electricity in dying thunderstorms we made one balloon sounding of the electric field in each of two dying thunderstorms both balloons were launched 20 min after the storm s last lightning flash each sounding revealed substantial residual electrification the peak magnitudes of electric field in the two dying storms were 35 and 71 kv m the charge structure in both storms appeared to consist of one internal negative charge region with positive screening layers at the upper and lower cloud boundaries one or two negative charge regions were found below cloud base the main charge regions had inferred charge densities of about 1 nc m3 and were hundreds of meters thick

three dimensional deformation measured in an alaskan glacier measurements of movement along 28 boreholes reveal the three dimensional flow field in a 6 million cubic meter reach of worthington glacier a temperate valley glacier located in alaska sliding at the bed accounted for 60 to 70 percent of the glacier s surface motion strain rates in the ice were low from the surface to a depth of about 120 meters but then increased rapidly toward the bed ice deformation was not affected by temporal changes in the sliding rate the three dimensional pattern of motion indicates that plane strain which is often assumed by models is a poor approximation of this viscous flow

measurements of the extension required for crustal breakup on the magma poor iberia newfoundland conjugate margins abstract rifted margins have been extensively studied however there are still many important questions related to their formation processes we use the iberia newfoundland conjugate rifted margins as a natural laboratory to investigate how much extension is required to produce crustal rupture and separation to achieve our aim we use i gravity anomaly inversion to measure continental crustal thinning ii subsidence analysis to measure continental lithosphere thinning and iii fault heave summation from seismic observations to obtain brittle continental crust extension integration of thinning from gravity anomaly inversion and subsidence analysis is used to determine continental crust extension and continental lithosphere extension respectively these measurements have been made between the proximal continental crust and the distal limit of contiguous continental crust lccc on the northern screech1 and ise01 and the southern screech2 and tgs lg12 conjugate seismic profiles for the iberia newfoundland conjugate rifted margins extension values determined from the integration of crustal and lithosphere thinning are similar and suggest that on average 172 km of extension is required to produce crustal breakup in contrast measured extension from fault heave summation is on average 128 km which indicates an apparent extension discrepancy at the scale of the whole conjugate margin system when observed brittle continental crust extension is compared with lithosphere and crust extension fault population analysis shows that sub seismic resolution faulting significantly contributes to an under estimate of brittle crustal extension by up to 25 allowing for fault extension at sub seismic resolution indicates that an extension discrepancy at the scale of the whole conjugate margin system does not exist fault geometries and the location of the distalmost contiguous continental crust are also important issues that should be carefully considered when comparing fault heave summation and extension from crust and lithosphere thinning the absence of an extension discrepancy between fault extension crustal extension and lithosphere extension at the scale of a whole conjugate margin system should not be confused with the presence of depth dependent stretching and thinning at a given location on the margin which is both expected and observed at rifted continental margins

correlated lightning mapping array and radar observations of the initial stages of three sequentially triggered florida lightning discharges 1 correlated lightning mapping array and vertical scan radar images are presented for three rocket and wire triggered lightning flashes that occurred sequentially within 17 min in the presence of a decaying multicellular convective storm system over north central florida the initial stage is of each flash propagated generally vertically to the altitude of the 0 c melting level about 5 km and then subsequently propagated for many kilometers horizontally along the melting level contour radar images suggest that the propagation paths of the is channels below and above the melting level were heavily influenced by precipitation gradients flash uf 11 24 exhibited a 12 6 km unbranched is channel the longest unbranched channel observed in the study by a factor of three during flash uf 11 25 119 ms following the cessation of the measured is current at ground and prior to the first return stroke a natural cloud to ground discharge perhaps induced by the is initiated between 2 5 and 4 km altitude and struck ground 5 to 7 km from the launching facility the is of flash uf 11 26 propagated upward through a descending precipitation packet and apparently induced a naturally appearing bi level intracloud discharge via an upward negative leader that initiated within the is breakdown region 3 5 km from the launching facility the upward negative leader propagated from 5 6 to 9 3 km altitude in a time of 11 ms the electrical current measured at ground during the is of flash uf 11 26 exhibited a 57 ms polarity reversal transferring 19 c of positive charge to ground

lithospheric extension from rifting to continental breakup at magma poor margins rheology serpentinisation and symmetry the symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years with various authors proposing asymmetric simple shear models others advocating more symmetric pure shear models and some combinations of the two the unroofing of vast expanses of sub continental mantle at non volcanic margins has led some authors to argue in favour of simple shear models but supporting evidence is lacking subsidence evidence from conjugate margin pairs is equivocal and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions in the porcupine basin where the final stages of break up are preserved the development of structural asymmetry is demonstrable and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust this agrees with theoretical modelling results which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled however whether such asymmetry is maintained during continued exhumation of the mantle is unclear

geometry bed topography and drainage system structure of the haut glacier d arolla switzerland as part of an integrated study of the hydrology meltwater quality and dynamics of the haut glacier d arolla switzerland the glacier s drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier this structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by a the subglacial hydraulic potential surface and b the subglacial bedrock surface these reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio echo sounding techniques the two reconstructions simulate the drainage system structures expected for a closed channels in which water is pressurized by the overlying ice and b gravity driven open channel flow the closed channel model provides the best fit to the observed structure even though theoretical calculations suggest that under summer discharge conditions open channel flow may be widespread beneath the glacier possible reasons for this apparent discrepancy are discussed

investigating the role of the galicia bank on the formation of the north west iberian margin using deformable plate tectonic models abstract the kinematic evolution of the west iberian margin during the opening of the southern north atlantic has been of interest for several decades and provides a well constrained environment to study and develop new concepts related to the formation of non volcanic magma poor passive margins building upon previous rigid and deformable plate tectonic modelling studies the aim of this work is to create deformable plate tectonic models of the west iberian margin using previously published constraints in gplates to investigate the role of the galicia bank as an independent continental ribbon during deformation of the west iberian margin from 200 ma to present day a comparison of present day crustal thickness results calculated from deformable plate tectonic reconstructions with those obtained from gravity inversion has allowed the plate kinematics of the galicia bank relative to the flemish cap to be investigated additionally this comparison provided detailed insights into the most probable kinematic scenarios for the galicia bank continental ribbon including its impact on the deformation experienced within the galicia interior basin two preferred kinematic models for the galicia bank are proposed one that implies synchronous motion with the flemish cap until 160 ma after which the galicia bank begins to move southward causing crustal thinning within the galicia interior basin the other preferred model despite its discrepancies with gravity inversion results implies synchronous motion of the galicia bank and southeastern flemish cap until breakup between the southeastern newfoundland and iberian margins initiating 140 ma in addition to the deformation induced by the motion of the galicia bank this study demonstrates the influence of inherited variscan structures and their impact on rift segmentation experienced within the galicia interior basin this is inferred through attempts to discretize triangular meshes within regions of deformation in areas where inherited structures have been previously interpreted

on the rotation and frictional lock up of normal faults explaining the dip distribution of normal fault earthquakes and resolving the low angle normal fault paradox abstract classical rock mechanics predicts that normal faults should form at 60 65 rotating to 30 40 before locking so widespread slip at around 20 is considered paradoxical furthermore the dip distribution of normal fault earthquakes has a distinct unexplained peak at 45 for both problems some combination of low friction high fluid pressures stress rotation and efficiency of low angle slip have been suggested but provide at best a partial solution a simple quantitative model for normal fault rotation iterating between slip on faults and distributed strain predicts that faults spend more time at lower angles combining this result with mohr coulomb analysis of the range where seismogenic normal faults should lock up predicts the dip distribution of seismogenic normal faults and matches both the range and 45 peak observed for normal fault earthquakes as even the lowest dips of normal fault ruptures fall within the limits of fault reactivation slip on seismogenic low angle normal faults is not paradoxical a new mohr griffith solution to the limits of fault reactivation extends the analysis into the top few km where low angle slip is best constrained by field observations and shows that low cohesion normal faults can remain active at

evolution of subglacial water pressure along a glacier s length observations from along the length of bench glacier alaska usa show that the subglacial water pressure field undergoes a multiphase transition from a winter mode to a summer mode data were collected at the glacier surface the outlet stream and in a network of 47 boreholes spanning the length of the 7 km long glacier the winter pressure field was near overburden with low magnitude centimeter to meter scale and long period days to weeks variations during a spring speed up event boreholes showed synchronous variations and a slight pressure drop from prior winter values diurnal pressure variations followed the speed up with their onset associated with a glacier wide pressure drop and flood at the terminus stream diurnal variations with swings of up to 80 of overburden pressure were typical of mid summer several characteristics of our observations contradict common conceptions about the seasonal development of the subglacial drainage system and the linkages between subglacial hydrology and basal sliding 1 increased water pressure did not accompany high sliding rates 2 the drainage system showed activity characteristic of the spring season long before abundant water was available on the glacier surface 3 the onset of both spring activity and diurnal variations of the drainage system did not show a spatial progression along the length of the glacier

water pressure and basal sliding on storglaciaren northern sweden the subglacial hydrology of the ablation area of storglaciaren a small valley glacier in northern sweden is dramatically affected by a subglacial ridge or riegel water pressures above this riegel are relatively constant while down glacier from it they vary significantly the lower part of the glacier accelerates in response to peaks in basal water pressure the upper part may be weakly coupled to the lower part during these peaks a power law fit of observed basal water pressures and measured surface velocities yields where u s is the surface velocity and p e is the effective water pressure ice overburden pressure minus subglacial water pressure data from findelengletscher reported by iken and bindschadler 1986 yield an identical exponent and a coefficient one order of magnitude larger the similar exponent implies that the process producing the velocity variations on both glaciers is similar the variations in velocity are inferred to be due to hydraulic jacking on both glaciers

electrical evolution during the decay stage of new mexico thunderstorms 1 the electric field e at the ground beneath a thunderstorm often exhibits an end of storm oscillation eoso during the storm s decay phase in which e typically undergoes three polarity changes over a period of 30 75 min to determine why the surface e oscillates the timing of the eoso polarity changes of a multicellular storm in new mexico has been compared to a series of balloon radar and lightning data three in situ balloon soundings provide vertical profiles of e and inferred charge regions just before and during the eoso these data have been compared to the evolving precipitation structure determined from orthogonal range height scans of two radars and to several other parameters including lightning mapping array data before the eoso began the storm had the typical electrical structure of the late mature stage estimates of the total charge magnitudes in each of the principal regions before and during the eoso ranged from 7 to 33 c the comparisons indicate that surface e polarity changes during the eoso were 1 primarily owing to the successive fallout of three of the four principal charge regions in the storm s mature stage charge structure modified by 2 the growth or decay of the charge regions as they descended and 3 changes in the screening charges at the upper and lower cloud boundaries more limited data from two other decaying storms indicate that similar in cloud evolution was probably occurring during their eosos

seasonal reorganization of subglacial drainage inferred from measurements in boreholes the effect of the formation of a major subglacial drainage channel on the behaviour of the subglacial drainage system of haut glacier d arolla switzerland was investigated using measurements of borehole water level and the electrical conductivity and turbidity of basal meltwaters electrical conductivity profiles were also measured within borehole water columns to identify the water sources driving water level changes and to determine patterns of water circulation in boreholes prior to channel formation boreholes showed idiosyncratic and poorly coordinated behaviour diurnal water level fluctuations were small and driven by supraglacial englacial water inputs even when boreholes were connected to a subglacial drainage system this system appeared to consist of hydraulically impermeable patches interspersed with storage spaces and transmitted a very low water flux drainage reorganization which occurred around 31 july 1993 in response to rapidly rising meltwater and rainfall inputs seems to have involved the creation of a connection between an incipient channel and a well established channelized system located further down glacier once a major channel existed within the area of the borehole array borehole water level fluctuations were forced by discharge related changes in channel water pressure although a diversity of responses was observed these included i synchronous ii damped and lagged iii inverse and iv alternating inverse lagged responses synchronous responses occurred in boreholes connected directly to the channel while damped and lagged responses occurred in boreholes connected to it by a more resistive drainage system pressure variations within the channel resulted in diurnal transfer of mechanical support for the ice overburden between connected and unconnected areas of the bed producing inverse and alternating patterns of water level response 1998 john wiley sons ltd

waveform inversion of the s reflector west of spain fine structure of a detachment fault the s reflection west of iberia has been interpreted as a low angle detachment fault separating crustal fault blocks from partially serpentinized mantle we apply full waveform inversion to investigate the fine structure of s our results confirm that s is largely a step increase in velocity and density probably from crustal rocks to partially serpentinized mantle peridotites a 50 m thick low velocity zone above s might represent a main fault zone of highly serpentinized peridotites or hydrofractured and altered crustal rocks above the main fault zone both interpretations imply focused fluid flow along s raising the possibility that low angle movement along s was aided by the development of local transient high fluid pressures

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

serial profiles of electrostatic potential in five new mexico thunderstorms 1 profiles of electric potential v integrated from balloon borne electric field e measurements are used to investigate the electrical evolution of thunderstorms over the mountains in central new mexico with sequential soundings through multiple storms the time skew associated with obtaining v from a noninstantaneous sounding is also studied the data show that a basic v profile with a maximum above a minimum forms in the early stage of the storm and is maintained throughout its mature stage series of soundings from individual storms show only a gradual evolution in the v profile from the early through the mature stage as the extrema descend in altitude and become shallower and vertically closer together more evolution occurs in the late stage when the shape of the v profile reverses to have a minimum above a maximum the 17 v z profiles from the mature stage of five different storms are also very similar in overall shape suggesting that the basic shape is not significantly affected by differences in lightning flash rate among these storms the findings indicate that the potential profile during a typical sounding in the mature stage is relatively stable and the overall shape of the mature stage v profile does not change markedly on the time scale of a particular balloon sounding 10 30 min through new mexico mountain storms thus time skew problems in the v profiles are minor during a storm s mature stage

migration of activity within normal fault systems examples from the quaternary of mainland greece abstract we examine five areas of mainland greece where active extension occurs on sub parallel systems of normal faults and where geomorphological and stratigraphic evidence indicates that the faulting has migrated basinwards into the original hanging walls in several cases within the late quaternary by comparing fault slip rates estimated from geomorphological data with current extension rates known from geodetic measurements it appears that the newest faults can account for effectively all the present day motions fault migration of this sort is easy to recognize in young systems close to sea level because vertical movements of footwalls and hanging walls are obvious and reveal which faults are currently most active but is less easy to confirm away from reliable reference levels or in older terrains with poorer time resolution it is probably more common than is appreciated and has a profound effect on syn rift sedimentation and erosion patterns fault migration is probably an inevitable consequence of the interplay between stresses generated by the fault related topography and the ultimate strength of major faults it is likely to be further encouraged in places where lower crustal flow or rotations about a vertical axis are important however it is not clear why migration should preferentially occur into the hanging walls as observed in central greece

flemish cap goban spur conjugate margins new evidence of asymmetry we present the combined results of deep multichannel reflection and refraction seismic surveys across the flemish cap goban spur conjugate margin pair north atlantic which we use to infer rifting style and breakup profiles on both margins cross magnetic anomaly 34 and extend into oceanic crust making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust the deep multichannel seismic mcs reflection data have previously been used to support a model of symmetric pure shear extension followed by asymmetric breakup and a sharp continent ocean boundary using both types of seismic data our results indicate instead that asymmetric structures are formed during all stages of rifting breakup and complex transition to oceanic spreading the differing nature of the two ocean continent transition zones is particularly striking for flemish cap our reprocessed image of the mcs profile clearly shows tilted fault blocks beneath back tilted sediment packages consistent with a wide region of highly thinned continental crust inferred from wide angle seismic data in contrast normal incidence and wide angle seismic data for the goban spur transition zone indicate the presence of exhumed serpentinized mantle

development of interconnected talc networks and weakening of continental low angle normal faults fault zones that slip when oriented at large angles to the maximum compressive stress i e weak faults represent a significant mechanical problem here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite abundant talc along a low angle normal fault within the fault core talc forms an interconnected foliated network that deforms by frictional sliding along 50 200 nm thick talc lamellae the low frictional strength of talc combined with dissolution precipitation creep can explain slip on low angle normal faults in addition the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures the development of phyllosilicates such as talc by fluid assisted processes within fault zones cutting mg rich carbonate sequences may be widespread leading to profound and long term fault weakness

geometry and flow conditions of subglacial water at south cascade glacier washington state u s a an analysis of tracer injections tracers were injected into south cascade glacier to determine the flow condition and geometry of the subglacial water system results indicate that two distincts drainage basins on the glacier feed the two main streams flowing from the glacier in the largest basin two parallel drainage networks exist one englacial and the other subglacial

flash propagation and inferred charge structure relative to radar observed ice alignment signatures in a small florida mesoscale convective system a series of vertical cross sections taken through a small mesoscale convective system mcs observed over florida by the dual polarimetric smart radar were combined with vhf radiation source locations from a lightning mapping array lma to examine the lightning channel propagation paths relative to the radar observed ice alignment signatures associated with regions of negative specific differential phase kdp additionally charge layers inferred from analysis of lma sources were related to the ice alignment signature it was found that intracloud flashes initiated near the upper zero kdp boundary surrounding the negative kdp region the zero kdp boundary also delineated the propagation path of the lightning channel with the negative leaders following the upper boundary and positive leaders following the lower boundary very few lma sources were found in the negative kdp region we conclude that rapid dual polarimetric radar observations can diagnose strong electric fields and may help identify surrounding regions of charge

resolving the fine scale velocity structure of continental hyperextension at the deep galicia margin using full waveform inversion continental hyperextension during magma poor rifting at the deep galicia margin is characterised by a complex pattern of faulting thin continental fault blocks and the serpentinisation with local exhumation of mantle peridotites along the s reflector interpreted as a detachment surface in order to understand fully the evolution of these features it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible travel time tomography models have revealed the long wavelength velocity structure of this hyperextended domain but are often insufficient to match accurately the short wavelength structure observed in reflection seismic imaging here we demonstrate the application of two dimensional 2d time domain acoustic full waveform inversion to deep water seismic data collected at the deep galicia margin in order to attain a high resolution velocity model of continental hyperextension we have used several quality assurance procedures to assess the velocity model including comparison of the observed and modelled waveforms checkerboard tests testing of parameter and inversion strategy and comparison with the migrated reflection image our final model exhibits an increase in the resolution of subsurface velocities with particular improvement observed in the westernmost continental fault blocks with a clear rotation of the velocity field to match steeply dipping reflectors across the s reflector there is a sharpening in the velocity contrast with lower velocities beneath s indicative of preferential mantle serpentinisation this study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite observed as a systematic decrease in seismic velocities consistent with increased serpentinisation our results confirm the feasibility of applying the full waveform inversion method to sparse deep water crustal datasets

short term variations in glacier flow controlled by subglacial water pressure at lauteraargletscher bernese alps switzerland short term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in lauteraargletscher bernese alps switzerland horizontal surface flow speed oscillated diurnally showing a correlation with the water level in a borehole flow speed increased as a function of the water level with an asymptote at the ice overburden level this observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions detailed examination of the diurnal variations however showed that the speed was larger when the pressure was increasing than when it was decreasing greater speed with increasing pressure was interpreted by subglacial watercavity opening and or longitudinal stress coupling with the upper reaches of the glacier upward surface movements were observed when the glacier flow speed increased simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources vertical straining of ice and volume increase of subglacial water cavities the vertical surface movement was largely affected by the vertical strain and the uplift events could not be simply attributed to cavity opening

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

diurnal fluctuations in borehole water levels configuration of the drainage system beneath bench glacier alaska usa water levels were measured in boreholes spaced along the entire length of bench glacier alaska usa for a period in excess of 2 years instrumented boreholes were arranged as nine pairs along the center line of the glacier and an orthogonal grid of 16 boreholes in a 3600 m 2 region at the center of the ablation area diurnal fluctuations of the water levels were found to be restricted to the late melt season pairs of boreholes spaced along the length of the ablation area often exhibited similar fluctuations and diurnal changes in water levels three distinct and independent types of diurnal fluctuations in water level were observed in clusters of boreholes within the grid of boreholes head gradients suggest water did not flow between clusters and a single tunnel connecting the boreholes could not explain the observed pattern of diurnal water level fluctuations inter borehole and borehole cluster connectivity suggests the cross glacier width of influence of a segment of the drainage system connected to a borehole was limited to tens of meters a drainage configuration whereby boreholes are connected to a somewhat distant tunnel by drainage pipes of differing lengths often hundreds of meters is shown with a numerical test to be a plausible explanation for the observed borehole behavior

evidence for fault weakness and fluid flow within an active low angle normal fault determining the composition and physical properties of shallow dipping active normal faults dips 35 with respect to the horizontal is important for understanding how such faults slip under low resolved shear stress and accommodate significant extension of the crust and lithosphere seismic reflection images1 and earthquake source parameters2 show that a magnitude 6 2 earthquake occurred at about 5 km depth on or close to a normal fault with a dip of 25 30 located ahead of a propagating spreading centre in the woodlark basin here we present results from a genetic algorithm inversion of seismic reflection data which shows that the fault at 4 5 km depth contains a 33 m thick layer with seismic velocities of about 4 3 km s 1 which we interpret to be composed of serpentinite fault gouge isolated zones exhibit velocities as low as 1 7 km s 1 with high porosities which we suggest are maintained by high fluid pressures we propose that hydrothermal fluid flow possibly driven by a deep magmatic heat source and high extensional stresses ahead of the ridge tip have created conditions for fault weakness and strain localization on the low angle normal fault

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

borehole video analysis of a temperate glacier englacial and subglacial structure implications for glacier flow models video observations made in 16 boreholes drilled through a deforming valley glacier affirm that temperate glacier ice may be reasonably well represented as homogeneous in glacier flow models but raise warnings about the complexities of basal boundary conditions and glacier sliding discrete englacial structures including clear ice layers voids and water conduits compose a total of 3 of the ice mass planar features clear ice layers are oriented near vertical and are not aligned with the sense of shear strain meaning that the layers probably do not influence the homogeneity of the strain both direct observations of the ice and analysis of its light reflectance suggest an increase in crystal size and decrease in bubble content with depth however previous laboratory work indicates that such changes are unimportant in terms of the viscosity of the ice observations of the basal boundary or sliding surface indicate that there are areas of both hard bedrock and soft deformable till which should cause spacial and temporal gradients in sliding rate

wave field migration as a tool for estimating spatially continuous radar velocity and water content in glaciers 1 normal moveout velocity analysis can lead to significant overestimates of the velocity structure of temperate glaciers since most englacial reflectors approximate point scatters and violate the assumption of planar flat lying reflectors migration velocity analysis mva is a tool that does not depend on the assumption of flat lying reflectors mva can provide laterally and vertically continuous velocity estimates from conventional common offset radar sections in a study of temperate bench glacier alaska we used mva coupled with dielectric modeling to estimate the distribution of water content along a cross section of the glacier we found the glacier has two layers an upper layer with relatively low water content and lower layer with relatively high water content the ability to quantify hydrostratigraphy is important in understanding water storage and routing within glaciers

two simultaneous charge structures in thunderstorm convection 1 a pair of simultaneous horizontally displaced thunderstorm electric field soundings are used to investigate an existing conceptual model of thunderstorm charge structure the soundings show that two different vertical charge structures one in the updraft and one outside the updraft can exist at the same time inside a storm as depicted in the existing model the data suggest that the two charge structures can be relatively close together since the soundings are separated by only 3 to 8 km horizontally in the convective region of the storm horizontal extent of the charge structures is estimated to be at least 4 km 4 km the differences in the two charge structures are coincident with substantial differences over this same 3 to 8 km horizontal distance in the reflectivity structure updraft speed and lightning activity of the convection the nonupdraft sounding was made through the intense reflectivity core found at middle levels in the convection additional electric field complexity and additional charge regions were found in the nonupdraft sounding and the lightning related electric field changes tended to be larger in magnitude and more numerous in that sounding than in the updraft sounding

borehole drainage and its implications for the investigation of glacier hydrology experiences from haut glacier d arolla switzerland studies of glacier hydrology rely increasingly on measurements made in boreholes as a basis for reconstructing the character and behaviour of subglacial drainage systems in temperate glaciers in which boreholes remain open to the atmosphere following drilling the interpretation of such data may be complicated by supraglacial or englacial water flows to and from boreholes we report on a suite of techniques used to identify borehole water sources and to reconstruct patterns of water circulation within boreholes at haut glacier d arolla switzerland results are used to define a number of borehole drainage types examples of each drainage type are presented along with the manner in which they influence interpretations of borehole water levels borehole water quality data and borehole dye traces the analysis indicates that a full understanding of possible borehole drainage modes is required for the correct interpretation of many borehole observations and that those observations provide an accurate indication of subglacial conditions only under relatively restricted circumstances copyright 2001 john wiley sons ltd

rapid spatiotemporal variations in rift structure during development of the corinth rift central greece the corinth rift central greece enables analysis of early rift development as it is young 5 ma and highly active and its full history is recorded at high resolution by sedimentary systems a complete compilation of marine geophysical data complemented by onshore data is used to develop a high resolution chronostratigraphy and detailed fault history for the offshore corinth rift integrating interpretations and reconciling previous discrepancies rift migration and localization of deformation have been significant within the rift since inception over the last circa 2 myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift but this transition did not occur synchronously along strike isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620 340 ka characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at 30 m kyr since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin demonstrated by lateral growth of discrete depocenters at 40 m kyr a single central depocenter formed by circa 130 ka indicating full fault linkage these results indicate that rift localization is progressive not instantaneous and can be synchronous once a rift border fault system is established this study illustrates that development processes within young rifts occur at 100 kyr timescales including rapid changes in rift symmetry and growth and linkage of major rift faults

diurnal water pressure fluctuations timing and pattern of termination below bench glacier alaska usa observations from basal water pressure sensors along the length of bench glacier alaska usa show that diurnal fluctuations of water pressure are seasonal and restricted to summer most notable about these fluctuations is their disappearance in the late summer and early autumn long before the seasonal end of diurnal meltwater input here we present data documenting the end of diurnal water pressure fluctuations during the 2002 and 2003 melt seasons the end of diurnal fluctuations occurred abruptly in multiple boreholes spaced meters to kilometers apart there was no obvious spatial progression of termination events and a clear correlation with meteorological forcing or discharge in the outlet stream was not apparent after diurnal pressure fluctuations ended basal water pressure returned to a high generally steady value either in an irregular pattern or by a distinct increase this high water pressure was interrupted by episodic acyclic events throughout the autumn before becoming stable and high in winter

seasonal changes in the morphology of the subglacial drainage system haut glacier d arolla switzerland dye tracing techniques were used to investigate the glacier wide pattern of change in the englacial subglacial drainage system of haut glacier d arolla during the ablation seasons of 1990 and 1991 analysis of breakthrough curve characteristics indicate that over the course of a melt season a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system by the end of the melt season this channel system extended at least 3 3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier the upper limit of the channel system closely followed the retreating snowline up glacier rates of headward channel growth reached c 65 m d 1 although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer it appears that the removal of snow with its high albedo and significant water storage capacity from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins and in the peakedness of daily runoff cycles this induced transient high water pressures within the distributed drainage system which caused it to evolve rapidly into a channelised system it is therefore likely that at a local scale channel growth occurred down glacier from moulins and that the overall up glacier directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice derived meltwater damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier 1998 john wiley sons ltd

slip heterogeneity on a corrugated fault abstract slip heterogeneity reflects the fundamental physics of earthquake rupture and has been attributed to strong fault patches termed asperities or barriers we propose that variations in fault surface orientation due to slip parallel corrugations may act as geometric asperities and barriers generating variations in incremental i e due to a single earthquake slip across a fault surface we evaluate this hypothesis using observations from the arkitsa normal fault exposure in central greece a scan of the arkitsa fault surface with 1 m spatial resolution and mm scale precision reveals corrugations made up of 1 5 m wide synforms antiforms and nearly planar fault sections with long axes that extend in the slip direction across the entire exposed surface the surface is thus more than an order of magnitude smoother in the direction of slip than in the perpendicular direction slip perpendicular profiles exhibit nearly self similar scaling across the range of observed wavelengths 2 50 m whereas slip parallel profiles are significantly smoother at shorter wavelengths

comment on spider lightning in intracloud and positive cloud to ground flashes by vladislav mazur xuan min shao and paul r krehbiel

sequential faulting explains the asymmetry and extension discrepancy of conjugate margins rifted continental margins are the sites that record the thinning and break up of continents to form new oceans cesar ranero and marta perez gussinye use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this to crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy in which crustal thinning appears to be greater than the extension caused by brittle faulting by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins these authors use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this with crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy where crustal thinning seems to be greater than the extension caused by brittle faulting during early extension cold continental lithosphere thins and subsides creating rift basins if extension continues to final break up the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins although basins and margins are ubiquitous structures the deformation processes leading from moderately extended basins to highly stretched margins are unclear as studies consistently report that crustal thinning is greater than extension caused by brittle faulting1 2 3 4 this extension discrepancy might arise from differential stretching of brittle and ductile crustal layers2 but that does not readily explain the typical asymmetric structure of conjugate margins5 6 in cross section one margin displays gradual thinning accompanied by large faults and the conjugate margin displays abrupt thinning but smaller scale faulting5 whole crust detachments active from early in the rifting could in theory create both thinning and asymmetry1 but are mechanically problematical furthermore the extension discrepancy occurs at both conjugate margins leading to the apparent contradiction that both seem to be upper plates to a detachment fault7 8 alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic resolution faulting9 invisible deformation along top basement 100 km scale detachments8 or the structural complexity of cross cutting arrays of faults3 here we use depth migrated seismic images to accurately measure fault extension and compare it with crustal thinning the observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins contrary to current wisdom the observations support the idea that thinning is to a first degree explained by simple andersonian faulting that is unambiguously visible in seismic data

statistical evaluation of glacier boreholes as indicators of basal drainage systems between 1988 and 1992 closely spaced arrays of boreholes were drilled at small river glacier british columbia the borehole arrays have been used to investigate the interannual and spatial consistency of patterns of basal hydraulics beneath the glacier a simple robust classification was devised identifying unconnected high standing low standing and dry base water levels in boreholes spatial and interannual comparisons were made using a simple nearest neighbour statistic corrected for differences in frequency of different borehole types and evaluated using monte carlo confidence intervals to compensate for array form arrays in the lower ablation zone showed spatial and interannual coherence with three distinct areas characterized by low water pressure till associated non connection and high pressure there was no indication of a dominant conduit slightly higher up glacier borehole patterns were less coherent and varied from year to year probably a result of subglacial karst capturing basal waters at a number of low pressure points at the bed therefore both the upper and lower arrays at small river glacier appear to encompass unusual drainage conditions the nearest neighbour analysis provides valuable constraints on more specific interpretation

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 1 observations the data presented in part 1 of this paper meier et al this issue are here used to assess the role of water input output water storage and basal water pressure in the rapid movement of columbia glacier alaska consistently high basal water pressures mostly in the range from 300 kpa below to 100 kpa above the ice overburden pressure are responsible in an overall way for the high glacier flow velocities 3 5 9 m d 1 which are due mainly to rapid basal sliding caused by the high water pressure diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site the water pressure fluctuation tracks the time integrated water input less a steady withdrawal as expected for the diurnally fluctuating storage of water in the glacier far from the terminus at the downglacier site the situation is more complex diurnal peaks in water level which are directly related to intraglacial water storage as well as to basal water pressure are shifted forward in time by 4 hours probably as a result of the effect of diurnal fluctuation in water output from the glacier which affects the local water storage fluctuations near the terminus because of the forward shift in the basal water pressure peaks which at the downglacier site lead the velocity peaks by 6 hours a mechanical connection between water pressure and sliding there would have to involve a 6 hour quarter period delay however the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single consistent control mechanism at both sites the velocity variations in nondiurnal speed up events caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier it appears that small variations in water pressure 100 kpa sufficient to produce the observed velocity variations 15 30 are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity at the achieved level of observation the clearest though not complication free control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

nature of the s reflector beneath the galicia banks rifted margin preliminary results from prestack depth migration beneath the tilted fault blocks of the galicia rifted margin a bright reflection termed s has been interpreted as the master detachment fault controlling the extension of the margin however on time migrated sections s appears to be locally offset by the steep faults of the upper section we investigate the true geometry of s and its relation to the overlying faults through iterative prestack depth migration we conclude that it is not significantly offset by the faults which instead appear to stop abruptly at s thus we interpret s as the master detachment and consider its role during extension

basal conditions and glacier motion during the winter spring transition worthington glacier alaska u s a observations of the motion and basal conditions of worthington glacier alaska u s a during late winter and spring melt seasons revealed no evidence of relationship between water pressure and sliding velocity measurements included borehole water levels used as a proxy for basal water pressure surface velocity englacial deformation sliding velocity and time lapse videography of subglacial water flow and bed characteristics the boreholes were spaced 10 15 m apart six were instrumented in 1997 and five in 1998 in late winter the water pressure field showed spatially synchronous fluctuations with a diurnal cycle the glacier s motion was relatively slow and non cyclic in spring the motion was characterized by rapid diurnally varying sliding the basal water pressure displayed no diurnal signal but showed high magnitude fluctuations and often strong gradients between holes this transition in character of the basal water pressure field may represent a seasonal evolution of the drainage system from linked cavities to a network of isolated patches and conduits these changes occurred as the glacier was undergoing a seasonal velocity peak the apparent lack of correlation between sliding velocity and water pressure suggests that local scale water pressure does not directly control sliding during late winter or early in the melt season

electrical structure in thunderstorm convective regions 2 isolated storms electric field e soundings through the convective regions of two types of isolated thunderstorms are examined analysis of seven soundings through strong updrafts of isolated supercell storms show that the basic e structure there has three e peaks a positive peak near 8 km height a negative peak near 10 5 km and a positive peak near 12 km strong updraft soundings are those with average balloon ascent rates in excess of 10 m s 1 the basic charge structure in strong updrafts of supercells has four charge regions of alternating polarity the lowermost charge is positive between about 4 and 8 km and the uppermost region is negative all the supercell updraft soundings are incomplete due to balloon burst or lightning induced instrument destruction below cloud top six supercell soundings that ascended outside strong updrafts are used to show that the e and charge structures there are more complex than within the strong updrafts analysis of 15 new or previously published soundings through small new mexican mountain thunderstorms indicates that the basic e structure in or near their convective cores consists of three e peaks a lower positive peak at about 5 km height a midlevel negative peak near 6 5 km and an upper positive peak near 9 5 km the basic charge structure near the center of new mexican storm convection has four charge regions alternating in polarity with a positive charge region lowest soundings in new mexican convection that did not ascend near the convective cores show more complex e profiles and charge structures

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

successive detachment faults and mantle unroofing at magma poor rifted margins seismic depth images show that magma poor rifted margins between iberia and newfoundland are characterized by two different types of detachment that led to the unroofing of a broad expanse of mantle low angle detachments develop in serpentinites at the base of pre thinned crust and control further crustal thinning these detachments are cut by large offset faults rooting at a steep angle but with an exhumed slip surface and footwall flexurally rotated to a low angle during unroofing successive generations of this second type of detachment lead to the roughly symmetric unroofing of a broad expanse of mantle as new detachments repeatedly cut through the footwall of the preceding detachment leaving the abandoned root zone as landward dipping reflectors within exhumed mantle on both sides of the developing rift

electrical structure and updraft speeds in thunderstorms over the southern great plains we have acquired 11 balloon soundings of the electric field and thermodynamics in large and sometimes severe and tornadic thunderstorms over the southern great plains the ascent rates of the balloons were used to divide the soundings into weak and strong updrafts all 11 soundings had magnitudes of charge densities and thicknesses of charge regions similar to those reported previously in small thunderstorms the five weak updraft soundings had complex charge structures of 7 9 charge regions below 10 km altitude the magnitude of the maximum measured electric field in these soundings was 100 150 k v m 1 the four soundings in strong updrafts had balloon ascent rates of at least 15 m s 1 for at least 1 km these soundings had simpler electrical structures of 3 5 charge regions below 10 km altitude their maximum measured electric fields ranged from 85 to 130 kv m 1 the data may support an earlier hypothesis that charge is elevated in strong updrafts in addition the data may suggest that simpler charge structures are associated with proximity to strong updrafts rather than with different storm types

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

continental breakup and the onset of ultraslow seafloor spreading off flemish cap on the newfoundland rifted margin prestack depth migrated seismic reflection data collected off flemish cap on the newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system within continental crust there is no clear evidence for detachment surfaces analogous to the s reflection off the conjugate galicia bank margin demonstrating a first order asymmetry in final rift development anomalously thin 3 4 km magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized this indicates that initial accretion of the oceanic crust was in a magma limited setting similar to present day ultraslow spreading environments seaward oceanic crust thins to 1 3 km and exhibits an unusual highly reflective layering we propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep marine sheet flows to form this layering subsequent seafloor spreading formed normal 6 km thick oceanic crust this interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow spreading system was established

models of the development of the west iberia rifted continental margin at 40 30 n deduced from surface and deep tow magnetic anomalies the ocean continent transition oct on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading the nature of the oct off western lberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by leg 149 of the ocean drilling program here we concentrate on the interpretation of a new digital magnetic anomaly chart for the iberia abyssal plain between 39 5 o and 42 2 o n the most striking anomaly is a trough which exists immediately east of and locally parallel to anomaly j and is closely coincident with a basement peridotite ridge we conclude from modeling surface magnetic anomalies and a deep tow magnetometer profile that seafloor spreading began about 129 9 m y ago barremian at a rate of 10 0 mm yr consistent with drilling and seismostratigraphic results from the southern lberia abyssal plain a second feature is the existence of linear low amplitude isochron parallel magnetic anomalies east of the peridotite ridge trough and a clear change in the trend of these anomalies east of about 11 o 15 w it is possible to model these anomalies by a series of 7 to 30 km wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust the west to east anticlockwise change in trend at 11 o 15 w suggests either a major change in stress regime and or in the type of magnetic anomaly source we speculate that the region between the peridotite ridge and 11 o 15 w represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading we explain the change in trend at 11 o 15 w by a model featuring a rift which propagated through continental crust from south to north we consider three scenarios for the formation of the transitional zone crust first continental crust is surrounded and impregnated by intrusive and extrusive material formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate second such crust is surrounded by unroofed upper mantle material or third the zone is the product of ultraslow seafloor spreading present evidence tends to support the first scenario in this scenario the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust fill the space caused by additional extension and form the transitional crust already described over a broad zone at least several tens of kilometers wide at breakup upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization in a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading initially producing thin oceanic crust because of a restricted melt supply the width of continuous tectonically extended continental crust in the southern iberia abyssal plain is less than was previously supposed

stress field rotation and rooted detachment faults a coulomb failure analysis several well known mechanical models have shown that unusual boundary or loading conditions can alter principal stress orientations into configurations consistent with low angle normal faulting such models however have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low angle surfaces we present the results of simple coulomb failure analyses to determine whether and where such models predict frictional slip assuming geologically plausible boundary stresses pore pressures and rock strengths models that invoke a sizable shear traction at the base of the upper crust or spatially varying loads on the upper crust reorient principal stresses and failure planes but do not produce frictional failure on crustal scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures models that reorient stresses by midcrustal dike intrusion produce slip on low angle surfaces at relatively deep crustal levels but only in the area of the dike tip the low angle failure surfaces curve into a high angle orientation a short distance from the dike all of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest that in any system in which stresses are allowed to evolve over time failure and stress release will occur on high angle faults before low angle ones have developed these assertions are true even when near lithostatic pore pressures are assumed unless there is an inhomogeneous extraordinarily fortuitous distribution of pore pressures and rock strengths at the time of initiation of a new detachment fault one model we tested for example required pore pressures exceeding 0 96 times lithostatic in the area of the hypothesized low angle normal fault with lower pore pressures both above and below the detachment to prevent slip and stress release on high angle normal faults in the upper part of the modeled region and on low angle thrust faults in the lower part

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

3d development of detachment faulting during continental breakup abstract the developing asymmetry of rifting and continental breakup to form rifted margins has been much debated as has the formation mechanics and role of extensional detachments bespoke 3d seismic reflection data across the galicia margin west of spain image in unprecedented detail an asymmetric detachment the s reflector mapping s in 3d reveals its surface is corrugated proving that the overlying crustal blocks slipped on s surface during the rifting crucially the 3d data show that the corrugations on s perfectly match the corrugations observed on the present day block bounding faults demonstrating that s is a composite surface comprising the juxtaposed rotated roots of block bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward changes in the orientation of the corrugations record the same oceanward migration however in contrast to previous rolling hinge models the slip of the crustal blocks on s occurred at angles as low as 20 requiring that s was unusually weak consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust as the crust only becomes entirely brittle once thinned to 10 km the asymmetric s detachment and the hyper extension of the continental crust only developed late in the rifting process which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs the 3d volume allows analysis of the heaves and along strike architecture of the normal faults whose planes laterally die or spatially link together implying overlaps in faults activity during hyper extension our results thus reveal for the first time the 3d mechanics and timing of detachment faulting growth the relationship between the detachment and the network of block bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins

a low angle detachment fault revealed three dimensional images of the s reflector fault zone along the galicia passive margin abstract a new 3 d seismic reflection volume over the galicia margin continent ocean transition zone provides an unprecedented view of the prominent s reflector detachment fault that underlies the outer part of the margin this volume images the fault s structure from breakaway to termination the filtered time structure map of the s reflector shows coherent corrugations parallel to the expected paleo extension directions with an average azimuth of 107 these corrugations maintain their orientations wavelengths and amplitudes where overlying faults sole into the s reflector suggesting that the parts of the detachment fault containing multiple crustal blocks may have slipped as discrete units during its late stages another interface above the s reflector here named s is identified and interpreted as the upper boundary of the fault zone associated with the detachment fault this layer named the s interval thickens by tens of meters from se to nw in the direction of transport localized thick accumulations also occur near overlying fault intersections suggesting either non uniform fault rock production or redistribution of fault rock during slip these observations have important implications for understanding how detachment faults form and evolve over time 3 d seismic reflection imaging has enabled unique insights into fault slip history fault rock production and redistribution

the extension of lightning flashes from thunderstorms near cape canaveral florida the horizontal distance that intracloud ic and cloud to ground cg lightning flashes extend from thunderstorms is presented for warm season nonsevere storms near cape canaveral fl the warning decision support system integrated information wdss ii software allowed us to ingest display and make distance calculations using radar reflectivity data from nearby national weather service offices and lightning information from the national lightning detection network and the local lightning detection and ranging network the flash distances were determined manually using a measuring tool within wdss ii six categories of flashes were investigated a cg flashes measured from the edge of the convective core 30 dbz to the ground strike location that is beneath the attached anvil b cg bolt from the blue flashes measured from cloud edge 0 dbz to the ground strike point that contains only 0 dbz overhead c cg bolt from the blue flashes as in category b but measured from the edge of the precipitation 18 dbz d ic flashes that exit a portion of the cloud that is not an anvil measured from cloud edge 0 dbz e ic flashes within anvils measured from the edge of convective precipitation 30 dbz to the termination point within the anvil and f ic flashes exiting the edge 0 dbz of anvils results for each category are presented in the forms of frequency distributions of distances and their cumulative frequency distributions

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

unravelling the architecture and evolution of the inverted multi stage north iberian bay of biscay rift abstract current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension through the stacking of deformation modes several rifted margins show however evidence for multiple and out of sequence rift events which developed within different kinematic frameworks with a complex spatial and temporal evolution in this work we address the problem of multi stage rifting based on the study of the central north iberian margin located at the southern bay of biscay triangular oceanic domain this magma poor rifted margin recorded three major mesozoic rift events and a subsequent alpine compressional reactivation representing a unique setting to study the architecture of multi stage rift systems and the exerted control on subsequent reactivation relying on a dense set of 2d seismic reflection profiles boreholes and published velocity models we map and describe structural domains and major extensional and compressional structures and we construct depth and thickness maps of syn rift units these new maps display the geometry and spatial distribution of major rift basins and bounding structures distinctive scenarios resulting from this tectono stratigraphic approach led us to define three rift systems a first diffuse and widespread triassic system with classical fault bounded half graben basins a second narrow deep and localised late jurassic to barremian transtensional system including laterally confined pull apart basins and a third widely distributed aptian to cenomanian hyperextended system our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation resulting in a complex 3d template deciphering the tectono stratigraphic architecture of such multi stage systems provide key insights on the spatial temporal and thermal evolution of divergent plate boundaries it is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation

movement along a low angle normal fault the s reflector west of spain 1 the existence of normal faults that moved at low angles less than 20 has long been debated one possible low angle fault is the s detachment at the west galicia spain margin and thought to occur at the top of serpentinized mantle it is unlikely that s was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts it appears to have rooted beneath the conjugate flemish cap margin and it is similar to structures elsewhere that also appear to be rooted detachments here we analyze depth images to identify synrift sediment packages above s and use the geometry of these synrift packages to constrain the angle at which s both formed and remained active we find that s must have remained active at angles below 15 too low to be explained simply by the low friction coefficient of partially serpentinized peridotites instead we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed enabling moderately high fluid pressures to develop

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes

corrugated megathrust revealed offshore from costa rica exhumed faults are rough often exhibiting topographic corrugations oriented in the direction of slip such features are fundamental to mechanical processes that drive earthquakes and fault evolution however our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth especially at subducting plate boundaries here we present three dimensional seismic reflection data of the costa rica subduction zone that image a shallow megathrust fault characterized by corrugated and chaotic and weakly corrugated topographies the corrugated surfaces extend from near the trench to several kilometres down dip exhibit high reflection amplitudes consistent with high fluid content pressure and trend 11 18 oblique to subduction suggesting 15 to 25 mm yr 1 of trench parallel slip partitioning across the plate boundary the corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits in contrast weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up section forming a nascent fault surface the variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up section transiently increases fault roughness analogous corrugations may exist along significant portions of subduction megathrusts globally mature parts of the shallow megathrust beneath costa rica are characterized by striking corrugations that may channel fluids according to seismic images nascent sections of the subduction zone plate boundary appear only weakly corrugated