short term viscous effects on measurements of remanance summary the magnetic remanence of some rock specimens when measured in mu metal shielded magnetometers appear to include a rapidly acquired 100 s component that has a characteristic direction for each magnetometer which seems to be related to the sequence of specimen positions during measurement this spurious remanence can be mistaken for a stable linear geologically meaningful magnetic vector the effect can be partially reduced by passing a small current through a coil wound around the mu metal opening and largely but not completely eliminated by placing the magnetometer in field free space

the south western branch of the variscan belt evidence from morocco abstract this work is based on the compilation and re evaluation of the most significant data either personal or from the literature concerning the moroccan variscides the latter constitute the only moderately disturbed or even undisturbed part of the south western branch of the variscan belt facing directly nw gondwana they include two orogenic segments namely the northern mauritanides and the meseta domain exposed in the saharan and atlas meseta regions respectively and a foreland belt cropping out essentially in the anti atlas the eastward thrust units of saharan morocco oulad dlim mostly originate from the west african craton wac border in an area of thin palaeozoic sedimentation thin skinned fold thrust foreland arcs develop progressively northward zemmour at the expense of the increasingly thick palaeozoic series whereas thick skinned deformation characterizes the inverted proximal paleomargin in the anti atlas domain as suggested by the meseta and anti atlas stratigraphic similarities the meseta domain corresponds to a collage of moderately displaced thinned crustal blocks from the distal gondwana paleomargin variscan deformation is dominated by nw verging thrusts and metamorphism developed in the thickened tectonic prism in relation with crustal anatexis at depth the meseta anti atlas boundary is a major ene trending transpressional dextral fault referred to as the south meseta fault smf discussing the correlations between the variscan segments of morocco and sw iberia allows us to suggest that a latitudinal transform zone similar to the smf separated these segments during the late palaeozoic subduction of the rheic ocean crust would have been directed se ward along both the iberian and moroccan meseta and nw ward south of the smf i e along the wac

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

geochemical and sr nd isotopic constraints on the petrogenesis and geodynamic significance of the jebilet magmatism variscan belt morocco in the variscan fold belt of morocco the jebilet massif is characterized by palaeozoic metasedimentary rocks intruded by syntectonic magmatism that includes an ultramafic granitoid bimodal association and peraluminous granodiorites emplaced c 330 ma intruded by younger leucogranites c 300 ma the mafic ultramafic rocks belong to a tholeiitic series and display chemical and isotopic signatures consistent with mixing between mantle derived and crust derived magmas or assimilation and fractional crystallization the granites within the bimodal association are mainly metaluminous to weakly peraluminous microgranites that show characteristics of a2 type granites the peraluminous calc alkaline series consists mainly of cordierite bearing granodiorites enclosing magmatic microgranular enclaves and pelitic xenoliths detailed element and isotope data suggest that the alkaline and the peraluminous granitoids were formed in the shallow crust 30 km by partial melting of tonalitic sources at high temperatures up to 900 c and by partial melting of metasedimentary protoliths at relatively low temperatures c 750 c respectively mixing between the coeval mantle derived and crust derived magmas contributed to the large variation of initial end values and initial sr isotopic ratios observed in the granitoids further contamination occurred by wall rock assimilation during ascent of the granodioritic plutons to the upper crust the ultramafic granitoid association has been intruded by leucogranites that have high initial sr isotopic ratios and low initial end values indicating a purely crustal origin the heating events that caused emplacement of the jebilet magmatism are related to cessation of continental subduction and convective erosion thinning of the lithospheric mantle during plate convergence

hydrothermal alteration fluid flow and volume change in shear zones the layered mafic ultramafic kettara intrusion jebilet massif variscan belt morocco ecole des mines de nancy parc de saurupt 54042 nancy cedex franceabstract during emplacement and cooling the layered ma c ultrama c kettara intrusion jebilet morocco underwent coeval effects of deformation and pervasive uid in ltration at the scale of the intrusion inthe zones not affected by deformation primary minerals olivine plagioclase clinopyroxene werepartially or totally altered into ca amphibole mg chlorite and caal silicates in the zones of activedeformation centimetre scale shear zones focused uid ow transformed the metacumulates peridotites and leucogabbros into ultramylonites where insoluble primary minerals ilmenite spineland apatite persist in a ca amphibole rich matrix mass balance calculations indicate that shearing wasaccompanied by up to 200 volume gain the ultramylonites being enriched in si ca mg and fe anddepleted in na and k the gains in ca and mg and losses in na and k are consistent with uid ow inthe direction of increasing temperature when the intrusion had cooled to temperatures prevailing in the country rock lower greenschistfacies deformation was still active along the shear zones intense intragranular fracturing in the shearzone walls and subsequent uid in ltration allowed shear zones to thicken to metre scale shear zones withtime the inner parts of the shear zones were transformed into chlorite rich ultramylonites in the shearzone walls muscovite crystallized at the expense of ca al silicates while calcite and quartz weredeposited in en echelon veins mass balance calculations indicate that formation of the chlorite richshear zones was accompanied by up to 60 volume loss near the centre of the shear zones the ultra mylonites being enriched in fe and depleted in si ca mg na and k while the shear zones walls areenriched in k and depleted in ca and si the alteration observed in and adjacent to the chlorite shearzones is consistent with an upward migrating regional uid which ows laterally into the shear zone walls isotopic sr o signatures inferred for the uid indicate it was deeply equilibrated with host lithologies key words uid ow gabbros morocco shear zones volume change introduction

paleosecular variation record of geomagnetic full vector during late miocene from the nayarit area mexico abstract we have sampled a sequence of 45 late miocene consecutive lava flows in the tepic area nayarit state mexico the age of the volcanic units lies between 8 and 9 million years ma according to available radiometric data all samples were stepwise demagnetized partly with alternating field af partly thermally with very similar results most of the rocks exhibited well defined one component remanent magnetisation with high unblocking temperatures mostly above 525 c and high median destructive fields mdf 40 50 mt rock magnetic experiments combined with microscopy show that in most cases the main magnetic mineral is ti poor titanomagnetite associated with exsoluted ilmenite continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with curie points close to that of magnetite and pseudo single domain characteristics characteristic remanent magnetisations chrm isolated after the first steps of demagnetisation are all normal polarity according to the dispersion of virtual geomagnetic pole vgp directions paleosecular variation was abnormally lower than the one observed in general during miocene considering our paleomagnetic results together with available radiometric data it seems that the volcanic units have been emplaced during a very short time span of about 0 08 million years the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene thirty one samples from eight individual flows yielded acceptable paleointensity estimates the site mean paleointensities range from 27 8 0 9 to 42 0 7 9 t the virtual dipole moments vdm range from 5 9 to 9 5 1022 am2 this corresponds to a mean value of 7 6 1 4 1022 am2 which is higher than the average vdm value for late miocene altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations these results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core with the inner core tending to stabilise core convection and hence the field when intensity is high some fluctuation of absolute intensity was detected within the same directional group dg indicating that the intensity of the geomagnetic field varies faster than its direction

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

greenland ice sheet motion coupled with daily melting in late summer 1 we use ground based and satellite observations to detect large diurnal and longer period variations in the flow of the greenland ice sheet gris during late summer that are strongly coupled with changes in its surface hydrology the diurnal signals are associated with periodic changes in surface melting and the longer period signals are associated with the episodic drainage of supra glacial lakes ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards demonstrating an intimate link between the surface and basal hydrology during late summer the ice sheet accelerates by 35 per positive degree day of melting the observed link between surface melting and enhanced flow is typical of alpine glaciers which may provide an appropriate analogue for the evolution of the gris in a warming climate

undercutting of marine terminating glaciers in west greenland marine terminating glaciers control most of greenland s ice discharge into the ocean but little is known about the geometry of their frontal regions here we use side looking multibeam echo sounding observations to reveal that their frontal ice cliffs are grounded deeper below sea level than previously measured and their ice faces are neither vertical nor smooth but often undercut by the ocean and rough deep glacier grounding enables contact with subsurface warm salty atlantic waters aw which melts ice at rates of meters per day we detect cavities undercutting the base of the calving faces at the sites of subglacial water sgw discharge predicted by a hydrological model the observed pattern of undercutting is consistent with numerical simulations of ice melt in which buoyant plumes of sgw transport warm aw to the ice faces glacier undercutting likely enhances iceberg calving impacting ice front stability and in turn the glacier mass balance

diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica 1 recent gps observations from a spatially extensive network across ronne ice shelf show significant daily ice flow variations at all sites the almost synchronous horizontal displacements occur at diurnal and semidiurnal tidal periods during spring tides displacements velocities and strains near the ice front have superimposed oscillations that are 300 of their mean values and occur over a six hour period resulting in regular ice shelf flow reversals close to ice stream grounding lines however the horizontal diurnal and semidiurnal signals decay and almost vanish from our analysis we conclude that ice shelves respond primarily elastically to tidal tilting thus accounting for the observed diurnal and semidiurnal flow variations and their amplification toward the ice shelf front our findings suggest that detailed modeling of these data could provide improved ice shelf and ice stream models for correctly simulating ice shelf flow and predicting future ice sheet evolution citation makinson k m a king k w nicholls and g hilmar gudmundsson 2012 diurnal and semidiurnal tide induced lateral movement of ronne ice shelf antarctica geophys res lett 39 l10501 doi 10 1029 2012gl051636

evidence for non tidal diurnal velocity variations of helheim glacier east greenland we have used tide gauge data from near helheim glacier east greenland and gps data acquired on the glacier to investigate the spectra of tidal forcing and flow response for both the tide gauge and gps time series we calculated amplitudes and phases for a harmonic expansion using a limited set of harmonic constituents we find that the semidiurnal constituents of the glacier flow are well modeled using a single admittance and lag with respect to the tide gauge data however diurnal variations in the glacier flow cannot be simply described using this model we find an additional signal in glacier position in phase with the s1 solar tide with some modulations at other frequencies these non tidal variations account for a peak to peak variation in glacier flow speed at a site close to the terminus of 0 7md 1 compared with a mean flow rate at this location of 22md 1 the speed variations reach their daily maximum value 6 hours after local noon we hypothesize that these additional diurnal variations are associated with peaks in lubrication of the glacier bed due to surface melting driven by diurnal solar heating

mass balance of the greenland ice sheet from 1958 to 2007 we combine estimates of the surface mass balance smb of the greenland ice sheet for years 1958 to 2007 with measurements of the temporal variability in ice discharge d to deduce the total ice sheet mass balance during that time period we find a robust correlation r2 0 83 between anomalies in smb and in d which we use to reconstruct a continuous series of total ice sheet mass balance we find that the ice sheet was losing 110 70 gt yr in the 1960s 30 50 gt yr or near balance in the 1970s 1980s and 97 47 gt yr in 1996 increasing rapidly to 267 38 gt yr in 2007 multi year variations in ice discharge themselves related to variations in smb cause 60 20 more variation in total mass balance smb and therefore dominate the ice sheet mass budget copyright 2008 by the american geophysical union

radar interferogram filtering for geophysical applications the use of sar interferometry is often impeded by decorrelation from thermal noise temporal change and baseline geometry power spectra of interferograms are typically the sum of a narrow band component combined with broad band noise we describe a new adaptive filtering algorithm that dramatically lowers phase noise improving both measurement accuracy and phase unwrapping while demonstrating graceful degradation in regions of pure noise the performance of the filter is demonstrated with sar data from the ers satellites over the jakobshavns glacier of greenland

geology and wall rock alteration at the hercynian draa sfar zn pb cu massive sulphide deposit morocco abstract draa sfar is a siliciclastic felsic volcanogenic massive sulphide vms zn pb cu deposit located 15 km north of marrakesh within the jebilet massif of the western moroccan meseta the draa sfar deposit occurs within the sarhlef series a volcano sedimentary succession that hosts other massive sulphide deposits e g hajar kettara within the dominantly siliciclastic sedimentary succession of the lower central jebilet at draa sfar the footwall lithofacies are dominated by grey to black argillite carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes associated in situ and transported autoclastic deposits and lesser dykes of aphanitic basalt and gabbro thin to thick bedded black carbonaceous argillite minor intercalated siltstone and a large gabbro sill dominate the hanging wall lithofacies the main lithologies strike nne ssw parallel to a pronounced s1 foliation and have a low grade chlorite muscovite quartz albite oligoclase metamorphic assemblage the draa sfar deposit consists of two stratabound sulphide orebodies tazakourt to the south and sidi m barek to the north both orebodies are hosted by argillite in the upper part of the lower volcano sedimentary unit the tazakourt and sidi m barek orebodies are highly deformed sheet like bodies of massive pyrrhotite up to 95 pyrrhotite with lesser sphalerite galena chalcopyrite and pyrite the draa sfar deposit formed within a restricted sediment starved fault controlled anoxic volcano sedimentary rift basin the deposit formed at and below the seafloor within anoxic pelagic muds the argillaceous sedimentary rocks that surround the draa sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns in the more proximal volcanic area to the south the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the tazakourt orebody chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of fe and mg and a loss of k and na in the volcanically distal and northern sidi m barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite a calcite quartz muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire draa sfar deposit the sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow domes within the footwall succession

short term surface ice motion variations measured with a ground based portable real aperture radar interferometer we report measurements using a portable real aperture radar gamma portable radar interferometer gpri for interferometric imaging of the surface ice motion on gornergletscher switzerland during the drainage of the adjacent ice marginal lake gornersee the gpri tracked the surface ice motion in line of sight over an area of 3k m 2 down glacier of gornersee almost continuously during the drainage event the displacement maps derived from the acquired interferograms capture the spatial distribution of the surface ice motion due to fast acquisition times of the microwave images the gpri was able to record sub daily variations of the ice displacements most likely caused by the impact of the gornersee drainage on the ice motion of gornergletscher in situ point measurements of the ice displacement agree reasonably well with the results obtained by the gpri and highlight the use of the gpri for high resolution measurements of glacier surface ice motion motion of gornergletscher these variations in surface ice motion were triggered by the subglacial drainage of the adjacent marginal ice dammed lake gornersee we present ice displacement maps derived from the gpri interfero grams and analyze the performance of the radar device for remote sensing of surface ice motions of a glacier on spatial and temporal scales lower than in conventional d insar applications we compare the radar displacements against local in situ ice displacement measurements to complete the study a brief discussion of the observed displacements in relation to the drainage phenomena is given

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

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

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons