zones1 to steep inclination shear zones2 the shear zones show typical features that can be explained by reactivation of pre existing joints and planar compositional heterogeneities palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening the kinematics of individual shear zones depends on the orientation of the original heterogeneities acting as nucleation planes and by partitioning of strain components at the kilometre scale with concentration of the flattening component to the piantonetto low strain domain the strain geometry and the kinematics of individual shear zones within piantonetto are not directly connected to the top to west sense of tectonic transport observed elsewhere in the gran paradiso nappe however the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the gran paradiso massif we conclude that the shear zone network of the piantonetto valley is representative of the incipient stages of ductile deformation of a granite nappe even if its architecture is determined by the arrangement of pre existing structural and compositional heterogeneities aspects of the large scale bulk strain can be derived from this local shear zone pattern

recognizing episodic lithospheric thinning along a convergent plate margin the example of the early oligocene alps large scale tectonic mode switches from lithospheric shortening to lithospheric extension may play a fundamental role in the tectonometamorphic evolution of orogens located along convergent plate margins short lived episodes of lithospheric extension can be inferred from the concomitant occurrence of bimodal magmatism rapid exhumation of rock units activity of extensional shear zones steep geothermal gradients and possibly topographic collapse of the orogenic belt here we provide a synthesis of existing data from the european alps supporting the hypothesis on tectonic mode switches data are interpreted to suggest that at ca 38 34 ma a period of n s orogen perpendicular shortening led to the formation of medium to ultra high pressure mineral assemblages by lithosphere scale thrust imbrication a tectonic mode switch took place at ca 34 ma marked by the onset of e w extension during this extensional event internal units of the western and central alps were rapidly exhumed to mid crustal level in the footwall of regional scale extensional shear zones tectonic thinning was also accomplished through pervasive e w stretching of the nappe pile lithospheric stretching resulted in the onset of an anomalous thermal regime which was responsible for the overprint of the older medium to ultra high pressure mineral assemblages by barrovian type mineral assemblages and by migmatization at the same time attenuation of the isotherms led to partial melting of mantle lithosphere and to bimodal magmatism contemporaneous topographic collapse of the orogen is suggested by the deposition in the foreland of flysch units rich in minerals that were probably derived from erosion of the early oligocene calc alkaline volcanics erupted in the hinterland this short lived episode of lithospheric stretching took place in no more than 4 myr from ca 34 ma to ca 32 30 ma renewed lithospheric thickening which is observed in seismic tomography studies resulted in large scale folding of the extensional structures and the in rapid generation of topographic relief feeding abundant clastic sedimentation in the peripheral basins

volume loss and tectonic flattening strain in granitic mylonites from the blue ridge province central appalachians abstract granitic mylonites from the blue ridge province in the central appalachians demonstrate the contribution of various deformation mechanisms to the strain geometry that developed during greenschist facies deformation in thrust related shear zones strain accumulated through cataclasis in feldspar crystal plastic processes in quartz and a significant contribution of pressure solution in both feldspar and quartz strain analysis was performed using r f f techniques on quartz grain shapes and by measuring stretched and boudinaged feldspars petrographic and geochemical evidence indicates that deformation ranged from isovolumetric to upwards of 60 bulk rock volume loss quartz and feldspar strain values plot within the field of apparent flattening feldspars in both xz and yz sections are boudinaged and separated by transverse veins and record a true tectonic flattening strain strain data indicate that the bulk deformation path in shear zones throughout the blue ridge province significantly deviated from plane strain

underplating related finite strain patterns in the gran paradiso massif western alps italy heterogeneous ductile strain superimposed on a nappe stack a finite strain study in the central and northern gran paradiso massif of the italian western alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the alps the r f and fry methods were used on feldspar porphyroclasts from 98 samples of the gran paradiso unit upper tectonic unit of the gran paradiso massif and 12 samples from the underlying erfaulet unit lower unit of the gran paradiso massif microstructures and thermobarometric data show that feldspar ductility at temperatures higher than c 450 c occurred only during high pressure metamorphism when the rocks were underplated beneath the overriding adriatic plate therefore the finite strain data can be related to high pressure metamorphism in the alpine subduction zone the augen gneiss was heterogeneously deformed and axial ratios of the strain ellipse in xz sections range from 2 1 to 69 8 the long axes of the finite strain ellipsoids trend west wnw and the short axes are subvertical associated with a subhorizontal foliation the strain magnitudes do not increase towards the nappe contacts the data indicate flattening strain type in the gran paradiso unit and constrictional strain type in the erfaulet unit and prove deviations from simple shear we conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that ductile strain was superimposed on and modified the nappe structure during high pressure underplating in the alpine subduction zone the accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical shortening which caused the subhorizontal foliation in the gran paradiso massif

evolution of quartz microstructure and c axis crystallographic preferred orientation within ductilely deformed granitoids arolla unit western alps abstract we have studied quartz microstructures and the c axis crystallographic preferred orientations cpos in four granitoid samples representative of increasing ductile shear deformation from a weakly deformed granitoid stage 1 to a mylonitic granitoid stage 4 the quartz c axis cpo measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area all the samples belong to the austroalpine arolla unit western alps and were deformed at greenschist facies conditions the quartz c axis cpo was analyzed using a u stage and the optical orientation imaging technique the magmatic plagioclase forming more than 50 of the volume of the granitoid is extensively replaced by a mica rich aggregate even in weakly deformed samples of stage 1 these aggregates flow to form an interconnected weak matrix with increasing deformation wrapping relatively less strained quartz grains that undergo dominantly coaxial strain recrystallization of quartz ranges from less than 1 in the weakly deformed granitoid to up to 85 in the mylonitic granitoid with average grain strain of 41 and 64 respectively with increasing strain and recrystallization quartz grains in the granitoids show a sequence of transient microstructures and cpos crystal plastic deformation is initially accomplished by dislocation glide with limited recovery and at 50 grain strain it results in a cpo consistent with dominantly basal a slip at 60 grain strain recrystallization is preferentially localized along shear bands which appear to develop along former intragranular cracks and the recrystallized grains develop a strong c axis cpo with maxima orthogonal to the shear band boundaries and independent of the host grain orientation within the granitoid mylonite at an average quartz grain strain of 64 recrystallization is extensive and the c axis cpo of new grains displays maxima overlapping the host c axis orientation and therefore unrelated to the bulk sense of shear the host controlled cpo is inferred to reflect pervasive recrystallization by progressive subgrain rotation the switch from shear band control to host control on c axis cpo occurred between 40 and 70 of recrystallization in the quartz mylonite the quartz c axis cpo develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions this study indicates that the cpo evolution of quartz may significantly differ in cases of polymineralic vs monomineralic rocks under the same deformation conditions if quartz in the polymineralic rock behaves as a strong phase

finite element modelling of shear zone development in viscoelastic materials and its implications for localisation of partial melting the development of shear zones initiating on random weaker initial perturbations is modelled numerically for low deborah number viscoelastic materials considering the influence of effective viscosity contrast power law rheology strain softening and different imposed bulk deformation geometries ranging from pure to simple shear conjugate shear zones initiate at 90 to one another and rotate with increasing bulk deformation the basic pattern not being markedly influenced by the vorticity of imposed deformation the rate of propagation of individual conjugate shear zones is little affected by increased effective viscosity contrast between matrix and inclusion but is promoted by power law rheology however the most marked effect is observed for strain softening behaviour where rapid propagation produces straighter and narrower shear zones the localisation of strain is reflected in a correspondingly heterogeneous stress distribution in particular mean stress or pressure is higher in the extending near planar weaker zones of localised shear melting of gneissic or pelitic compositions is pressure dependent with free water present increased pressure promotes melting whereas the opposite is true for water absent melting for water present conditions a positive feedback could develop between localised shearing increased pressure and partial melting this is potentially more effective in concentrating melt in shear zones than shear heating where melt related softening has a negative feedback effect

strain localization in homogeneous granite near the brittle ductile transition a case study of the kellyland fault zone maine usa abstract we present a detailed case study of a strike slip shear zone that cut homogeneous granite near the brittle ductile transition this zone contains three distinct strain facies 1 a 2 3 km wide belt of foliated granite 2 a 100 300 m wide belt of small localized shear zones and 3 a 200 400 m wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites the foliated granite formed via dislocation creep of quartz and minor dissolution precipitation creep and dislocation creep of feldspar under amphibolite facies conditions shortly after granite crystallization the localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite and they deformed by granular flow grain boundary sliding coupled with dissolution precipitation creep these deformation features reveal a three phase rheologic evolution in granite near the brittle ductile transition phase 1 is recorded by the foliated granite and the rheology was governed by dislocation creep of quartz phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone phase 3 was a long lived period of ductile deformation and the rheology was governed by the granular flow mechanism strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

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

dating deformation in the gran paradiso massif nw italian alps implications for the exhumation of high pressure rocks in a collisional belt the gran paradiso massif situated in the internal part of the western italian alps records a complex tectono metamorphic history involving high pressure metamorphism and subsequent exhumation dur

small scale ductile shear zones neither extending nor thickening nor narrowing abstract the length thickness and strain gradients of small scale 10 3 10 1 m thick ductile shear zones are pre determined by the presence of surface precursors e g fractures or compositional layers and associated fluid rock interaction fractures with their surrounding host rock damage zone and concurrent tectonic under pressure during dilation provide efficient fluid pathways and networks with diffusive fluid rock interaction at their margins the fluid induced gradational to zoned compositional haloes that symmetrically surround a fracture control the strain gradients of developing shear zones and result in a diversity of geometric types including single homogeneous to heterogeneous shear zones and paired shear zones as a consequence geochemical differences between shear zone and host rock may reflect fluid rock interaction during the precursor brittle history rather than during slip especially considering that shear zones with even a small component of stretch may be over pressured and therefore unable to drain fluids from the surrounding rocks support from field observation for this interpretation is given by 1 the lack of correlation between shear zone thickness and accumulated displacement and 2 the similar thickness of shear zones and locally unexploited alteration haloes surrounding fracture precursors most small scale shear zones in massive rocks granitoids are initially neither thickening nor narrowing with increasing strain this concept may also apply to more foliated rocks but does not necessarily hold for larger scale shear zones

fem simulation of interseismic and coseismic deformation associated with the 2008 wenchuan earthquake abstract we make use of two step modeling to simulate inter and coseismic deformation within one seismic cycle by means of visco elastic finite element method fem the first step is to perform inversion to find the optimum modeling parameters by fitting the observed interseismic deformation then the model continues to produce a major seismic event to simulate the wenchuan earthquake the model results show that in order to fit the observed interseismic deformation it needs a soft lower crust and upper mantle beneath the eastern tibet and a very strong lithosphere of the sichuan basin the computed interseismic strain accumulation in the lower crust beneath the eastern tibet is much faster than that in the other regions especially the elastic strain energy density rate esed accumulates very rapidly in and around the longmen shan fault in the depth above 30 km that may explain why the great wenchuan earthquake occurs in the region of such a slow surface deformation rate simulations of coseismic processes reveal relationships among slip acceleration normal stress and shear stress changes on surface of the seismogenic fault coseismic slip appears to initiate in the gently dipping section of the fault but primary slips coseismic rupture on both gently dipping and steeply dipping sections take place simultaneously minor fault slip accelerations decrease normal stress on fault surface to reduce frictional strength of fault and in general increase shear stress on fault surface to enhance tectonic stress on the fault to drive the fault further slips both processes facilitate major slip or rupture on the listric reverse fault coseismic slip distribution also suggest that an important role of high angle listric reverse faulting is to transfer overwhelming horizontal deformation of the eastern tibet into significant vertical displacement in the longmen shan during the 2008 wenchuan earthquake

crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins abstract quartz veins within rieserferner pluton underwent deformation during post magmatic cooling at temperature around 450 c different crystallographic orientations of cm sized quartz vein crystals conditioned the evolution of microstructures and crystallographic preferred orientations cpo during vein parallel simple shear up to high shear strains 10 for glide in ribbons with c axis sub parallel to the shear zone vorticity y axis ribbons with c axis orthogonal to y xz type ribbons were stronger and hardened more quickly they show lower aspect ratios and fine grain size 10 20 m recrystallization along sets of microshear zones szs exploiting crystallographic planes distortion of xz type ribbons and recrystallization preferentially exploited the slip systems with misorientation axis close to y new grains of szs initiated by subgrain rotation recrystallization sgr and thereupon achieved high angle misorientations by a concurrent process of heterogeneous rigid grain rotation around y associated with the confined shear within the sz dauphine twinning occurred pervasively but did not play a dominant role on sz nucleation recrystallization became widespread at 2 and pervasive at 10 ultramylonitic quartz veins are fine grained 10 m similar to new grains of sz and show a cpo banding resulting in a bulk c axis cpo with a y maximum as part of a single girdle about orthogonal to the foliation and orientations at the pole figure periphery at moderate to high angle to the foliation this bulk cpo derives from steady state sgr associated with preferential activity in the different cpo bands of slip systems generating subgrain boundaries with misorientation axes close to y the cpo of individual recrystallized bands is largely inherited from the original crystallographic orientation of the ribbons and therefore vein crystals from which they derived high strain and pervasive recrystallization were not enough to reset the initial crystallographic heterogeneity and this cpo memory is explained by the dominance of sgr this contrast with experimental observation of a rapid erasure of a pristine cpo by cannibalism from grains with the most favourably oriented slip system under dominant grain boundary migration recrystallization

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

semi brittle deformation of granitoid gouges in shear experiments at elevated pressures and temperatures abstract the physical and chemical processes acting in the brittle to plastic transition are of great interest for a better understanding of fault rheology we performed a series of experiments on granitoid gouge material under high confining pressures pc 500 1500 mpa temperatures t 300 c and 500 c and fast shear strain rates 1 8 10 4 s 1 where the material deforms by semi brittle flow samples deformed at 500 c are systematically weaker than samples deformed at 300 c over the whole examined confining pressure range indicating a non frictional component of the deformation all samples develop an s c fabrics and deformation localizes in slip zones containing amorphous feldspar material with an intermediate composition na ca and k rich further we observe changes in composition of feldspars enrichment in the albite component in the highly fragmented but crystalline regions with increasing finite shear strain our results indicate that mass transfer processes keep pace with frictional deformation even at high strain rates and together with viscous flow of the amorphous material are responsible for the observed strength difference

syn convergent exhumation implied by progressive deformation and metamorphism in the valle dell orco transect nw italian alps structural metamorphic and geochronological data from the valle del9orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the alpine orogeny deformation generally migrated westwards in this section and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain the data suggest a dynamic crustal stacking system lasting from 70 to 35 ma rather than continental collision following long lived b type subduction and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear bounded slices within the orogenic wedge the easternmost internal unit of the sesia lanzo zone the eclogitic micaschist complex was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late cretaceous during the development of the major preserved structures the eastern eclogitic micaschist complex was undergoing general non coaxial shear with an overall top down to the east vorticity suggesting a potential mechanism of exhuming an eclogitic 9pip9 slices of pre alpine basement the vasario sparone subunit were incorporated into the lower western parts of this unit during the latest part of this phase but do not show penetrative effects of the early alpine deformation they were passively folded during the early decompressional part of the p t history prior to the unit being juxtaposed against the underlying gneiss minuti complex under blueschist greenschist facies conditions the gneiss minuti complex itself was then recrystallized at high strain in the greenschist facies in the latest eocene fabrics developed at this stage indicate top down to east extension which may have accommodated the exhumation of eclogitized piemont oceanic material and the european continental material of the gran paradiso massif evidence of northwest directed thrusting within the gran paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse sense shear zone that presumably links to the more external thrusts now exposed to the west high pressure mineral lineations within the piemont zone cannot be unequivocally assigned to either the burial or exhumation paths but are consistent with the generally east west transport directions observed in the structurally higher units

the development of discrete shear zones in a granite stress strain and changes in deformation mechanisms abstract the development of millimetre scale continuous shear zones in the sidi ali bou nab algeria granite and their progressive transformation into discrete shear zones are presented increases in strain stress and strain rate magnitudes as well as changes of deformation mechanisms are suggested by an evolution of the microstructure across these narrow shear zones temperatures are assumed to be constant from boundaries to core the most striking aspects of this evolution are 1 1 a decrease in the angle between the foliation and the shear zone 2 2 a decrease in the recrystallized grain size of quartz together with the development of a crystallographic orientation characterized by a strong concentration of c axes around the y axis of the finite strain ellipsoid 3 3 the development of very fine grained feldspathic aggregates or layers probably through dynamic recrystallization of original large crystals and 4 4 the replacement of original biotite by muscovite and a new generation of biotite in the continuous shear zones the deformation is controlled by the behaviour of quartz this mineral undergoes extensive dynamic recrystallization which probably favours a higher strain rate and a change in the active slip system from a dominant basal one in the outermost part of the structures to a dominant prismatic one in its core paleopiezometry and strain computations show a shear strain up to 10 and an inferred steady state differential stress to 100 130 mpa respectively the high stress level is suspect because the estimates are derived from recrystallized grain size which may be sensitive to strain and variations of chemical environment discrete shear zones develop in relation with dynamic recrystallization of feldspars in the core of continuous shear zones dynamic recrystallization of feldspars produces very fine grained aggregates grain size of 6 7 m which coalesce and form a continuous layer at the central part of the shear zones this fabric may result from 1 1 a change in the driving flow mechanism from dislocation creep of quartz to dislocation creep or and grain boundary sliding of feldspar neoblasts and 2 2 a large strain softening that would favour strain concentration in the fine grained matrix and the development of sharp shear zone boundaries

modeling the influence of tectonic extrusion and volume loss on the geometry displacement vorticity and strain compatibility of ductile shear zones oblate strains are often observed in meso scale ductile shear zones and this is generally taken to indicate narrowing across the shear zone during formation volume loss is one mechanism that could produce shear zone narrowing however not all shear zones display characteristics consistent with volume loss and in such cases the narrowing must be accomplished by the extrusion of material from within the shear zone to explore the relationship between shear zone geometry volume loss and extrusion shear zones were mathematically modeled results demonstrate the important influence of pure shear and volume loss on controlling the geometry displacement and vorticity of ductile shear zones further volume loss does not preclude extrusion unless for a given volume loss the strain is of a specific geometry extrusion is a likely mechanism important in shear zone development even if volume loss occurs extrusion presents strain compatibility problems because unlike crustal scale shear zones meso scale ductile shear zones do not possess a free surface if extrusion occurs bulk strain compatibility can be maintained if shear zones interlink in anastomosing arrays or change in thickness though not all shear zone systems display such characteristics modeling results elucidate the deformation style of shear zone in the northwest adirondacks in ny and in the kebnekaise region in northern sweden

impact of uncertainty in rainfall estimation on the identification of rainfall thresholds for debris flow occurrence abstract estimation of rainfall intensity duration thresholds used for the identification of debris flow landslide triggering rainfall events has been traditionally based on raingauge observations the main drawback of using information from gauges is that the measurement stations are usually located far away from the debris flow initiation areas in complex terrain where debris flows take place the spatial variability of rainfall can be very high and this translates in large uncertainty of raingauge based estimates of debris flow triggering rainfall this work focuses on the assessment of the impact of rainfall estimation uncertainty on identification and use of rainfall thresholds for debris flow occurrence the upper adige river basin northern italy is the area of study a detailed database of more than 400 identified debris flow initiation points during the period 2000 2010 and a raingauge network of 100 stations comprise the database used for this work the methodology examines the intensity duration thresholds derived from a set of raingauges that are assumed to be located at debris flow initiation points dfrs and an equivalent set of raingauges assumed to have the role of the closest to debris flow available measurement mrs a set of reference rainfall thresholds is used to identify the rainfall events at dfrs that triggered debris flows i e exceed the threshold for these same events the corresponding rainfall thresholds are derived from mr observations comparison between the rainfall thresholds derived from dfrs and mrs revealed that uncertainty in rainfall estimation has a major impact on estimated intensity duration thresholds specifically the results showed that thresholds estimated from mr observations are consistently underestimated evaluation of the estimated thresholds for warning procedures showed that while detection is high the main issue is the high false alarm ratio which limits the overall accuracy of the procedure overall performance on debris flow prediction was shown to be good for low rainfall thresholds and poor for high rainfall thresholds examined lastly it was found that filtering out rainfall events with duration

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

the peristerite gap in low grade metamorphic rocks coexisting na plagioclases from greenschists both in the thermal aureole of the kasugamura granite japan and in the low p metamorphic zone of yap island western pacific were analyzed in great detail the peristerite solvus was determined for each suite the asymmetric solvus has steep albite rich and gentle oligoclase rich limbs that are similar to those for higher pressure series the present results together with those from vermont new zealand and the sanbagawa belt indicate that the peristerite solvus shifts toward the albite component and higher temperature with increasing pressure with increasing pressure albite co existing with oligoclase an 100 ca ca na 20 varies in composition from an 8 9 in kasugamura through an 3 in yap island and vermont to an 1 in new zealand and an less than 0 5 in the sanbagawa belt the consolute temperatures for the peristerite solvus estimated from available geothermometry are 420 c in kasugamura 450 550 c in vermont and 550 600 c in the sanbagawa belt the variation of plagioclase composition in progressive metamorphic zones is explained by intersection of a plagioclase forming reaction and the peristerite immiscibility gap in an isobaric t xan diagram the greenschist zone is characterized by albite the transition zone by occurrence of peristerite pairs and the amphibolite zone by plagioclase of an 20 50

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization