tourmaline microboudinage an indicator of its host rheology abstract columnar tourmaline in cenozoic granitic mylonites from the chongshan metamorphic zone cmz yunnan china has deformed at 450 300 c by tensile fractures normal to the foliation and lineation sequential fracturing and separation of the fragments during progressive cooling and exhumation upward the brittle ductile transition produced a series of microboudins with decreasing length width ratios here we present a method to apply the microboudinage paleopiezometry combined with strain analyses microstructural observations and thermochronological data to provide constraints on strain rates e differential stresses and their correlations with temperature for the polycrystalline quartz matrix in which tourmaline grains have been fragmented into microboudins the results yield a flow law of e a nexp q rt with n 4 q 184 kj mol and a 7 180 10 7 mpa ns 1 for naturally deformed quartzite the differential stresses calibrated by this flow law are very close to the geometrical mean of the lower and upper bounds given by luan and paterson 1992 and gleason and tullis 1995 respectively utilization of this flow law to the present day crust of the cmz provides a reliable interpretation for the distribution of focal depths and electrical resistivity structure the microboudinage piezometer also permits to estimate the mechanical strengths of rocks deformed in the elastic brittle regime

tectonothermal evolution of the badajoz cordoba shear zone sw iberia characteristics and 40ar 39ar mineral age constraints field characteristics indicate that the badajoz cordoba shear zone has had a long polyphase tectonothermal evolution 40ar 39ar cooling ages recorded within central ductile sectors of the badajoz cordoba shear zone range from ca 370 to 360 ma amphibole and 340 to 330 ma muscovite these are interpreted to date post metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant variscan tectonothermal overprint these results combined with field relationships indicate that the badajoz cordoba shear zone experienced at least 15 km of uplift relative to adjacent areas during late paleozoic sinistral transpression the data also indicate that individual structural units within the shear zone experienced contrasting uplift histories both in total amount and rate the 40ar 39ar cooling ages indicate complete variscan rejuvenation of all older intracrystalline argon systems within central sectors of the badajoz cordoba shear zone

tourmaline kar ages compared to other radiometric dating systems in alpine anatectic leucosomes and metamorphic rocks cyclades and southern spain abstract k ar ages of tourmalines from alpine anatectic leucocratic rocks in the migmatite dome on naxos greek cyclades and along ultramafic intrusions in southern spain are older than the corresponding mineral ages they are interpreted as cooling ages closely approaching the age of magmatism suggesting a closure temperature exceeding that of hornblende rb sr whole rock analyses from metagranites and gneisses of the palaeozoic suite of the mulhacen complex of the betic cordilleras southern spain yield errorchrons of between 275 and 191 ma several k ar tourmaline ages of these rock units are higher than the corresponding whole rock ages pointing to the incorporation of excess radiogenic ar of the young tourmalines three of them give ages of 80 85 ma indicating possible examples of incomplete degassing and or possible incorporation of excess radiogenic ar in the mineral during the alpine metamorphism the tourmaline ages are compared with the ages of the coexisting micas these have a range of k ar ages consistently higher than the corresponding young alpine rb sr ages of 15 6 12 5 ma pointing to a complex behavior for radiogenic ar during the alpine metamorphism

non linear responses of rutford ice stream antarctica to semi diurnal and diurnal tidal forcing modulation of the flow of rutford ice stream antarctica has been reported previously at semi diurnal diurnal fortnightly and semi annual periods a model that includes non linear response to tidal forcing has been shown to fit closely observations at fortnightly frequencies here we examine that model further and test its fit at a larger set of observed frequencies including the large semi annual displacement we show analytically that when forced by major tidal terms the model using a basal shear stress exponent m 3 predicts several discrete response periods from 4 hours to 0 5 years we examine a 1 5 year gps record from rutford ice stream and find that the model when forced by a numerical tide model is able to reproduce the reported semi annual signal we confirm that about 5 of the mean flow is due solely to the m 3 non linear response to tidally varying basal shear stress our best fitting set of model parameters is similar to those originally reported using a much shorter data record although with noticeably improved fit suggesting these parameters are robust we find that m 3 fits the data well but that m 2 does not furthermore we find that a small variation in flow over the 18 6 year lunar node tide cycle is expected fits to semi diurnal and diurnal terms remain relatively poor possibly due to viscoelastic effects that are not included in the model and reduced gps data quality at some discrete periods for comparison we predict the response of bindschadler ice stream and lambert glacier and show given identical model parameters a similar response pattern but with 1 2 orders of magnitude smaller variability these may still be measurable and hence useful in testing the applicability of this model to other locations

constructing improved decadal records of antarctic ice shelf height change from multiple satellite radar altimeters abstract antarctica s ice shelves the floating extensions of the ice sheet exert an important dynamic constraint on the flow of ice from the grounded ice sheet to the ocean and therefore on changes in global sea level thinning of an ice shelf reduces its ability to restrain the ice discharge from the grounded ice sheet however our understanding of how ice shelf processes couple ice sheet changes to climate variability is still rudimentary in part this is due to the brevity and low temporal resolution of surveys of ice shelf thickness relative to the broad range of time scales on which ice sheet mass fluctuates here we present improved procedures to construct 18 year 1994 2012 time series of antarctic ice shelf surface height at high spatial resolution 30 km by merging data from three overlapping satellite radar altimeter missions ers 1 ers 2 and envisat we apply an averaging scheme to enhance the signal to noise ratio of height changes over the floating ice shelves and extract low order polynomial trends using a robust approach regularized regression with cross validation that accounts for both bias and variance in the fit we construct formal confidence intervals by bootstrap resampling of the residuals of the fit the largest source of height error arises from the interaction of the radar signal with the snow and firn surface on annual time scales changes in surface and sub surface scattering and radar penetration lead to apparent height changes that are larger than the true surface height change arising from densification our 18 year time series of surface height provide an insight into how ice shelves respond to the changing atmospheric and oceanic conditions our methods could also be applied to grounded portions of the ice sheets both in antarctica and greenland

geological relationships and u pb zircon and 40 ar 39ar tourmaline geochronology of gneisses and tourmalinites from the nevado filabride complex western sierra nevada spain tectonic implications abstract the nevado filabride complex the lowest complex in the betic hinterland forms a stack of alpine nappes the tectonic units consist of metasedimentary sequences whose ages are not well constrained gneiss bodies included in the sequences have been one of the few sources of geochronological data in this metamorphic complex new radiometric data from u pb zircon dating on gneisses from the western sierra nevada confirm the presence of late carboniferous intrusive rocks in the betic hinterland these results combined with available data from the literature and a detailed structural analysis suggest that the gneisses represent a single late variscan magmatic event evidence for a close genetic relation between gneisses and tourmalinites is provided by field and petrographic observations in conjunction with geochemical data u pb zircon 314 7 304 23 ma and 40 ar 39 ar tourmaline 319 85 5 81 317 85 3 67 ma geochronology a pre late carboniferous age for the basal formation of the nevado filabride sequence can be inferred the gneiss protolith and the graphite schist are considered to be the boron source and the precursor of the tourmalinites respectively superposed tectonic units rather than a continuous palaeozoic sequence is supported by the occurrence of palaeozoic rocks at the top of the nevado filabride complex the nappe tectonics as evidenced from the contractional character of the unit boundaries and the superposition of higher grade on lower grade metamorphic rocks is additionally supported by the superposition of older on younger rocks as revealed from radiometric data

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

tide induced lateral movement of brunt ice shelf antarctica 1 accurate position data show that brunt ice shelf has an irregular three dimensional motion driven by the ocean tide a 30 day gps record from halley station shows not only the expected vertical tidal displacement but also a significant variation in horizontal velocity the maximum forward velocity occurs about four hours before the maximum tidal height a phase relationship that suggests ocean currents may be responsible however values of ice water friction coefficients needed to make this explanation plausible appear to be too high to be physically reasonable alternative explanations invoke interactions between the ice shelf and a grounded area at the ice front our findings suggest that significant errors might result if these motions are ignored when interpreting double difference satellite sar interferograms where the common assumption is that the horizontal velocity of an ice shelf has no tidal dependence

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

future sea level rise due to projected ocean warming beneath the filchner ronne ice shelf a coupled model study a general ocean circulation model is coupled with a 3d thermodynamical ice sheet shelf model to simulate the response of the filchner ronne ice shelf fris antarctica and coastal parts of its catchment basin to a postulated inflow of warm deep water into the ice shelf cavity on a 1000 yr timescale prescribed ocean warming based on climate projections enters the ice shelf cavity in the up to 1500 m deep filchner trough and penetrates deep into the sub ice cavity increasing basal melt rates induce geometry changes of the cavity which in turn have an impact on the ocean circulation and therefore the modelled melt rates highest melt rates of about 20 m yr 1 follow the up to 180 km retreating grounding line basal mass loss reaches about 250 km3 yr 1 doubling the present day value the most vulnerable areas below the fris are the bailey ice stream and the area between the institute and moeller ice streams where the increased melting accounts for about 80 km of the modelled grounding line retreat on the backward sloping bedrock the potential additional contribution to the eustatic sea level rise due to the grounded ice loss simulated in an ensemble approach against a transient control experiment is about 0 05 mm yr 1 during the first 500 yr and about 0 17 mm yr 1 thereafter

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

sphalerite 40ar 39ar progressive crushing and stepwise heating techniques abstract it is difficult to determine the mineralization ages of sulfide deposits because no suitable minerals can be selected for the traditional isotopic dating methods the 40ar 39ar method rarely applies well to sulfide minerals because of their trace potassium content and strong radioactivity after irradiation this preliminary study investigates the possibility of dating the hydrothermal sphalerites by 40ar 39ar using crushing and heating techniques the 40ar 39ar crushing experiments of two sphalerite samples from the fankou lead zinc deposit yield well defined and concordant isochron ages of 265 8 1 0 1 ma and 269 7 6 2 ma corresponding to initial 40ar 36ar ratios of 290 7 2 4 and 294 6 8 1 respectively the crushed powders of the later sphalerite yields with stepwise heating a flat age spectrum with 40ar 39ar plateau age of 267 2 2 5 ma which define a good isochron with an age of 271 3 5 4 ma and initial 40ar 36ar ratio of 294 0 1 7 this is the first report to distinguish secondary and primary fluid inclusions within sphalerite to obtain their ages using the crushing technique and to obtain good concordant ages both of fluid inclusions by crushing and heating of k containing minerals we expect progressive crushing and stepwise heating techniques will become an effective approach to 40ar 39ar dating of the mineralization ages of hydrothermal sulfide deposits

tourmaline rich pseudomorphs in sillimanite zone metapelites demarcation of an infiltration front textural features combined with mineral chemistry preserved in metamorphic rocks provide insights into metamorphic reaction mechanisms as well as open vs closed system processes prograde tourmaline rich muscovite pseudomorphs after staurolite develop in sillimanite zone metapelites adjacent to peraluminous granitoid intrusives in nw maine tourmalines occur in discrete domains restricted to central regions of muscovite rich quartz poor pseudomorphs with biotite rich margins these tourmaline grains are relatively large 1 0 mm lack detrital cores and exhibit only minor compositional zoning in sharp contrast to matrix tourmaline from other samples these features suggest fluid infiltration as the causative mechanism for the formation of these tourmaline rich mica pseudomorphs after staurolite irreversible thermodynamic models of local reactions and material transport in combination with mineral chemistry allow evaluation of reaction mechanisms that produced these pseudomorphs thermodynamic models in the nkcmtfashob system mimic the observed textural features if a threestage process is used stage 1 staurolite replacement is initiated by infiltration of an aqueous phase that adds k na h2o to the rock with the concomitant removal of al fe because the system is initially undersaturated with respect to tourmaline a pseudomorph containing muscovite with minor biotite develops at the expense of staurolite stage 2 with continued infiltration concentration of b increases tourmaline saturation is exceeded tourmaline nucleates and grows local material transport constraints mandate that tourmaline precipitation be spatially restricted to regions of staurolite dissolution consequently tourmaline forms in clusters at sites containing the last vestiges of staurolite in the pseudomorph core also evidenced by staurolite inclusions within several tourmaline grains resultant domains of staurolite replacement during this stage contain about equal amounts of muscovite and tourmaline typical staurolite poikiloblast pseudomorphing reactions require silica transport matrix quartz dissolves from the surrounding host resulting in a local enrichment of biotite and plagioclase at the pseudomorph margin stage 3 small amounts of sillimanite nucleate and grow throughout the rock late stage aqueous fluids from the adjacent monzonitic intrusive are likely to be the primary b source theoretical textural and compositional modeling combined with observational data indicate that boron must have been derived externally from the rock that the modal amount of tourmaline is very sensitive to the b content of the fluid that tourmaline is stable throughout the sillimanite zone depending on other cation activities and ph of the fluid and that these pseudomorphs provide insight into b contents of metamorphic fluids and the timing of the b influx the outer geographic extent of the tourmaline bearing pseudomorphs marks the boundary of a reactive geochemical front and thus defines an advective isograd interpretation of subtle textural features preserved in the rock in conjunction with irreversible textural modeling provides a powerful tool with which to understand the chemical evolution of metamorphic rocks and the fluids involved

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

tides and the flow of rutford ice stream west antarctica 1 surface speeds of rutford ice stream west antarctica are known to vary by around 10 20 depending on location with a fortnightly periodicity corresponding to a springneap tidal cycle the reasons for these periodic variations in flow are unclear here the possible role of tidal stress transmission upstream of the grounding line in affecting rates of basal motion is investigated it is found that nonlinear rheological effects within the till when coupled with transmission of tidal stresses within the ice that are linearly related to tidal amplitude can give rise to the type of periodic oscillations in flow observed this nonlinear interaction between tidal forcing and till deformation increases the mean ice flux across the grounding line by a few percent above what might be expected in the absence of tidal forcing periodic velocity fluctuations of this type have not been observed on other ice streams however modeling suggests that this may be due to lack of data and that such flow variations are likely to be common features of active ice streams draining into the ronne ice shelf as well as of other ice streams subjected to similar tidal forcing

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

granite migmatite genetic link the example of the manaslu granite and tibetan slab migmatites in central nepal in central nepal the tibetan slab is made up of biotite gneisses metapelites and metagreywackes orthogneisses metagranites and migmatites melanosomes are generally biotite muscovite bearing but locally they may be tourmaline rich when associated with boron rich granitic material leucosomes occur as lenses conformable with the foliation veins patches or as fillings in shear zones and extensional structures field relationships and mineralogical and chemical data show that three processes may have contributed to the formation of the tibetan slab leucosomes metamorphic differentiation or disequilibrium partial melting low zr tonalitic leucosomes in situ equilibrium partial melting high zr leucosomes and some granitic leucosomes and injection of externally derived melts most granitic and some tonalitic leucosomes the manaslu pluton belongs to the high himalayan leucogranite belt and was emplaced at the top of the tibetan slab it corresponds to a muscovite biotite leucogranite that has been assumed to derive from melting of the tibetan slab gneisses formation i phase relationships a more magnesian chemistry of the ferromagnesian minerals from the tibetan slab migmatites as compared to the manaslu leucogranite the microtextures of accessory phases and trace element compositions lower u li f and higher sr eu y yb contents in the migmatite leucosomes show that the in situ himalayan migmatites at the crustal level presently exposed have not been produced under the same p t xh2o conditions as the manaslu leucogranite magma while the formation i was the probable source for the manaslu granite migmatites within the formation are not the remanants of a melting process from which the manaslu granite was derived both the tibetan slab migmatites and the manaslu leucogranite may be considered as evidence of dehydration and melting at deeper crustal levels and of percolation of melts and hydrothermal fluids through the crust

secondary airflow and sediment transport in the lee of a reversing dune lee side windspeed and sediment transport were measured over a small 1 2 m transverse ridge in the silver peak dunefield west central nevada usa using an intensive array of 25 cup anemometers and seven total flux traps during crest transverse and transporting flow conditions u0 3crest 8 4 m s 1 windspeed near the surface of the lee slope averaged half 48 per cent that of crest speeds dimensionless speeds in the separation zone ranged from 0 2 to 0 8 that of the outer flow u12 along the boundary of the separation cell windspeed increased by 10 per cent of the crest speed before separation equilibrium of upper and lower wake regions was not observed by the documented eight dune heights suggesting that wake recovery may not occur over closely spaced dunes sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs this suggests that a significant amount c 70 95 per cent of sediment transported over the crest moved as fallout for this data set flux was approximately proportional to the cube of the near surface windspeed u0 3 and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection beyond the interdune transport direction progressed from oblique to crest transverse this indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest transverse flows regarding the grain size and sorting properties of transported sediment there was no significant difference in mean grain size over the dune although in general particles were finer and more poorly sorted in the lee copyright 1999 john wiley sons ltd

boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite abstract in order to understand the behavior of boron b and its isotope fractionation during subduction zone metamorphism b contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the sambagawa metamorphic belt central shikoku japan no systematic changes in whole rock b content and isotope composition of the metasediments were observed among the different metamorphic grades indicating the lack of a bulk fluid rock b isotope fractionation as a result of devolatilization both modal abundance and grain size of tourmaline increase with increasing metamorphic grade in contrast b contents in muscovite and chlorite decrease with increasing metamorphic grade these observations combined with mass balance calculations of b suggest the formation of tourmaline during progressive metamorphism from metamorphic fluids containing b mainly derived from muscovite and subordinately from chlorite without allowing significant net removal of b from the metasedimentary rocks tourmalines in the higher grade metasedimentary rocks have zonal structure of b isotope and major element composition with decreasing 11 b and increasing mg mg fe from the inner rim core to the outer rim the change of mg mg fe in the tourmalines with increasing grade is paralleled by similar variation in chlorite these observations suggest that the growing tourmalines record the progressive evolution of the b isotopic composition of the metamorphic fluid in the outermost rims preserving the isotope signature of peak metamorphic p t fluid conditions based on the above observations the 11 b of the tourmaline is thought to have been nearly identical to that of the metamorphic fluid resulting in the apparent b isotopic fractionation factor between metamorphic fluid and whole rock 11 b 10 b fluid 11 b 10 b whole rock which decreases from 1 007 0 003 to 1 001 0 003 from chlorite to biotite zone metamorphism such results together with the formation of tourmaline from and sequestering of b in metamorphic fluids may lead to less b isotopic fractionation as a result of subduction zone devolatilization than noted in suites containing less tourmaline this therefore makes it possible to transport b isotopic signatures which ultimately reflect earth s surface materials to the deep mantle perhaps resulting in mantle b isotope anomalies near convergent margins

surface structure and stability of the larsen c ice shelf antarctic peninsula a structural glaciological description and analysis of surface morphological features of the larsen c ice shelf antarctic peninsula is derived from satellite images spanning the period 1963 2007 the data are evaluated in two time ranges a comparison of a 1963 satellite image photomosaic with a modern digital mosaic compiled using 2003 04 austral summer data and an image series since 2003 showing recent evolution of the shelf we map the ice shelf edge rift swarms crevasses and crevasse traces and linear longitudinal structures called flow stripes or streak lines the latter are observed to be continuous over distances of up to 200 km from the grounding line to the ice shelf edge with little evidence of changes in pattern over that distance integrated velocity measurements along a flowline indicate that the shelf has been stable for 560 years in the mid shelf area linear longitudinal features may be grouped into 12 units each related to one or a small group of outlet feeder glaciers to the shelf we observe that the boundaries between these flow units often mark rift terminations the boundary zones originate upstream at capes islands or other suture areas between outlet glaciers in agreement with previous work our findings imply that rift terminations within such suture zones indicate that they contain anomalously soft ice we thus suggest that suture zones within the larsen c ice shelf and perhaps within ice shelves more generally may act to stabilize them by reducing regional stress intensities and thus rates of rift lengthening

tourmaline breakdown in the migmatite zone of the ryoke metamorphic belt sw japan a sharp line delimitating the distribution of tourmaline termed as a tourmaline out isograd is defined in the migmatite zone of the ryoke metamorphic belt japan the trend of the tourmaline out isograd closely matches that of the isograds formed through the regional metamorphism suggesting that it represents the breakdown front of tourmaline during regional metamorphism this is confirmed by the presence of the reaction textures of tourmaline to sillimanite and cordierite near the tourmaline out isograd the breakdown of tourmaline would release boron into associated melts or fluids and be an important factor in controlling the behaviour of boron in tourmaline bearing high temperature metamorphic rocks near the tourmaline out isograd large tourmaline crystals occur in the centre of interboudin partitions containing leucosome in the melanosome of the intervening matrix reaction textures involving tourmaline are locally observed these observations imply that tourmaline breakdown is related to a melting reaction and that the boron in the leucosome is derived from the breakdown of tourmaline in the melanosome during prograde metamorphism boron released by tourmaline breakdown lowers both the solidus temperature of the rock and the viscosity of any associated melt considering that the tourmaline out isograd lies close to the schist migmatite boundary these effects might have enhanced melt generation and segregation in the migmatite zone of the ryoke belt the evidence for the breakdown of tourmaline and the almost complete absence of any borosilicates throughout the migmatite zone suggest that boron was effectively removed from this region by the movement of melt and or fluid this implies that the tourmaline out isograd can reflect a significant amount of mass transfer in the anatectic zones

flow of the ross ice shelf antarctica is modulated by the ocean tide the ice streams feeding the ross ice shelf antarctica have large tidally modulated sinusoidal and stick slip flow but the interaction with the ice shelf is poorly understood we show that the flow of the ross ice shelf front up to 650 km from the ice streams exhibits smooth sinusoidal motions corresponding to tidal modulation these observations suggest a possible linking of the ice shelf with the ice streams to form a unified system that responds to small perturbations in stresses associated with ocean tides if this is the case the presence of the sinusoidal motion but the absence of stick slip motion suggests there is damping of very high frequency signals the dissimilar signatures of the motions observed in the ice streams and at the front of the ice shelf present challenges to model development aimed at understanding the dynamics of coupled ice stream ice shelf flow and the movement of ice across grounding lines

rifting processes and ice flow modulation observed on mertz glacier east antarctica we investigated the evolution of two major rifts cutting across mertz glacier tongue east antarctica using a combination of satellite images and 60 day sets of gps data from two stations deployed either side of the western rift in 2007 the eastern rift began to open in the early 1990s and the western rift initiated in 2002 in conjunction with the collision of a large iceberg with the tongue velocity time series derived from the 2007 gps data exhibited strong variations at tidal periods modulated by sea surface height and sea surface slope and reproduced here with a conceptually simple model we found that opening of the western rift in 2002 leads to a dramatic change in behavior of the tongue as the large range in velocity 700 2400 m a 1 observed in 2000 was largely reduced 1075 1225 m a 1 in 2007 opening of the western rift decoupled the glacier from the transverse loading on the tongue driven by east west tidal circulation this loading previously induced time varying lateral drag which caused the large velocity range our results suggest changes in the mechanical behavior of an ice tongue impact the dynamics of the outlet glacier system and should be considered in longer term mass balance evaluations

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

clarification of some misconceptions about antidune geometry and flow character estimates of palaeoflow conditions based on antidune geometries generally are obtained using equations derived from potential flow analysis unfortunately in several previous studies and review articles misunderstanding and improper substitution of density terms has resulted in two misconceptions a third is discussed in detail elsewhere both concern the commonly derived ratio of antidune wavelength to critical flow depth l d previously published results which suggest that for a given set of flow conditions in a two layer system l d increases with increasing density contrast are wrong correct substitutions indicate the opposite there is a slightly inverse relationship between the value of l d and the second misconception concerns the two values reported for critical flows l d 6 3 or 2 and l d 12 6 or 4 despite insinuations that the latter is wrong by a factor of two both are correct in this case what has been overlooked is that the former value is derived for antidunes on free surface flows a two layer system in which 2 1 and the latter for antidunes at the interface of a two layer system in which p 0 i e 2sime 1

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

multistage boron metasomatism in the alamo complex central iberian zone spain evidence from field relations petrography and 40ar 39ar tourmaline dating the alamo complex consists of structural metamorphic domes surrounded by low grade metasedimentary rocks of upper proterozoic to lower cambrian age that form part of the schist graywacke complex central iberian zone spain tourmaline is ubiquitous throughout the domes in which it occurs in tourmalinites psammo pelitic schists quartzites gneisses migmatites leucogranites aplo pegmatites and quartz veins overall tourmaline compositions can be described within the four component system schorl dravite foitite magnesiofoitite with k2o 0 23 low ca contents mg mg fe 0 25 0 71 and x x na 0 18 0 43 where x vacancies in the x site field relations and petrographic observations combined with tourmaline 40ar 39ar data provide evidence of intense boron metasomatism affecting this region tourmaline 40ar 39ar step heating spectra for cambrian ordovician orthogneisses are complex yielding a pseudo plateau age of 370 ma that is interpreted to reflect variscan rejuvenation of older tourmaline tourmaline 40ar 39ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo plateau ages of 355 342 ma that are unsuitable for precise age determination these ages however are consistent with published ages 340 350 ma for the second variscan deformation d2 tourmaline from fine layered tourmalinite and metasedimentary rocks yield well defined plateau ages of 317 and 315 ma respectively recording an additional metasomatic event concomitant with anatexis and evolution of b bearing granites pegmatites and hydrothermal fluids the different tourmaline forming stages reflect significant boron cycling within the continental crust of the central iberian zone driven by deformation metamorphism and magmatism during the variscan orogeny boron rich aqueous fluids related to cambro ordovician magmatism are considered to be the primary source of boron

evolution of compositional polarity and zoning in tourmaline during prograde metamorphism of sedimentary rocks in the swiss central alps the compositional evolution of tourmaline from diagenetic to lower amphibolite facies conditions was investigated in two metasedimentary units redbed and black shale formations along a traverse across the central alps with increasing metamorphism three distinct rim zones grew around detrital tourmaline cores simultaneously with the development of prismatic neoblasts detrital cores and all successive rim zones are preserved in the amphibolite facies rocks as indicated by whole rock data the b required for tourmaline growth was released from illite and muscovite and was not introduced from external sources the studied samples of tourmaline exhibit compositional polarity i e their compositions are different at the opposite ends of the c axis the compositional difference between the two poles can be expressed by the substitutions na mg 0 al and 2al ti mg with the positive pole always richer in al and having more x site vacancies both chemical and optical polar effects are most pronounced in the internal rim zones and become less prominent toward the external zones the systematic prograde compositional trends are depicted in a vector space for li poor tourmaline at low grade conditions the tourmaline composition is clearly controlled by the host rock composition but with increasing metamorphic grade the compositions of tourmaline from a variety of rocks converge the data show that in the central alps increasing metamorphic grade is reflected by increases in ca ca na and mg mg feh a decrease in fe3 and an increase in the occupancy of the x site of tourmaline

syn collisional extensional collapse parallel to the orogenic trend in a domain of steep tectonics the salamanca detachment zone central iberian zone spain the area south of the town of salamanca is a zone of the spanish variscan belt characterized by a first compressional event which gave rise to steep structures followed by the development of an extensional ductile shear zone more than 4 km thick that is described as the salamanca detachment zone the strain associated to the detachment is analyzed using quartz pebbles in conglomerates comparing the measured strain values with theoretical strain paths leads to the conclusion that the deformation was approximately of plane strain type quartz c axis fabrics and kinematic criteria indicate that simple shear was a very important component of the deformation though coaxial components were probably involved the extensional character of the detachment is indicated by the geometry of the metamorphic zones and the metamorphic evolution taken together the data indicate that the extensional event was related to gravitational collapse induced by the thickening of the continental crust and that it was syn collisional the movement of the hangingwall unit was to the southeast parallel to the trend of the fold belt and the translation was of the order of one to a few dozens of km though important this is not exceedingly large and allowed the preservation of low grade metamorphic conditions in the hangingwall preventing it from being affected by extensive brittle tectonic processes

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

antarctic analog for dilational bands on europa europa s surface shows signs of extension which is revealed as lithospheric dilation expressed along ridges dilational bands and ridged bands ridges the most common tectonic feature on europa comprise a central crack flanked by two raised banks a few hundred meters high on each side together these three classes may represent a continuum of formation in tufts dilational model ridge formation is dominated by daily tidal cycling of a crack which can be superimposed with regional secular dilation the two sources of dilation can combine to form the various band morphologies observed new gps data along a rift on the ross ice shelf antarctica is a suitable earth analog to test the framework of tufts dilational model as predicted by tufts dilational model tensile failures in the ross ice shelf exhibit secular dilation upon which a tidal signal can be seen from this analog we conclude that tufts dilational model for europan ridges and bands may be credible and that the secular dilation is most likely from a regional source and not tidally driven

nonlinear interaction between ocean tides and the larsen c ice shelf system 1 we report on three gps records of flow of the larsen c ice shelf spanning 2 months to 2 years variations in speed are evident at periods from a few hours to 182 days including semi diurnal diurnal and 14 76 days at fortnightly periods the ice shelf varies by 10 from its long term speed but at diurnal timescales it is up to 100 a nonlinear relationship between ocean tides and velocity is required to explain the observed variations in flow as an initial examination we model flow as a non linear function of basal shear stress including tidal perturbations in the ice shelf grounding zone and show that the major features of the observations are reproduced notably the long period signal largely absent from the vertical tidal signal alternative explanations are discussed these observations demonstrate that the ice shelf system is highly sensitive to relatively modest changes in forcing at its boundaries

structural controls on metamorphism and syn tectonic magmatism the portuguese hercynian collision belt upper palaeozoic collisional movements along the ibero armorican arc were accommodated on oblique strike slip ductile shear zone systems in north central portugal these shear zones are coincident with long linear belts of high temperature low pressure metamorphism and s type granitic plutons a model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12 15 km this was achieved by a combination of i increased heat flow ii lowering of melting temperatures caused by fluxing of hydrous fluids and iii a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts shear zones are proposed as the most important influence in controlling the site of development of such belts anatexis of metasedimentary crust and emplacement of the resulting s type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source