strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

forecasting change of the magnetic field using core surface flows and ensemble kalman filtering 1 accurate forecasting of the change of the earth s internal magnetic field over short intervals of time e g less than five years has many applications for government academic and commercial users forecasting can be achieved by making a number of reasonable assumptions about how the main field interacts with the flow in the liquid outer core in particular the magnetic field can be considered to be entrained in the large scale flow along the core mantle boundary surface over short time periods giving rise to measurable change at the earth s surface the observed change or secular variation at or above the surface of the earth can thus be inverted to produce flow models these can be used to propagate fluid parcels threaded by the field forwards in time to forecast the non linear change of the magnetic field in addition to prediction of field change by flow models it would be advantageous to include observations of the field from satellite measurements or ground based observatories we therefore present a method using ensemble kalman filtering enkf to produce an optimal assimilation between magnetic field change as forecast from core flow models and direct observations of the field we show by assuming a steady flow and assimilating field observations annually it is possible to produce a forecast over five years with less than 30nt root mean square difference from the true field within an assumed error budget the enkf method also allows sensitivity analysis of the field models to noise and uncertainty within the physical representation

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

structural controls of lode gold mineralization by mafic dykes in late paleozoic granitoids of the kochkar district southern urals russia the kochkar gold district in the east uralian zone of the southern urals is located in late paleozoic granite gneisses of the plast massif gold mineralization is associated with tabular quartz lodes that are preferentially developed along the margins of easterly trending mafic dykes fabric development indicates that dykes had a profound influence on the development of shear zones in granitoids ene and se trending dykes have been reactivated as dextral and sinistral oblique strike slip shear zones respectively forming a set of approximately conjugate shear zones related to the permian regional scale e w directed shortening dyke shear zone relationships in the plast massif are the result of strain refraction due to the presence of biotite rich incompetent dykes in more competent granite gneisses deformation and the formation of associated gold quartz lodes occurred close to peak metamorphic upper greenschist to lower amphibolite facies conditions strain refraction has resulted in partitioning of the bulk strain into a component of non coaxial mainly ductile shear in mafic dykes and a component of layer normal pure shear in surrounding granitoids where deformation was brittle ductile brittle fracturing in granitoids has resulted in the formation of fracture permeabilities adjacent to sheared dykes that together with the layer normal dilational component promoted the access of mineralizing fluids both ore controlling dykes and gold quartz lodes were subsequently overprinted by lower greenschist facies mainly brittle fault zones and associated hydrothermal alteration that post date gold mineralization

geomagnetic evidence for fluid upwelling at the core mantle boundary summary previous studies both geomagnetic and seismic have been unable to show conclusively whether or not there is fluid upwelling at the core mantle boundary here a new method is developed in which an attempt is made to invert geomagnetic secular variation data measured at the earth s surface for a frozen flux purely toroidal core mantle boundary cmb velocity field under the assumption that the mantle is electrically insulating and flux is frozen in at the cmb these data have previously been inverted for the core mantle boundary radial secular variation from which the appropriate fit between model and data is known two different main field models were used to assess the effect of uncertainty in its radial component at the cmb the conclusions were the same in both cases frozen flux purely toroidal motions provide a poor fit a statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen flux velocity fields at the cmb if upwelling and down welling is included the inherent non uniqueness in the velocity field obtained is not of concern since only their statistical properties are utilized and no physical significance is attached to the flows obtained

grimm the gfz reference internal magnetic model based on vector satellite and observatory data summary in this paper the new gfz reference internal magnetic model grimm is presented the model has been derived from nearly 6 yr of champ satellite data and 5 yr of observatory hourly means at high latitudes full vector satellite data are used at all local times which allows a separation between on one hand the fields generated by ionosphere and field aligned currents and on the other hand the fields generated in the earth s core and lithosphere this selection technique leads to a data set without gaps during the polar summers resulting in a core field model that has an unprecedented time resolution the modelled static core field secular variation and lithospheric field are all in good agreement with previously published magnetic field models order five b splines are used to model the variation in time of the core field the energy in the secular acceleration has therefore a smooth behaviour in time and increases continuously from 2003 5 mapping the field acceleration from 2001 5 to 2005 5 reveals its rapid and complex evolution over this time period at the earth s surface due to the applied regularization technique the acceleration energy in spherical harmonics 6 11 is significantly larger than for other models and we show that such a spectrum is acceptable

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

can core surface flow models be used to improve the forecast of the earth s main magnetic field 1 geomagnetic main field models used for navigation are updated every 5 years and contain a forecast of the geomagnetic secular variation for the upcoming epoch forecasting secular variation is a difficult task the change of the main magnetic field is thought to be principally due to advection of the field by flow at the surface of the outer core on short timescales and when large length scales are considered with accurate secular variation sv and secular acceleration sa models now available from new satellite missions inverting for the flow and advecting it forward could lead to a more accurate prediction of the main field however this scheme faces two significant challenges the first arises from the truncation of the observable main field at spherical harmonic degree 13 this can however be handled if the true core flow is large scale and has a rapidly decaying energy spectrum the second is that even at a given single epoch the instantaneous sv and sa cannot simultaneously be explained by a steady flow nevertheless we find that it may be feasible to use flow models for an improved temporal extrapolation of the main field a medium term 10 years hindcast of the field using a steady flow model outperforms the usual extrapolation using the presently observed sv and sa on the other hand our accelerated toroidal flow model which explains a larger portion of the observed average sa over the 2000 2005 period fails to improve both the short term and medium term hindcasts of the field this somewhat paradoxical result is related to the occurrence of so called geomagnetic jerks the still poorly known dynamical nature of which remains the main obstacle to improved geomagnetic field forecasts

the vilalba gold district a new discovery in the variscan terranes of the nw of spain a geologic fluid inclusion and stable isotope study abstract this work defines the vilalba gold district a new gold region in the nw of spain comprising three mineralized areas castro de rei valina azumara and arcos and two different types of mineralization 1 w au skarn in castro de rei and 2 au as ag pb zn cu sb vein disseminated mineralization in valina azumara and arcos mineralization is mainly hosted by impure limestone and black slates lower cambrian and spatially related to variscan and late variscan structures rhyolite dikes occur in arcos 299 6 ma and castro de rei both consisting of peraluminous high potassium and calc alkaline rocks these dikes represent the fractionated expression of unexposed post tectonic granitoids located at depth the castro de rei area exhibits a contact aureole with biotite cordierite and calc silicate hornfels prograde skarn consists of garnet pyroxene quartz wollastonite and scheelite the retrograde alteration took place in two stages and mainly consists of amphibole epidote zoisite quartz calcite chlorite and sulfides gold mineralization in the skarn occurs as invisible gold in sulfoarsenides and as electrum related to bi te s minerals the mineralization of valina azumara and arcos comprises two stages the first consists of as rich pyrite and arsenopyrite with invisible gold the second stage mainly consists of base metal sulfides and ag pb zn cu sb sulfosalts hydrothermal fluids involved in the skarn formation are low salinity up to 6 6 wt nacl eq aqueous carbonic fluids h2o co2 ch4 n2 evolving into aqueous fluids during the last stage of gold mineralization stable isotope geochemistry indicates the importance of a magmatic fluid in the early stages 18osmow from 7 5 to 11 34scdt from 2 7 to 5 9 that evolved through interaction with host rocks and mixing with metamorphic and or meteoric fluids the 34scdt values 7 8 10 9 of second retrograde stage sulfides suggest the incorporation of 34s enriched from host rocks in valina azumara and arcos the fluid inclusion and isotope studies indicate similar hydrothermal fluids to those associated with the second retrograde skarn stage taking into account the fluid inclusion study and mineral geothermometry an attempt was made to determine the p t conditions of ore formation the prograde skarn of castro de rei formed at temperatures and pressures ranging from 520 to 560 c and from 1 7 to 2 5 kb the mineralization of valina azumara formed at minimum p t conditions of 1 7 kb and 355 c the estimated temperatures for the arcos mineralization are similar to those of valina azumara up to 383 c but at lower pressures we propose the model of intrusion related gold systems irgss to explain the mineralizations of the vilalba gold district in this model the castro de rei skarn and the valina azumara mineralization represent a proximal and a distal deposit respectively relative to an unexposed granitoid situated at depth we also consider arcos as a distal deposit in relation to another unexposed granitoid located further south in the district

shear related gold mineralization in northwest ghana the julie deposit abstract the julie deposit is currently the largest gold prospect in nw ghana it is hosted in sheared granitoids of ttg composition of the paleoproterozoic julie greenstone belt the main mineralization consists of a corridor of gold bearing quartz veins forming a network of a few tens of metres in thickness trending e w and dipping 30 60 n contained within the main shear zone that affects these rocks the core of this vein corridor is altered by sericite quartz ankerite calcite tourmaline and pyrite and is surrounded by an outer halo consisting of albite sericite calcite chlorite pyrite and rutile a second set of veins conjugate to the first set occurs in the area these veins have alteration halos with a similar mineralogy as the main corridor however their extent as well as the size of the mineralization is less important in the main corridor gold forms micron sized grains that occur in pyrite as inclusions on its edges and in fractures crosscutting it silver tellurium bismuth copper and lead commonly accompany the gold pyrite occurs disseminated in the veins and in the surrounding rocks up to several ppm au occurs in the structure of pyrite from the main mineralization

role of shape and numbers of ridges and valleys in the insulating effects of topography on the rayleigh wave characteristics this research work is inspired by the recently accepted concept that high frequency rayleigh waves are generated in the epicentral zone of shallow earthquakes such high frequency rayleigh waves with large amplitude may develop much of spatial variability in ground motion which in turn may cause unexpected damage to long span structures like bridges underground pipelines dams etc in the hilly regions further it has been reported that topography acts as an insulator for the rayleigh waves ma et al bssa 97 2066 2079 2007 the above mentioned scientific developments stimulated to quantify the role of shape and number of ridges and valleys falling in the path of rayleigh wave in the insulating effect of topography on the rayleigh waves the simulated results reveals very large amplification of the horizontal component of rayleigh wave near the top of a triangular ridge which may cause intensive landslides under favorable condition the computed snapshots of the wave field of rayleigh wave reveals that the interaction of rayleigh wave with the topography causes reflection splitting and diffraction of rayleigh wave in the form of body waves which in turn provides the insulating capacity to the topography insulating effects of single valley is more than that of single ridge further this effect was more in case of elliptical ridge valley than triangular ridge valley the insulating effect of topography was proportional to the frequency of rayleigh wave and the number of ridges and valleys in the string the obtained level of insulation effects of topography on the rayleigh wave energy of rayleigh wave reduced to less than 4 after crossing a topography of span 4 5 km calls for the consideration of role of hills and valleys in seismic hazard prediction particularly in case of shallow earthquakes

the kalana vein hosted gold deposit southern mali abstract the kalana gold deposit occurs within metamorphosed paleoproterozoic volcano sedimentary series of the birimian baoule mossi domain of the west african craton in the south of mali these formations are intruded successively by small dioritic bodies as well as andesitic and tonalitic dykes which exhibit volcanic arc setting signatures mineralization is hosted by two sets of quartz veins that intersect the regional schistosity the first set of veins is the most important in terms of grades and size and consists of thick veins up to meter size that range in trend from n s to e w with shallow plunges the second set consists of much thinner centimeter size sub vertical quartz veins oriented ne sw the two sets of veins are interpreted to have formed during the evolution of late gently dipping thrust faulting two episodes of gold precipitation are recognized a first episode during early stages of vein growth formed micron size native gold inclusions in arsenopyrite a later episode during vein shearing and fracturing of the quartz lodes precipitated native gold in free form in quartz in fractured arsenopyrite and associated with chalcopyrite galena sphalerite bi sulfides and native bismuth all evidence suggests that the kalana deposit represents orogenic gold mineralization formed during a relatively long lived hydrothermal system at a late stage of the tectonic history of the greenstone belt

rapidly changing flows in the earth s core variations in the earth s magnetic field over a span of a few months can be resolved despite the potential filtering effects of the electrically conducting mantle and are indicative of rapid flow in the earth s outer core

a new approach to directly determine the secular variation from magnetic satellite observations 1 observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location unfortunately determination of the global pattern of changes using observatory data is hampered by their uneven distribution satellite data provide excellent global coverage but the spacecraft movement makes direct comparisons of satellite and observatory data difficult to investigate short period secular variation in observatory and satellite data we developed an approach to extract satellite monthly means for virtual observatories at 400 km altitude using champ magnetic measurements comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well correlated signal at time scales of months to years which is beyond the temporal resolution limit of recent global models here we describe this newly developed approach its validation and discuss how it can be used to understand short period changes of the recent geomagnetic field

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

thrust related dome structures in the karibib district and the origin of orthogonal fabric domains in the south central zone of the pan african damara belt namibia abstract the high grade metamorphic south central zone scz exposes an oblique crustal section through the internal parts of the pan african damara belt in namibia the structural pattern of the south central zone is characterized by kilometer scale northeast trending dome structures for which a number of different origins have been proposed detailed structural mapping of the karibib and usakos domes in the karibib district indicates an origin of the two domes as large tip line folds located above blind thrusts the two domes are overridden from the southeast by a crystalline thrust sheet along the mon repos thrust zone and the thrusts and associated folds form part of a deeply eroded foreland northwest vergent fold and thrust belt intrusive relationships of syn and post tectonic granitoids constrain the timing of thrusting to between 550 and 540 ma corresponding to the main phase of collisional tectonics in the damara belt on a regional scale the south central zone is underlain by two orthogonal fabric domains northwest vergent tectonic transport documented in this study dominates the lower amphibolite facies northeastern parts of the south central zone in contrast a southwest directed orogen parallel extrusion is widely documented for the lower structural levels of the upper amphibolite to granulite facies southwestern parts of the south central zone that contain voluminous synkinematic granites and widespread migmatization fabric development in the two orthogonal domains can be shown to be contemporaneous and related to the main collision between the congo and kalahari cratons at ca 540 550 ma we suggest that the formation of the two orthogonal fabric domains represents the different crustal response of the rheologically layered crust to the late proterozoic crustal shortening at shallower crustal levels shortening of sufficiently strong crust in the karibib district resulted in vertical thickening and orogen normal folding and thrust transport shortening of the deeper levels of the scz resulted in pure shear deformation of the rheologically weakened and partially molten crust and the lateral orogen parallel displacement of rocks

third generation of the potsdam magnetic model of the earth pomme the potsdam magnetic model of the earth pomme is a geomagnetic field model providing an estimate of the earth s core crustal magnetospheric and induced magnetic fields the internal field is represented to spherical harmonic sh degree 90 while the secular variation and acceleration are given to sh degree 16 static and time varying magnetospheric fields are parameterized in geocentric solar magnetospheric gsm and solar magnetic sm coordinates and include disturbance storm time dst index and interplanetary magnetic field imf by dependent contributions the model was estimated from five years of champ satellite magnetic data all measurements were corrected for ocean tidal induction and night side ionospheric f region currents the model is validated using an independent model from a combined data set of orsted and sac c satellite measurements for the core field to sh degree 13 the root mean square rms vector difference between the two models at the center of the model period is smaller than 4 nt at the earth s surface the rms uncertainty increases to about 100 nt for the predicted field in 2010 as inferred from the difference between the two models

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

the relationship between the structural orientation and the gold mineralisation of quartz sulphide veins in the navachab gold deposit namibia abstract this study uses a representative dataset of more than 500 vein samples from the navachab gold deposit containing information on the gold concentration structural orientation vein thickness alteration and host rock type to investigate small scale controls on the grade distribution in the veins in order to obtain a better understanding of the architecture of the navachab ore bodies we calculated mean gold concentrations for each vein set in different parts of the mine we then examine the preferential mineralisation of certain vein sets by setting this grade information into context to other geological parameters such as the structural geology of that particular part of the mine as well as the above mentioned factors structural orientation of the vein vein thickness alteration and host rock type our main findings are that vein sets oriented favourably to the stress field and at high angles to the bedding are better mineralised than veins sets oriented at low angles to the bedding other factors do not exert a great influence on the gold concentration in the veins we propose that the gold grade in the veins is influenced by the effectiveness of fluid flow into the veins and by time rates during vein propagation and fluid flow