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

marine isotope stage 3 paleotemperature inferred from reconstructing the die shan ice cap northeastern tibetan plateau glacial extent mapping and dating indicate that the local last glacial maximum llgm of the northeastern tibetan plateau occurred during mid marine isotope stage mis 3 this is asynchronous with the global last glacial maximum lgm that occurred during mis 2 the causes underlying this asynchronicity are the subject of ongoing debate and paleoclimatic reconstructions are a key to advancing understanding of the climatic influence on the spatial and temporal patterns of paleoglaciation we used multiple methods to reconstruct the equilibrium line altitude ela of the die shan paleo ice cap on the northeastern tibetan plateau and to infer past temperature for ice maximum positions believed to be mid mis 3 in age based on regional correlation geomorphic ela reconstructions combined with an energy and mass balance model yield a paleo ela of 4117 31 m asl 786 m lower than present with temperature depressions of 3 8 to similar to 4 6 degrees c compared to the present this is less than the lgm reconstruction of temperature depression inferred from other climatic proxy records on the tibetan plateau and suggests that the llgm glacial advance was a product of lower temperatures and slightly reduced precipitation compared to present whereas the lgm was a more restricted advance in which much colder conditions were combined with much lower precipitation

present and former equilibrium line altitudes in the taiwanese high mountain range abstract previously reported values of former equilibrium line altitudes ela for taiwan are based on sparse field data or extrapolations from japan or china this paper presents new calculations based on recently obtained geological data hebenstreit r bose m 2003 geomorphological evidence for a late pleistocene glaciation in the high mountains of taiwan dated with age estimates by optically stimulated luminescence osl zeitschrift fur geomorphologie n f suppl bd 130 31 49 hebenstreit r murray a s bose m this vol late pleistocene and early holocene glaciations in taiwanese mountains quaternary international from three taiwanese mountain massifs nanhuta shan hsueh shan and yushan using established calculation methods benn d i lehmkuhl f 2000 mass balance and equilibrium line altitudes of glaciers in high mountain environments quaternary international 65 66 15 29 porter s c 2001 snowline depression in the tropics during the last glaciation quaternary science reviews 20 10 1067 1091 the relationship of annual precipitation and summer jja temperature ohmura a kasser p funk m 1992 climate at the equilibrium line of glaciers journal of glaciology 38 130 397 411 obtained from a 30 year meteorological database is used to estimate the present theoretical virtual equilibrium line altitude ela t at about 4000 m over the southern and 3900 m over the northern part of taiwan the ela was 3350 3500 m during late glacial to early holocene time 11 8 ka including a minimum post glacial mountain uplift rate of 5 mm a lundberg n dorsey r j 1990 rapid quaternary emergence uplift and denudation of the coastal range eastern taiwan geology 18 7 638 641 lin c h 1998 tectonic implications of an aseismic belt beneath the eastern central range of taiwan crustal subduction and exhumation journal of the geological society of china 41 3 441 460 in the calculation the ela depression ela was 400 550 m during this period data from lgm or earlier are still rare but glacial landforms and deposits indicate the ela position at 2775 3195 m and the ela of 705 1125 m

timing and style of late pleistocene glaciation in the queer shan northern hengduan mountains in the eastern tibetan plateau glacial landforms and sediments provide evidence for the existence of two late pleistocene major glacial advances in the queer shan northern hengduan mountains in the eastern tibetan plateau in the current study optically stimulated luminescence and electron spin resonance dating results reveal that the two glacial advances occurred during marine isotope stage mis 3 and the last glacial maximum lgm in mis 2 respectively geomorphic evidence shows that the glacial advance during mis 3 was more extensive than that in mis 2 this glacial advance is synchronous with other glaciated areas in the himalaya and tibet but contrasts with global ice volumes that reached their maximum extent during the lgm glaciers in the queer shan are of the summer accumulation type and are mainly fed by precipitation from the south asian monsoon palaeoclimate proxies show that during mis 3 the south asian monsoon strengthened and extended further north into the tibetan plateau to supply more precipitation as snow at high altitudes this in turn led to positive glacier mass balances and caused glaciers to advance however during the lgm despite cooler temperature than in mis 3 the weakened south asian monsoon and the associated reduced precipitation were not as favourable for glacier expansion as in mis 3 copyright c 2010 john wiley sons ltd glacial landforms and sediments provide evidence for the existence of two late pleistocene major glacial advances in the queer shan northern hengduan mountains in the eastern tibetan plateau in the current study optically stimulated luminescence and electron spin resonance dating results reveal that the two glacial advances occurred during marine isotope stage mis 3 and the last glacial maximum lgm in mis 2 respectively geomorphic evidence shows that the glacial advance during mis 3 was more extensive than that in mis 2 this glacial advance is synchronous with other glaciated areas in the himalaya and tibet but contrasts with global ice volumes that reached their maximum extent during the lgm glaciers in the queer shan are of the summer accumulation type and are mainly fed by precipitation from the south asian monsoon palaeoclimate proxies show that during mis 3 the south asian monsoon strengthened and extended further north into the tibetan plateau to supply more precipitation as snow at high altitudes this in turn led to positive glacier mass balances and caused glaciers to advance however during the lgm despite cooler temperature than in mis 3 the weakened south asian monsoon and the associated reduced precipitation were not as favourable for glacier expansion as in mis 3 copyright c 2010 john wiley sons ltd

relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation field observations of mafic dykes intruded into felsic rocks from the lewisian complex nw scotland suggest that the mafic rocks are weaker than the felsic ones contrary to experimental results in order to resolve this conflict samples were studied to constrain the deformation mechanisms active under amphibolite facies conditions crystallographic preferred orientation cpo in plagioclase is used to infer deformation by dislocation creep on the 112 11 0 11 2 110 001 11 0 and 001 11 0 slip systems with increasing strain this cpo became weaker due to grain boundary sliding that accompanied diffusion creep in rocks where grain size reduction of plagioclase occurred by chemically dominated recrystallisation there is no cpo suggesting deformation was wholly accommodated by diffusion creep and grain boundary sliding in metamorphosed dykes plagioclase grains have orientations that are not consistent with dislocation creep deformation amphibole has a cpo consistent with either dislocation creep on 100 or deformation by diffusion creep with anisotropic dissolution and precipitation rates it is inferred that metamorphism of the dykes lead to the production of fine grained amphibole and plagioclase both of which deform by diffusion creep but that anisotropic dissolution and precipitation in the amphibole produced a cpo the mafic dykes are weaker than the felsic gneisses because grain size reduction is more extreme in the dykes even though both may be deforming primarily by grain size sensitive mechanisms this work highlights the variation in processes active in polyphase rocks and how these can lead to variations in cpo of the same mineral among different rock types

grain boundary sliding and development of grain boundary openings in experimentally deformed octachloropropane abstract in situ observations of grain boundary sliding and associated accommodation mechanisms in experimentally pure and simple sheared octachloropropane at 75 85 of its absolute melting temperature and at strain rates of 10 5 10 6 s 1 are discussed discontinuities in the strain rotation and or translation components of deformation across the grain boundary induce grain boundary sliding the influence of crystallographic orientation of grains on grain boundary sliding is so strong that grains unfavorably oriented for basal slip contribute to deformation mainly by grain boundary sliding grain boundary diffusion and intragranular plastic deformation are observed to accommodate grain boundary sliding grain boundary diffusion also causes grain boundary migration types ii and iii that is different from conventional grain boundary migration type i in being non conservative and in the details of boundary movement with respect to material points within grains grain boundary openings filled with fluid octachloropropane vapor are strongly associated with grain boundary sliding openings develop preferentially along grain boundaries at low angles to the shortening direction once openings grow they are closed by thrusting of sliding grains and by diffusion in faster strain rate experiments and entirely by diffusion in slower strain rate experiments an approximately steady openings ratio of 0 5 3 of the sample volume persists without the development of any large scale fracture all grain boundary openings disappear during static readjustment of the microstructure after deformation

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

do stable isotope data from calcrete record late pleistocene monsoonal climate variation in the thar desert of india late pleistocene terrestrial climate records in india may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete petrography shows that calcrete nodules in quaternary sediments of the thar desert in rajasthan are pedogenic with little evidence for postpedogenic alteration the calcrete occurs in four laterally persistent and one nonpersistent eolian units separated by colluvial gravel thermoluminescence and infrared and green light stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units i iv of ca 70 000 60 000 ca 60 000 55 000 ca 55 000 43 000 and ca 43 000 25 000 yr respectively the youngest eolian unit v is 10 000 yr old and contains no calcrete stable oxygen isotope compositions of calcretes in most of eolian unit i in the upper part of eolian unit iv and in the nonpersistent eolian unit range between 4 6 and 2 1 pdb these values up to 4 4 greater than values from eolian units ii and iii are interpreted as representing nonmonsoonal18o enriched normal continental waters during climatic phases when the monsoon weakened or failed conversely 25 000 60 000 yr old calcretes eolian units ii and iii probably formed under monsoonal conditions the two periods of weakened monsoon are consistent with other paleoclimatic data from india and may represent widespread aridity on the indian subcontinent during isotope stages 2 and 4 the total variation in 13c is 1 7 0 0 1 7 and 13c covaries positively and linearly with 18o 13c values are highest when 18o values indicate the most arid climatic conditions this is best explained by expansion of c4grasses at the expense of c3plants at low latitudes during glacial periods when atmosphericpco2was lowered c4dominance was overridingly influenced by global change in atmosphericpco2despite the lowered summer rainfall

using lagrangian particle saltation observations for bedload sediment transport modelling a lagrangian model for the saltation of sand in water is proposed simulated saltation trajectories neglecting particle rotation and turbulence effects compare fairly well with experimental observations the model for particle motion is coupled with a stochastic model for particle collision with the bed such that a number of realizations of the saltation process can be simulated numerically model predictions of mean values and standard deviations of saltation height length and streamwise particle velocity agree fairly well with experimental observations model predictions of the dynamic friction coefficient are also in good agreement with experimental observations but they underestimate the value of 0 63 proposed by bagnold for this coefficient the saltation model is applied to the estimation of bedload transport rates of sand using a bagnoldean formulation modelled values of the bedload transport rates overestimate those predicted by commonly used bedload formulae which appears to be a consequence of problems in the definition of the dynamic friction coefficient these results seem to indicate a few problems with the bagnoldean formulation particularly regarding the continuum assumption for the bedload layer which would be valid only for very high particle concentrations and small particle diameters and also regarding the evaluation of the shear stress exerted on the bed by the saltating particles 1998 john wiley sons ltd

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

fluid assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone semail ophiolite this study reports on feedback mechanisms between fluid migration ductile deformation and strain localization processes in an incipiently forming mantle wedge the basal banded unit of the semail ophiolite these peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction during this stage they were affected by coeval ductile deformation forming proto mylonites at 900 800 c to ultramylonites at 700 c and interaction with subduction fluids from the petrological and microstructural study of these hydrated peridotites and their protolith preserved lenses of porphyroclastic tectonites we show that peridotite interaction with hydrous fluids triggered dissolution precipitation processes dissolution of coarser grains and precipitation of new and smaller ones mainly pyroxenes spinel and amphibole resulted in a drastic grain size reduction phase mixing and a switch from olivine dislocation to grain size sensitive creep in proto mylonites data also evidence a feedback process of fluid focusing in actively deforming shear zones this rapid switch in olivine deformation mechanism driven by subduction fluid peridotite interaction triggered an intense weakening of the peridotites at 900 800 c and a transition from a mechanically decoupled to coupled plate interface as witnessed by the detachment and underplating of a high temperature metamorphic sole to the base of the ophiolite it also explains the intense strain localization 1 km thick shear zone along this ductile portion of the plate interface considering that similar mechanisms take place in mature subduction zones they may explain plate interface coupling at subarc depths in worldwide subduction zones

timing and climatic drivers for glaciation across semi arid western himalayan tibetan orogen abstract mapping and forty seven new 10 be ages help define the timing of glaciation in the ladakh and pangong ranges in northwest india five new local glacial stages are defined for the ladakh range from oldest to youngest these include the ladakh 4 glacial stage at 81 20 ka the ladakh 3 glacial stage not dated the ladakh 2 glacial stage at 22 3 ka the ladakh 1 glacial stage not dated and the ladakh cirque glacial stage at 1 8 0 4 ka three local glacial stages are defined for the pangong range which include the pangong 2 glacial stage at 85 15 ka the pangong 1 glacial stage at 40 3 ka and the pangong cirque glacial stage at 0 4 0 3 ka the new 10 be ages are combined with 645 recalculated 10 be ages from previous studies to develop the first regional framework of glaciation across the dryland regions of the greater himalaya transhimalaya pamir and tian shan at the western end of the himalayan tibetan orogen nineteen regional glacial stages are recognized that are termed semi arid western himalayan tibetan stages swhts these include swhts 9 at 311 32 ka swhts 7 at 234 44 ka tentative swhts 6 at 146 18 ka swhts 5e at 121 11 ka swhts 5a at 80 5 ka swhts 5a at 72 8 ka swhts 4 at 61 5 ka swhts 3 at 46 4 ka swhts 2f at 30 3 ka swhts 2e at 20 2 ka swhts 2d at 16 9 0 7 ka swhts 2c at 14 9 0 8 ka swhts 2b at 13 9 0 5 ka swhts 2a at 12 2 0 8 ka swhts 1e at 8 8 0 3 ka tentative swhts 1d at 6 9 0 2 ka tentative swhts 1c at 3 8 0 6 ka swhts 1b at 1 7 0 2 ka and swhts 1a at 0 4 0 1 ka regional glacial stages older than 21 ka are broadly correlated with strong monsoons swhts that are 21 ka or younger have smaller uncertainties and broadly correlate with global ice volume given by marine oxygen isotope stages and northern hemisphere climatic events oldest dryas older dryas younger dryas roman humid period and little ice age

the strain dependent spatial evolution of garnet in a high pressure ductile shear zone from the western gneiss region norway a synchrotron x ray microtomography study reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono metamorphic processes however they are usually analysed in two dimensions omitting important information regarding the third spatial dimension we applied synchrotron based x ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite garnet eclogite in four dimensions where the 4th dimension is represented by the degree of strain in the investigated samples which cover a strain gradient into a shear zone from the western gneiss region norway we focused on the spatial transformation of garnet coronas into elongated garnet clusters with increasing strain the microtomographic data allowed quantification of garnet volume shape and spatial arrangement evolution with increasing strain the microtomographic observations were combined with light microscope and backscatter electron images as well as electron microprobe empa and electron backscatter diffraction ebsd analysis to correlate mineral composition and orientation data with the x ray absorption signal of the same mineral grains with increasing deformation the garnet volume almost triples in the low strain domain garnet grains form a well interconnected large garnet aggregate that develops throughout the entire sample we also observed that garnet coronas in the gabbros never completely encapsulate olivine grains in the most highly deformed eclogites the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics we interpret the aligned garnet aggregates to direct synkinematic fluid flow and consequently influence the transport of dissolved chemical components ebsd analyses reveal that garnet shows a near random crystal preferred orientation that testifies no evidence for crystal plasticity there is however evidence for minor fracturing neo nucleation and overgrowth microprobe chemical analysis revealed that garnet compositions progressively equilibrate to eclogite facies becoming more almandine rich we interpret these observations as pointing to a mechanical disintegration of the garnet coronas during strain localization and their rearrangement into individual garnet clusters through a combination of garnet coalescence and overgrowth while the rock was deforming

magnetostratigraphic dating of river terraces rapid and intermittent incision by the yellow river of the northeastern margin of the tibetan plateau during the quaternary up to 23 paleosols have been identified in loess sequences that overlie fluviatile sediments on seven terraces of huang he the yellow river and daxia he in the linxia basin at the northeastern margin of the tibetan plateau the magnetostratigraphic record in the oldest sequence appears to be fairly complete and spans the entire quaternary it includes evidence for the jaramillo and olduvai normal polarity subchrons within the matuyama chron as well as the cobb mountain and stage 54 formerly gilsa events age constraints within the brunhes are provided by 14c and thermoluminescence optical stimulated luminescence dating the magnetostratigraphic chronology of the quaternary sediments indicates that terraces were formed at about 1 66 1 4 1 1 0 6 0 12 0 055 and 0 01 ma the terraces may represent climatically induced pauses in long term tectonically induced incision by the rivers on the basis of terrace heights and ages incision rates were highest from 1 66 to 1 4 ma 1 m kyr decreased from 1 4 to 0 12 ma 0 1 m kyr and increased from 0 12 ma to present 0 75 m kyr if river incision is attributed solely to uplift of the tibetan plateau then this uplift appears to have been rapid as well as intermittent following the first evidence of the existence of huang he in this area at about 1 75 ma however the effects of uplift cannot be fully separated from incision caused by changes in climate copyright 1997 by the american geophysical union

a 2 d numerical model of snow ice energy balance and ice flow for paleoclimatic interpretation of glacial geomorphic features abstract standard methods of interpreting the paleoclimates recorded in alpine glacial geomorphic features rely on estimates of paleoglacier shape commonly assume that relationships between mass balance gradient and equilibrium line altitude of modern glaciers apply equally well to glaciers of in many cases dramatically different climates and do not account for the varying influence of topography on glacial extent while those methods may be adequate for many studies there are an increasing number of areas in which that level of analysis does not match the level of detail of the glacial chronologies being developed as an alternative we present a physically based 2 d glacier model that can be used to determine steady state glacier shapes and distributions for a wide range of climatic conditions the model requires only a modest amount of data beyond that which would be required for simpler methods and includes a relatively accurate representation of the effects of topography on the largest component of the surface energy balance shortwave radiation it calculates 2 d in the horizontal plane distribution of snow accumulation using a surface mass and energy balance approach and calculates the resultant glacier shapes with a 2 d flow model in addition to offering a potentially more accurate reconstruction of paleoclimatic conditions the model provides a means of performing detailed sensitivity analysis establishing relationships between similar deposits in basins of different shape and aspect and of predicting glacier shape terminus position in areas not yet explored

an sem study of porosity and grain boundary microstructure in quartz mylonites simplon fault zone central alps a backscattered and secondary electron sem study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde simplon fault zone established three characteristic components 1 fine isolated pores 1 m diameter are scattered across two grain interfaces preferentially concentrated on surfaces in extension pores are uncommon on three grain junctions and there is no evidence for fluid interconnectivity along three and four grain junctions the fine porosity may develop by accumulation of original mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding dynamic cavitation implies that the ductile mylonitic deformation is at least locally dilatant and therefore pressure sensitive 2 large vug like pores up to mm scale extend along multi grain boundaries observed in all samples they are most common in the higher initial temperature coarse grained samples with a microstructure dominated by grain boundary migration recrystallization grains bordering this connected porosity develop perfect crystal faces undecorated by fine pores or pits the irregular lobate optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces the coarse porosity is probably due to accumulation during dynamic recrystallization of co2 rich fluid with a high wetting angle against quartz 3 in one sample interconnected sinuous ridges 0 2 m high are observed to follow three and four grain junctions and disjoint into more isolated worms and spheroidal globules on two grain interfaces these are transitional to more branching vein like or convoluted brain like forms the brain like and globular forms have been observed with varying frequency through the range of samples with the globules attaining sizes of up to 60 m vein structures have also been observed on intragranular fractures these topologies do not match across adjoining surfaces and must have developed into free space the ridge vein brain spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established they could represent unstable meniscus necking of a thin grain boundary phase of low viscosity developed due to quasi adiabatic shear and or local stress induced dilatancy during microcracking

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

effects of frictional viscous oscillations and fluid flow events on the structural evolution and re os pyrite chalcopyrite systematics of cu rich carbonate veins in northern norway abstract mesothermal chalcopyrite pyrite magnetite bearing calcite dominated vein deposits in the repparfjord tectonic window northern norway have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation the veins cut across paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured coarse grained margins and the fine grained mylonitic cores they formed under overall viscous conditions although cyclically increased fluid pressures caused transient embrittlement as mineral precipitation sealed the fractures deformation was accommodated again viscously leading to mylonitization of the vein s core local brecciation of the calc mylonite demonstrates the cyclicity of this process re os chalcopyrite pyrite and k ar fault gouge dates define an almost continuous age range from 2540 ma to 460 ma regression of three re os analyses yields a 2069 14 ma age 187os 188os 0 18 0 04 interpreted as the age of vein emplacement sulfide precipitation and initial frictional viscous deformation k ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 ma hence indicating structural reactivation connected with silurian caledonian orogenic compression the spread in re os model ages reflects this reactivation wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing dynamic recrystallization and isotopic disturbance of the sulfides the study provides evidence for fluid flow during viscous deformation and demonstrates that strain and flow of oxidizing fluids can have a significant yet localized control on the integrity of the re os systematic in pyrite and chalcopyrite

a preliminary study on timing of the oldest pleistocene glaciation in qinghai tibetan plateau abstract the qinghai tibetan plateau consists of broad planation surfaces basins and mountain ranges research in the past several decades has argued that quaternary glaciations developed only around the high mountains on the plateau not the whole plateau surface in recent years we and other researchers have dated the oldest moraines in several mountain ranges using esr the relative age of these moraines was first confirmed based on their geomorphological position and degree of weathering the results show that the oldest glaciation the zhonglianggan glaciation in the qilian mountains occurred at about 463 ka and the high elevation moraines of the wangfeng glaciation the oldest ones in the tianshan mountains was dated at 477 and 460 ka the oldest glaciation in the hengduan mountains the daocheng glaciation which is characterized by moraines with deeply weathered red soils was dated at 570 ka this is reconfirmed by an age of 590 ka on the oldest moraine at yulong shan in the southern hengduan mountains the wangkun glaciation the oldest in the kunlun shan pass was dated at 710 ka using esr this is similar to the 36cl age of the bottom ice layer of a 309 m ice core of guliya ice cap western kunlun shan these dates of the oldest moraines in several mountain ranges suggest that glaciers began to develop during mis 12 in the qilian and tianshan mountains mis 16 in the hengduan mountains and perhaps mis 18 in the kunlun and nyainqentanglha mountains a widespread rapid uplift of qinghai tibetan plateau kunlun huanghe tectonic movement was determined to have occurred between 1 1 and 0 6 ma thus it is reasonable that uplift of mountain ranges on the tibetan plateau during this time initiated widespread mountain glaciations in ice ages

temperature variability and vertical vegetation belt shifts during the last 50 000 yr in the qilian mountains ne margin of the tibetan plateau china abstract a 13 94 m long sediment core collected from a medium sized lake in the qilian mountains ne tibetan plateau china was analysed palynologically at 81 horizons the interpretation of indicator taxa yielded various vertical shifts of the vegetation belts these palaeovegetation results have been checked with lake surface pollen spectra from 8 lakes representing different altitudinal vegetation belts our main findings are the following a short period of the late marine isotope stage 3 around 46 000 yr ago was characterized by interglacial temperature conditions with a tree line above its present day altitude during the lgm the vicinity of the lake was not covered by ice but by sparse alpine vegetation and alpine deserts indicating that the climate was colder by 4 7 c than today markedly higher temperatures were inferred from higher arboreal pollen frequencies between 13 000 and 7 000 yr ago with a holocene temperature optimum and a maximal picea betula mixed forest expansion between 9 000 and 7000 yr ago when temperatures exceeded the present day conditions by at least 1 2 c alpine steppes and meadows and sub alpine shrub vegetation dominated around the lake since the middle holocene suggesting that vegetation and climate conditions were exceptionally stable in comparison to previous periods

a cavitation seal mechanism for ultramylonite formation in quartzofeldspathic rocks within the semi brittle field vivero fault nw spain abstract bands of multi phase mixtures with an extreme grain size reduction called ultramylonites are a common feature in shear zones from the middle crust to upper mantle conditions indeed they are one of the most typical manifestations of strain localization in high strained polymineralic rocks in the lithosphere especially in the semi brittle field yet many questions remain on their origin and existing models either do not explain all the observations or require the introduction of physical processes that are difficult to prove particularly for the inter grain mixing of phases here we present a case study of ultramylonite bands developed in a coarse grained granite deformed at conditions near the base of the seismogenic zone i e showing coseismic deformation microstructures overprinted by thermally activated ones we found that in the mylonite to ultramylonite transition two deformation mechanisms coexist promoting inter grain phase mixing through kfs precipitation 1 dislocation accommodated grain boundary sliding in quartz and 2 diffusion accommodated grain boundary sliding in feldspar the model proposed to explain grain mixing is based on a cavitation seal mechanism which is strongly dependent on grain boundary sliding for the opening of transient micro cavities and on the diffusivity of potassium feldspar for their progressive sealing

mid mis3 climate inferred from reconstructing the dalijia shan ice cap north eastern tibetan plateau the local last glacial maximum lgml on dalijia shan occurred during mid marine isotope stage 3 mis3 which is consistent with some tibetan plateau records but asynchronous with the mis2 global last glacial maximum lgmg paleoclimatic reconstructions are key to advancing understanding such spatial and temporal patterns of paleoglaciation using multiple methods we reconstructed mid mis3 equilibrium line altitude ela on dalijia shan and inferred temperature depressions geomorphic ela reconstructions combined with empirical data on current ela temperature and precipitation yield a mid mis3 ela of 4000 78 m a s l 618 851 m lower than current and a temperature depression of 3 3 6 8 c independent glacial modeling using the degree day approach suggests a mid mis3 ela between 3971 and 4024 m a s l and a temperature depression of 4 2 5 6 c the consistency of these results indicate that mid mis3 temperature depressions in the dalijia shan region were 3 3 6 8 c which is less than the lgmg temperature depression inferred from regional climatic proxy data combined with other proxy data this suggests that the lgml glacial advance in dalijia shan resulted from lower temperatures and slightly reduced precipitation compared with present whereas the lgmg advance was more restricted because much colder conditions were combined with much lower precipitation

creep cavitation bands control porosity and fluid flow in lower crustal shear zones shear zones channelize fluid flow in earth s crust however little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone this study investigates the deformation mechanisms fluid rock interaction and development of porosity in a monzonite ultramylonite from lofoten northern norway the rock was deformed and transformed into an ultramylonite under lower crustal conditions temperature 700 730 c pressure 0 65 0 8 gpa the ultramylonite consists of feldspathic layers and domains of amphibole quartz calcite which result from hydration reactions of magmatic clinopyroxene the average grain size in both domains is 25 mm microstructural observations and electron backscatter diffraction analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains festoons of isolated quartz grains define c type bands in feldspathic layers these quartz grains do not show a crystallographic preferred orientation the alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites as evidenced from synchrotron x ray microtomography such c type bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding mass balance calculation indicates a 2 volume increase during the protolith ultramylonite transformation which is consistentmore with synkinematic formation of creep cavities producing dilatancy thus this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain size sensitive creep thereby stabilizing strain localization in the polymineralic ultramylonites less

application of a mass balance model to a himalayan glacier a mass balance model based on the energy balance at the snow or ice surface is formulated with particular attention paid to processes affecting absorption of radiation the model is applied to a small glacier glacier ax010 in the nepalese himalaya and tests of its mass balance sensitivity to input and climatic parameters are carried out calculated and observed area averaged mass balances of the glacier during summer 1978 june september show good agreement namely 0 44 and 0 46 m w e respectively results show the mass balance is strongly sensitive to snow or ice albedo to the effects of screening by surrounding mountain walls to areal variations in multiple reflection between clouds and the glacier surface and to thin snow covers which alter the surface albedo in tests of the sensitivity of the mass balance to seasonal values of climatic parameters the mass balance is found to be strongly sensitive to summer air temperature and precipitation but only weakly sensitive to relative humidity

defining modern day area altitude balance ratios aabrs and their use in glacier climate reconstructions abstract with the increasing use of digital elevation models in palaeo glacier reconstructions and the availability of freeware spreadsheets the area altitude balance ratio aabr and balance ratio br methods are becoming increasingly used in palaeo glacier reconstruction for estimating equilibrium line altitudes ela and subsequently deriving quantitative estimates of palaeo climate while there are many data detailing contemporary accumulation area ratios there are still only a few studies that have established from contemporary environments aabr br ratios publicly available glacier mass balance world glacier monitoring service us geological survey and norwegian water directorate and spatial extent datasets provided the basis for this research from a time series of mass balance regressing specific net balance against ela allows the zero net balance ela to be identified once the zero balance ela is established the glacier hypsometry above and below the ela is defined the aabr br is calculated by using the right hand side the following aabr b n a b b n a c z a c a a c z a b a a b where b nab and b nac are the net mass balance gradients in the ablation and accumulation zones respectively z a c and z a b are the area weighted mean altitudes of the accumulation and ablation areas respectively and a ac and a ab are the areas of the accumulation and ablation areas respectively aabrs are calculated for a suite of glaciers located across a range of climatic zones and glacier types with antarctica being excluded the following representative aabrs are found a global aabr 1 75 0 71 mid latitude maritime 1 9 0 81 high latitude 2 24 0 85 north america west coast 2 09 0 93 north america eastern rockies 1 11 0 1 canadian arctic 2 91 0 35 svalbard 2 13 0 52 western norway 1 5 0 4 european alps 1 59 0 6 central asia 1 75 0 56 kamchatka 3 18 0 16 this study provides an empirically derived dataset characterising aabr ratios which may be used for ela estimation in palaeo glacier reconstructions and for palaeo climate quantification

a note on the extent of glaciation throughout the himalaya during the global last glacial maximum abstract quantitative chronologies for the impressive glacial successions that occur throughout the himalaya have until recently been almost totally lacking within the last decade two new techniques have promised to remedy this situation these techniques optically stimulated luminescence and cosmogenic nuclide surface exposure dating enable the age of many glacial features to be determined and have allowed us to study the extent and timing of himalayan glaciation in the late quaternary new data show that the local last glacial maximum lgm occurred during the early part of the last glacial cycle in most areas during marine isotope stage 3 mis 3 mis 3 was a time of increased insolation when the south asian summer monsoon strengthened and penetrated further north into the himalaya the concomitant increased precipitation occurring as snow at high altitudes produced positive glacier mass balances thereby allowing glaciers to advance on the other hand during the global lgm 18 24 ka himalayan glaciation was very restricted in extent generally extending

asynchronous maximum advances of mountain and continental glaciers the last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the wisconsinan continental ice sheets 20 000 years b p published evidence from widely spaced localities in north and south america europe asia and hawai i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation roughly 115 000 30 000 years b p than at its end when low global temperature and high precipitation presumably were most favorable for the growth of glaciers variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions their study might provide sensitive tests for theories of global climate change glacial deposits are often used as time markers in neotectonic and geomorphic studies and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change undated geomorphic features such as moraines sharply incised streams and river terraces which commonly have been associated with the last maximum glacier advance 20 000 years b p or last deglaciation 13 000 years b p may actually be older by a factor of 2 to 4 or more conversely deposits assigned to the maximum advances of the penultimate global glaciation 130 000 years b p may actually date from early in the most recent glacial cycle and be younger by a comparable factor rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great or too small by corresponding factors

seismic anisotropy and its impact on imaging the shallow alpine fault an experimental and modeling perspective the transpressional alpine fault in new zealand has created a thick shear zone with associated highly anisotropic rocks low seismic velocity zones lvz and high seismic reflectivity are recorded in the alpine fault zone but no study has explored the underlying physical rock parameters of the shallow crust that control these observations protomylonites are the volumetrically dominant lithology of the fault zone here we combine experimental measurements of p wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged numerical models that account for the porosity free real samples fabric elastic tensors from ebsd are calculated by mtex and a finite element model fem while microfractures are modeled with differential effective media dem theory at effective pressures representative of the alpine fault brittle zone experimental wave speeds are lower than those predicted by mtex fem a possible dem model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures 70 mpa such micro porosity in the form of foliation and mica basal plane parallel microfractures and grain boundaries is validated with synchrotron x ray microtomography and tem images finally by modeling anisotropy of seismic reflection coefficients with angle of incidence we demonstrate that the high reflectivity and lvz observed at the alpine fault can only be explained if this microporosity is accounted for throughout the brittle fault zone even at depths of 7 10 km

evaluating the role of climate and tectonics during non steady incision of the yellow river evidence from a 1 24 ma terrace record near lanzhou china abstract the competing roles of bedrock uplift and climatic change in the formation of fluvial terraces remain uncertain most of recent studies have attributed terrace formation to climatic changes and held that even in tectonically active settings climate variations control cycles of terrace planation and abandonment based on field investigations of loess paleosol sequences magnetostratigraphy and optically stimulated luminescence osl dating we develop a new chronology for a spectacular flight of terraces along the yellow river near lanzhou china over past 1 24 ma all the terraces are strikingly similar in that they have several meters of paleosol developed directly above fluvial deposits on the terrace treads suggesting that the abandonment of each terrace due to river incision occurs during the transition from glacial to interglacial climates however the ages of terraces cluster in two relatively short time periods 1 24 0 86 ma and 0 13 ma present during the intervening time between 0 86 ma and 0 13 ma terraces either did not form or were not preserved we suggest that this record indicates that rock uplift rates varied through time and influenced terrace formation preservation thus our results demonstrate the utility of deep chronologic records from fluvial terraces for deconvolving the effects of tectonics and climate on fluvial incision

climate during the last glacial maximum in the wasatch and southern uinta mountains inferred from glacier modeling abstract recent improvements in understanding glacial extents and chronologies in the wasatch and uinta mountains and other mountain ranges in the western u s call for a more detailed approach to using glacier reconstructions to infer paleoclimates than commonly applied aar ela at methods a coupled 2 d mass balance and ice flow numerical modeling approach developed by plummer m a phillips f m 2003 a 2 d numerical model of snow ice energy balance and ice flow for paleoclimatic interpretation of glacial geomorphic features quaternary science reviews 22 1389 1406 allows exploration of the combined effects of temperature precipitation shortwave radiation and many secondary parameters on past ice extents in alpine settings we apply this approach to the little cottonwood canyon in the wasatch mountains and the lake fork and yellowstone canyons in the south central uinta mountains results of modeling experiments indicate that the little cottonwood glacier required more precipitation during the local last glacial maximum lgm than glaciers in the uinta mountains assuming lapse rates were similar to modern model results suggest that if temperatures in the wasatch mountains and uinta mountains were 6 c to 7 c colder than modern corresponding precipitation changes were 3 to 2 modern in little cottonwood canyon and 2 to 1 modern in lake fork and yellowstone canyons greater amounts of precipitation in the little cottonwood canyon likely reflect moisture derived from the surface of lake bonneville and the lake may have also affected the mass balance of glaciers in the uinta mountains

deformation mechanisms and reaction of hornblende examples from the bergell tonalite central alps abstract hornblendes in the bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization during the crystallization of the magma deformation commences by magmatic flow in this stage the hornblendes behave as rigid particles in a viscous matrix melt the rotation and alignment of hornblende as elongate rigid particles have produced a strong preferred orientation of both particle long axes and crystallographic directions with progressive solidification of the melt there is a gradual transition from magmatic flow to solid state deformation in the bergell tonalite the crystallographic and shape preferred orientation that have developed during solid state deformation are identical to those of magmatic flow this identical fabric development during solid state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods i e hornblendes have always acted as rigid particles in a viscous matrix tem observations show no evidence for intracrystalline plasticity in hornblende grains the modal abundance of hornblende decreases progressively with increasing solid state deformation to yield more biotite epidote and quartz small hornblende matrix grains form by cataclastic processes the observed changes in grain size shape and abundance of hornblende occur mainly by fracturing dissolution and reaction so that the solid state deformation of the tonalite is a complex process to which fracturing dissolution of hornblende and metamorphic reactions all contribute the relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction it is inferred that generally hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650 750 c in the presence of an aqueous fluid crystal plasticity could become dominant at higher temperatures and or lower aqueous fluid activities

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

episodic tremor and slip explained by fluid enhanced microfracturing and sealing episodic tremor and slow slip events at the deep extension of plate boundary faults illuminate seismic to aseismic processes around the brittle ductile transition these events occur in volumes characterized by overpressurized fluids and by near failure shear stress conditions we present a new modeling approach based on a ductile grain size sensitive rheology with microfracturing and sealing which provides a mechanical and field based explanation of such phenomena we also model pore fluid pressure variation as a function of changes in porosity permeability and strain rate dependent fluid pumping the fluid enhanced dynamic evolution of microstructures defines cycles of ductile strain localization and implies increase in pore fluid pressure we propose that slow slip events are ductile processes related to transient strain localization while nonvolcanic tremor corresponds to fracturing of the whole rock at the peak of pore fluid pressure our model shows that the availability of fluids and the efficiency of fluid pumping control the occurrence and the p t conditions of episodic tremor and slip

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

comparison of microstructures in superplastically deformed synthetic materials and natural mylonites mineral aggregation via grain boundary sliding we conducted compressional tensile and torsional creep experiments on fine grained forsterite plus ca bearing pyroxene aggregates a distinct microstructure with aggregation of the same phase in the direction of compression was formed in our samples after all the experiments the stress strain rate relationship grain size dependent flow strength and the achievement of large tensile strain all indicate that samples underwent creep due to grain boundary sliding gbs as a result of gbs grain switching events allow dispersed phases to contact grains of the same phase and orient in the direction of compression we identify similar aggregated microstructures in previously reported micrographs of polymineralic granite origin ultramylonites mineral phase mixing through gbs which helps to retain fine grain size in rocks due to grain boundary pinning has been speculated to occur during formation of mylonites however our results contradict this hypothesis because mineral aggregation through gbs promotes demixing rather than mixing of the mineral phases gbs processes alone will not promote a transformation of well developed monomineralic bands to polymineralic bands during mylonitization

diffusion pathways of fe 2 and fe 3 during the formation of ferrian chromite a µxanes study the alteration of chromian spinel is a key process during serpentinization and metamorphism of ultramafic rocks controlled by oxygen fugacity fo2 and fe2 fe3 exchange during fluid rock interaction chromian spinel alteration is better recorded in less permeable chromitite than in peridotites where extensive fluid rock interaction frequently overprints the record of earlier stages of alteration to shed light on that process we have studied the distribution of fe2 and fe3 in variably altered chromian spinel grains from a set of chromitite samples from the same mining district using synchrotron based microscopic chemical imaging and spatially resolved x ray absorption near edge structure spectroscopy our results show that early stages of alteration do not involve changes in cr3 and fe2 contents or in fe speciation but only depletion in al3 and mg2 resulting in the formation of porous chromite with ongoing alteration fe3 migrates into porous chromite mainly along fracture walls and fracture zones as well as along grain boundaries sheared type chromitites record the maximum rates of fluid rock interaction because in these chromitite types the accommodation of deformation on porous chromite allowed higher rates of diffusion of fe3 and fe2 a magnetite component with fe3 fetotal 0 66 into the newly formed neoblasts in porous chromite type texture all the original chromite grains fully transformed to porous chromite the deformation and accompanying diffusion processes result in the formation of homogenous ferrian chromite grains in contrast in partly altered type texture chromite grains with original cores surrounded by porous chromite such processes are only restricted to the porous rims giving rise to zoned chromian spinel ferrian chromite grains

holocene monsoon climate documented by oxygen and carbon isotopes from lake sediments and peat bogs in china a review and synthesis there has been much recent debate about holocene climate variation in the monsoon region of china especially the temporal pattern of variations in precipitation the time transgressive nature of the holocene precipitation maximum and the extent to which variations in regions influenced by the indian summer monsoon ism and the east asian summer monsoon easm have been synchronous we summarize and compare carbonate oxygen isotope records delta o 18 carb from ten lakes within the present day ism region we discuss their paleoclimate significance considering the present day moisture source isotopic composition of precipitation and the hydrological setting the delta o 18 carb records are controlled mainly by the isotopic composition of lake water which in turn is a function of regional precipitation evaporation p e balance and the proportion of precipitation that is monsoon derived we normalized the delta o 18 carb data and used these records to generate an integrated moisture index this index along with oxygen isotope records from speleothems and carbon isotope records delta c 13 org from peats within the monsoon region suggests that holocene climate was broadly synchronous across the monsoon region and within the limits of accuracy of the existing age models provides no strong evidence for previously proposed anti phasing of the ism and the easm stable isotope records from lake sediments and peat bogs have excellent potential for providing high quality paleoclimate data for monsoon asia and complement high resolution speleothem sequences which are only found in certain localities c 2011 elsevier ltd all rights reserved

weaker than weakest on the strength of shear zones thin laterally continuous ultramylonites within kilometer scale ductile shear zones may control mid lower crustal strength where deformation is localised interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach yet fine grained polyphase mixtures are commonly found in the cores of high strain zones we study a continental strike slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite to greenschist facies conditions a brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively whereas the ultramylonites comprise a fine grained mixture of the host rock minerals the localisation of strain in ultramylonites demonstrates how fine grained polyphase mixtures can be weaker than and supersede interconnected phyllosilicate networks with increasing shear strain this contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach

the role of deformation reaction interactions to localize strain in polymineralic rocks insights from experimentally deformed plagioclase pyroxene assemblages abstract in order to study the mutual effect of deformation and mineral reactions we have conducted shear experiments on fine grained plagioclase pyroxene assemblages in a griggs type solid medium deformation apparatus experiments were performed at a constant shear strain rate of 10 5 s 1 a confining pressure of 1 gpa and temperatures of 800 850 and 900 c while the peak stress of plagioclase orthopyroxene assemblages reaches values between those of the end member phases the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase plagioclase this weakening correlates with the coeval development of high strain shear zones where new phases are preferentially produced including new pyroxene plagioclase and amphibole the reaction products mostly occur as intimately mixed phases within fine grained and interconnected shear bands together with different compositions with respect to the starting material this indicates that deformation significantly enhances the kinetics of mineral reactions which in turn strongly weaken the deforming sample here attributed to a switch to grain size sensitive diffusion creep through phase nucleation and grain size reduction such an interplay between deformation and mineral reactions may have strong implications for the initiation development and durability of shear zones in the lower crust

last glacial maximum climate based on cosmogenic 10be exposure ages and glacier modeling for the head of tashkurgan valley northwest tibetan plateau abstract fully understanding the timing and climate of last glacial maximum lgm is still lack in the head of the tashkurgan valley northwest tibetan plateau tp the recent improvements in cosmogenic 10 be exposure dating and glacier modeling present an opportunity to estimate the lgm climatic conditions in the paper we investigated the moraines and used the 10 be exposure dating on the moraine boulders to constrain the timing of the lgm the kuzigun glacial stage in the head of the tashkurgan valley then we used a coupled mass balance and ice flow model to quantify the paleoclimate from past glacier extents constrained by mapped and dated kuzigun glacial moraines in the area we showed that the glaciers terminated at an elevation of 4100 4200 m asl during the kuzigun glacial stage and the glacial stage occurred during or slightly before 15 23 ka corresponding to the timing of the global lgm the modeled mass balances indicated that present day equilibrium line altitude ela ranges from 5150 to 5250 m asl and lgm ela is between 4550 and 4600 m asl in the area suggesting an ela lowering of 600 m during the lgm model results suggested that the temperature during the lgm in the head of the tashkurgan valley is 5 8 c colder than today considering a 30 70 reduction in precipitation compared to today the paleoclimate estimates are generally consistence with previous studies that showed the lgm climate was colder and drier than today on the tp

rock strength and texture evolution during deformation in the earth s ductile lithosphere a two phase thermodynamics model building upon the two phase and grain damage theory we propose a new formulation allowing to track the evolution of phase mixing segregation during ductile deformation of a two phased aggregate our model is based on a set of variables characterizing a rock texture the mean grain sizes and the phase proportion during ductile deformation activation of different micro mechanical processes impacts the aggregate texture dislocation and diffusion creep are the two main deformation processes considered we only account for the effect of zener pinning in slowing down grain growth and allow for active grain size reduction mechanisms in the diffusion creep domain for this purpose an equation is proposed to track the phase mixing evolution during ductile deformation numerical application using anorthite rheology shows that any grain reduction mechanisms that could be active in the diffusion creep regime requires a very high partition fraction in order to reach the grain size size predicted by the feldspar piezometer application of this model to gabbroic composition relevant for the ductile crust demonstrates that the strong coupling between phases grain sizes and interface evolution results in steady state grain sizes far below the field boundary this effect is coeval with an important increase of mixing between the two phases in addition accounting for the phase mixing results in a drop of the global aggregate stress during deformation this model allows for further comparison of mylonitized textures evolution with natural shear zones at the local and regional scales

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

conductivity response to intraplate deformation evidence for metamorphic devolatilization and crustal scale fluid focusing we present two dimensional electrical resistivity models of two 40km magnetotelluric mt profiles across the frome embayment to the east of the northern flinders ranges south australia the lower crust shows low resistivity of 10 m at around 30km depth the middle crust is dominated by resistive 1000 m basement rocks underlying the flinders ranges adjacent to the ranges conductive lower crust is connected to vertical zones of higher conductivity extending to just below the brittle ductile transition bdt at 10 km depth the conductive zones narrow in the brittle upper crust and dip at roughly 45 beneath the surface zones of enhanced conductivity coincide with higher strain due to topographic loading and sparse seismicity we propose that fluids are generated through neotectonic metamorphic devolatilization low resistivity zones display areas of fluid pathways along either pre existing faults or an effect of crustal compression leading to metamorphic fluid generation the lower crustal conductors are responding to long wavelength flexure induced strain while the upper crustal conductors are responding to short wavelength faulting in the brittle regime mt is a useful tool for imaging crustal strain in response to far field stresses in an intraplate setting and provides important constraints for geodynamic modeling and crustal rheology

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

climate at the equilibrium line of glaciers the relationships between temperature precipitation and radiation on glacier equilibrium lines are investigated using 70 glaciers for which the mass balance and meteorological observations have been carried out for sufficiently long periods it is found that the characteristic climate at glacier equilibrium lines can be described using the summer 3 months temperature in a free atmosphere annual total precipitation and the sum of global and long wave net radiation all of these are measured at or very near the equilibrium line altitudes then it is shown how the shift of the equilibriumline will occur as a result of a climatic change finally the effect of the shift of the equilibrium line on the annualmean specific mass balance is analytically derived and compared with observations the present results make it possible to identify the altitudes in climate models where glacierization should begin and to evaluate the mass balance changes as a result of possible future changes in the climate

instability of the atlantic overturning circulation during marine isotope stage 3 variations in the strength of the atlantic meridional overturning circulation amoc were involved in the occurrences of dansgaard oeschger d o events during marine isotope stage 3 mis3 the stability of the amoc to north atlantic freshwater perturbations is studied using a comprehensive climate model under mis3 boundary conditions an amoc stability diagram constructed from a series of equilibrium freshwater perturbation experiments reveals a highly nonlinear dependence of amoc strength on freshwater forcing the mis3 baseline state is remarkably unstable with respect to minor perturbations the global climate signal associated with a change in amoc strength is consistent with a transition from an interstadial to a stadial state including an annual mean surface air temperature drop of 8 k in central greenland we suggest that minor freshwater perturbations in the hydrologic cycle e g related to ice sheet processes had the potential to trigger d o type climate shifts associated with a threshold in the atmosphere ocean system

mass balance and equilibrium line altitudes of glaciers in high mountain environments abstract the mass balance characteristics of glaciers in high mountain environments complicate the relationship between glacier equilibrium line altitudes elas and climatic variables such as precipitation and air temperature therefore methods of ela reconstruction employed in low relief environments are commonly not applicable in high mountains or require some modification we review the concept of the ela with reference to the mass balance of a range of glacier types found in high mountain regions we examine the applicability of several commonly used methods of ela reconstruction for different glacier types and propose some general principles to guide the choice of appropriate methods

cosmogenic nuclide constraints on late quaternary glacial chronology on the dalijia shan northeastern tibetan plateau abstract cosmogenic nuclide cn apparent exposure dating has become a widely used method for determining the age of glacial landforms on the tibetan plateau with 1200 published ages we present the first 10be exposure ages from the dalijia shan the most northeastern formerly glaciated mountain range on the tibetan plateau the moraine groups identified from field and remote sensing imagery mapping record four glacial events at 37 07 3 70 to 52 96 4 70 ka mis 3 20 17 1 79 to 26 99 2 47 ka mis 2 16 92 1 49 to 18 76 1 88 ka mis 2 and 11 56 1 03 to 11 89 1 06 ka younger dryas these ages indicate that glaciation in the northeastern tibetan plateau is much younger than previously thought in addition this record is consistent with many other regions on the tibetan plateau with a local last glacial maximum during mis 3 asynchronous with northern hemisphere last glacial maximum during mis 2 the dalijia shan might also include an event of younger dryas age but this needs to be tested in future studies

how ductile are ductile shear zones three premises of ductile deformation in the middle and lower crust are widely accepted 1 rocks flow with power law viscous rheology 2 localization in ductile shear zones involves strain softening and 3 fluid flows into and is channelized within ductile shear zones ductile viscous shear zones should therefore initially develop along planes of maximum shear stress strain soften and rotate as material planes into the field of progressive extension however the mean stress or pressure within a weak elongate viscous band being stretched is higher than the surrounding matrix and this is difficult to reconcile with premise 3 in contrast mohr coulomb brittle faults always have lower pressure within the fault zone flow of fluid and melt into high temperature shear zones therefore implies that ductile shear zones are not perfectly viscous but have a pressure dependent viscoplastic rheology the continued pressure dependence may reflect significant microcracking on the grain scale even when localized deformation does not produce larger scale discrete fractures

climates during late quaternary glacier advances glacier climate modeling in the yingpu valley eastern tibetan plateau abstract the last glacial maximum lgm featured a major cooling of earth s climate after which the climate evolved in the largest reconfiguration of the past 100 ka despite its significance full understanding of the climate history during and since the lgm is still lacking on the eastern tibetan plateau recent improvements in understanding glacial extents and chronologies in the yingpu valley eastern tibetan plateau present an opportunity to estimate the glacial climatic conditions during and since the lgm using a relatively new glacier climate model this study reconstructs glacier advances in the yingpu valley and quantifies the related climate conditions during the lgm lateglacial and late holocene glacial stages the model results show that the yingpu valley contained ice volumes of 1 65 km3 1 03 km3 and 0 29 km3 with equilibrium line altitude ela lowering values of 500 m 410 m and 150 m in the three successive glacial stages respectively by examining other independent paleoclimatic reconstructions it is concluded that the temperature decreased by 4 0 5 9 c 3 4 3 7 c 0 3 0 6 c with the precipitation amounts being 40 80 80 100 and 100 110 of modern values during the lgm lateglacial and late holocene glacial stages respectively the climate estimates for the three glacial stages are generally in agreement with other climatic proxy records on the tibetan plateau and atmospheric circulation modeling results

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

tracking metamorphic dehydration reactions in real time with transmission small and wide angle synchrotron x ray scattering the case of gypsum dehydration we present a review of a unique non destructive method for the real time monitoring of phase transformations and nano pore evolution in dehydrating rocks transmission small and wide angle synchrotron x ray scattering saxs waxs it is shown how saxs waxs can be applied to investigating rock samples dehydrated in a purpose built loading cell that allows the coeval application of high temperature axial confinement and fluid pressure or flow to the specimen because synchrotron sources deliver extremely bright monochromatic x rays across a wide energy spectrum they enable the in situ examination of confined rock samples with thicknesses of 1 mm at a time resolution of order seconds hence fast kinetics with reaction completion times of about hundreds of seconds can be tracked with beam sizes of order tens to hundreds of micrometres it is possible to monitor multiple interrogation points in a sample with a lateral extent of a few centimetres thus resolving potential lateral spatial effects during dehydration and enlarging sample statistics significantly therefore the saxs waxs method offers the opportunity to acquire data on a striking range of length scales for rock samples with thicknesses of 10 3 m and widths of 10 2 m a lateral interrogation point spacing of 10 5 m can be achieved within each irradiated interrogation point volume information concerning pores with sizes between 10 9 and 10 7 m and the crystal lattice on the scale of 10 10 m is acquired in real time this article presents a summary of the physical principles underpinning transmission x ray scattering with the aim of providing a guide for the design and interpretation of time resolved saxs waxs experiments it is elucidated 1 when and how saxs data can be used to analyse total porosity internal surface area and pore size distributions in rocks on length scales from 1 to 300 nm 2 how waxs can be employed to track lattice transformations in situ and 3 which limitations and complicating factors should be considered during experimental design data analysis and interpretation to illustrate the key capabilities of the saxs waxs method we present a series of dehydration experiments on a well studied natural gypsum rock volterra alabaster our results demonstrate that saxs waxs is excellently suited for the in situ tracking of dehydration kinetics and the associated evolution of nano pores the phase transformation from gypsum to bassanite is correlated directly with nano void growth on length scales between 1 and 11 nm for the first time a comparison of the saxs waxs kinetic results with literature data emphasises the need for future dehydration experiments on rock specimens because of the impact of rock fabric and the generally heterogeneous and transient nature of dehydration reactions in nature it is anticipated that the saxs waxs method combined with in situ loading cells will constitute an invaluable tool in the ongoing quest for understanding dehydration and other mineral replacement reactions in rocks quantitatively

palaeoglaciology of bayan har shan ne tibetan plateau exposure ages reveal a missing lgm expansion the bayan har shan a prominent upland area in the northeastern sector of the tibetan plateau hosts an extensive glacial geological record to reconstruct its palaeoglaciology we have determined

spatial patterns of holocene glacier advance and retreat in central asia glaciers in the southern himalayas advanced in the early holocene despite an increase in incoming summer solar insolation at the top of the atmosphere these glacier advances are in contrast to the smaller alpine glaciers in the western and northern regions of central asia two different glacier mass balance models are used to reconcile this holocene glacier history with climate by quantifying the change in equilibrium line altitudes ela for simulated changes in holocene climate both ela models clearly show that the lowering of elas in the southern himalayas is largely due to a decrease in summer temperatures and that an increase in monsoonal precipitation accounts for less than 30 of the total ela changes the decrease in summer temperatures is a dynamic response to the changes in solar insolation resulting in both a decrease in incoming shortwave radiation at the surface due to an increase in cloudiness and an increase in evaporative cooling in the western and northern zones of central asia both ela models show a rise in elas in response to a general increase in summer temperatures this increase in temperatures in the more northern regions is a direct radiative response to the increase in summer solar insolation

palaeoclimatic reconstruction from loch lomond readvance glaciers in the west drumochter hills scotland geomorphological mapping in the west drumochter hills provides evidence of a readvance of locally nourished glaciers during the loch lomond younger dryas stade in the form of an icefield 67 7 km2 in area drained by outlet glaciers the icefield limits accord broadly with those proposed by sissons 1980 but all geomorphic stratigraphic and sedimentological evidence conflicts with a recent proposal that the landforms in the area reflect southwestwards retreat of the last ice sheet up valley continuity of recessional moraines indicates that the ice remained active and close to climatic equilibrium during the earlier stages of glacier retreat consistent with slow warming following the coldest part of the stade the pattern of equilibrium line altitudes elas across the icefield is consistent with transfer of snow by westerly and southerly winds the ela of the reconstructed icefield as a whole is 622 629 m although this figure is likely to be lower than the regional climatic ela because the icefield probably received additional snow blown from adjacent plateau surfaces and slopes inclusion of potential snow blow areas in the ela calculation yields a value of 648 656 m the climatic ela is therefore likely to have lain between 622 and 656 m mean june to august temperature at the ela based on chironomid assemblages at two sites falls within the range 4 0 0 7 c empirical relationships between temperature and precipitation at modern glacier elas indicate that mean annual precipitation map at the ela was 1977 464 mm statistically indistinguishable from modern values comparison with precipitation values calculated for the isle of mull on the west coast suggest that the precipitation gradient across the central highlands of scotland was steeper during the loch lomond stade than at present probably as the result of efficient scavenging of precipitation from westerly airflows by the west highland icefield copyright 2005 john wiley sons ltd

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

fluid present deformation aids chemical modification of chromite insights from chromites from golyamo kamenyane se bulgaria abstract chemical signatures of chromitites are commonly used to track the evolution of the earth s mantle however chemical modification during deformation may have important implications for the interpretation of chromites signatures here we describe the details of how deformation promotes chemical modification in chromite physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using electron back scattered diffraction ebsd and electron microprobe analysis emp chromites show porphyroclastic textures with coarse grained porphyroclasts ca 0 2 5 mm and fine grained neoblasts 3 and cr and lower mg values than the cores of large grains we interpret f1 and f2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization respectively crystallographic preferred orientation cpo and misorientation data on the well developed low angle subgrain boundaries in coarse grains and f2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the 111 slip system the retrograde p t exhumation path predicted by thermodynamic and chemical modeling suggests that these fine grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism 1 0 gpa and 500 700 c our results show that deformation in the dislocation creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization this close physicochemical link offers new avenues of interpreting the chemical signatures of chromites utilizing their microstructurally controlled variation or lack thereof

deformation at the frictional viscous transition evidence for cycles of fluid assisted embrittlement and ductile deformation in the granitoid crust abstract mid crustal deformation is classically characterized by the transition from ductile to brittle deformation defining the frictional to viscous transition fvt here we investigate an exhumed continental mid crustal basement section in order to envisage the relationship between ductile and brittle deformation at the fvt our detailed study from km to micro scale shows that under greenschist metamorphic conditions deformation is accommodated by a dense network of highly localized ductile shear zones in the investigated case it is not quartz which defines the overall ductile deformation behavior but the viscous granular deformation in shear zones with an ultrafine grained polymineralic matrix consisting of quartz feldspar sheet silicates and epidote during viscous granular flow mass transfer processes under the presence of fluids promote a chemo mechanical mixing resulting in grain size reduction and reaction softening coeval with this ductile deformation fluid assisted embrittlement occurs as indicated by biotite coated fractures cataclasites and injection of non cohesive polymineralic gouge material into secondary fractures inside the host rock the embrittlement during predominant ductile deformation occurs in cycles i e prolonged periods of slow viscous granular flow are interrupted by rapid brittle deformation we interpret this fluid assisted cyclic embrittlement evidenced by injection of the fluidized material into off fault fractures as an alternative equivalent to pseudotachylites and as a microstructural indicator for paleo seismic activity with exhumation and associated cooling localized deformation persists in the ultrafine grained polymineralic shear zones but progressively transitions to cataclastic flow and finally to pressure dependent frictional flow always showing cycles of slow interseismic flow and fast seismic injection events overall in the granitic crust of the aar massif brittle and ductile deformation coexist up to deformation temperatures of minimum 450 c indicating that the fvt has to be placed in a rather wide range from 8 km up to 18 20 km in the granitoid crust

new polygonal grains formed by dissolution redeposition in quartz mylonite abstract arrays of polygonal grains are common along original grain margins subgrain boundaries and intragranular fractures of deformed quartz porphyroclasts we investigated these internal polygonal grains within quartz porphyroclasts of a partially recrystallized quartz mylonite deformed in the presence of fluid in a greenschist facies shear zone from quadrilatero ferrifero southeastern brazil optical observation showed profuse fluid inclusions and muscovite flakes inside the deformed porphyroclasts observation by sem revealed the presence of crystal faces on the internal polygonal grains domainal c axis analysis showed that the internal polygonal grains have a preferred orientation similar to the host porphyroclasts c axes at low angles to the stretching lineation but different from the fabric of recrystallized matrix grains type 1 cross girdle based on these observations we suggest a mechanism of in situ solution reprecipitation where the new grains are nucleated in the intergranular fluid filled pores present along the internal discontinuities of the porphyroclasts the grains grow with their boundaries largely in contact with fluid allowing the development of crystal faces at least during the early growth stages after the new grain reaches the opposite pore wall further grain growth should occur through ordinary grain boundary migration mechanisms driven by the strain energy difference between the new grain and the host porphyroclast we conclude that in these rocks in situ solution reprecipitation operated in conjunction with crystal plastic processes accounting for the nucleation of new grains during deformation recrystallization in the presence of fluid under low grade metamorphic conditions

timing and climatic drivers for glaciation across monsoon influenced regions of the himalayan tibetan orogen abstract mapping and thirty eight 10 be terrestrial cosmogenic nuclide tcn ages help define the timing of glaciation in the monsoon influenced greater himalaya in central garwhal india glacial landforms in central garwhal are present only within a few kilometers of the present glaciers and all date to less than 12 5 ka this suggests that the lateglacial and or holocene glacial advances were more extensive than their predecessors and hence destroyed or buried evidence for earlier glaciation or that other processes such as intensive fluvial erosion and or hillslope mass movements have destroyed evidence of earlier glaciation prominent laterofrontal moraines date to the lateglacial the early holocene and the neoglaciation moraines next to the active ice and boulders on contemporary glaciers date to 10 1 10 2 years before present this suggests only a minor glacial advance during the little ice age occurred in central garhwal these young ages indicate that inheritance of tcns in areas that were recently glaciated is very small and likely has little effect when considering tcn ages on moraines older than the global last glacial maximum the new 10 be ages are combined with 1081 recalculated 10 be ages from previous studies to develop a regional framework of glaciation across the monsoon influenced and adjacent regions of the himalayan tibetan orogen we separate what appears to be continuous glaciation into 27 regional glacial stages plus 2 tentative glacial stages that are termed monsoonal himalayan tibetan stages mohits the regional glacial stages cover a wide chronologic range that includes five regional glacial stages older than the last glacial cycle mohits 13 at 483 38 ka to mohits 5e at 122 15 ka thirteen regional glacial stages within the last glacial cycle mohits 5b at 91 15 ka to mohits 2a at 12 9 0 9 ka and eleven regional glacial stages during the holocene mohits 1k at 11 4 0 7 ka to mohits 1a at 0 4 0 1 ka there are strong correlations both with periods of strong monsoons and northern hemisphere events throughout the entire chronologic range with 16 stages linked to the monsoon 11 stages linked to the mid latitude westerlies and two unassigned because of large age uncertainties overall we see a complex pattern of glaciation influenced by two climatic systems throughout the mid late quaternary