influence of sediment source on the shapes and surface textures of glacial quartz sand grains a study of the shapes and surface textures of glacial quartz sand grains that were largely derived from sediment sources of different rock types was conducted in order to evaluate the influence of the sediment source on these textures the results show that the roundness and surface textures of glacial quartz sand grains are largely dictated by their sources and that surface textures may not always be reliable indicators of ancient glacial deposits

carbon hydrogen oxygen and sulfur isotope and fluid inclusion study of the weolag tungsten molybdenum deposit republic of korea fluid histories of metamorphic and ore forming events molybdenite wolframite and scheelite were deposited at temperatures between 380 degrees and 200 degrees c from fluids with salinities between 3 7 and 11 1 equivalent wt percent nacl at pressures below 180 bars late stage carbonates and fluorite were deposited at temperatures between 190 degrees and 150 degrees c the delta 34 s values of sulfides within the mine have the following ranges and values molybdenite 3 0 to 5 7 per mil pyrite 4 5 to 7 6 per mil pyrrhotite 4 8 to 5 1 per mil chalcopyrite 4 8 per mil sphalerite 3 0 to 5 7 per mil and galena 3 9 per mil an early pervasive flow of water rich fluids which took place uniformly throughout the carbonate rocks during metamorphism is suggested for nonmineralized regions channeled flow within structurally controlled aquifers and fracture systems during metamorphism high water to rock ratios is suggested for mineralized regions later ore fluids followed these same restricted pathways while depositing ore minerals the geochemical and isotope data suggest that the overall hydrothermal system was characterized by low water to rock ratios but local areas with channeled fluid flow were characterized by high water to rock ratios modified journal abstract

a stable isotope study of serpentinization and metamorphism in the highland border suite scotland u k abstract d h and 18 o 16 o ratios have been measured for whole rock samples and mineral separates from the mafic and ultramatic rocks of the cambro ordovician highland border suite the h and o isotopic compositions of these rocks record individual stages in a relatively complex 500 myr old hydrothermal metamorphic history lizardite serpentinites d 105 18 o 6 2 record a premetamorphic history and indicate that parent harzburgites dunites and pyroxenites were serpentinized through low temperature interaction with meteoric waters during cooling the other rocks of the highland border suite record subsequent interaction with metamorphic fluids amphibolite facies hornblende schists were produced through thrust related dynamothermal metamorphism of spilitic pillow lavas during dehydration d enriched fluids were driven off from the spilites thus leaving the hornblende schists to equilibrate with a relatively d depleted internal fluid reservoir d 45 the expelled d enriched fluids may have mixed with more typical dalradian metamorphic waters which then exchanged with the remaining mafic rocks and lizardite serpentinites during greenschist facies regional metamorphism to produce antigorite serpentinites d 62 18 o 8 and greenschist metaspilites d 57 18 o 7 3 with similar h and o isotopic compositions serpentinites which have been only partially metamorphosed show intermediate h isotopic compositions between that of metamorphic antigorite d 62 and non metamorphic lizardite d 105 end members

characterization of glacial silt and clay using automated mineralogy glacial erosion produces vast quantities of fine grained sediment that has a far reaching impact on earth surface processes to gain a better understanding of the production of glacial silt and clay we use automated mineralogy to quantify the microstructure and mineralogy of rock and sediment samples from 20 basins in the st elias mountains yukon canada sediments were collected from proglacial streams while rock samples were collected from ice marginal outcrops and fragmented using electrical pulse disaggregation for both rock fragments and sediments we observe a log normal distribution of grain sizes and a sub micrometer terminal grain size we find that the abrasion of silt and clay results in both rounding and the exploitation of through going fractures the abundance of inter versus intragranular fractures depends on mineralogy and size unlike the relatively larger grains where crushing and abrasion are thought to exploit and produce discrete populations of grain sizes the comminution of fines leads to a grain size composition and rounding that is continuously distributed across size and highly dependent on source rock properties

petrology of glacial sand implications for the origin and mechanical durability of lithic fragments the petrology of first cycle unmodified and second cycle reworked sand at the termini of eleven valley glaciers eroding coarse to fine grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands first cycle sands are coarse to medium grained poorly sorted fine skewed non modal lithic sands with an average composition of q21f6l73 grain size distributions do not vary with composition or source rock types although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources by contrast second cycle sands are medium grained poorly sorted fine to coarse skewed arkosic to lithic sands with an average composition of q19f40l41 and contain fewer lithic fragments than do first cycle sands we propose that the origin mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness and the particular stress field of the transporting medium as a result of abrasion glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point loading during saltation in addition to shear stress point loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness so the lithic fragments break into smaller monomineralic grains and or lithic fragments lithic fragments of very coarse sand size are abundant in first cycle sands thus refuting at least for glacial sands the widely held view that grains of this size are deficient in nature the presence of fewer grains of this size in second cycle sands indicates that very coarse sand size lithic fragments can survive recycling but in greatly reduced proportions due to breakage

scanning electron micrographs of quartz sand grains from cold environments examined using fourier shape analysis abstract the use of grain shape to identify sedimentary environments assumes that grain morphology reflects environmental history scanning electron microscopy permits detailed examination of quartz sand grain surface morphology particle shape from gross form to surface texture is quantified using closed form fourier analysis of whole grain outlines from micrographs of 158 quartz sand grains representing subglacial transport and subaerial weathering in cold environments mean harmonic amplitude spectra and roughness coefficients are calculated and three problems are investigated yielding the following results findings are preliminary due to the relatively small number of grains analyzed 1 two dimensional particle micrographs are a suitable surrogate for three dimensional grain roundness a d texture 2 the particle size of a sand grain sample affects its roundness and texture and only samples of similar size range should be compared 3 fourier shape analysis distinguishes significant differences between basal and englacial grains the latter being more angular and rough in texture this finding is relevant to the definition of debris transport paths through glaciers the technique may also prove useful in the interpretation of marine sediments from cold regions and pre quaternary diamictites of controversial or unknown origin

ground motion prediction equation for small to moderate events at short hypocentral distances with application to induced seismicity hazards abstract the evaluation of seismic hazard from induced seismicity requires the development of ground motion prediction equations gmpes that are tuned to the key magnitude distance range for such applications i use events of m 3 6 at hypocentral distances less than 40 km drawn from the next generation attenuation west 2 nga west 2 database to develop a gmpe that accounts correctly for point source scaling in both magnitude and distance space for such events the developed gmpe is in demonstrable agreement with the nga west 2 database and with the predictions of a stochastic point source simulation model the database is sparse at close distances implying epistemic uncertainty of as much as a factor of 2 in ground motion amplitudes within 10 km of the hypocenter an important conclusion from this study is that the ground motion amplitudes for moderate induced events could be much larger near the epicenter than predicted by most of the nga west 2 gmpes the potential for large motions is a consequence of the shallow depth of induced events which places the earthquake fault only a short distance beneath the epicenter

microfracturing paleostress and the growth of faults abstract we have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip we have also recorded the orientation of microfractures as a function of distance from the associated fault surface in order to control as many parameters as possible shallow faults with similar lithologies were chosen they showed the same kind of deformation namely microfracturing without block rotation recrystallization extensive granulation or crystal plasticity the results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip we interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip the angular relationship between the shear plane and microfractures was commonly found to be between 5 and 20 significantly less than the 30 expected between the far field maximum principal stress and the shear surface this low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode ii fault tip other possible explanations for the small angle for tensional microfractures and the shear surface are 1 the effective confining pressure was very low or 2 shear strength at failure was much larger than expected

lineaments and basement domains an alternative view of dalradian evolution this paper examines the evidence for basement structures of predominantly non caledonoid trend which appear to have influenced the development of the dalradian belt in scotland a number of domains or blocks separated by dominantly trans caledonoid lineaments are recognized on the basis of geophysical stratigraphical and geochemical patterns these are thought to reflect fault bounded depositional basins controlled by basement lineaments during the extensional phases of the caledonian 9orogeny9 in addition the recognition of a number of these domains and lineaments in the structural metamorphic and igneous data sets for the dalradian belt shows the continuing influence of these basement controlled features throughout the compressional and uplift phases of the orogenesis a possible tectonic model is proposed which envisages the depositional basins forming in a setting dominated by caledonoid trending transcurrent faults and a major pull apart basin forming on a trans caledonoid axis

pore water controlled grain fracturing as an indicator for subglacial shearing in tills genetic classification of glacial deposits still proves to be highly controversial in many cases it remains unclear whether a particular sediment has been sub glacially deformed in the present paper it is suggested that micromorphological research is helpful where current techniques fail to lead to unambiguous interpretations it is argued that the occurrence of fractured grains in glacial sediments is indicative of subglacial shearing and that such grains may be used as diagnostic evidence of glaciotectonism deformational mechanisms associated with the fracturing process are outlined and explained using a series of thin sections from wijnjewoude the netherlands

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

fractal dimensions of diamictic particle size distributions simulations and evaluation it has been proposed that deformation tills have fractal particle size distributions reflecting scale invariant debris comminution processes numerical simulations however show that fractal dimensions m typical of deformation tills can be produced by the mixing of gaussian parent populations analogous to the mechanical mixing of coarse medium and fine grained sorted sediments the simulated distributions have high r2 values 0 98 and narrow 95 confidence limits indicating that fractal analysis cannot distinguish diamictons that have been created by shearing from those formed by other mechanisms such as the mixing of preexisting sediments the fractal dimensions of diamictons reflect the relative amounts of coarse and fine material in the distribution but are not uniquely diagnostic of particular genetic processes

role of grain boundaries and micro defects on the mechanical response of a crystalline rock at multiscale the paper examines the cracks initiation propagation and coalescence until failure in a crystalline rock under stress states involving rock volumes varying in size from nano berkovich indentation in continuous stiffness measurement mode to micro vickers indentation scratching and macro rockwell indentation rock impact hardness number scales the experimental work was performed on a calcitic marble characterized by two different textures xenoblastic and granoblastic the grain boundaries microcracks were deeply studied with focused ion beam scanning electron microscopy technique innovative approach in rock mechanics the experimental results show that at the three investigation scales the microcracks give rise to the rock failure with fractures propagation mechanisms varying with the microstructure in the granoblastic texture fracturing is ruled by grain boundaries microcracks which often appear almost open conversely the fracture behavior of the xenoblastic marble seems dominated by the grains cleavage even at the macroscale

correlations of suspended sediment size with bedrock lithology and glacier dynamics the hypothesized link between glacier surging and bedrock geology motivates this study of the suspended sediment size distributions sssd from surge type and non surge type glaciers we analyze sssds from proglacial streams in 20 individual basins comprising various fractions of metasedimentary ms and felsic plutonic rocks we compare the size distributions by performing tests of significance on the distribution statistics and a principal component analysis on discrete grain sizes we find that surge type and non surge type glaciers underlain solely by ms rocks have significantly different sssds while surge type glaciers as a whole have remarkably similar sssds regardless of the underlying bedrock geology these observations hint at a relationship between sediment characteristics and glacier surging though causation in either direction cannot be established without additional data

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

silt 2 62 μm 9 4φ the particles in finer grained detrital sediments are usually composed of quartz they fall into two size grades sand 2 mm 62 m and silt 62 m 2 m these size gradings conceal important geological processes since there are geological controls on both the quartz sand and silt populations sand nature is largely controlled by geochemical reactions for example in cooling granite from which it is eventually released by weathering action the quartz is among the last part of the rock system to solidify and the eutectic like reaction which occurs ensures that the quartz exists as small crystalline units each suffering considerable cooling and recrystallisation stresses silt is broken quartz and has traditionally been considered to lack a specific geological control the situation is confused by the large range of materials that fall into the size category but there do seem to be distinguishable modes and possibly comminution limits within the 60 m span the controls operating in this size range are probably the critical concentration of moss defects in the quartz particles moss postulated the formation of specific crystal defects in the quartz formed in granites these affect sand formation and it is possible that they also control the mode size of silt particles within the silt range there may be several usefully definable populations as moss proposed for sand the quaternary appears to be a silt rich period due to tectonic and glacial activity but silt production is apparent throughout the sedimentary record very long term silt producing processes are required to produce silt in nature on a large scale very energetic processes are required many processes that are believed to generate silt particles have been listed however large scale production is essentially due to glacial grinding or to intense weathering processes in high cold tectonically active mountain regions the region of high asia of crustal overlap is a major generator of silt particles these form the productive alluvial soils of north india most of the nation of bangladesh the loess deposits along the syr darya and amu darya rivers in central asia and the great loess deposits of north china some claims have been made for silt production in hot deserts large amounts of very fine aerosolic dust are produced but in terms of loess sized particles this is a small scale process and leads to modest disputed deposits the loess deposits around the sahara usually have smaller mode sizes e g nigeria or larger mode sizes e g libya tunisia than the true chinese mode at 25 m this paper aims to introduce the distinguishing properties of sand and silt discuss in detail the possible processes of silt formation in relation to the evidence of what is produced describe the major silt populations and conclude on the most likely formation process es in particular three silt ranges are discussed in relation to the r size diagram c 2 5 m d1 20 30 m and d2 60 m these can represent canadian quickclay qq chinese loess and the ideal loughborough loess ll and north african loess nal it is intended that this paper will put into context the diverse work in the area and in so doing shed light on what is an often overlooked subject

controls on the mechanisms of fluid infiltration and front advection during regional metamorphism a stable isotope and textural study of retrograde dalradian rocks of the sw scottish highlands vein controlled retrograde infiltration of h2o co2 fluids into dalradian epidote amphibolite facies rocks of the sw scottish highlands under greenschist facies conditions resulted in alteration of calcite rich marble bands to dolomite and spatially associated 18o enrichment of about 10 on a scale of metres fluid inclusion data indicate that the retrograde fluid was an h2o salt mixture with a low co2 content and that the temperature of the fluid was about 400d c detailed petrographic and textural backscattered electron imaging studies at one garnet grade locality show that advection of fluid into marbles proceeded by a calcite calcite grain edge flow mechanism while alteration of non carbonate wall rock is associated with veinlets and microcracks stable isotopic analysis of carbonates from marble bands provides evidence for advection of isotopic fronts through carbonate wall rocks perpendicular to dolomite veins and fluid fluxes in the range 2 4 28 6 m3 m2 have been computed from measured advection distances coincidence of isotope and reaction fronts is considered to result from reaction enhanced kinetics of isotope exchange at the reaction front front advection distances are related to the proportion of calcite to quartz in each marble band with the largest advection distance occurring in nearly pure calcite matrix this relationship indicates that fluid flow in carbonates is only possible along fluid calcite calcite grain edges however experimental constraints on dihedral angles in calcite fluid systems require that pervasive infiltration occurred in response to calcite dissolution initiated at calcite calcite grain junctions rather than to an open calcite pore geometry the regional extent of the retrograde infiltration event has been documented from the high 18o of dolomite ankerite carbonates from veins and host rocks over an area of least 50 50 km in the sw scottish highlands isotopically exotic 18o rich retrograde fluids have moved rapidly upwards through the crust inducing isotopic exchange and mineral reaction in wall rocks only where lithology pore geometry or mineral solubilities pressure and temperature have been appropriate for pervasive infiltration to occur

silica mobility and fluid movement during metamorphism of the connemara schists ireland quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of connemara and attest to considerable migration of silica contrary to common assumptions there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower p t down the regional geothermal gradient under amphibolite facies conditions a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated while cooling over the temperature interval from 650 to 500 c the absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through flowing externally derived fluids the oxygen isotopic composition of matrix quartz shows considerable differences between different rock types quartzite pelite and marble with a range of 18osmow from c 11 5 quartzite to 18 5 marble in each rock type vein quartz compositions closely match those of the matrix quartz these results demonstrate the importance of local segregation processes in the formation of veins and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations if at all both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies with only a relatively minor component of through flow of externally derived fluid veins are clearly not the major pathways of metamorphic dewatering it is proposed that abundant veins in the predominantly pelitic ballynakill formation formed during peak metamorphic d3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins while irreversible loss through the rest of the succession was a much less important process in the central mountains of connemara rather pure unreactive quartzites are cut by widely spaced laterally extensive quartz veins that are axial planar to d3 folds these veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile but here too oxygen isotope data indicate considerable segregation of locally derived quartz reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow

oxygen and carbon isotopic composition of ordovician brachiopods implications for coeval seawater one hundred and two articulate brachiopods and twenty one marine cements from ordovician strata were analyzed for oxygen and carbon isotopes these together with 200 previously published analyses of ordovician brachiopods mostly of upper ordovician age define a general trend of increasing 18o values with decreasing age from a minimum of 10 5 in the earliest ordovician to a maximum of 1 5 at the ordovician silurian boundary the 13c values show a general positive correlation with 18o and increase from 2 5 in the earliest ordovician to as high as 7 at the ordovician silurian boundary the magnitudes of ordovician excursions for both 18o and 13c are similar to those reported previously for the entire phanerozoic different genera of brachiopods from the same stratigraphie level did not yield any large differences in their isotopic compositions suggesting that vital effects did not influence decisively the isotope values of the studied skeletal parts petrographic examination and trace element analyses indicate that brachiopod shells from llandeilo to ashgill strata are well preserved and thus may have retained their primary isotopic signatures the same criteria for the tremadoc arenig and some llanvirn brachiopods suggest partial diagenetic alteration of 18o values in these samples but this does not preclude their utility as a recorder of a primary 13c signal the observed 18o trend for the well preserved portion of the record may reflect progressive cooling during ordovician perhaps complemented by a changing 18o values of seawater the latter could have been a consequence of a net increase in the rate of high temperature water rock interaction relative to its low temperature counterpart as indicated also by the large coeval decrease of marine 87sr86sr superimposed on the above long term 18o trend is a positive excursion of about 2 at the end of the ordovician probably reflecting a large expansion of polar ice caps the magnitude of this excursion indicates a degree of glaciation comparable to that at the height of the quaternary glacial episodes the secular 13c trend may reflect a progressive increase in marine organic productivity and or enhanced organic deposition in the ordovician oceans particularly noticeable at the time of the terminal ordovician glaciation

microstructural control of quartz sand grain shape and texture implications for the discrimination of debris transport pathways through glaciers abstract previous studies of the surface textures of glacially transported quartz sand grains have rarely distinguished between the influence which quartz grain microstructure and transport history independently exert on the appearance of grains in this paper we identify two distinct categories of quartz sand grain both of which can be found in subglacial and supraglacial debris translucent grains are derived from monocrystalline quartz which is largely free of dislocation structures while opaque grains are derived from polycrystalline quartz which displays a wide range of dislocation structures whilst subglacially derived grains show no differences in terms of their angularity fourier shape or surface texture from supraglacially derived grains of the same category translucent grains from both environments are consistently more angular and less rough than opaque grains and their surfaces are delimited primarily by planar fractures rather than blocky surfaces these differences in the appearance of translucent and opaque grains appear to reflect the manner in which the process of brittle fracture operates on undeformed and deformed quartz microstructure may therefore exert a greater influence on the appearance of glacially transported quartz sand grains than their specific transport history

retrogression of staurolite schists and the sources of infiltrating fluids during metamorphism staurolite kyanite schists from the dalradian of scotland and ireland show two types of retrograde alteration in the irish examples staurolite and kyanite are replaced by muscovite and paragonite often with margarite and sometimes chloritoid plagioclase in these samples may be albitized in contrast one scottish sample shows replacement of both staurolite and plagioclase by muscovite retrogression resulted from infiltration of fluid but while in some samples the large number of retrograde phases internally buffered the fluid composition and cation metasomatism cannot be proven in others the fluid composition was externally controlled and there was cation metasomatism indicating more extensive infiltration it is demonstrated by the use of activity diagrams showing the relationships between al silicate white mica and alkali feldspar that wholesale alteration to muscovite was most likely caused by fluids moving down temperature from a granitic or arkosic source in contrast if growth of retrograde albite is actually accompanied by na metasomatism then the infiltrating fluids are likely to have been moving up temperature through pelitic lithologies large fluid rock ratios of around 10 1 are needed to achieve significant alkali metasomatism except in extreme situations of large temperature changes and highly concentrated fluids

an investigation of subglacial processes at the microscale from briksdalsbreen norway the subglacial processes at briksdalsbreen norway are examined by a combination of sedimentology thin section and scanning electron microscope sem analysis of till samples from an exposed subglacial surface and from beneath the glacier studies of a fluted surface indicate that subglacial deformation is occurring on a field scale with flutes forming behind most clasts 0 6 m high at the thin section scale 0 014 2 0 mm it is seen that deformation is by rotation and attenuation and is dependent on till texture at the sem scale 0 1 0 4 mm it is seen that erosion is controlled by abrasion and percussion which produces distinct grain styles as part of an erosional continuum overall it is shown that rotation and attenuation is a dominant process at all scales and that the clast interactions associated with different scale perturbations within the shear zone control erosion and deposition as well as landform and fabric production

an oxygen and carbon isotopic study of multiple episodes of fluid flow in the dalradian and highland border complex stonehaven scotland the carbon and oxygen systematics of rocks north of stonehaven scotland were studied to provide new constraints on the nature and timing of metamorphic and post metamorphic fluid infiltration carbon and oxygen isotopic data were collected from carbonated spilites and carbonate bearing veins in the highland boundary fault from dalradian metacarbonate layers and pervasively carbonated schists and from three generations of carbonate bearing veins and a quartz porphyry dyke within the dalradian isotopic evidence for syn metamorphic fluid infiltration in the dalradian is preserved in quartz calcite veins in the upper chlorite zone and in staurolite zone metacarbonates 18 o values for coexisting quartz 18 o quartz 13 6 and calcite 18 o calcite 11 6 in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c 375 c oxygen isotopic data from staurolite zone metacarbonate layers 18 o calcite 10 4 to 12 9 suggest major isotopic exchange with an external fluid equilibrated with a quartzo feldspathic reservoir such as the dalradian metasediments at metamorphic temperatures low negative carbon isotope data from these metacarbonate layers 13 c calcite 12 4 to 17 7 may indicate a component of oxidized organic material in the infiltrating fluid post metamorphic fluids derived from or equilibrated with a high temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post metamorphic vein carbonate carbonation of pelitic schists and alteration of metamorphic feldspar porphyroblasts from barrow s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow quartz from this generation of veins preserves a metamorphic isotopic signature 18 o quartz 11 2 to 13 2 whereas dolomite and calcite are considered to be post metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals 18 o dolomite 23 7 to 24 2 18 o calcite 25 3 18 o quartz 11 2 to 13 2 variably depleted carbon isotopes from carbonate minerals in these veins 13 c dolomite 8 1 to 13 4 13 c calcite 12 2 may reflect input of oxidized organic material in the mineralizing fluid isotopic data from carbonate veins and variably carbonated spilites from the highland boundary fault are consistent with hydrothermal alteration of basalts by seawater which had its carbon isotope systematics modified by addition of magmatic co 2 and or oxidation of organic carbon such alteration may have occurred on the seafloor or during emplacement in the highland boundary fault

thermochronology in southeast alaska and southwest yukon implications for north american plate response to terrane accretion abstract this study presents the first comprehensive dataset of low temperature thermochronology from 43 bedrock samples collected north of the active yakutat north american plate boundary our apatite and zircon u th he and fission track data reveal the cooling history of the inboard wrangellia composite terrane that is dominated by rapid cooling after late jurassic to early cretaceous arc magmatism followed by very little cooling and exhumation until today deformation resulting in rock exhumation due to the collision of the yakutat microplate is spatially very limited 20 30 km and is concentrated mainly in the chugach prince william terrane and rocks near the border ranges fault focused exhumation from greater depths of ca 10 km with very high rates 5 km myr is localized at the syntaxis region starting ca 10 ma and shifted south through time the rapid exhumation rates are explained by the development of strong feedbacks between tectonically driven surface uplift and erosion which started already before glaciation of the area the shift in the location towards the south is a consequence of continuous readjusting between tectonics and climate which is changing on local and global scales since the late miocene

subglacial till formation microscale processes within the subglacial shear zone abstract this was a study of subglacial deformation till genesis from a modern temperate glacier at skalafellsjokull iceland detailed microscale properties of till samples from scanning electron microscope sem and thin section analysis were examined from a glacial site with in situ subglacial process monitoring and an exposed subglacial surface in the foreland two lithofacies were examined a grey sandy till derived from the ash and basalt and a silty reddish brown till derived from oxidized paleosols and or tephra layers these also represented a clay content continuum from low 0 3 to high 22 3 the evolution from debris to subglacial till was investigated this included a reduction in grain size 21 for grey lithology 13 reddish brown lithology and reduction in rounding ra 32 for the grey lithology 26 for the reddish brown lithology and the quantification and analysis of the different grain erosion comminution processes in the resultant till it was shown that the microstructures within a till were dependent on shear strain and glaciological conditions deformation history the low clay content tills were dominated by linear structures lineations and boudins and anisotropic microfabric whilst the higher clay content tills were dominated by rotational structures turbates and plaster and isotropic microfabric these results are important in our understanding of the formation of both modern and quaternary tills and informs our reconstruction of past glacial dynamics

subglacial comminution in the deforming bed inferences from sem analysis this study investigates the variations in erosional processes beneath briksdalsbreen a norwegian valley glacier with a thin coarse grained deforming bed the subglacial zone was investigated in situ close to the central glacial flow line site a and close to the valley wall site b via boreholes the till was collected using two different sediment samplers and the bed was examined using a borehole video camera in order to evaluate how representative borehole sampling was of the subglacial environment the results from in situ subglacial samples were compared with random samples taken from an exposed subglacial surface in the glacier foreland as well as a sample taken from the site after glacier retreat the results indicated that the texture of the exposed subglacial surface was similar to in situ samples from subglacial site b close to the valley wall and that the samples collected with the small sediment sampler had the closest fit sem analysis revealed eroded quartz grains at both sites as a result of rotation abrasion and fracture the samples from close to the valley wall were very poorly sorted with evidence for rounding and preservation of pre weathered surfaces which suggest that a relatively simple pattern of erosion from crushing to rotation dominated however the till from the centre of the glacier was better sorted more angular and with few pre weathered surfaces we suggest that this results from a more complex glacial erosional history associated with greater strain and longer residence time within the deforming layer when compared with other deformation tills it was confirmed that there is a relationship between grain size and erosional style clay rich tills show little comminution fine sand rich tills generate a significant silt component whilst the coarse sand rich tills at briksdalsbreen showed complex deformational styles but no significant silt component

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

mechanics of grain size reduction in fault zones 1 recent observations of nanometer scale particles in the cores of exhumed fault zones raise questions about how such small particles are formed and how they survive especially if significant shear heating is produced during an earthquake commercial crushing and grinding operations encounter a grind limit near 1 mm below which particles deform plastically rather than fracturing a fragmentation model and low temperature plasticity mechanics indicate that it is not possible to produce under compressive loading and short timescale significantly smaller particles at any strain rate however shock loading and subcritical crack growth can produce nanometer scale fragments in compression under tensile loading the fragment size is determined by a competition between the nucleation of cracks and stress relaxation in their neighborhoods therefore higher tensile strain rates produce smaller fragments the ultimate limit is determined by the availability of elastic strain energy which does not place a significant constraint on the minimum grain size grain growth kinetics suggests that survivability of grains is very temperature sensitive a 10 nm quartz fragment will double its size in 0 1 s at 1000c in 20 s at 800c in 14 h at 600c and in 10 years at 400c the observation of grains smaller than 10 nm places meaningful constraints on the dynamic fields and permeability of the fault zone during a large earthquake microstructural analysis of the grains and rock damage may be used to infer whether fragmentation occurred under macroscopic tension or compression

a hypothesis on the relative roles of crushing and abrasion in the mechanical genesis of a glacial sediment relatively new techniques of optical microscopy and interferometry have been used combined with fractal and modal analyses to study possible deformation mechanisms in glacial sediment from langjokull iceland previous studies have shown that the langjokull till has reached an ultimate grading during deposition and transport the results presented here add information on the deformation mechanisms that it may have suffered until and after reaching this ultimate grading the modal and fractal analyses suggest high mechanical weathering this is supported by the shape and texture of particles determined using the different imaging techniques of laser scanning optical microscopy combined with three dimensional image construction interferometry and scanning electron microscopy the larger particles have highly weathered surfaces indicating severe abrasion to their surface during transport the finer particles show less abrasion stamp and higher angularity they also bear evidence of relatively fresh fractures this seems to indicate that as the sediment became better distributed during transport breakage moved from the larger to the smaller particles with the larger grains only suffering abrasion this supports the hypothesis of fractal breakage in this type of soil

indentor corner tectonics in the yakutat st elias collision constrained by gps the yakutat st elias collision in se alaska and adjacent canada represents a prime example of present day tectonics associated with an indentor corner its eastern syntaxis is marked by high exhumation a sharp structural bend and strain concentration at the transition from shortening to oblique transpression here we present gps velocity and strain rate fields that cover the syntaxis including 11 new stations in the core of the st elias mountains these data are corrected for transient deformation glacial isostatic adjustment and postseismic and interseismic loading to produce residual velocities and strain rates representative of long term tectonics the main features of these velocity and strain rate fields are a peak in strain rates strain knot in the syntaxis at the junction between the main fault systems and a rapid rotation from convergence parallel to convergence normal orientations of the velocities and shortening axes around the syntaxis leading to shortening across the southern denali fault these features are consistent with the strain and tectonic patterns expected near an indentor corner at the transition from shortening to transpression with a combination of diffuse and localized deformation the gps velocities and strain rates show diffuse deformation around the syntaxis from pure convergence parallel shortening in the orogenic wedge to oblique extension that accommodates the strain rotation at the front of the syntaxis this indentor corner model also results in a near zero strike slip rate on the southern denali fault and shows no clear evidence for a throughgoing fault hypothesized to link the fairweather and totschunda faults

micromechanics of quartz sand breakage in a fractal context from a quaternary science perspective sand sized quartz as well as silt sized quartz is often acknowledged as final products of glacial abrasion through different evolution mechanisms this view challenges the existence of any universal comminution process which may relate the formation of detrital quartz sand and silt the contribution of grain size energy input and crystalline integrity in the scale of quartz crushability has long been the matter of much debate the present empirical work examines the micromechanics of sand to silt size reduction in the quartz material a series of grinding experiments was performed on leighton buzzard lower greensand using a high energy agate disc mill analogous conditions to glacial abrasion are provided due to the combined abrasion between grains asperity tips and also between grains and rotating smooth tungsten carbide pestle simulation of discontinuous grain breakage allowed the examination of grains crystalline defects to enable an objective assessment of micromechanics of size reduction measurements of particle and mode size distribution fractal indexes and micro morphological signatures were made the crushing approach was probed through varied grinding times at a constant energy input as well as varied energy inputs at constant grinding time breakage pathway was inspected via laser diffraction spectroscopy and transmission light microscopy results suggested that the grain breakdown is not necessarily an energy dependent process non crystallographically pure amorphous quartz sand and silt are inherently breakable materials through a fractal breakdown process results also revealed that the internal defects in quartz are independent from size and energy input

regional fluid and metal mobility in the dalradian metamorphic belt southern grampian highlands scotland a prominent set of veins was formed during post metamorphic deformation of the caledonian dalradian metamorphic belt these veins are concentrated in dilational zones in fold hinges but apophyses follow schistosity and fold axial surface fractures the veins are most common in the cores of regional structures especially the dalradian downbend and consist of quartz calcite chlorite and metallic sulphides and oxides metals including gold have been concentrated in the veins the fluid which formed the veins was low salinity 1 5 wt nacl and kcl co2 bearing 3 16 wt co2 water of metamorphic origin the fluid varies slightly in composition within and between samples but is essentially uniform in composition over several hundred km2 vein formation occurred at about 350 50 c and 200 300 mpa pressure further quartz mineralization occurred in some dilational zones at lower temperatures 160 180 c this later mineralization was accompanied by co2 immiscibility dilution and oxidation of the metamorphic fluid occurred due to mixing with meteoric water as the rocks passed through the brittle ductile transition a similar metamorphic fluid is thought to have been responsible for gold mineralization in the nearby tyndrum fault at a later stage in the dalradian uplift

particle size and mineralogical composition of inorganic colloids in glacier melting water and overlying ice in an alpine glacier oberaargletscher switzerland

architecture kinematics and exhumation of a convergent orogenic wedge a thermochronological investigation of tectonic climatic interactions within the central st elias orogen alaska abstract the kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate but little research has been devoted to the long term effects of glacial erosion on orogenesis here we use low temperature apatite and zircon u th he and fission track thermochronometry along with subsidiary structural relationships and seismicity to develop a new architectural model of the st elias orogen in southern alaska which is one of the best examples of a glaciated orogenic wedge worldwide these data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding thin skinned fold and thrust belt on the windward flank a structure beneath the bagley ice field separates these distinct deformational domains which we propose is a backthrust that makes the orogen doubly vergent thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion long term exhumation at rates of up to 4 mm yr is concentrated within a narrow zone along the windward flank where glacier equilibrium lines intersect the orogenic wedge the onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge accelerated backthrust motion and a major shift in deformation away from the north american yakutat terrane suture chugach st elias fault we propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation which in turn are systematically truncated by the backthrust this focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially dominated orogen

composite suspended sediment particles and flocculation in glacial meltwaters preliminary evidence from alpine and himalayan basins research over the last decade has shown that the suspended sediment loads of many rivers are dominated by composite particles these particles are also known as aggregates or flocs and are commonly made up of constituent mineral particles which evidence a wide range of grain sizes and organic matter the resulting in situ or effective particle size characteristics of fluvial suspended sediment exert a major control on all processes of entrainment transport and deposition the significance of composite suspended sediment particles in glacial meltwater streams has however not been established existing data on the particle size characteristics of suspended sediment in glacial meltwaters relate to the dispersed mineral fraction absolute particle size which for certain size fractions may bear little relationship to the effective or in situ distribution existing understanding of composite particle formation within freshwater environments would suggest that in stream flocculation processes do not take place in glacial meltwater systems because of the absence of organic binding agents however we report preliminary scanning electron microscopy data for one alpine and two himalayan glaciers that show composite particles are present in the suspended sediment load of the meltwater system the genesis and structure of these composite particles and their constituent grain size characteristics are discussed we present evidence for the existence of both aggregates or composite particles whose features are largely inherited from source materials and flocs which represent composite particles produced by in stream flocculation processes in the absence of organic materials the latter may result solely from electrochemical flocculation in the meltwater sediment system this type of floc formation has not been reported previously in the freshwater fluvial environment further work is needed to test the wider significance of these data and to investigate the effective particle size characteristics of suspended sediment associated with high concentration outburst events such events make a major contribution to suspended sediment fluxes in meltwater streams and may provide conditions that are conducive to composite particle formation by flocculation copyright 2002 john wiley sons ltd

genesis and mobility of the h2o co2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism a petrological and stable isotope study in the scottish dalradian during high pressure low temperature greenschist and epidote amphibolite facies metamorphism in dalradian rocks of the sw scottish highlands mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids consequently mineral assemblages capable of buffering the fluid phase composition were rare equilibrium prograde h 2 o co 2 fluids usually contained less or much less than about 1 2 mol co 2 three fluid infiltration events are recognized during prograde greenschist facies metamorphism metabasic sills were infiltrated by large volumes of co 2 bearing hydrous fluid carbon isotope studies indicate that the co 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments this may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f o 2 initiated by thermal expansion of water during heating decompression and consequent hydraulic fracturing in the epidote amphibole facies garnet zone dehydration reactions in metabasites generated large quantities of water which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular diopside k feldspar amphibole clinozoisite and sphene a late retrograde infiltration of co 2 bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing k feldspar chlorite rutile dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation large scale infiltration of fluid into greenschist facies metadolerite sills was intimately related to and possibly controlled by penetrative deformation and in the absence of a penetrative deformation grain boundary diffusion by itself was an ineffectual mechanism of fluid transport

the highland border rocks and their relation to marginal basin development in the scottish caledonides it has long been known that the highland border includes an assemblage of altered serpentinites gabbros spilitic lavas ferruginous and manganiferous cherts black phyllites quartzose clastics and amphibolites new work confirms that many of these rocks represent a dismembered tectonically emplaced ophiolite of caledonian age which originated in a narrow oceanic basin within or bordering the dalradian sedimentary pile the ophiolitic rocks were expelled from the basin as thrust sheets transecting d1 structures studies in the sw highlands suggest emplacement during d2 synchronous with se translation of the tay nappe this implies that the ophiolite originated in the n the highland border rocks are seen as marginal to a long lived northward dipping subduction zone within the southern uplands the evolution of the ballantrae ophiolitic complex the midland valley and the dalradian sedimentary basin are considered within this framework

comparison of thermal structure and evolution between neighboring subarctic glaciers 1 the distribution of cold and temperate ice within glaciers and ice sheets affects processes relevant to englacial and basal hydrology sliding and material rheology thermal regimes in turn are shaped by glacier and ice sheet dynamics as well as environmental setting we investigate the thermal structures of two small 7 km2 neighboring glaciers in the st elias mountains of southwestern yukon canada using ice penetrating radar and borehole temperature measurements our data reveal polythermal regimes in both glaciers that are strongly influenced by accumulation zone meltwater entrapment suggesting a climatic control on thermal structure differences in hypsometry and glacier dynamics nevertheless result in observed variations in the distribution of temperate ice between the two sites experiments with a thermomechanically coupled flow band model corroborate the strong control of meltwater entrapment on thermal structure and suggest a generally minimal role for strain heating an exception to this occurs where localized basal sliding produces lateral shearing and thus enhanced heat generation time dependent model simulations suggest that the future thermal evolution of the two glaciers may differ and therefore simple parameterizations of thermal response based on regional climate may not capture realistic variability between individual glaciers despite these differences both glaciers are ultimately expected to become fully cold prior to disappearing under negative mass balance conditions

processes of debris comminution in the glacial environment and implications for quarts sand grain micromorphology abstract studies of till composition rock crushing and abrasion experiments and detailed consideration of the mechanics of the comminution processes which occur in the subglacial environment suggest that monomineral quartz sand grains are mostly produced by the brittle fracture of larger particles abrasive wear is an inefficient mechanism for producing and modifying quartz sand grains because of the relatively great hardness of quartz viewed under the scanning electron microscope surface textures of subglacially derived quartz sand grains are typically those associated with brittle fracture of quartz since the release of quartz sand grains from bedrock by mechanical weathering may also involve brittle fracture grains which have been passively transported through a glacier may exhibit similar textures to those actively produced in the glacial environment hence the examination of the surface textures of quartz sand grains under the scanning electron microscope is unlikely to be a satisfactory technique for the discrimination of active and passive transport paths through glaciers

microfracture spacing distributions and the evolution of fracture patterns in sandstones abstract natural fracture patterns in sandstone were sampled using scanning electron microscope based cathodoluminescence sem cl imaging all fractures are opening mode and are fully or partially sealed by quartz cement most sampled fractures are too small to be height restricted by sedimentary layers at very low strains 100 datasets show spacings that are best fit by log normal size distributions compared to exponential power law or normal distributions the clustering of fractures suggests that the locations of natural factures are not determined by a random process to investigate natural fracture localization we reconstructed the opening history of a cluster of fractures within the huizachal group in northeastern mexico using fluid inclusions from synkinematic cements and thermal history constraints the largest fracture which is the only fracture in the cluster visible to the naked eye among 101 present opened relatively late in the sequence this result suggests that the growth of sets of fractures is a self organized process in which small initially isolated fractures grow and progressively interact with preferential growth of a subset of fractures developing at the expense of growth of the rest size dependent sealing of fractures within sets suggests that synkinematic cementation may contribute to fracture clustering

analysis of antarctic glacigenic sediment provenance through geochemical and petrologic applications abstract the number of provenance studies of glacigenic sediments in antarctica has increased dramatically over the past decade providing an enhanced understanding of ice sheet history and dynamics along with the broader geologic history such data have been used to assess glacial erosion patterns at the catchment scale flow path reconstructions over a wide range of scales and ice sheet fluctuations indicated by iceberg rafted debris in circumantarctic glacial marine sediments it is notable that even though most of the bedrock of the continent is ice covered and inaccessible provenance data can provide such valuable information about antarctic ice and can even be used to infer buried rock types along with their geo and thermochronologic history glacigenic sediments provide a broader array of provenance analysis opportunities than any other sediment type because of their wide range of grain sizes and in this paper we review methods and examples from all size fractions that have been applied to the antarctic glacigenic sedimentary record interpretations of these records must take careful consideration of the choice of analytical methods uneven patterns of erosion and spatial variability in sediment transport and rock types which all may lead to a preferential identification of different elements of sources in the provenance analyses because of this we advocate a multi proxy approach and highlight studies that demonstrate the value of selecting complementary provenance methods

studies of the base metal sulfide deposits at mcarthur river northern territory australia iii the stable isotope geochemistry of the h y c ridge and cooley deposits new measurements of s c and o isotopic ratios in the cooley ridge and h y c deposits at mcarthur river place important constraints on the formation of the deposits s isotope ratios in the discordant cooley and ridge mineralization and concordant ridge mineralization have absolute values and distributions similar to those of the h y c mineralization with galena and sphalerite having relatively constant values and pyrite having widely varying values galena sphalerite isotopic fractionations indicate higher equilibrium isotopic temperatures for the cooley deposits 290 degrees to 275 degrees c than for the ridge and h y c deposits 260 degrees to 100 degrees c correlating with a systematic temperature decrease away from the emu fault c and o isotope ratios for carbonates in the discordant and concordant deposits are related describing a single linear trend on a delta 13 c vs delta 18 o plot ratios for carbonates from unmineralized sediments do not plot on the line suggesting that the line does not reflect mixing of a hydrothermal and a sedimentary carbonate but rather reflects equilibrium between carbonates and a hydrothermal fluid over a temperature range similar to that indicated by the s isotopic fractionations in the discordant mineralization the isotopic equilibration occurred during the precipitation of ore stage carbonates whereas in the concordant mineralization the equilibration occurred during chemical exchange between preexisting carbonates and the mineralizing fluid and possibly also during the precipitation of some ore stage carbonates the new data provide additional support for the hypothesis that the discordant and concordant deposits are related and that both types of deposits formed from a common mineralizing fluid which flowed from the emu fault zone the data do not place unequivocal constraints on whether the concordant deposits formed syngenetically or epigenetically but they do indicate that in the concordant deposits the mineralizing process was operating at least in part below the sediment water interface

structure and kinematic evolution of the duke river fault southwestern yukon in southwest yukon the boundary between the alexander terrane and wrangellia corresponds with the duke river fault in this paper we report on observations of the duke river fault from four localities in southwest yukon and provide new constraints on 1 permian regional metamorphism within the alexander terrane 2 cretaceous ductile deformation along the duke river fault and 3 post miocene brittle deformation along the fault within these areas the duke river fault juxtaposes imbricated pervasively foliated and folded greenschist facies rocks of the alexander terrane southwest of the fault against sub greenschist facies less deformed rocks of wrangellia multiple lines of evidence from this region indicate the alexander terrane has been juxtaposed against wrangellia along a southwest dipping thrust fault 40ar 39ar dates from muscovite which grew during faulting or have been reset by motion along the duke river fault range from 79 to 105 ma suggesting that ductile movement along the fault i

glacier sliding seismicity and sediment entrainment the evolution of glaciers and ice sheets depends on processes in the subglacial environment shear seismicity along the ice bed interface provides a window into these processes such seismicity requires a rapid loss of strength that is typically ascribed to rate weakening friction i e decreasing friction with sliding or sliding rate many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds to our knowledge however these studies have all found rate strengthening friction here we investigate the possibility that rate weakening rock on rock friction between sediments frozen to the bottom of the glacier and the underlying water saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds we test this entrainment seismicity hypothesis using targeted laboratory experiments and simple models of glacier sliding seismicity and sediment entrainment these models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity we propose that stagnation at the whillans ice stream west antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins our results suggest that basal shear seismicity may indicate geomorphic activity

18o16o and 13c12c in lower paleozoic articulate brachiopods implications for the isotopic composition of seawater three hundred and eleven articulate brachiopods with extensive geographic coverage spanning the cambrian to silurian stratigraphic interval were analyzed for oxygen and carbon isotopic composition cambrian samples have 18o 7 ordovician samples 2 4 and silurian samples 1 9 confirming the previously established trend towards lighter isotopic compositions with increasing age throughout the lower paleozoic forty nine best preserved ordovician and silurian samples were identified based on scanning electron microscopy and trace element analysis they were found to bracket the isotopic compositions of over 85 of specimens from these stratigraphic intervals supporting widespread preservation of the isotopic signal some latest ordovician and lower ludlovian samples associated with shale sequences are apparently enriched in 18o these are interpreted as an environmental phenomenon perhaps related to water temperature and glaciation a number of silurian samples of varying genera and stratigraphic levels are highly enriched in 13c up to 6 some are shale related but some are associated with carbonate bearing basins these are also thought to represent near original compositions but a single environmental cause is unknown the present data show that luminescence is not a decisive criterion for evaluating the degree of brachiopod preservation whole shell values were isotopically similar to their nonluminescent portions for both oxygen and carbon

subglacial comminution in till evidence from microfabric studies and grain size distributions quartz feldspar and chlorite the principal minerals in a basal till from the halasi river catchment in the altay mountains northwestern china are present in approximately equal concentrations in the coarse fraction of the till 1 0 0 125 mm quartz concentrations are significantly higher than those of the other two minerals in the 0 125 0 016 mm size range feldspar and chlorite concentrations are higher than those of quartz in the finest fraction quartz has a strong preferred mode at 0 063 0 032 mm feldspar and chlorite have two weak modes in the silt size range one between 0 063 and 0 032 mm and the other between 0 016 and 0 004 mm thin sections of oriented impregnated samples were used to study crushing and abrasion over 2700 daughter particles were identified as products of comminution of 925 parent grains quartz and feldspar are most likely to be broken into two particles of roughly equal size as are fine chlorite grains however owing to their weakness and cleavage larger grains of chlorite tend to be split into more than two daughter particles sizes of the daughter grains were measured and sizes of the original parent grains were estimated mean parent grain sizes for quartz feldspar and chlorite are 0 129 0 078 and 0 059 mm respectively whereas mean daughter grain sizes are 0 068 0 041 and 0 024 mm respectively the greater percentage reduction in the size of chlorite reflects its tendency to break into more than two daughter particles most grains tend to be crushed only a few large particles seem to have suffered from abrasion

glacial comminution of quartz sand grains and the production of loessic silt a simulation study abstract traditional views have attributed the formation of loess sized quartz silt particles to glacial processes glacial grinding was considered to be the only mechanism capable of producing loessic silt in the quantities necessary for the formation of loess deposits this idea has been strengthened by the apparent close association between present or former glacierised regions and major global loess deposits however some doubts exist as to whether or not glacial grinding is capable of producing sufficient quantities of quartz silt because of the difficulties entailed in the observations and direct measurement of subglacial abrasion and crushing this paper describes and discusses the results of a simulation study designed to examine abrasion and size reduction of quartz grains under grinding conditions analogous to those experienced in a subglacial environment the findings from these experimental runs suggest that although particle breakage and comminution does occur little silt sized material is actually produced the amount of silt produced ranges from approximately 9 of the original sample in one experimental run to less than 1 for the other seven runs it is suggested that the results obtained may show considerably less abrasion and loessic silt production than may be expected to occur within a natural subglacial environment because 1 subglacial grains experience much greater overburden pressures for much longer periods of time 2 the use of sand sized grains may represent a stable component resistant to further size reduction through crushing or abrasion and 3 the use of a monomineralic sample of relatively sound quartz grains although care must be taken when relating results obtained from these experimental runs to silt production within a glacial environment findings from this work do question whether large amounts of quartz silts are produced within subglacial environments the importance of glacial processes in the formation of loess deposits may lie in the reworking of pre existing sediments by ice rather than the actual production of vast quantities of silt this in turn suggests that care should be taken in the interpretation of the provenance of particles comprising loess deposits

meltwater intensive glacial retreat in polar environments and investigation of associated sediments example from pine island bay west antarctica abstract modern pine island and thwaites glaciers which both drain into pine island bay are among the fastest changing portions of the cryosphere and the least stable ice streams in antarctica here we show that the uppermost sediment unit in pine island bay was deposited from a meltwater plume a plumite during the late stages of ice sheet retreat 7 8 6 k cal yr bp and argue that this deposit records episodes of meltwater intensive sedimentation the plumite is a hydraulically sorted glacially sourced draping deposit that overlies proximal glacimarine sediments and thickens towards the modern grounding line the uppermost sediment unit is interpreted as a product of non steady state processes in which low background sedimentation in large bedrock carved basins alternates with episodic purging of sediment laden water from these basins the inner part of pine island bay contains several basins that are linked by channels with a storage capacity on the order of 70 km 3 of stagnant water and significant sediment storage capacity purging of these basins is caused by changes in hydraulic potential and glacial reorganization the sediment mobilized by these processes is found here to total 120 km 3 this study demonstrates that episodes of meltwater intensive sedimentation in pine island bay occurred at least three times in the holocene the most recent episode coincides with rapid retreat of the grounding line in historical time and has an order of magnitude greater flux relative to the entire unit we note that the final phase of ice stream retreat in marguerite bay was marked by a similar sedimentary event and suggest that the modern thwaites glacier is poised for an analogous meltwater intensive phase of retreat