correction to strong ferrimagnetic resonance signal and magnetic susceptibility of the nojima pseudotachylyte in japan and their implication for coseismic electromagnetic changes 1 pseudotachylyte is a glassy fault rock providing certain evidence of ancient seismic fault movement i examine the nojima pseudotachylyte found along the nojima fault which caused the 1995 kobe earthquake m 7 2 in japan using the electron spin resonance technique the nojima pseudotachylyte has a strong ferrimagnetic resonance fmr signal derived from bulky trivalent iron ions in ferrimagnetic iron oxides this fmr signal appears by heating the surrounding fault gouge that is the source material of the nojima pseudotachylyte furthermore the magnetic susceptibilities of the nojima pseudotachylyte are 10 2 10 3 times larger than those of the surrounding fault gouge and are proportional to the fmr signal intensities here i reconstruct the magnetization process of fault gouge during ancient seismic fault slip using the fmr signal the results indicate that the instantaneous magnetization of the nojima fault gouge may have induced instantaneous geomagnetic changes and that the instantaneous geomagnetic changes may have further induced geoelectric changes by electromagnetic induction

the role of ice thickness and bed properties on the dynamics of the enhanced flow tributaries of bailey ice stream and slessor glacier east antarctica

uplift and tilting of the shackleton range in east antarctica driven by glacial erosion and normal faulting unravelling the long term evolution of the subglacial landscape of antarctica is vital for understanding past ice sheet dynamics and stability particularly in marine based sectors of the ice sheet here we model the evolution of the bedrock topography beneath the recovery catchment a sector of the east antarctic ice sheet characterized by fast flowing ice streams that occupy overdeepened subglacial troughs we use 3d flexural models to quantify the effect of erosional unloading and mechanical unloading associated with motion on border faults in driving isostatic bedrock uplift of the shackleton range and theron mountains which are flanked by the recovery slessor and bailey ice streams inverse spectral free air admittance and forward modeling of topography and gravity anomaly data allow us to constrain the effective elastic thickness of the lithosphere te in the shackleton range region to 20 km our models indicate that glacial erosion and the associated isostatic rebound has driven 40 50 of total peak uplift in the shackleton range and theron mountains a further 40 50 can be attributed to motion on normal fault systems of inferred jurassic and cretaceous age our results indicate that the flexural effects of glacial erosion play a key role in mountain uplift along the east antarctic margin augmenting previous findings in the transantarctic mountains the results suggest that at 34 ma the mountains were lower and the bounding valley floors were close to sea level which implies that the early ice sheet in this region may have been relatively stable

reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

permeability dependence of streaming potential in rocks for various fluid conductivities streaming potentials have been measured on sandstone and limestone samples in a large range of permeabilities the electrokinetic coupling coefficient increases with permeability and we explain this effect by the related variation of surface conductivity a model is proposed to study this effect for various fluid conductivities and it is shown that the dependence of the electrokinetic coupling coefficient on permeability is stronger for high fluid resistivity and is weaker for lower fluid resistivity when fluid resistivity is below 1 m permeability and streaming potential are no more related

geomagnetic anomaly maps of central victoria land east antarctica from ground measurements abstract the results of an extensive ground geomagnetic survey over the central victoria land east antarctica between the transantarctic mountains and the ross sea are used to plot the residual field map of the area under consideration the data indicate that intense positive anomalies occur along two corridors one near mt nansen and the other near mt melbourne further elaboration of our measurements has produced three other geomagnetic maps 1 a high pass filter cut off wavelength of 20 km map 2 a low pass filter cut off wavelength of 80 km map and 3 an upward continuation of the geomagnetic field up to 12000 ft 3660 m map the residual field map and the high pass filter geomagnetic map indicate that all the intense anomalies both positive and negative are of shallow origin the low pass filter map and the upward continuation map show that the land mass of the central victoria land is characterized by a weak negative geomagnetic anomaly whereas the near shore area close to terra nova bay displays a weak positive anomaly our measurements indicate that all rock types characterized by high susceptibility produce geomagnetic anomalies even if their outcrop is not extensive these rocks belong to the mt melbourne volcanics jurassic dolerites and granite harbour plutons none of the formations of the metamorphic complex are responsible for intense magnetic signals

the 2004 parkfield earthquake test of the electromagnetic precursor hypothesis 1 a controversy has existed for 30 years concerning the possibility of earthquake prediction using electromagnetic precursors long term electromagnetic monitoring prior to during and after the m6 0 earthquake at parkfield california on 28 september 2004 now provides a definitive test of this hypothesis during the earthquake our instruments recorded clearly documented electrical signals from an earthquake impulsive changes of up to 2 5 mv at an electrode located 250 m from the rupture zone followed by a transient decay lasting at least 3 1 4 hours similar signals 1 5 mv were recorded with the only two m 5 aftershocks but their transient decays lasted for only 17 min these signals are unambiguously a result of the earthquake and can be useful in studies of fluid flow in faults a major current topic in earthquake physics patterns in the distribution of transient voltages differ from static stress changes but are consistent with a small coseismic pressure drop on the fault and fluid flow inward signals at a distance of 1 km from the fault are less than 1 mv and are statistically no different from zero in our analysis transient voltages for the aftershocks have opposite polarity from those recorded with the main shock and are again maximal close to the fault suggesting subsequent outward flow from the fault finally there is no evidence of any precursory signal strongly calling into question previously suggested signals preceding smaller or more distant earthquakes

seismic electric signals ses and earthquakes a review of an updated van method and competing hypotheses for ses generation and earthquake triggering abstract electromagnetic phenomena are sometimes associated with seismic events but earthquake prediction using seismic electric signals ses has not been seriously considered since the early 1990s there are several causes 1 false alarms that have created panics in greece and 2 a strong critique of the varotsos alexopoulos nomicos van method used there an updated van method that includes time series analysis has made successful medium range predictions and short range predictions when coupled with seismic time series analysis four types of potential mechanisms to create precursory ses relying on deformation effects temperature effects ore mineral movement or groundwater variation are reviewed data relevant to each are insufficient to favor a single mechanism records of the number of false positive and missed events for an updated van method have not been well maintained false positives appear to be abundant and render the method unusable co seismic groundwater fluctuations are addressed and two novel hypotheses of earthquake trigger mechanism are also presented ionospherically induced electric and resultant magnetic fields to weaken rock thereby causing failure and volatiles liberated from minerals additionally promoting rock failure recommendations for further study are included

improved magnetic anomalies of the antarctic lithosphere from satellite and near surface data summary the antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60 s through 1999 and used magsat data to help fill in the regional gaps between the surveys orsted and champ satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the antarctic have now supplanted the magsat data we combined the new satellite observations with the near surface survey data for an improved magnetic anomaly map of the antarctic lithosphere specifically we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite derived free air gravity observations regional gaps in the near surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near surface and satellite crustal anomalies comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models we also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near surface crustal anomaly estimates

laboratory studies of electrical potential during rock failure we have investigated electrical potential and acoustic emissions signals associated with rock deformation five rock types were studied clashach bentheim and darley dale sandstones all quartz rich and a seljadur basalt and portland limestone both quartz free both air dry and the rocks were tested in distilled water shallow crustal conditions were simulated in a triaxial rock deformation cell with a confining pressure simulating depth of 40 mpa pore pressures ranging 5 35 mpa and strain rates 10 7 10 4 s 1 precursory electric potential signals prior to failure were observed in both saturated and dry samples of the quartz rich sandstones but only observed in the water saturated quartz free rocks co seismic electrical signals were obtained in all tests providing strong evidence that two of the main sources for precursory and co seismic signals are the piezoelectric and electrokinetic phenomena lowering the strain rate resulted in an increase in the number of acoustic emissions the pore volume changes during compaction and dilatancy remained approximately constant for all strain rates streaming potential generated by fluid flow across the sample was also measured at different stages of deformation the potential signals increased with the pore pressure gradient c 2005 elsevier ltd all rights reserved

the british siluro devonian palaeofield the great glen fault and analytical methods in palaeomagnetism comments on paper by k me storetvedt et al summary late silurian early devonian palaeomagnetic poles throughout the british isles lie in a coherent group about 1 s and 314 e a95 9 6 the clustering of these poles which are derived from 11 individual studies of siluro devonian rocks of all the major tectonic elements of britain carries two important tectonic implications 1 the british sector of the iapetus ocean recognizable from ordovician poles had closed by late silurian times and 2 any postulated mega shear whether related to acadian or hercynian deformation is below the limit of palaeomagnetic resolution the collective rejection of all siluro devonian results by storetvedt et al 1990a b is demonstrated to be unfounded

subglacial geology and geomorphology of the pensacola pole basin east antarctica the east antarctic ice sheet eais is underlain by a series of low lying subglacial sedimentary basins the extent geology and basal topography of these sedimentary basins are important boundary conditions governing the dynamics of the overlying ice sheet this is particularly pertinent for basins close to the grounding line wherein the eais is grounded below sea level and therefore potentially vulnerable to rapid retreat here we analyze newly acquired airborne geophysical data over the pensacola pole basin ppb a previously unexplored sector of the eais using a combination of gravity magnetic and ice penetrating radar data we present the first detailed subglacial sedimentary basin model for the ppb radar data reveal that the ppb is defined by a topographic depression situated 500 m below sea level gravity and magnetic depth to source modeling indicate that the southern part of the basin is underlain by a sedimentary succession 2 3 km thick this is interpreted as an equivalent of the beacon supergroup and associated ferrar dolerites that are exposed along the margin of east antarctica however we find that similar rocks appear to be largely absent from the northern part of the basin close to the present day grounding line in addition the eastern margin of the basin is characterized by a major geological boundary and a system of overdeepened subglacial troughs we suggest that these characteristics of the basin may reflect the behavior of past ice sheets and or exert an influence on the present day dynamics of the overlying eais

near lithostatic pore pressure at seismogenic depths a thermoporoelastic model summary a model is presented for pore pressure migration through a transition layer separating a meteoric aquifer at hydrostatic pressure from a deeper reservoir at lithostatic pressure this configuration is thought to be pertinent to the south iceland seismic zone sisz and to other tectonically active regions of recent volcanism where volatiles are continuously released by ascending magma below the brittle ductile transition poroelastic parameters are computed for basaltic rock the model is 1 d the fluid viscosity is temperature dependent and rock permeability is assumed to be pressure dependent according to a dislocation model of a fractured medium environment conditions are considered pertinent to basalt saturated with water at shallow depth case i and at mid crustal depth case ii if the intrinsic permeability of the rock is high no significant effects are observed in the pressure field but advective heat transfer shifts the brittle ductile transition to shallower depths if the intrinsic permeability is low the pressure dependent permeability can propagate near lithostatic pore pressures throughout most of the transition layer while the temperature is practically unaffected by advective contributions so that the rock in the transition layer remains in brittle condition geometrical parameters characterizing the fracture distribution are important in determining the effective permeability in particular if an interconnected system of fractures develops within the transition layer the effective permeability may increase by several orders of magnitude and near lithostatic pore pressure propagates upwards these modelling results have important bearings on our understanding of seismogenic processes in geothermal areas and are consistent with several geophysical observations in the sisz in connection with the two 2000 june m 6 5 earthquakes including i fluid pressure pulses in deep wells ii low resistivity at the base of the seismogenic layer iii low vp vs ratio and time dependent seismic tomography iv heterogeneity of focal mechanisms v shear wave splitting vi high b value of deep foreshocks vii triggered seismicity and viii radon anomalies

influence of subglacial conditions on ice stream dynamics seismic and potential field data from pine island glacier west antarctica 1 we interpret seismic reflection and airborne potential field data acquired on pine island glacier west antarctica and find variations in the subglacial geology which correlate with variations in ice dynamics immediately beneath the glacier is a mixture of soft deforming sediments and harder non deforming sediments beneath this a sedimentary basin lies under part of the main glacier with another under one of its slower moving tributaries a tectonic boundary underlies the main trunk of the glacier separating these sedimentary basins to the north from crystalline rocks to the south which also include a thick rift related magmatic intrusion the boundary correlates with changes in the basal roughness ice flow speed and basal drag smoother bed faster flow and lower basal drag characterize the thicker sedimentary sequences to the north but there is no corresponding lateral change in the acoustic properties of the bed changes in the sub bed i e deeper than the ice bed interface lithology appear to account for the contrasting basal drag and ice velocity patterns over the glacier subglacial erosion could remove a thin layer of soft sediments to the south of the geological boundary leading to increased basal drag and reduced ice flow in the future we conclude that the subglacial geology plays a significant role in controlling the spatial pattern of present day ice flow and that the consequences of subglacial erosion may be reflected in temporal changes to the ice dynamics in the past and perhaps also in the near future

physical properties of the variations of the electric field of the earth preceding earthquakes ii determination of epicenter and magnitude abstract as reported in the preceding paper a transient change of the electric field of the earth seismic electric signal hereafter called ses appears many hours before an earthquake eq by measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction measurements in two directions e w and n s give the relative signal intensity jrel at the station under consideration by measuring jrel at a number of stations and considering that it attenuates according to a 1 r law the epicenter can be determined with an accuracy usually around 100 km once the epicenter has been determined the product jrel r can be evaluated so that the magnitude m can be estimated by resorting to an empirical log jrel r versus m plot the uncertainty of m is around 0 5 units following sobolev 1975 and for the statistics to be beyond any doubt predictions were officially documented before the eq occurrence for 23 earthquakes with a magnitude equal or greater than ms 5 0 two events were missed the present method is compared to other electrical methods used in china japan and soviet union a number of problems concerning the origin of the effect its directivity and the attenuation with distance remain open for further studies

electric potential changes prior to shear fracture in dry and saturated rocks electric potential changes before shear rupture were measured using darley dale sandstone quartz rich and icelandic basalt quartz free on both dry specimens and in the presence of pore fluid we find that electric potential changed markedly just prior to dynamic rupture in dry and saturated sandstones and saturated basalt but we did not detect precursory signals in dry basalt the absence of signals in dry basalt provides strong evidence that the piezoelectric effect and electrokinetic effect are dominant sources for precursory signals moreover we find that the amplitude of the precursory signals due to electrokinetic effect in saturated sandstone were as large as the coseismic signals we propose that this signal is caused by accelerating evolution of dilatancy as cracks grow in the rock before rupture resulting in water flow into the dilatant region with an electric current produced concurrently

role of pore fluids in the generation of seismic precursors to shear fracture a systematic study of temporal changes in seismic b values defined as the log linear slope of the earthquake frequency magnitude distribution has shown that large earthquakes are often preceded by an intermediate term increase in b followed by a decrease in the months to weeks before the earthquake1 the onset of the b value increase can precede earthquake occurrence by as much as 7 years a recently proposed fracture mechanics model of the earthquake source2 explains these temporal fluctuations in b in terms of the underlying physical processes of time varying applied stress and crack growth the model predicts two minima in b separated by a short lived maximum here we report the results of controlled laboratory deformation experiments done in simulated upper crustal conditions on both air dried and water saturated rock specimens as found in previous experiments3 5 shear fracture in dry specimens is characterized by a decline in b during anelastic deformation to a single minimum reached just before failure but in water saturated specimens when pore fluid volume is kept constant by servo control we also observe a second intermediate term b value minimum so reproducing the double b value anomaly predicted by the model2

time resolved study of charge generation and propagation in igneous rocks electrical resistivity changes ground potentials electromagnetic em and luminous signals preceding or accompanying earthquakes have been reported many times in addition to ground uplift and tilt and other parameters however no concept exists that would tie these diverse phenomena together into a physically coherent model using low to medium velocity impacts to measure electrical signals with microsecond time resolution it is observed that when gabbro and diorite cores are impacted at relatively low velocities 100 m s highly mobile charge carriers are generated in a small volume near the impact point they spread through the rocks causing electric potentials exceeding 400 mv em and light emission as the charge cloud spreads the rock becomes momentarily conductive when a granite block is impacted at higher velocity 1 5 km s the propagation of the p and s waves is registered through the transient piezoelectric response of quartz after the sound waves have passed the surface of the granite block becomes positively charged suggesting the same charge carriers as observed during the low velocity impact experiments expanding from within the bulk during the next 2 3 ms the surface potential oscillates indicating pulses of electrons injected from ground and contact electrodes the observations are consistent with positive holes e g defect electrons in the o2 sublattice traveling via the o 2p dominated valence band of the silicate minerals the positive holes propagate as charge clouds rather than as classical em waves before activation they lay dormant in form of electrically inactive positive hole pairs php chemically equivalent to peroxy links o3x oo xo3 with x si4 al3 etc phps are introduced into the minerals by way of hydroxyl o3x oh which all nominally anhydrous minerals incorporate when crystallizing in h2o laden environments the fact that positive holes can be activated by low energy impacts and their attendant sound waves suggests that they can also be activated in the crust by microfractures during the dilatancy phase depending on where in the stressed rock volume the charge carriers are activated they will form rapidly moving or fluctuating charge clouds that can account for earthquake related electrical signals and em emission wherever such charge clouds intersect the surface high fields are expected causing electric discharges and earthquake lights

using a 3d gravity inversion technique to image the subsurface density structure in the lunayyir volcanic field saudi arabia abstract an earthquake of magnitude m 5 4 occurred on 19 may 2009 in the lunayyir volcanic field lvf saudi arabia the lvf is covered by recent basaltic rocks the earthquake caused damages to the surrounding areas and resulted in surficial rapture trending in nnw sse direction with about 8 km length due to concern in future damaging earthquakes or volcanic activities the government evacuated the residents for over 3 months seismologists from the saudi geological survey sgs studied the recorded seismicity data around the damaged area to locate the epicenter of the m 5 4 earthquake after extensive studies they suggested that the main cause of the m 5 4 event was magma dike intrusion at depth of about 5 km however the dike did not reach the surface and no eruption took place detailed ground and airborne gravity surveys around the earthquake area was carried out in an attempt to understand the reason of the earthquake and imaging the subsurface geology in that regard 380 gravity stations were measured in an area of about 600 km2 covering the surficial rupture the gravity data was analyzed using edge detection tools and a 3d inversion technique the results indicate that the 8 km surficial rupture extends toward the se direction into the sub surface which suggested a future seismic activity along this extension additionally the dike intrusion location has low density values compared with the surrounding area which imply that this dike intrusion was created in the past cooled and solidified consequently we assign the m 5 4 earthquake and the associated seismic swarm to simultaneous red sea rift parallel faulting and magmatic intrusion

electric potential changes associated with slip failure of granite preseismic and coseismic signals electric potential changes were measured for stick slip events in granite samples with a three block direct shear arrangement at 8 mpa normal stress two electrodes were mounted on the left and right hand blocks and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from dc to 100 hz as well as coseismic electric signals of about 1 5 v which appeared the moment of the dynamic slip event preseismic signals were detected just before the slip event the coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of e s where e is the permittivity and s is the conductivity of the rock sample we conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal this suggests that the electric signal is generated by the piezoelectric effect we proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response which is in agreement with experimental data the preseismic signal appears about 2 3 s before the dynamic event with an amplitude of about 50 mv the local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone when the growth of the rupture nucleation zone occurred on the left sliding surface a clear preseismic signal was detected at the electrode mounted on the left granite block when the growth occurred on the right hand surface a signal was detected at the electrode on the right block this shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone these preseismic and coseismic signals were also detected with an antenna which was placed away from the sample surface

short time electrical effects during volcanic eruption experiments and field measurements laboratory experiments on the fragmentation and expansion of magmatic melt have been performed using remelted volcanic rock at magmatic temperatures as magma simulant a specially designed dc amplifier in combination with high speed data recording was used to detect short time electrostatic field effects related to the fragmentation and expansion history of the experimental system as documented by simultaneous force and pressure recording as well as by high speed cinematography it was found that 1 the voltage time ratio of electrostatic field gradients 100 to 104 v s reflects different physical mechanisms of fragmentation and expansion and 2 the maximum voltage measured in 1 m distance 0 1 to 180 v can be correlated with the intensity of the respective processes based on these experimental results a field method was developed and tested at stromboli volcano in italy a 0 8 m rod antenna was used to detect the dc voltage against local ground i e the electrostatic field gradient at a distance of 60 to 260 m from the respective vent upwind position of the detection site was chosen to prevent interference caused by contact of charged ash particles with the antenna a standard 8 hz geophone was used to detect the accompanying seismicity three types of volcanic activity occurred during the surveillance operation two of these could be clearly related to specific electrical and seismical signals a typical delay time was found between the electrical and the seismical signal corresponding to the seismic velocity within the crater deposits using a simple first order electrostatic model the field measurements were recalibrated to the laboratory scale comparison of field and laboratory data at first approximation revealed striking similarities thus encouraging the further development of this technique for real time surveillance operation at active volcanoes

self potential observations during hydraulic fracturing 1 the self potential sp response during hydraulic fracturing of intact sierra granite was investigated in the laboratory excellent correlation of pressure drop and sp suggests that the sp response is created primarily by electrokinetic coupling for low pressures the variation of sp with pressure drop is linear indicating a constant coupling coefficient cc of 200 mv mpa however for pressure drops 2 mpa the magnitude of the cc increases by 80 in an exponential trend this increasing cc is related to increasing permeability at high pore pressures caused by dilatancy of microcracks and is explained by a decrease in the hydraulic tortuosity resistivity measurements reveal a decrease of 2 prior to hydraulic fracturing and a decrease of similar to 35 after fracturing an asymmetric spatial sp response created by injectate diffusion into dilatant zones is observed prior to hydraulic fracturing and in most cases this sp variation revealed the impending crack geometry seconds before failure at rupture injectate rushes into the new fracture area where the zeta potential is different than in the rock porosity and an anomalous sp spike is observed after fracturing the spatial sp distribution reveals the direction of fracture propagation finally during tensile cracking in a point load device with no water flow a sp spike is observed that is caused by contact electrification however the time constant of this event is much less than that for transients observed during hydraulic fracturing suggesting that sp created solely from material fracture does not contribute to the sp response during hydraulic fracturing

a combined complex electrical impedance and acoustic emission study in limestone samples under uniaxial loading abstract in the present work complex electrical impedance measurements in the frequency range of 10 mhz to 1 mhz were carried out in conjunction with acoustic emission monitoring in limestone samples subjected to linear and stepped like uniaxial loading up to ultimate failure cole cole plots of the complex impedance during the stepped loading of limestone have been used to discriminate the contributions of grains interior grain boundaries and electrode polarization effects to the overall electrical behavior the latter is well described with an equivalent circuit model which comprises components of constant phase elements and resistances in parallel connection electrical conductivity increases upon uniaxial loading giving rise to negative values of effective activation volume this is a strong experimental evidence for the generation of transient electric signals recorded prior to seismic events and may be attributed to charge transfer proton conduction due to cracks generation and propagation as a result of the applied stress the time series of ac conductivity at two distinct frequencies 10 khz 200 khz during linear loading of limestone samples exhibits a strong correlation with the acoustic emission activity obeying the same general self similar law for critical phenomena that has been reported for the energy release before materials fracture

palaeomagnetic re examination of the basal caithness old red sandstone aspects of local and regional tectonics abstract a palaeomagnetic re examination of the basal strata of the caithness old red sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region after structural correction the dominant remanence component has d 205 i 3 95 6 4 n 27 the existence of this shallow inclined magnetization in the middle devonian strata of caithness invalidates the model proposed by van der voo and scotese 1981 involving a ca 2000 km sinistral offset along the great glen fault in the carboniferous however the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone however the possibility of there having been a much larger transcurrent shift between europe and north america in late post devonian times accumulated along various fracture zones within the caledonian fold belt is discussed on the basis of an inferred overprinted magnetization it is tentatively concluded that the tectonic deformation of the old red sandstone of caithness has a mid jurassic or younger age

exploring the recovery lakes region and interior dronning maud land east antarctica with airborne gravity magnetic and radar measurements long range airborne geophysical measurements were carried out in the icegrav campaigns covering hitherto unexplored parts of interior east antarctica and part of the antarctic peninsula the airborne surveys provided a regional coverage of gravity magnetic and ice penetrating radar measurements for major dronning maud land ice stream systems from the grounding lines up to the recovery lakes drainage basin and filled in major data voids in antarctic data compilations such as antgp for gravity data admap for magnetic data and bedmap2 for ice thickness data and the sub ice topography we present the first maps of gravity magnetic and ice thickness data and bedrock topography for the region and show examples of bedrock topography and basal reflectivity patterns the 2013 recovery lakes campaign was carried out with a british antarctic survey twin otter aircraft operating from the halley and belgrano ii stations as well as a remote field camp located at the recovery subglacial lake b site gravity measurements were the primary driver for the survey with two airborne gravimeters lacoste and romberg and chekan am providing measurements at an accuracy level of around 2 mgal r m s supplementing goce gravity field and steady state ocean circulation explorer satellite data and confirming an excellent sub milligal agreement between satellite and airborne data at longer wavelengths

rifted crust at the east antarctic craton margin gravity and magnetic interpretation along a traverse across the wilkes subglacial basin region abstract early geophysical studies hypothesized a continental rift structure beneath the wilkes subglacial basin recent models favour a flexural origin for the basin linked to transantarctic mountains uplift and to east antarctic craton lithospheric rigidity flexural modelling predicts crustal thickening beneath the basin gravity modelling along the international trans antarctic scientific expedition traverse 1998 99 however reveals crustal thinning beneath the basin at 75 s the crust thins from 37 km beneath the transantarctic mountains to 31 2 km beneath the wilkes basin the western flank of the basin features a sharp magnetic break this signature may arise from a fault separating highly magnetic precambrian craton crust from weakly magnetic neoproterozoic crust much later crustal extension may have focussed along the craton margin the eastern flank of the wilkes basin exhibits a prominent aeromagnetic signature potential field modelling predicts 1 4 km thick sedimentary infill within the wilkes extended terrane interpreted mainly as beacon supergroup intruded by jurassic ferrar tholeiites the adjacent adventure subglacial trench is a narrow rift basin with 25 5 km thick crust and a 10 4 km sedimentary infill

the mount feather diamicton of the sirius group an accumulation of indicators of neogene antarctic glacial and climatic history abstract a paucity of data from the antarctic continent has resulted in conflicting interpretations of neogene antarctic glacial history much of the debate centres on interpretations of the glacigene sirius group strata that crop out as discrete deposits along the length of the transantarctic mountains and in particular on its age and the origin of the siliceous microfossils it encloses pliocene marine diatoms enclosed within sirius group strata are inferred to indicate a dynamic east antarctic ice sheet that was much reduced compared with today in the early middle pliocene and then expanded again in the late pliocene however the geomorphology of the dry valleys region is interpreted to represent a relatively long lived middle miocene recent and stable polar climatic regime similar to that of today the mount feather diamicton infills a palaeovalley at ca 2500 m on the ne flank of mount feather in the dry valleys region and has been included within the sirius group we obtained four shallow cores comrac 8 9 10 and 11 from beneath the permafrost boundary in the mount feather diamicton in order to understand its origin and relationship with the surrounding landscape detailed studies of these cores stratigraphy sedimentology palaeontology micromorphology petrography and fabric have yielded new data that demonstrate a much more complex climatic and glacial history for the mount feather diamicton than in previous interpretations the data indicate that the mount feather diamicton was deposited beneath a wet based glacier fed from a larger ice sheet behind the transantarctic mountains it is however unlikely that this ice sheet overtopped mount feather 2985 m a near in situ non marine diatom assemblage was recovered from 90 cm depth in comrac 10 and indicates a maximum depositional age of late miocene for the mount feather diamicton a subsequent glacial episode has distributed a boulder blanket across the surface of the diamicton other post depositional processes include drying infilling of surface layers with aeolian sediment and the development of melt water runnels we interpret these combined data to indicate the persistence of more temperate climatic and glacial conditions in the vicinity of mount feather until at least the late miocene

subglacial geology in coats land east antarctica revealed by airborne magnetics and radar sounding during the austral summer of 2001 02 five thousand line kilometres of airborne radio echo sounding and aeromagnetic data were collected in the region of three tributaries of slessor glacier east antarctica which drains into the filchner ice shelf basal topography and roughness estimates were obtained from the echo sounding data which have been combined here with the subglacial geological units inferred from an analysis of the aeromagnetic data this analysis included euler and werner deconvolution to determine the location and depth of sills dykes and faults in addition we high pass filtered the magnetic anomaly profiles to obtain information about the near surface subglacial geology this revealed a prominent area of low magnetic susceptibility lying in a deep topographic basin underneath the northern tributary of slessor glacier forward and inverse magnetic models indicate that this area is likely underlain by a 3 km thick sedimentary basin ice flow modelling suggests that this sedimentary basin influences the ice dynamics in the tributary these results have important implications for understanding the dynamics of this and other east antarctic flow features that may be underlain by a weak sedimentary bed

basal roughness of the east antarctic ice sheet in relation to flow speed and basal thermal state basal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water to date basal motion represents one of the largest uncertainties in ice flow models it is that component of the total flow velocity that can change most rapidly and can therefore facilitate rapid variations in dynamic behaviour in this study we investigate the subglacial properties of the east antarctic ice sheet by statistically analysing the roughness of the bed topography inferred from radio echo sounding measurements we analyse two sets of roughness parameters one derived in the spatial and the other in the spectral domain with two roughness parameters each this enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet we further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet we find that one of the roughness parameters the hurst exponent derived in the spatial domain coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity

inland extent of the weddell sea rift imaged by new aerogeophysical data the weddell sea rift was a major focus for jurassic extension and magmatism during the early stages of gondwana break up and underlies the weddell sea embayment which separates east antarctica from a collage of crustal blocks in west antarctica newly collected aerogeophysical data over the catchments of institute and moller ice streams reveal the inland extent of the weddell sea rift against the ellsworth whitmore block and a hitherto unknown major left lateral strike slip boundary between east and west antarctica aeromagnetic and gravity anomalies define the regional subglacial extent of proterozoic basement middle cambrian rift related volcanic rocks jurassic intrusions and sedimentary rocks of inferred post jurassic age 2d and 3d magnetic depth to source estimates were used to help constrain joint magnetic and gravity models for the region the models reveal that proterozoic crust similar to that exposed at haag nunataks extends southeast of the ellsworth mountains to the margin of the coastal basins thick granitic jurassic intrusions are modelled at the transition between the ellsworth whitmore block and the thinner crust of the weddell sea rift and within the pagano shear zone the crust beneath the inland extension of the weddell sea rift is modelled as being either 4 km thinner compared to the adjacent ellsworth whitmore block or as underlain by an up to 8 km thick mafic underplate

microlevelling procedures applied to regional aeromagnetic data an example from the transantarctic mountains antarctica the extensive application of digital enhancement and filtering as a powerful tool for aeromagnetic interpretation not only of high resolution but also of regional data requires an improved levelling two microlevelling techniques were thus compared in order to find an effective but relatively simple procedure to remove or at least to reduce residual magnetic errors remaining after standard levelling processes this study was carried out on regional aeromagnetic data recently acquired at high magnetic latitudes along the transantarctic mountains in antarctica where it is particularly critical to remove time dependent magnetic variations two dimensional fft filters applied to the gridded data namely the butterworth and a directional cosine filter proved to be more effective than previously proposed one dimensional space domain filters in the reduction of the residual corrugation not removed by statistical levelling tectonic interpretation of trends detected in the total field magnetic anomaly map and in the 3d analytic signal improved after application of frequency domain microlevelling however we also show that when interpreting microlevelled data two factors must be considered i the possible presence of real geological trends aligned along the flight lines ii modifications in the results yielded by depth estimates of magnetic sources due to the fft filters applied during the microlevelling procedure such changes were seen both in the well established 2d fft method based on the slope of the energy spectrum and in the more recent 3d euler deconvolution technique overall our results indicate that microlevelling could profitably be applied to older gridded aeromagnetic data sets in antarctica thus improving the accuracy and geological significance of future regional magnetic compilations as already seen in other continents

air and shipborne magnetic surveys of the antarctic into the 21st century the antarctic geomagnetics community remains very active in crustal anomaly mapping more than 1 5 million line km of new air and shipborne data have been acquired over the past decade by the international community in antarctica these new data together with surveys that previously were not in the public domain significantly upgrade the admap compilation aeromagnetic flights over east antarctica have been concentrated in the transantarctic mountains the prince charles mountains lambert glacier area and western dronning maud land dml coats land additionally surveys were conducted over lake vostok and the western part of marie byrd land by the us support office for aerogeophysical research projects and over the amundsen sea embayment during the austral summer of 2004 2005 by a collaborative us uk aerogeophysical campaign new aeromagnetic data over the gamburtsev subglacial mountains 120 000 line km acquired within the ipy antarctica s gamburtsev province project reveal fundamental geologic features beneath the east antarctic ice sheet critical to understanding precambrian continental growth processes roughly 100 000 line km of magnetic data obtained within the international collaboration for exploration of the cryosphere through aerogeophysical profiling promises to shed light on subglacial lithology and identify crustal boundaries for the central antarctic plate since the 1996 97 season the alfred wegener institute has collected 90 000 km of aeromagnetic data along a 1200 km long segment of the east antarctic coast over western dml recent cruises by australian german japanese russian british and american researchers have contributed to long standing studies of the antarctic continental margin along the continental margin of east antarctica west of maud rise to the george v coast of victoria land the russian polar marine geological research expedition and geoscience australia obtained 80 000 and 20 000 line km respectively of integrated seismic gravity and magnetic data additionally us expeditions collected 128 000 line km of shipborne magnetic data in the ross sea sector

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

study on electrical potential inversion imaging of abnormal stress in mining coal seam during the mining activities coal rock dynamic disasters have caused grievous casualties and massive property losses it is the severe problem for regional monitoring of abnormal stress electrical potential ep can be produced on coal rock under loading and its response is related closely with the loading stress and damage evolution meanwhile electric field inversion has the advantage to realize regionalization monitoring in the space to identify the abnormal stress localization characteristics in mining coal seam after theoretical derivation the strategy of ep inversion imaging on is studied under bilateral model further simulation experiment of coal rock under loading is conducted the abnormal probability zones of sample can be identified with ep inversion imaging while it corresponds with severe damage zones with significant crack propagation it can be utilized to reveal the localized characteristics of damage and failure of coal rock spatially finally the results of ep inversion in the mining coal seam indicate that the abnormal probability zones can be considered as stress concentration and dynamic hazard areas its effectiveness is verified by microseismic monitoring and rock burst hazard assessment the study provides a new idea to monitor abnormal stress zone regionally and forecast dynamic disasters in the field

major neogene fluctuations of the east antarctic ice sheet stratigraphic evidence from the lambert glacier region stratigraphic and sedimentological data are presented from a region likely to directly record the evolution of the east antarctic ice sheet along the western margin of the lambert graben which now drains 1 106 km2 or one eighth of the total area of the east antarctic ice sheet extensive remnants of cenozoic glaciomarine fjordal sediment known as the pagodroma group are preserved as far as several hundred kilometers inland from the open coast the sediments resemble those produced by tidewater glaciers in the arctic rather than those of modern antarctica four separate formations of the pagodroma group spanning the interval from early miocene or older to pliocene or early pleistocene are preserved the oldest and highest crop out at nearly 1500 m above sea level each formation was deposited in close proximity to an ice margin grounded on the fjord bottom during major recessional phases of the lambert glacier underlying each formation is a glacially cut erosion surface each surface records a separate stage of fjord excavation when ice expanded onto the continental shelf in prydz bay evidence from offshore drilling in prydz bay and these data indicate fluctuations of the lambert glacier terminus in excess of 500 km in neogene time thereby reflecting large changes in the volume of the ice sheet depositional episodes were succeeded by phases of uplift totalling more than 1500 m

convection current generated prior to rupture in saturated rocks laboratory experiments were performed to study the generation mechanism of electric signals during deformation and rupture of rock with the focus on the effects of pore water movement we have developed a triaxial apparatus that is specially designed for this purpose within the framework of the international frontier program on earthquake research of the institute of physical and chemical research riken this apparatus can deform a rock specimen that is electrically isolated from the surroundings at a very slow strain rate using a cylindrical specimen of intact rock inada granite makabe granite muroto gabbro and fujieda sandstone saturated with distilled water we carried out triaxial deformation tests under a constant pore pressure at room temperature during a deformation test we continuously measured the electric current generated in the specimen in addition local strains pore pressure and pore fluid movement from the intensifier were recorded at a sampling frequency up to 1 khz the volume change due to dilatancy was obtained from the average of the local strain measurements at four positions we found that the convection current flowed before the main fracture showing good correlation with the dilatancy rate and the water flow rate the current density of the signal was about 1 ma m2 this result demonstrated that the electric current is caused by an electrokinetic effect due to the water flow associated with accelerating evolution of dilatancy before the fracture

error propagation in electromagnetic transfer functions what role for the magnetotelluric method in detecting earthquake precursors summary whether electromagnetic precursors exist is a contentious issue that should be considered in the context of the focal depth and epicentral distance at which earthquakes nucleate and with due regard to externally driven changes in the electromagnetic fields recorded at the earth s surface if high frequency electromagnetic signals are generated by earthquakes then these signals may not be detectable because of attenuation occurring between the focal depth of the earthquake and the earth s surface this is particularly true in seismotectonic regions e g subduction zones where saline fluids including seawater raised heat flow and partial melts often lead to enhanced conductivities a technique for examining temporal variations in magnetotelluric mt spectra is presented which i considers both non inductive and inductive parts of the mt transfer function ii allows electromagnetic skin depths and adjustment distances to be calculated iii takes account of error propagation in the transfer functions and and iv acknowledges possible nonearthquake sources of observed variations in transfer functions and their errors data processing tends to eliminate non stationary electromagnetic disturbances of the source field hence the method is more suited to detecting hypothesized changes in subsurface conductivity than short lived electromagnetic bursts the technique is applied to mt data from seismotectonic regions in new zealand mt ruapehu the andes western cordillera and the mediterranean montecristo island and to a culturally quiet control site situated on a stable craton in central australia we demonstrate that the mt method can resolve apparent conductivity changes of the order of a few percent over 48 hr time intervals and give thresholds for other interval lengths comparison of the results obtained from seismotectonic regions with those from central australia and consideration of external source field effects highlight how temporal variations of electromagnetic fields might be speciously interpreted as earthquake precursors

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

fluid flow near reservoir lakes inferred from the spatial and temporal analysis of the electric potential electric self potential sp variations have been monitored continuously from 1995 to 1998 at 14 points on a ridge separating the roselend and la gittaz reservoir lakes in the french alps the lakes have level variations of at least 50 m over yearly cycles seasonal variations of sp associated with lake level variations are observed on five points of the array for three points located on the banks of the lakes a positive correlation to the lake level variations is observed with a maximal amplitude of about 180 mv corresponding to an average response of 2 4 mv per meter of water for two points located on the bottom of each lake the correlation is negative with a maximal magnitude of about 50 mv corresponding to an average response of 1 1 mv per meter of water two independent temporary electrical arrays located on the banks of each lake confirm these measurements and allow a better spatial characterization of the sources associated with the observed sp variations in particular near the roselend lake the electrical response to lake level variations is increasing for decreasing altitude the measured sp variations are proposed to result from the electrokinetic coupling associated with a vertical groundwater flow connecting a constant pore pressure source to the bottom of the lakes numerical modeling indicates that the spatial variation of the response and the nonlinear response observed at one point can be explained by leakage currents in the conductive lake water the values of the streaming potential coefficient spc measured in the laboratory with crushed rock samples from the site range from 14 to 50 mv 0 1 mpa for an electrolyte resistivity of 40 m and are compatible to first order with the magnitude of the observed seasonal sp variations a detailed quantitative electrokinetic modeling is currently limited mainly by the poor knowledge on the contribution of electrical leakage currents and the local variability of the spc this experiment indicates that spatial and temporal variations of the electric potential are promising tools to characterize and monitor shallow groundwater flow and provide practical data for the investigation of groundwater flow associated with volcanic or tectonic activity

melting and freezing beneath filchner ronne ice shelf antarctica 1 we use remote sensing data sets to evaluate the spatial distribution of melt beneath the filchner ronne ice shelf fris the net melt rate of 83 4 24 8 gtons yr is 2 5 5 times lower than previous glaciological estimates but is similar to existing oceanographic estimates the spatial distribution however differs significantly from standard conceptual and numerical models in which most melt occurs along the grounding lines our results suggest most grounding line melt is refrozen while the dominant ice shelf water isw source is melting near the ice shelf front probably associated with tidal action this suggests that changes in ice shelf extent can impact isw production rates in the weddell sea

a deep crustal model to explain regional aeromagnetic and gravity anomalies in southwest england abstract regional aeromagnetic anomaly maps of southwest england produced by upward continuation trend surface analysis and low pass wavenumber filtering of the published aeromagnetic map show a pronounced gradient zone of an east west trend crossing central devon across a 60 km wide zone regional field values reduce southwards by about 150 nt this gradient zone can be modelled as the edge effect of a 6 km thick permanently magnetised layer at the base of the crust a dense layer of similar geometry can explain the high regional gravity field that characterises southwest england

basal topography and ice flow in the bailey slessor region of east antarctica an airborne radio echo sounding campaign carried out in the upper reaches of bailey ice stream and slessor glacier in coats land east antarctica has revealed that tributaries of enhanced flow lie within well defined basal troughs and are separated from each other by bed highs these new data indicate significant differences in ice thickness compared with those estimated in the bedmap database a numerical modeling study has revealed that driving stresses are high enough to account for flow by ice deformation alone in intertributary areas most flow in the enhanced flow tributaries of slessor glacier may also be explained by ice deformation alone however although ice deformation is also significant in the bailey ice stream tributary a large amount of basal motion is also required to fully explain flow velocities here it is proposed that the trough in which bailey tributary lies is sufficiently deep that marine sediments may have accumulated here in preglacial times along with water produced by geothermal heating frictional heating and through a reduction in the pressure melting point basal motion may therefore be facilitated by the presence of a deformable saturated till layer at the bed

streaming potential and permeability of saturated sandstones under triaxial stress consequences for electrotelluric anomalies prior to earthquakes the streaming potential due to fluid circulation in rock was measured on saturated sediments fontainebleau sandstones the electrokinetic coupling coefficient which is the ratio of the streaming potential and the excess pore pressure is proportional to the fluid resistivity additionally for a fluid conductivity of 10 3 s m the electrokinetic coupling coefficient varies from 10 to 6642 mv 0 1 mpa for sample permeability in the range of permeabilities from 0 15 10 15 to 1220 10 15 m2 the different values of the electrokinetic coupling coefficient have been explained by the effect of increasing surface conductivity which becomes nonnegligible compared to fluid conductivity for low permeability when the sample is deformed under triaxial stress up to failure the vertical permeability along the principal stress drops by about 0 20 0 1 mpa when failure occurs the typical variation of the electrokinetic coupling coefficient is a large increase beginning with the onset of the localization of the shear band at about 75 of the yield stress and stopping at the failure this increase of the electrokinetic coupling coefficient is due to an increase of potential in the shear zone when new surfaces are created and connected possible consequences of our results are given concerning the electrical fields which could appear during the preparation of an earthquake it is shown that in some cases self potential anomalies reported in the deformed zone preceding an earthquake occurrence could be due to an increase of the electrokinetic coupling coefficient from 75 of the yield stress to rupture in the vicinity of one of the electrodes any variation of fluid resistivity or permeability in the vicinity of one electrode could change the electrokinetic coupling coefficient inducing a surface electrokinetic potential anomaly in regard to the interpretation of the electrokinetic effect which occurs at large distance from the epicenter a larger electrokinetic potential anomaly could be measured between electrodes situated along a vertical fluid flow for instance in a shallow borehole an electrokinetic potential anomaly up to 30 mv for a fluid conductivity of 0 01 s m and a rock permeability of 10 12 m2 could be observed with a change of the underground water table level as slight as 50 cm 50 mbar moreover if the permeability between the electrodes is increased by a factor of 8 103 the electrokinetic coupling coefficient could be enhanced by a factor up to 650

earthquake prediction research and the earthquakes of 2000 in the south iceland seismic zone abstract the purpose of this paper is to describe and to model long and short term processes that preceded two large earthquakes that occurred in 2000 in the south iceland seismic zone sisz the results are based on some key findings from multinational earthquake prediction research projects in the sisz since 1988 it involves a fusion of significant old and new results the paper presents a new hypothesis for earthquake build up processes in the region followed by a discussion on how interseismic and preseismic observations can be explained by this model and a discussion about a plausible earthquake cycle in light of the new hypothesis the research described here started in 1988 with the south iceland lowland sil project a significant outcome was the development of a seismic acquisition and evaluation system the sil system retrieving source information from earthquakes down to magnitude zero the research continued with the use of more multidisciplinary and multinational projects revealing new information about crustal processes related to large earthquakes in the area the validity of the work was tested in 2000 when two magnitude 6 6 m s earthquakes occurred in the central part of the sisz the earthquakes had a long term assessment of place a short term warning within short about the location and size of a probable impending earthquake was issued 25 hours before the second earthquake

a new magnetic map of the weddell sea and the antarctic peninsula abstract as part of the antarctic digital magnetic mapping project admap workers from vniiokeangeologia russia the british antarctic survey uk and the naval research laboratory usa have brought together almost all of the available magnetic data in the area 0 120 w 60 90 s the final map covers the whole weddell sea and adjacent land areas the antarctic peninsula and the seas to the west an area comparable in size with that of the usa this paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it distinct magnetic provinces are associated with precambrian rocks of the east antarctic craton highly extended continental crust in the weddell sea embayment the arc batholith of the antarctic peninsula and oceanic crust of the northern weddell sea which was created as a direct consequence of south america antarctica plate motion and oceanic crust generated at the pacific antarctic ridge the magnetic anomaly map thus provides an overview of the fragmentation of south western gondwana and the tectonic development of the weddell sea sector of antarctica

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana

magnetic properties of the lower old red sandstone lavas in the midland valley scotland palaeomagnetic and tectonic considerations abstract thermal demagnetization studies of lavas in the strathmore area of the midland valley scotland support overall palaeomagnetic data found in previous studies of these rocks reduced directional scatter as compared to some earlier studies is attributed to more effective demagnetization resolving some of the directional complexity of previous studies combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism the existence of almost antiparallel directional groups and field tests give supporting evidence for a primary deuteric origin of the main magnetization of these rocks additionally a second remanence component having shallow reverse directions of magnetization is attributed to later remagnetization in old red sandstone time the midland valley results are seen in conjunction with other palaeozoic palaeomagnetic results and possible geodynamic implications are discussed

thickening of the ice stream catchments feeding the filchner ronne ice shelf antarctica 1 earlier ice sheet mass balance estimates disagree with satellite altimeter measured thickening of the catchment area that feeds the filchner ronne ice shelf antarctica in an effort to reconcile these differences we generated a new flux gate mass balance estimate using improved elevation and velocity data our results show a 39 26 gton yr thickening whereas earlier flux gate results indicated minor thinning our results are consistent with altimeter derived estimates of elevation change this suggests that altimeter observed thickening on the east antarctic plateau may be the result of some combination of a 20th century accumulation increase and a continuing response to the 50 accumulation increase that began in the early holocene