mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

seismicity following the 1985 eruption of nevado del ruiz colombia abstract over 200 of the best recorded earthquakes occurring near nevado del ruiz colombia between december 1985 and may 1986 are examined for spatial patterns information on local stress orientations and evidence for seismogenic structures that may be related to magma transport processes these earthquakes occurred following the devastating eruption of november 13 1985 earthquake activity that is probably related to the volcanic system occurs over a large area of the order of 50 km 2 a pattern of earthquakes beneath the southern half of ruiz may be the expression of a conduit system but evidence is inconclusive due to low spatial resolution of most hypocenters epicenter and first motion plots suggest most earthquakes occur on small faults of varying orientation most seismicity during this time period was unrelated to mapped faults including some with holocene offsets except for the villamaria fault near ruiz s northeast sector composite focal mechanisms show a variation of nearly 90 in the strike of t axes and indicate ruiz is in a zone of local extension

sand dunes as a major proximal dust source for late pleistocene loess in the negev desert israel grain size analyses of three hilltop primary eolian loess sequences in the negev desert southern israel show a bimodal grain size distribution at 50 60 m and 3 8 m using analyses of mineralogy and osl ages we demonstrate that the coarse mode is composed mostly of quartz grains and its relative magnitude increases regionally with time suggesting an enhancement of a time transgressive proximal dust source compared to a distal saharan fine grain dust the only proximal dust source for large amount of coarse silt quartz grains is the sands that advanced into sinai and the negev concurrently with the loess accretion during the late pleistocene as a result of the exposure of the mediterranean shelf we therefore propose that the coarse silt quartz grains were formed through eolian abrasion within the margins of an advancing sand sea this relationship between desert sand seas as a source for proximal coarse dust and desert margin loess deposits can be applicable to other worldwide deserts such as northern africa china and australia

resurfacing of deeply buried oceanic crust in naga hills ophiolite north east india petrofabric microstructure and seismic properties abstract we have studied the microstructure texture and seismic anisotropy of a foliated blueschist eclogite rock in the naga hills ophiolite nho complex north east india the rock is composed essentially of omphacite sodic amphibole glaucophane sodic calcic amphibole barroisite phengite and garnet and represents oceanic crust that experienced neo tethyan subduction and metamorphism before it resurfaced and accreted in the ophiolite omphacite and glaucophane show weak shape preferred orientation spo while phengite displays a strong spo the crystallographic preferred orientation cpo of omphacite is characterized by the 001 axes gridle within the foliation and the 010 poles concentrated sub perpendicular to the foliation for glaucophane the 001 axis aligns parallel to lineation and the 100 axis and 110 pole plunge perpendicular to foliation these cpos correspond to sl type fabrics related to plane strain deformation geometry based on our analysis we propose that the observed texture had developed from plastic deformation through dislocation creep employing the textural data we have presented the seismic anisotropies of the individual minerals blueschist eclogite domains and discussed their contributions in the bulk rock anisotropy the calculated bulk rock seismic anisotropies avp and avs of the rock are 12 8 and 8 1 respectively which correlate with the observed seismic anisotropy of the subducting oceanic crust

fluid ascent during the 2004 2005 unrest at mt spurr inferred from seismic tomography 1 we present a seismic model of the crust beneath mt spurr volcano in alaska based on tomographic inversion of local earthquake data recorded by temporary seismic network during the unrest of mt spurr in 2004 2005 beneath the northern vent of mt spurr where the most intensive fumarolic activity was observed during the last unrest we observe a finger shaped anomaly with a higher p and lower s velocity which corresponds to high vp vs ratio another anomaly is located to the south of crater peak approximately beneath the same location where an explosive eruption occurred in 1992 we propose that these two finger anomalies represent separated conduits of magma and volatiles the ne conduit extends to the surface and enables free volatile flux the se conduit seems to be blocked at a ca 20 km depth and in case of continuous pressure build up may become a source of an explosive eruption citation koulakov i m west and p izbekov 2013 fluid ascent during the 2004 2005 unrest at mt spurr inferred from seismic tomography geophys res lett 40 4579 4582 doi 10 1002 grl 50674

reconstruction of the late quaternary environment of the lower luni plains thar desert india geomorphological processes in the thar desert of india are largely climate driven in the lower reaches of the river luni the only major drainage system in the thar desert a fluvio aeolian sequence was located at a site called khudala sediments of this sequence represented a variety of depositional environments namely aeolian fluvially reworked aeolian overbank deposits gravels and occasional evidence of pedogenesis this provided a good opportunity to study aeolian fluvial interaction in the region and for deducing climatic records from the luminescence dating standpoint these sequences offered a good opportunity for a comparative study of thermoluminescence tl blue green light stimulated luminescence bgsl and infrared light stimulated luminescence irsl on different mineral separates of identical provenance but deposited under different environments broadly within experimental errors the tl ages agreed with bgsl and irsl ages on aeolian sands but differed substantially in the case of fluvially reworked and proximally deposited sands and silts the sequence provided a record spanning more than 100 ka with an aeolian phase at 100 ka a channel activation phase between 70 and 30 ka and a phase of climate instability between 13 and 8 ka this appears consistent with the records of monsoon performance during this period which includes the younger dryas it is also inferred that during the last glacial epoch geomorphological processes in the thar both aeolian and fluvial were dormant largely on account of their relationship with the southwest monsoon copyright 2001 john wiley sons ltd

preferred orientation developed by rigid markers in two dimensional simple shear strain a theoretical and experimental study abstract in a homogeneous two dimensional deformation linear markers embedded in a viscous matrix develop a preferred orientation along the principal axis of the strain ellipse theoretical analysis of marker rotation under two dimensional simple shear is used to derive an orientation distribution function this function shows that the relative density maximum increases with progressive strain its value being equal to the major minor strain ellipse axis ratio moreover the density is equal to the unity along the lines of no finite longitudinal strain experimentally observed preferred orientation is in good agreement with the expected theoretical distribution

high resolution holocene environmental changes in the thar desert northwestern india sediments from lunkaransar dry lake in northwestern india reveal regional water table and lake level fluctuations over decades to centuries during the holocene that are attributed to changes in the southwestern indian monsoon rains the lake levels were very shallow and fluctuated often in the early holocene and then rose abruptly around 6300 carbon 14 years before the present 14c yr b p the lake completely desiccated around 4800 14c yr b p the end of this 1500 year wet period coincided with a period of intense dune destabilization the major harrapan indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase

a 200 ka record of climatic change and dune activity in the thar desert india abstract an 18 4 m excavated dune section in the thar desert of india with a chronology based on 12 tl ages and a basal age of 190 ka has preserved 12 cycles of dune accretion soil formation calcrete development and subsequent erosion together with the presence of stone artefacts ranging in age from lower palaeolithic to mesolithic coeval with more humid climatic interludes phases of soil development and carbonate precipitation were relatively wet and phases of dune accretion relatively dry so that there were 12 significant moist intervals separated by 11 drier intervals during the past 190 ka the calculated time interval between successive phases of dune sand accumulation ranged from 22 2 ka to 15 8 ka with a mean of 19 0 ka these values are consistent with a precessional influence on dune activity and on the associated onset of early monsoonal activity in this region carbon isotopes measured on organic matter within the sand profiles show consistent values close to 21 6 1 pointing to deposition during a transitional climatic regime characterized by a change from open c3 grassland to c4 woodland or forest

eolian sediment responses to late quaternary climate changes temporal and spatial patterns in the sahara abstract this paper presents a compilation of eolian based records of late quaternary climate changes in the sahara although the data are relatively sparse when viewed as a whole they reveal a general pattern of widespread eolian sediment mobilization prior to 11 000 cal years bp eolian sediment stabilization from 11 000 to 5000 cal years bp and a return to widespread eolian sediment mobilization after 5000 cal years bp furthermore an eolian based record from southern tunisia reveals the existence of millennial scale changes in eolian sediment behavior these millennial scale variations provide examples of eolian sediment responses to climate changes at a scale intermediate between seasonal and orbital milankovitch changes and they are also coincident with abrupt atmospheric and oceanic changes the general synchroneity of the eolian stratigraphic records and their coincidence with various oceanic and atmospheric changes suggest that global forcing mechanisms have influenced late quaternary eolian sediment behavior in the sahara

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

holocene paleoclimates of india we present a comprehensive summary of the available palaeoclimate records from india and compare the results from different proxies the results indicate i fluctuating lake levels during the early holocene the period of relatively higher lake levels from increased precipitation efficiency was reached only 7 2 6 0 cal kyr bp possibly due to increased contribution from winter rainfall ii the onset of aridity in nw india could have begun as early as 5 3 cal kyr bp subsequently there were multiple wet events but of shorter duration and smaller magnitude than during the mid holocene iii there is evidence of several short term climate events in the proxy record however in the absence of a rigorous chronological framework a detailed regional correlation is not possible at this stage finally a comparison between marine and terrestrial records indicates that episodes of strongest and weakest monsoon winds were not always associated with wettest and driest episodes respectively in the nw indian lakes

luminescence dating of loess palaeosol sequences and coversands methodological aspects and palaeoclimatic implications abstract of the terrestrial archives loess palaeosol sequences provide a most complete record of climatic change this may be compared with the marine oxygen isotope stratigraphy and may help in the reconstruction of past atmospheric circulation patterns numerical chronometry of loess palaeosol sequences has generally been based on correlation of variations in climatic proxies such as magnetic susceptibility and particle sizes with marine isotopic data such chronometric assignments involve implicit assumptions about the constancy of sedimentation rates and particle fluxes through time this review presents a brief survey of the present status methodology outstanding problems and interpretational aspects of luminescence techniques and discusses the import of luminescence ages on global land sea correlations statistical analysis of the ages suggests episodicity of loess accumulation with extended periods of quiescence recent luminescence dating studies on closely spaced samples also lead to a similar inference luminescence ages imply high variability in loess sedimentation rates this conflicts with the assumption made in some current attempts to correlate loess records with marine records of almost constant particle fluxes a review of source proximal coversand deposits of northwest europe is also presented evidence of the onset of coversand deposition at 15 ka with a peak in accretion during the younger dryas and subsequent minor reactivation episodes is discussed

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

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

fluvial response to late quaternary climate changes western india we review here the fluvial response to late quaternary climatic changes in the semi arid regions of western india i e the thar desert and its margins and maharashtra the rivers in the thar desert show a spectrum of environments from gravel bedload braided to ephemeral sand bed streams to sheet flows and sheet floods while those in the thar margin and maharashtra show dominantly two modes i e braided and meandering rivers the streams became defunct and aeolian sands were deposited during periods of relatively greater aridity in the thar desert and its margins incisions occurred during the wet phases in ois 1 it is inferred that changes in the monsoon precipitation led to relative changes in the discharge and sediment supply which were routed through those in the vegetation cover and played a key role in determining the fluvial style an inter comparison of the late quaternary alluvial deposits in the semi arid regions of western india suggests a continuum of fluvial environments interrelated through a precipitation gradient further a global comparison indicates that the response of semi arid western indian rivers was synchronous with the rivers in other climatic settings

three dimensional velocity structure of the galeras volcano colombia from passive local earthquake tomography abstract a three dimensional estimation of the vp vs and vp vs ratio structure at galeras volcano was conducted by means of passive local earthquake tomography 14 150 volcano tectonic events recorded by 58 stations in the seismological network established for monitoring the volcanic activity by the colombian geological survey pasto volcano observatory between the years 1989 and 2015 were inverted by using the lotos code the seismic events are associated with shear stress fractures in solid rock as a response to pressure induced by magma flow tomography resolution tests suggest a depth of imaging that yield 10 km from the summit of the main crater illuminating a large portion of the volcanic structure and the interaction of tectonic features like the buesaco and silvia pijao faults full catalog tomographic inversion that represents the stacked image of the volcanic structure or the most permanent features underneath the volcano shows vertical structures aligned with seismicity beneath the main crater we hypothesize that these structures correspond to a system of ducts or fractures through which magma and fluid phases flow up from deeper levels toward the top and related with the intersection of the surface traces of the silvia pijao and buesaco faults

abrupt onset and termination of the african humid period rapid climate responses to gradual insolation forcing a detailed ca 100 yr resolution and well dated 18 ams 14 c dates to 23 cal ka bp record of latest pleistocene holocene variations in terrigenous eolian sediment deposition at odp site 658c off cap blanc mauritania documents very abrupt large scale changes in subtropical north african climate the terrigenous record exhibits a well defined period of low influx between 14 8 and 5 5 cal ka bp associated with the african humid period when the sahara was nearly completely vegetated and supported numerous perennial lakes an arid interval corresponding to the younger dryas chronozone punctuates this humid period the african humid period has been attributed to a strengthening of the african monsoon due to gradual orbital increases in summer season insolation however the onset and termination of this humid period were very abrupt occurring within decades to centuries both transitions occurred when summer season insolation crossed a nearly identical threshold value which was 4 2 greater than present these abrupt climate responses to gradual insolation forcing require strongly non linear feedback processes and current coupled climate model studies invoke vegetation and ocean temperature feedbacks as candidate mechanisms for the non linear climate sensitivity the african monsoon climate system is thus a low latitude corollary to the bi stable behavior of high latitude deep ocean thermohaline circulation which is similarly capable of rapid and large amplitude climate transitions

deformation and motion in the western alpine arc we attempt to reconstruct the kinematics deformation and motion of the western alpine arc between the mercantour and aar massifs first we compile a map showing principal extension x direction within three major structural units in the external zone x directions are broadly radial to the alpine arc in the internal zone they are uniform or smoothly varying in some domains whereas in the intermediate zone they often show much scatter next we use geochronological data and p t conditions estimated from metamorphic mineral reactions to time the deformation on each of a series of interval maps we show x directions for strain accumulated during a chosen time interval we argue that these incremental x directions are approximately equivalent to segments of particle paths showing displacements relative to stable europe for 120 100 ma the displacement direction was about n150 and coeval with eclogite facies metamorphism it is interpreted as a direction of overthrusting by the african plate around 40 ma the displacement in the south part of the arc became westerly finally between 25 and 15 ma displacement directions were generally radial to the arc

luminescence chronometry and late quaternary geomorphic history of the ganga plain india abstract flexing of the indian lithosphere due to the continent continent collision and thrust fold loading produced the gangetic foreland basin during the middle miocene the basin attained its present configuration during the late quaternary neotectonics and climate change since the last interglacial influenced the evolution of the present landscape of the gangetic ganga plains the regional geomorphic surfaces in these plains are 1 upland interfluve surface t2 2 marginal plain upland surface mp 3 megafan surface mf 4 piedmont fan surface pf 5 river valley terrace surface t1 6 active flood plain surface t0 minor geomorphic features comprise ponds alluvial ridges and small channels which occur on the upland interfluve surfaces the present contribution reviews geomorphic processes that sculpted the basin landscape and then using the blue green stimulated luminescence bgsl and infrared stimulated luminescence irsl ages provides a first order chronological framework for the relationship of major and minor geomorphic features to climate and tectonics luminescence dating of the upland interfluve surface t2 shows that formation of this surface started before 51 ka and continued at least up to 7 ka deposition in the marginal plain mp began at least at 76 ka and continued after 32 ka bracketing the evidence of a more humid climate during 50 40 ka sedimentation on the t2 and mp surfaces was synchronous the top of these surfaces gives variable ages ranging from 48 to 7 ka due to differential erosion of an undulatory topography caused by tectonic activity age upper bound for the initiation and termination of the final building phase of ganga megafan can be placed at 26 ka and 22 ka respectively deposition on piedmont fan surface continued between 8 and 3 ka the river valley terrace surface t1 of ganga river shows the activity of yazoo type channels during 3 1 1 ka the last phase of active accretion on the narrow poorly developed youngest active flood plain surface t0 was after 1 5 ka the ponds of t2 surface formed due to cessation of fluvial activity sometime during 8 6 ka a prominent tectonic activity at 7 5 ka and dry climate at 5 ka produced undulatory topography and high siltation rates in the lakes and ponds the peripheral bulge southern ganga plain has been tectonically active causing deep incision of rivers and a 40 ka seismic event and mid late holocene tectonic activity can be inferred

kinematics of austro alpine cover nappes changing translation path due to transpression upper and middle austro alpine nappes of the eastern alps the structural uppermost subunits of the alps suffered two major superposed deformations d1 d2 during the eo alpine cretaceous orogeny d1 is characterized by nw to w directed thrusting and nappe internal rock flow d2 by large scale folding and imbrication toward the north and northeast structures strain metamorphic history microfabrics and stretching trajectories indicate that the superposition of d2 over d1 structures can be interpreted as a progressive deformation it is concluded that the emplacement of the nappes follows a curved translation path a model which emphasizes non coaxial deformation structures explains the detachment of the nappes the configuration of the separating thrust planes and the distribution of intense deformation zones the overall kinematic history of the austro alpine nappes fits a model in which strike slip shearing horizontal shortening and vertical lengthening act together continuously over a finite deformation zone transpressional model this model emphasizing a gradually changing strain regime eliminates the space problems encountered in models which consider simple south over north imbrication during the alpine orogeny the study stresses two interpretations 1 1 stretching lineations indicate nappe transport directions 2 2 transpression may be a common feature of orogenic belts

estimation of viscosity contrast and finite strain from deformed elliptical inclusions a method of strain analysis is described which takes into account a possible competence difference between the strain markers and the rest of the rock using data consisting of the sectional shapes and orientations of groups of inclusions from conglomerate like rocks limits are placed on the possible effective viscosity contrast between the inclusions and the rock as a whole as well as on the bulk finite strain suffered by the rock these results are calculated using a computer program based on the theory for the deformation of elliptical cylinders the method can be considered an extension of the rf method but provides more information and involves less restrictive assumptions

microstructures and petro fabrics of lawsonite blueschist in the north qilian suture zone nw china implications for seismic anisotropy of subducting oceanic crust abstract we conducted a detailed study on the microstructures and petro fabrics of massive and foliated lawsonite blueschist lbs in north qilian suture zone nw china the lattice preferred orientation lpo of glaucophane and lawsonite in foliated lawsonite blueschist lbs is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid body rotation respectively the lpo of glaucophane is mainly characterized by the 001 axis aligning parallel to lineation and the 100 axis and 110 pole plunging perpendicular to foliation in contrast the lpo of lawsonite features the maximum 010 axis concentrated close to lineation and the 001 axis strongly clustered normal to foliation the preferred orientation of 010 axis of lawsonite parallel to lineation is supported by a two dimensional numerical modeling using the finite volume method fvm the mineral lpos are much stronger in foliated lbs than in massive lbs in addition a kinematic vorticity analysis suggests that both pure shear dominant wm 0 18 0 26 and simple shear dominant wm 0 86 0 93 deformation regimes are present in foliated lbs the 001 axis and 010 pole of glaucophane and the 100 and 010 axes of lawsonite tend to distribute in a foliation parallel girdle in the pure shear dominant samples but simple shear dominant samples display more lineation parallel concentrations of a 001 axis of glaucophane and a 010 axis of lawsonite because the whole rock seismic anisotropies in foliated lbs are significantly higher than those in massive lbs and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite the delay time of fast s wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle 45 60 is expected to range from 0 03 to 0 09 s lower bound for massive lbs and from 0 1 to 0 3 s upper bound for foliated epidote blueschist

delayed disequilibrium degassing in rhyolite magma decompression experiments and implications for explosive volcanism recent numerical models and analog shock tube experiments show that disequilibrium degassing during magma ascent may lead to violent vesiculation very near the surface in this study a series of decompression experiments using crystal free rhyolite melt were conducted to examine the development of large supersaturations due to delayed homogenous spontaneous bubble nucleation melts were saturated at 900 c and 200 mpa with either 5 2 wt dissolved h2o or with 4 2 wt h2o and 640 ppm co2 and isothermally decompressed at linear rates of either 0 003 0 025 or 8 5 mpa s to final pressures between 25 and 175 mpa additional isobaric saturation experiments 900 c 200 25 mpa using pure h2o or mixed h2o co2 fluids establish reference equilibrium solubility curves values homogenous nucleation is triggered in both h2o only and h2o co2 experiments once the supersaturation pressure pss reaches 120 150 mpa and the melt contains two times its equilibrium water contents bubble number density and nucleation rate depend on the supersaturation pressure with values on the order of 102 cm3 and 1 cm3 s for pss 120 mpa 106 cm3 and 103 105 cm3 s for pss 130 150 mpa and 107 cm3 and 106 cm3 s for pss 160 175 mpa nucleation rates are consistent with classical nucleation theory and infer an activation energy for nucleation of 1 5 10 18 j nucleus a critical bubble radius of 2 10 9 m and an effective surface tension for rhyolite at 5 2 wt h2o and 900 c of 0 10 0 11 n m the long nucleation delay limits the time available for subsequent diffusion such that disequilibrium dissolved h2o and co2 contents persist to the end of our runs the disequilibrium degassing paths inferred from our experiments contrast markedly with the equilibrium or quasi equilibrium paths found in other studies where bubble nucleation occurs heterogenously on crystals or other discontinuities in the melt at low pss homogenous and heterogenous nucleation rates are comparable however as are bubble number densities so that at a given decompression rate it appears that nucleation mechanism rather than nucleation rate determines degassing efficiency by fixing the pressure depth at which vesiculation commences and hence the time available for equilibration prior to eruption although real systems are probably never truly crystal free our results show that rhyolitic magmas containing up to 104 crystals cm3 and perhaps as high as 106 crystals cm3 are controlled by homogenous rather than heterogenous nucleation during ascent

deformation and metamorphism in the bard area of the sesia lanzo zone western alps during subduction and uplift the structure microstructure and petrology of a small area close to the village of bard in val d aosta italy has been studied in detail the area lies across the contact between the gneiss minuti gm and the eclogitic micaschist ems complexes of the lower element of the sesia portion of the sesia lanzo zone western alps both complexes have undergone high pressure metamorphism but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the gm to the ems therefore we interpret the contact as a thrust dividing the lower element of the sesia into two sub elements this interpretation is supported by structural evidence the early alpine 90 70 ma metamorphic history is best preserved in the ems and is one of increasing pressure associated with thrusting the maximum p t recorded in the ems is 1500 mpa 15kbar and 550 c and in the gm is 1500 1300 mpa 15 13 kbar and 500 550 c we suggest that the rocks were probably in an active benioff zone during this time from then on the histories of the gm and ems are the same deformation continued and the thrust and thrust slices were folded during decreasing pressure we interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the benioff zone had become inactive and a new benioff zone had developed further to the north west a still later deformation and the lepontine metamorphism 38 ma are related to continued uplift much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface

dust clouds rain types and climatic variations in tropical north africa abstract dust and processes of raindrop formation in the clouds play a very important role in the climatic evolution of tropical north africa sedimentologic stratigraphic pedologic geomorphologic and palynologic data converge to show that a major environmental change occurred in tropical africa about 7000 yr b p in the sudanian and sudano guinean zones wet tropical zone from 15 000 to 7000 yr b p rivers deposited mostly clay while from 7000 to 4000 yr b p they deposited mostly sand during the first period pedogenesis was vertisolic montmorillonite dominant associated with pollen belonging mostly to vegetation typical of hydromorphic soils while during the second period pedogenesis was of ferruginous type kaolinite dominant with pollen belonging mostly to vegetation typical of well drained soils the great change near 7000 yr b p is linked chiefly to a major hydrological change that appears related to a change in the size of raindrops from fine rains associated with considerable atmospheric dust raindrop diameter essentially less than 2 mm to the second period associated with thunderstorm rains raindrop diameter mostly greater than 2 mm the size of raindrops is related particularly to cloud thickness and dust concentration in the troposphere thunderstorm activity is influenced also by fluctuations of the atmospheric electricity modulated by the sun

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

vegetation response to south asian monsoon variations in konkan western india during the late quaternary evidence from fluvio lacustrine archives the palaeobotanical and palynological data along with physical proxies obtained from konkan western india have been addressed while ascertaining the vegetation response to monsoon variations since late pleistocene 53 000 yrs bp the retrieved plant fossil assemblage being a sensitive indicator of climate changes has important implications in palaeoclimate study of the late pleistocene as it is the first of its kind in the entire 7500 km long coastline of india the plant fossil assemblage of the lacustrine sediments forms potential evidence for the prevalence of wet evergreen forest in the late pleistocene 44 000 yrs bp this scenario was attributed to prevalence of an extended period of rainfall of both the southwest sw and northeast ne monsoon in konkan although the indian subcontinent was under the influence of active tectonic processes numerous fossil plant taxa are now confined to freshwater myristica swamps of southeast asia in general and that of southwestern india in particular suggesting that these areas maintain a similar monsoon trend to that existing prior to 44 000 yrs bp on the other hand the ne monsoon did not reappear during the early holocene when the sw monsoon became strong along the konkan coast many of the plant species of the fossil assemblage have been displaced from konkan as a response to weakening of the monsoons since the late pleistocene and towards the last glacial maximum lgm the aridity reduced rainfall since late pleistocene including the lgm and limited terrestrial accumulation in the early holocene suggests an ecological shift and subsequent replacement of evergreen forest cover and a possible break in sedimentation palynological evidence of the late holocene 3000 yrs bp fluvio lacustrine sediments reveals that the evergreen forest could not reappear rejuvenate in konkan due to such climatic stress in contrast to southwestern india and sri lanka where extended rainfall was still prevalent the hypothesis of an extended period of monsoonal rains prior to 44 000 yrs bp in the present humid tropical parts of konkan holds significance as data on paleosols and vertebrate fossils from the semi arid arid parts of western india indicate relatively wet climate and wetland flood plains

the behaviour of deformable ellipsoidal particles in three dimensional slow flows implications for geological strain analysis abstract eshelby s equations governing the motion of a viscous inclusion embedded in a matrix of contrasting viscosity are solved numerically for arbitrarily shaped and arbitrarily oriented ellipsoidal particles the theory has important implications for geological strains and strain analysis specifically it predicts that particles more competent than their matrix will have deformed shapes with larger k values than equivalently shaped and oriented passive markers the effect is significant even for viscosity ratios as low as r 1 5 and increases with increase in r for general applications numerical solutions must be sought nevertheless there are useful exceptions 1 1 the two dimensional deformed shape of viscous particles can be computed graphically provided the matrix strain is irrotational pure shear and the section ellipse is a principal plane of the particle ellipsoid for sections through a three dimensional system with arbitrarily oriented particles this method and those based on sections through an elliptic cylinder is inaccurate the reliability is further diminished for simple shear 2 2 irrotational deformations in three dimensions can be computed on transformed flinn plots if the particle or average particle axial orientations are parallel with the applied flow similar diagrams may be used to make independent estimates of viscosity contrasts in polymict conglomerates

asymmetric shape preferred orientations as shear sense indicators abstract mineral shape preferred orientations can be characterized by plotting aspect ratios against grain length orientation and such plots commonly display asymmetric patterns in xz sections perpendicular to foliation and parallel to the direction of maximum elongation in most cases the asymmetries cannot be discerned by superficial examination of thin sections the sense of asymmetry can be defined by a the shape of the highest aspect ratio per grain length orientation curve and b the relative orientations of the most elongate grain and the mean or median of the population as a whole observed senses of asymmetry display consistent relationships to known senses of shear and they therefore provide a useful alternative to or check on other methods of shear sense determination examples of quartz plagioclase microcline amphibole and biotite shape preferred orientations are described and they include recrystallized mosaics as well as populations of discrete mineral grains the examples include mineral preferred orientations that evolved by plastic deformation anisotropic crystal growth and mechanical rotation of inequidimensional grains

deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust abstract investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust and here we report on our systematic fabric analyses of glaucophane lawsonite and epidote in naturally deformed blueschists from the diablo range and franciscan complex in california and the hida mountains in japan glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations cpos characterized by a 1 0 0 0 0 1 pattern these characteristics together with a bimodal distribution of grain sizes from some samples possibly indicate the occurrence of dynamic recrystallization for glaucophane although lawsonite and epidote display high aspect ratios and a strong cpo of 0 0 1 0 1 0 the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism the p wave av p and s wave av s seismic anisotropies of glaucophane av p 20 4 av s 11 5 and epidote av p 9 0 av s 8 0 are typical of the crust consequently the fastest propagation of p waves is parallel to the 0 0 1 maxima and the polarization of s waves parallel to the foliation can form a trench parallel seismic anisotropy owing to the slowest v s polarization being normal to the subducting slab the seismic anisotropy of lawsonite av p 9 6 av s 19 9 is characterized by the fast propagation of p waves subnormal to the lawsonite 0 0 1 maxima and polarization of s waves perpendicular to the foliation and lineation which can generate a trench normal anisotropy the av s of lawsonite blueschist 5 6 9 2 is weak compared with that of epidote blueschist 8 4 11 1 calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench parallel and trench normal seismic anisotropy beneath ne japan but not to that beneath the ryukyu arc our results demonstrate therefore that lawsonite has a strong influence on seismic velocities in the oceanic crust and that lawsonite might be the cause of complex anisotropic patterns in subduction zones

the behavior of shape preferred orientations in metamorphic rocks amphiboles and jadeites from the monte mucrone area sesia lanzo zone italian western alps abstract in the eclogitic micaschists of the monte mucrone area sesia lanzo zone italian western alps we have measured in detail the eclogitic lineation at several outcrops we present observations at the markers scale this scale allowing the whole range of progressive deformations to be described populations of glaucophanes and jadeites show different statistical features which must be at least for a part produced by the distinct mechanical behavior for these markers during the alpine history the glaucophanes show a positive correlation between the intensity of preferred orientation and their aspect ratio due to a mainly rigid behavior the observed differences in shape and orientation of jadeites also show different behavior related to different retromorphic overprints we briefly discuss the implications of these observations on the significance of metamorphic lineations

late quaternary sediments minerals and inferred geochemical history of didwana lake thar desert india abstract variations in clastic sediment texture mineralogy of both evaporites formed at the surface and precipitates formed below the lake floor and the relative chemical activities of the major dissolved components of the chemical precipitates have allowed reconstruction of the history of salinity and water level changes in didwana lake thar desert india hypersaline conditions prevailed at about the last glacial maximum with little evidence of clastic sediments entering the lake between ca 13 000 and 6000 b p the lake level fluctuated widely the lake alternately hypersaline and fresh and clastic sediments were delivered to the lake at a low rate deep water conditions occurred ca 6000 b p and clastic influx increased abruptly the water level dropped towards 4000 b p when the lake dried briefly since 4000 b p the lake has been ephemeral with a lowered rate of sedimentation and mildly saline conditions rather like those of today this sequence of changes documented in the lake parallels changes in vegetation recorded in published pollen diagrams from both the thar and the arabian sea correlation of the various lines of evidence suggests that the climate of the last glacial maximum at didwana was dry and windy with a weak monsson circulation the monsson was re established between ca 13 000 and a little before 6000 b p and when winter rainfall increased ca 6000 b p the lake filled to its maximum depth

river response to quaternary climatic fluctuations evidence from the son and belan valleys north central india abstract the last glacial period was cold and dry in peninsular india in north central india the interval from 39 9 to 16 3 ka was associated with widespread and prolonged aggradation in the son and belan valleys the aggradation ended with sustained vertical incision after 16 3 ka and reflects a return to warmer wetter conditions in this region it would appear that terminal pleistocene to holocene river incision was broadly synchronous with a strong summer monsoon regime and higher levels of river discharge and the preceding river aggradation with lower discharge and a weaker or more variable summer monsoon regime two older phases of prolonged aggradation followed by vertical incision are evident in the son and belan valleys before 39 ka one of these phases is centred towards 73 4 ka when ash from the toba mega eruption in sumatra was deposited across peninsular india the following phase of aggradation has yielded infrared stimulated luminescence ages of 58 6 and 45 12 ka the youngest phase of aggradation began towards 5 5 ka and seems to mark a return to a weaker summer monsoon regime

three dimensional seismic velocity structure of mauna loa and kilauea volcanoes in hawaii from local seismic tomography we present a new three dimensional seismic velocity model of the crustal and upper mantle structure for mauna loa and kilauea volcanoes in hawaii our model is derived from the first arrival times of the compressional and shear waves from about 53 000 events on and near the island of hawaii between 1992 and 2009 recorded by the hawaiian volcano observatory stations the vp model generally agrees with previous studies showing high velocity anomalies near the calderas and rift zones and low velocity anomalies in the fault systems the most significant difference from previous models is in v p vs structure the high vp and high v p vs anomalies below mauna loa caldera are interpreted as mafic magmatic cumulates the observed low vp and high v p vs bodies in the kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments the high vp and moderate to low vp vs anomalies beneath kilauea caldera can be explained by a combination of different mafic compositions likely to be olivine rich gabbro and dunite the systematically low vp and low vp vs bodies in the southeast flank of kilauea may be caused by the presence of volatiles another difference between this study and previous ones is the improved vp model resolution in deeper layers owing to the inclusion of events with large epicentral distances the new velocity model is used to relocate the seismicity of mauna loa and kilauea for improved absolute locations and ultimately to develop a high precision earthquake catalog using waveform cross correlation data 2014 american geophysical union all rights reserved

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

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

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

late pleistocene and holocene dune activity and wind regimes in the western sahara desert of mauritania the western sahara desert in mauritania is dominated by extensive sand seas consisting largely of linear dunes analyses of landsat images geomorphic and stratigraphic studies and optically stimulated luminescence dating of dunes in the azefal agneitir and akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25 15 ka centered around the last glacial maximum 10 13 ka spanning the younger dryas event and after 5 ka the wind regimes that occurred during each of these periods were significantly different leading to the formation of dunes on three distinct superimposed trends northeast north northeast and north and the development of the sand seas as composite geomorphic features

asymmetric quartz c axis fabrics and flow vorticity a study using rotated garnets abstract the development of a type i crossed girdle c axis fabric with an asymmetric density distribution and an internally asymmetrical skeletal outline is examined in a specimen of garnet bearing mylonitic quartzite from the betic movement zone in southern spain the garnets are pre mylonitic skeletal porphyroblasts that rotated during mylonitic deformation but preserved a strain free equilibrium microstructure and a random c axis fabric it follows that the asymmetry of the density distribution must result from the deformation history itself the sense of asymmetry of the c axis population and that of the skeletal outline are consistent with the sense of vorticity inferred from extensional crenulations and asymmetric microboudinage and also with the sense of rotation of the garnets analysis of the rotation angles with aspect ratio of the garnet porphyroblasts demonstrates that a pure shear component was involved in an essentially non coaxial flow this may explain the internal asymmetry of the fabric skeleton and confirms the idea that a spectrum exists of type i fabrics intermediate between the theoretically predicted fabrics for pure and simple shear a tentative reconstruction of the kinematic framework suggests that the central segment of the fabric skeleton is linked to the extensional apophysis of the flow

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

fabric development in a middle devonian intraoceanic subduction regime the careon ophiolite northwest spain abstract a middle devonian suprasubduction zone ophiolite the careon unit northwest spain displays amphibolite facies ductile deformation fabrics related to the onset of the rheic ocean closure two different fabrics an early high t foliation and a subsequent lower t foliation each of which characterized by distinct deformation mechanisms have been identified in two distinct crustal scale shear zones of the same ophiolitic thrust sheet combined quantitative texture analysis by electron backscattered diffraction and time of flight neutron diffraction were carried out on the shear zones and correlated with micro and macrostructural data the results indicate that the regional lineation and shear zone kinematics east west top to the east represent fabrics developed essentially during the intraoceanic subduction of the rheic ocean and their orientation may be considered a reference vector for convergence models in this part of the variscan belt

constraints on geological strain rates arguments from finite strain states of naturally deformed rocks a summary of known finite strain states is presented longitudinal strains 1 e as measured in many rocks often range from 1 to 40 and 1 to 0 025 the time span available to produce such measureable strains in young orogenic zones seems to be less than 10 m y possibly less than 1 m y which constrains conventional strain rates into the range of 10 sup 13 s sup 1 to 10 sup 15 s sup 1 for both pure and simple shear the most efficient way and much less efficient way to accumulate incremental strains respectively the ellipticity of the finite strain ellipse increases in a nonlinear manner finite strain variations in adjacent layers which give rise to features such as cleavage refraction arise with only slight differences in the strain rates within these layers

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

continental record of the southwest monsoon during the last 130 ka evidence from the southern margin of the thar desert india abstract field stratigraphy sedimentology and luminescence dating of the exposed fluvial and aeolian sequences from the southern desert margin were used to discern the pattern of southwest monsoon variability during the last 130 ka the sedimentary architecture and textural attributes of the sediments belonging to the period 130 120 ka suggests the existence of a meandering river system consequent to enhanced southwest monsoon conditions a shift from a meandering to a braided pattern during 120 100 ka indicates a phase of reduced precipitation presence of flood plain fines and bedded calcretes during 100 70 ka suggest a period of enhanced monsoon with seasonality following this a phase of weak monsoon activity was observed during 70 60 ka regional flood plain aggradation and pedogenesis during the period termination of a regional fluvial phase around 30 ka and incision of dunes dated to 20 11 ka suggest that the present rivers occupied their course between 11 ka drowned valleys in the estuarine zones suggest that the incision began during the low sea stand around 20 ka when the rivers were graded to the exposed shelf the incision intensified with the re establishment of monsoon during the early holocene

palaeoenvironmental dynamics and palaeolithic occupation at katoati thar desert india late pleistocene palaeoenvironments in the thar desert india located at the eastern extent of the saharo arabian desert belt have fluctuated considerably as a result of the varying range and intensity of the indian summer monsoon phases of widespread pleistocene aridity are well documented in the thar desert but research focusing on humid proxies is critical to examine how the region may have facilitated population expansions across southern asia at katoati located on the northeast margin of the thar desert the combination of field recording of sediment sections with detailed analyses micromorphology stable isotope loss on ignition magnetic susceptibility and x ray fluorescence from an archaeological site identify a series of hominin occupations during phases of enhanced humidity between 96 and 60 ka a gradient of humidity on the eastern margin of the thar desert during the late pleistocene is identified with the periodic humidity evident at katoati occurring more frequently and with longer duration towards the southern margin this uneven distribution of humidity in the thar desert is likely to have strongly influenced when and where hominin populations could expand into and across the region

garnet chloritoid equilibria in eclogitic pelitic rocks from the sesia zone western alps their bearing on phase relations in high pressure metapelites abstract a detailed study of garnet chloritoid micaschists fom the sesia zone western alps is used to constrain phase relations in high pressure hp metapelitic rocks in addition to quartz phengite paragonite and rutile the micaschists display two distinct parageneses namely garnet chloritoid chlorite and garnet chloritoid kyanite talc has never been observed garnet and chloritoid are more magnesian when chlorite is present instead of kyanite the distinction of the two equilibria results from different bulk rock chemistries not from p t conditions or redox state estimated p t conditions for the eclogitic metamorphism are 550 600 c 15 18 kbar the presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible afm topologies for the sesia metapelites the paper describes a kfmash multisystem for hp pelitic rocks which extends the grid of harte hudson 1979 towards higher pressures and adds the phase talc observed parageneses in hp metapelites are consistent with predicted phase relations critical associations are gt ctd chl and gt ctd ky at relatively low temperatures and gl chl ky and gt tc ky at relatively high temperatures

the influence of matrix rheology and vorticity on fabric development of populations of rigid objects during plane strain deformation abstract the influence of vorticity and rheology of matrix material on the development of shape preferred orientation spo of populations of rigid objects was experimentally studied experiments in plane strain monoclinic flow were performed to model the fabric development of two populations of rectangular rigid objects with object aspect ratios rob 2 and 3 the density of the rigid object populations was 14 of the total area objects were dispersed in a newtonian and a non newtonian power law matrix material with a power law exponent n of 1 2 the kinematic vorticity number wn of the plane strain monoclinic flow was 1 0 8 and 0 6 with finite simple shear strain of 4 6 3 0 and 0 9 respectively in experiments with rob 3 the spo is strongly influenced by wn and the material properties of the matrix deformation of a power law matrix material and low wn resulted in a stronger spo than deformation of a linear viscous matrix and high wn strain localization coupled with particle interaction plays a significant role in the development of a shape preferred orientation high strain simple shear zones separate trains of rigid objects that are surrounded by low strain zones with wn lower than the bulk wn in fabrics involving populations of objects with rob 2 rheology of the matrix materials does not systematically influence the intensity of the spo

fluvial response to climate change inferred from sediment cores from the ghaggar hakra paleochannel in nw indo gangetic plains abstract in the indo gangetic plains igp the inter basinal area between the ganga in east and the indus in west is currently devoid of any major drainage a wide paleochannel belt the ghaggar hakra channel has been mapped in this region our previous studies along with others established that a large himalayan river flowed through this paleochannel and a large scale drainage reorganisation occurred during the last 10 ka this follow up study presents sedimentary facies and chrono stratigraphy of the alluvial deposits from the ghaggar hakra paleochannel 1 to understand the depositional environments of the river system 2 to reconstruct aggradation incision history and stratigraphic development and 3 to understand fluvial response to climate change at a regional scale sediment cores 35 50 m across the length and width of the paleochannel and optically stimulated luminescence osl based chronology are used to reconstruct alluvial stratigraphy developed during marine isotope stage mis 5 1 twelve sedimentary facies identified in the cores characterize floodplain channel and aeolian depositional settings four stratigraphic units were identified the oldest unit 1 150 ka representing aeolian deposition disconformably underlies the younger units 2 to 4 that represent four periods of fluvial aggradation and incision period i 86 64 ka comprises coarse grained grey micaceous channel sand and period ii 62 30 ka shows transition to fine to medium channel sand interlayered with yellow mud of floodplain deposition period iii 25 12 ka comprised of fine grained grey micaceous channel sand and very fine deposits of channel margin settings period iv 11 4 0 ka marks the termination of flow from the large himalayan river the sutlej into the ghaggar hakra system and fluvial sequence in this period red brown silty clay represents modern ghaggar deposits we suggest a strong climatic control on the stratigraphic development in this region based on the available proxy records we further compare our stratigraphic data with that of the igp and the adjoining western indian rivers for understanding the regional scale fluvial response to monsoonal fluctuations

compositional control on the polymorphism 2m 1 3t of phengitic white mica from high pressure parageneses of the sesia zone lower aosta valley western alps italy the polymorphism 2m1 or 3t modification of phengites coexisting with jadeite quartz k feldspar in gneisses of granitic composition from the internal part of the sesia zone correlates with their mg al fe mg ratio for values 0 015 the 2m1 modification for values 0 025 the 3t modification is formed for mg al fe mg values between these limits both polymorphs coexist as distinct grains less than 1 of a large number of separated grains show an intergrowth of both modifications in these cases the predominant phase boundaries are hko planes rational at least with respect to the 2m1 lattice boundaries parallel 001 occur as subordinate steps in hko boundaries extensive intergrowth parallel 001 is not observed as far as revealed by optical microscopy the stacking sequence once established in a nucleus is retained during growth of the crystal

multiple episodes of aggradation and calcrete formation in late quaternary aeolian sands central thar desert rajasthan india abstract a 12 m thick section in a dune sandy plain terrain of the central thar in rajasthan has provided a near continuous record of environmental change for the past 160 ka the site presently receives 280 mm rainfall almost entirely from the summer monsoon the base of this section comprises a gravel bed of an ephemeral stream and the overlying six litho units each with discrete boundaries comprise a succession of aeolian sands luminescence dating provided an estimate of the timing of the sand aggradation periods to 160 90 60 27 and 17 14 ka and helped constrain the timing of calcrete formation periods in each aggradation unit discrete nodular calcretes formed by the leaching of carbonate from the overlying solum this is analogous to present day conditions in sandy plains during periods of increased rainfall and landscape stability several of these calcretes are however devoid of their donor solum suggesting solum removal during a subsequent period of decreased rainfall and resultant surface instability this is supported by the presence of reworked nodules on the surfaces of some calcretes a prominent phase of calcrete development followed the aeolian sand aggradation at 60 ka suggesting climate amelioration that also caused the formation of groundwater related calcrete and mottling the study suggests that stage ii calcrete nodules form in a time frame of 10 20 ka and confirms limited data on the duration and stage of calcrete development in the literature the 13c values of calcrete carbonate lie in a narrow range 0 5 to 1 1 suggesting formation under soils with c4 vegetation this implies that even during phases of climatic amelioration the high temperatures and increased seasonality of rainfall did not permit significant development of c3 plants in the central thar

unrest of the udina volcano in kamchatka inferred from the analysis of seismicity and seismic tomography abstract the udina complex consisting of two stratovolcanoes bolshaya and malaya udinas is located at the southeastern edge of the klyuchevskoy volcano group kgv in kamchatka prior to 2017 it was considered to be extinct however since late 2017 continuing seismic activity beneath bolshaya udina has been recorded which may indicate the possible awakening of this volcano complex to perform a detailed investigation of this seismic process we installed four temporary seismic stations in the vicinity of the udinas0 volcanoes and operated them for two months from may to july 2018 using the data obtained from this network we detected and localized 559 local events in the area of the udina volcanoes furthermore we performed a tomographic inversion to improve the locations of the sources the tomography unexpectedly resulted in a stable model which was carefully verified via a series of different tests we found that the seismicity formed an elliptical cluster oriented in the north northeast south southwest direction beneath the volcano the seismicity occurred at depths of greater than 5 km from the surface in the tomography model we observed an anomaly of a high vp low vs and high vp vs ratio located directly beneath bolshaya udina these seismic properties may indicate the presence of magma intrusions with a high content of melts and fluids which may justify changing the current status of this volcano from extinct to active we also observed that the seismicity cluster connects bolshaya udina with the tolud zone where continuous seismicity has been observed at depths of approximately 10 20 km in previous years this zone is considered to be a magma storage in the lower crust that contributed to feeding the fissure eruptions in the tolbachinsky dol based on the results of this study we conclude that during 2018 the tolud magma source appeared to have built another pathway to bolshaya udina

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

stable isotopic composition of pedogenic carbonates from the eastern margin of the thar desert rajasthan india pedogenic carbonates in a 19 m thick dune section 16r at didwana and several shorter profiles from the eastern margin of the thar desert rajasthan were studied for their stable isotope composition carbon isotope results show the c4 biomass grasses has dominated local vegetation for most of the 250 000 years represented by the 16r section our results display a decrease in 18o and 13c values during the last interglacial and early glacial periods the higher 13c values in our dune records correlate to episodes in the strong upwelling and decreased sedimentation rates in indian ocean cores probably the result of a strong asian monsoon the high 13c values may therefore denote the expansion of c4 vegetation in response to warm wetter monsoon circulation conversely low 18o and 13c values mark the expansion of c3 vegetation probably caused by enhanced winter rains and lower temperatures

seismic structure changes beneath redoubt volcano during the 2009 eruption inferred from local earthquake tomography we present seismic velocity models of the area beneath redoubt volcano alaska corresponding to two time periods before and after the strong eruption that occurred in march 2009 the calculations were based on tomographic inversion of p and s arrival time data recorded by 19 stations using the local earthquake tomography code lotos we performed thorough analysis of the results based on synthetic tests in each of the time periods that allow real time variations in structure to be distinguished from artifacts caused by changes in the observation system configurations in the resulting images corresponding to the period before the eruption the summit area is characterized by higher values of both p and s velocities and moderate values of the vp vs ratio this may correspond to the rigid igneous rocks composing the body of the volcano that lack significant liquid content in the second episode corresponding to the time of the eruption the p velocity remained high whereas the s velocity became very low the anticorrelation of p and s anomalies in the summit area produced a vp vs ratio as high as 2 2 which is seen down to 2 3 km depth this indicates presence of mobile phase at shallow depth beneath the volcano which can be either in the form of partial melt or fluid saturated rocks

geochemical signatures of late holocene paleo hydrological changes from phulera and pokharan saline playas near the eastern and western margins of the thar desert india abstract stratigraphical mineralogical geochemical and optical dating methods were used to reconstruct paleo hydrological changes in two playas phulera 500 mm a and pokharan 200 mm a in near extremum climatic regions of the thar desert sediment successions in shallow profiles from phulera and pokharan contain three and four stratigraphic units respectively each with characteristic geochemical properties these units reflect changes in chemical weathering detrital input salinity and provide a measure of the changes in precipitation i e monsoon through time sediments from pokharan suggest short rainfall events during ca 6 6 4 ka relatively stable fresh water higher and persistent rainfall regime during 4 2 3 ka and a hyper saline low rainfall condition during 2 3 1 1 ka sediments at phulera record hyper saline low rainfall lacustrine conditions during 1 4 ka higher abundance of gypsum in pokharan 2 3 1 1 ka and proto dolomite in phulera 2 3 1 4 ka are nearly synchronous and reflect enhancement of salinity selenite crystals in pokharan and large desiccation cracks in buried horizons at phulera reflect desiccation of playas at ca 2 ka both playas progressively became less saline after 1 4 ka given the regional nature of this record these changes are attributed to fluctuation of the monsoon over the indian sub continent

climate and seismicity in the higher central himalaya during 20 10 ka evidence from the garbayang basin uttaranchal india abstract in the trans himalaya the trans himadri fault thf controlled the extent of past glaciers the rivers were impounded by glacial debris resulting in the formation of lakes that recorded regional climatic and seismic events one such lake formed at garbayang where the upper 28 m lacustrine succession spanned an age range of 20 10 ka magnetic susceptibility and sedimentological data suggest that the region experienced high amplitude low frequency millennial scale climate changes during the period 20 3 and 13 2 ka followed by low amplitude high frequency changes from 13 2 11 1 ka a sudden drop in magnetic susceptibility and elemental concentration of layers dated at 11 12 ka suggests cooling associated with the younger dryas occurrence of faulted strata seismites during 20 17 and 14 13 ka suggests recent activation of the thf this is noteworthy as the thf has been considered dormant since its last reactivation at 11 ma further the luminescence dates suggest that radiocarbon ages on organic carbon are grossly overestimated due to a hard water effect caused by the dominance of limestone in the lake catchment this implies that earlier radiocarbon based palaeoenvironmental reconstruction in the region will need revalidation

the three dimensional structure beneath the popocatepetl volcano mexico based on local earthquake seismic tomography abstract this paper presents a new seismic velocity model p and s wave velocities and vp vs ratio beneath the popocatepetl volcano to a depth of 4 km below sea level bsl this model is based on the tomographic inversion of p and s wave arrival time data from earthquakes in the region of the volcano these data were recorded by the 15 stations of a temporary seismic network that was deployed on the volcano in 1999 and 2000 the recording period was during a relatively quiet period between two strong eruptions which occurred before and after the experiment this period is characterized by low levels of volcano related seismicity most of the recorded events occurred across an area much larger than the network in this study we conducted several synthetic tests which validate the use of the out of network events to improve the resolution of the tomographic inversion beneath the stations in the resulting model we see that the main volcanic edifice is associated with high velocities that have a mushroom shape and that these high velocities are most prominent in the p wave model this feature may indicate the presence of overpressured solidified igneous rocks which comprise the edifice of popocatepetl below the summit of the volcano we observe a prominent high vp vs anomaly which reaches a value of 1 9 this anomaly probably indicates the existence of cracks and pores filled with melts and fluids and it may represent a fracture zone that serves as a conduit feeding the volcano this model characterizes the interior structure of the popocatepetl volcano prior to the strong september 2000 eruption which occurred a few months after the termination of the experiment

pb isotopic variability in the modern pleistocene indus river system measured by ion microprobe in detrital k feldspar grains abstract the western himalaya karakoram and tibet are known to be heterogeneous with regard to pb isotope compositions in k feldspars which allows this system to be used as a sediment provenance tool we used secondary ion mass spectrometry to measure the isotopic character of silt and sand sized grains from the modern sutlej and chenab rivers together with thar desert sands in order to constrain their origin the rivers show a clear himalayan provenance contrasting with grains from the indus suture zone but with overlap to known karakoram compositions the desert dunes commonly show 207 pb 204 pb and 206 pb 204 pb values that are much higher than those seen in the rivers most consistent with erosion from nanga parbat this implies at least some origin from the trunk indus probably reworked by summer monsoon winds from the sw a hypothesis supported by bulk nd and u pb zircon dating further data collected from holocene and pleistocene sands shows that filled and abandoned channels on the western edge of the thar desert were sourced from himalayan rivers before and at 6 8 ka but that after that time the proportion of high isotopic ratio grains rose indicating increased contribution from the thar desert dunes prior to 4 5 ka when flow ceased entirely this may be linked to climatic drying northward expansion of the thar desert or changes in drainage style including regional capture channel abandonment or active local thar tributaries our data further show a himalayan river channel east of the present indus close to the delta in the nara river valley during the middle holocene while this cannot be distinguished from the indus it is not heavily contaminated by reworking from the desert the pb system shows some use as a provenance tool but is not effective at demonstrating whether these nara sediments represent a ghaggar hakra stream independent from the indus our study highlights an important role for eolian reworking of floodplain sediments in arid rivers such as the indus

modulation and amplification of climatic changes in the northern hemisphere by the indian summer monsoon during the past 80 k y high frequency suborbital variations dansgaard oeschger cycles characterize the climatic history of the northern hemisphere as observed in greenland ice cores deep sea sediments of the north atlantic the californian borderland the arabian sea the south china sea and the chinese loess area paleoceanographic data from core kl126 from the bay of bengal in combination with data from the other asian monsoonal areas indicate that the feedback processes involving snow and dust of the tibetan plateau vary the summer monsoon capacity to transport moisture into central south asia and into the atmosphere we postulate that the summer monsoon initiates amplifies and terminates these cycles in the northern hemisphere

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

the initiation and development of metamorphic foliation in the otago schist part 2 evidence from quartz grain shape data shape size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the otago schist new zealand reveal the role of quartz in the progressive development of metamorphic foliation sedimentary compaction and diagenesis contributed little to the formation of a shape preferred orientation spo within the analysed samples metamorphic foliation was initiated at sub greenschist facies conditions as part of a composite s1 bedding structure parallel to the axial planes of tight to isoclinal f1 folds an important component of this foliation is a pronounced quartz spo that formed dominantly by the effect of dissolution precipitation creep on detrital grains in association with f1 strain with increasing grade the following trends are evident from the spo data i a progressive increase in the aspect ratio of grains in sections parallel to lineation and the development of blade shaped grains ii the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation lineation and the intermediate axes of the blades define a partial girdle about the lineation iii a slight thinning and reduction in volume of grains in the one transect and iv an actual increase in thickness and volume in the survivor grains of the second transect the highest grade samples within the chlorite zone of the greenschist facies record segregation into quartz and mica rich layers this segregation resulted largely from f2 crenulation and marks a key change in the distribution deformation and spo of the quartz grains the contribution of quartz spo to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment pressure solution now affects the margins of quartz rich layers rather than individual grains in higher grade samples therefore the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica spo

3d reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the alpine subduction system the example of mt mucrone sesia lanzo zone western alps geological mapping multiscale structural analysis and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3d quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the alpine subduction system and mainly structured under eclogite facies conditions before the continental collision the investigated and modelled rocks outcrop in the surroundings of mt mucrone in the central sesia lanzo zone the geomodeller software allowed a quantitative 3d estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress dfe and drp respectively for two of the seven structural and metamorphic imprints detected in this area the d2 eclogitic and d5 greenschist stages which are the most pervasive at km scale such a 3d modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that i dfe and drp are closely related regardless of the rock type ii the syn deformational thermal regime can influence the degree of metamorphic transformation if dfe remains below the 60 threshold iii the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution

quantitative texture analysis of glaucophanite deformed under eclogite facies conditions sesia lanzo zone western alps comparison between x ray and neutron diffraction analysis

magnetic fabric shape preferred orientation and regional strain in granitic rocks since granites do not preserve easily mappable foliations lineations and strain markers determining the degree of shape preferred orientation spo in them is challenging the aim of this paper is to present a methodology for determining variation in the degree of spo in granites and to test the feasibility of correlating the same with regional strain the case of the godhra granite located in the southern parts of aravalli mountain belt india is taken as an example degree of spo is determined using two different techniques a anisotropy of magnetic susceptibility ams and b strength of mineral lineation determined by calculating the concentration parameter k of von mises distribution by digital image analysis of biotite kbi and feldspar kf in thin sections prepared parallel to the magnetic foliation plane spo data obtained using the above two techniques from 20 samples distributed across the entire granite are analysed samples with a higher k also have a stronger magnetic fabric magnetic lineation l and degree of magnetic anisotropy p 0 also the degree of spo is greater for granites from the southern parts as compared to the northern parts it is argued that this variation in degree of spo cannot be attributed to rheological differences or strain resulting from difference in aspect ratios regional strain is inferred as the dominant factor since the emplacement of the godhra granite is synchronous with the tectonic rejuvenation along the central indian tectonic zone citz that lies to its south the higher degree of spo in the southern parts is attributed to its proximity to the citz whilst caution needs to be exercised to directly apply magnetic data as a measure of strain intensity on a regional scale it can be a useful guide to select samples for detailed spo analysis using some alternative technique especially for granites and other magmatic rocks that are devoid of mesoscopic strain markers 2006 elsevier ltd all rights reserved

the use of mohr circles to describe non coaxial progressive deformation abstract mohr circles are useful in strain analysis but are also potentially powerful tools to translate geological fabric data into tensors describing the deformation and to reconstruct flow parameters this paper shows how mohr circles can be used to visualize the effects of variable flow parameters such as stretching rate volume change rate and vorticity number on parameters of cumulative deformation as described by the position gradient tensor a mean vorticity number is introduced as a deformation parameter besides finite cumulative strain and volume change

spider texture and amphibole preferred orientations abstract foliation in blueschist facies chert from california is defined by layers of oriented alkali amphibole which consistently curve towards and converge on pyrite and possibly pyrrhotite crystals these foliation nodes not previously described are called here spider texture the texture is interpreted in terms of perturbations of the stress field in a matrix undergoing strain about rigid pyrite or pyrrhotite crystals and it has important implications for understanding the mechanisms of amphibole preferred orientation development geometrical relationships between spider texture pressure shadows and quartz preferred orientations suggest that amphiboles grew with a strong preferred orientation along planes of maximum shearing stress the mechanism of foliation and preferred orientation development probably involved competitive anisotropic growth of amphibole prisms within the small gaps that open at steps on shear planes followed by additional micro porphyroblastic growth the first stage of the mechanism is similar to slickenfibre development

the initiation and development of metamorphic foliation in the otago schist part 1 competitive oriented growth of white mica the 3d shape size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the otago schist new zealand reveal that metamorphic foliation as defined by mica shape preferred orientation spo developed rapidly at sub greenschist facies conditions early in the deformation history the onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition higher aspect ratios and a strong preferred orientation the metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation once initiated foliation progressively intensified by an increase in the aspect ratio size and alignment of grains although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in spo strength despite continued increase in grain size these trends are interpreted in terms of progressive competitive anisotropic growth of blade shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation the competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade from c 1000 relatively small mica grains per square millimetre of thin section at lower grades to c 100 relatively large grains per square millimetre in higher grade samples reversal of spo intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow

deformation microstructures of glaucophane and lawsonite in experimentally deformed blueschists implications for intermediate depth intraplate earthquakes a series of simple shear experiments on blueschist was performed at 400 500 c and 1 2 5 gpa to understand the deformation of seismically active subducting oceanic crust the experiments show that brittle microstructures are mainly found at pressures of 1 2 gpa whereas ductile microstructures form at 2 5 gpa j indices a measure of fabric intensity of glaucophane crystal preferred orientations change systematically with changing shear strain and confining pressure and the angle between the slip plane and the shear direction of samples deformed at 2 gpa is similar to that of a strain ellipsoid these results together with the variable orientations of fine grains in a selected area electron diffraction image at 2 gpa indicate that the brittle ductile transition for glaucophane occurs at 2 gpa in contrast to this lawsonite in the experiments show abundant fracturing in most specimens and a poor correlation between the j index shear strain and confining pressure this demonstrates that lawsonite deformed by brittle failure under all experimental conditions in the case of a starting material that has a strong fabric and deformed under dry experimental conditions the brittle ductile transition zone of glaucophane will be much shallower than 2 gpa therefore our initial experimental results on the deformation behavior of blueschist indirectly support the dehydration embrittlement of subducting oceanic crust glaucophane as an important factor in the origin of intraplate earthquakes