ionospheric electron density inversion for global navigation satellite systems radio occultation using aided abel inversions the abel inversion of ionospheric electron density profiles with the assumption of spherical symmetry applied for radio occultation soundings could introduce a greater systematic error or sometimes artifacts if the occultation rays trespass regions with larger horizontal gradients in electron density the aided abel inversions have been proposed by considering the asymmetry ratio derived from ionospheric total electron content tec or peak density nmf2 of reconstructed observation maps since knowledge of the horizontal asymmetry in ambient ionospheric density could mitigate the inversion error here we propose a new aided abel inversion using three dimensional time dependent electron density ne based on the climatological maps constructed from previous observations as it has an advantage of providing altitudinal information on the horizontal asymmetry improvement of proposed ne aided abel inversion and comparisons with electron density profiles inverted from the nmf2 and tec aided inversions are studied using observation system simulation experiment osse comparison results show that all three aided abel inversions improve the ionospheric profiling by mitigating the artificial plasma caves and negative electron density in the daytime e region the equatorial ionization anomaly crests in the f region become more distinct the statistical results show that the ne aided abel inversion has less mean and rmse of error percentage above 250 km altitudes and the performances for all aided abel inversions are similar below 250 km altitudes

effective tension shear relationships in extensional fissure swarms axial rift zone of northeastern iceland abstract the geometry of fracture systems in selected areas of the active krafla fissure swarm mid atlantic ridge northeastern iceland is analysed based on geodetic analysis of the present day topography at the top of holocene basaltic lava flows which fill the axial rift zone the deformation of this initially horizontal surface can be reconstructed extensional deformation is localised at all scales and block tilting though present remains minor using simple models of the surface expression of normal faults the geometrical characteristics of the topographic features related to active deformation during tectonic volcanic events are quantitatively analysed at crustal depths of about 1 km normal faults are present and have an average 70 dip comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone at palaeodepths of 1 2 km indicates that this dip determination is valid comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin newly formed brittle crust of the rift zone in the axial oceanic rift zone of ne iceland the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense their relative importance depending on depth absolute tension dominates in the uppermost several hundred metres of the crust resulting in the development of fissure swarms effective tension plays an important role at a deeper level 2 5 km because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections at crustal depths of about 1 km normal shear prevails along fault planes which dip 60 75 this importance of normal shear at moderate depth between upper and lower crustal levels where tension prevails is pointed out within the extensional context of rifting these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure

single and joint fault plane solutions for microearthquakes in south iceland abstract we obtain fault plane solutions for a swarm of twelve events and two single events ranging in magnitude between 0 3 and 3 1 the events were recorded by a local network in south iceland consisting of eight three component stations with approximately 30 km between the stations for events larger than m l 1 the solutions are well constrained the non uniqueness of the fault plane solutions increases with diminishing magnitude to enhance the uniqueness of the fault plane solutions for the earthquake swarm we search for mechanisms with a common fault plane but possibly varying slip directions in that plane data for all the events are consistent with faulting on a common plane relative location of the events shows that they all occurred less than 11 m from a single plane fault plane solutions for all the analysed events of the swarm as well as the two single events are consistent with strike slip dominated faulting on faults striking roughly north south the results are in agreement with the strikes of mapped surface fractures and focal mechanisms of larger earthquakes in the area this indicates that the microseismicity is related to the same pattern of regional deformation as the major earthquakes in south iceland

analogue modeling of overlapping spreading centers insights into their propagation and coalescence abstract the propagation and segmentation of mid ocean ridges is studied using centrifuged analogue models built with non linear materials the deformation of the brittle ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force this linear upwelling laterally stretches the model mantle that in turn induces failure in the upper layer simulating the brittle crust arrays of fractures initiate within zones of high stress concentration above the diapir fractures propagate laterally in a direction perpendicular to the maximum tensile the minimum principal stress secondary tension cracks initiate in the vicinity of parent fracture tips through going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns the overlap overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence non overlapping sub parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture if overlapping fractures are parallel they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material fracture curvature in this case results from crack crack interaction and is similar to that of overlapping spreading centers oscs observed along mid ocean ridges overlapping non parallel fractures tend to coalesce by one of their tips propagating sub parallel to the spreading direction toward the other fracture such offsets can serve as models for the development of the orthogonal ridge transform fault patterns common along mid ocean ridges

experiments of dike induced deformation insights on the long term evolution of divergent plate boundaries the shallow transport of magma occurs through dikes causing surface deformation our understanding of the effects of diking at the surface is limited especially on the long term for repeated intrusive episodes we use analogue models to study the upper crustal deformation induced by dikes we insert metal plates within cohesive sand with three setups in setup a the intrusion rises upward with constant thickness and in setups b and c the intrusion thickens at a fixed depth with final rectangular setup b or triangular setup c shape in section setup a creates a doming delimited by reverse faults with secondary apical graben without close correspondence in nature in setups b and c a depression flanked by two uplifted areas is bordered by inward dipping normal faults propagating downward and for deeper intrusions in setup b also by inner faults reverse at the surface this deformation is similar to what is observed in nature suggesting a consistent physical behavior dikes in nature initially propagate developing a mode i fracture at the tip subsequently thickened by magma intrusion without any host rock translation in the propagation direction as in setup a the deformation pattern in setups b and c depends on the intrusion depth and thickness consistently to what is observed along divergent plate boundaries the early deformation in setups b and c is similar to that from a single rifting episode i e lakagigar iceland and dabbahu afar whereas the late stages resemble the structure of mature rifts i e krafla iceland confirming diking as a major process in shaping divergent plate boundaries

structure of tertiary and pleistocene normal faults in iceland normal faults were measured in three selected areas of the late tertiary and pleistocene lava pile in iceland the arithmetic mean throw of 315 faults is 8 2 m the minimum being 0 5 m and the maximum 150 m about 93 of the faults have throws of less than 20 m the mean strike of the faults is n31 e but the mode is n45 e the faults dip from 42 to 89 nearly 80 dip between 65 and 79 and the mean dip is 73 fault breccia is common and varies in thickness from 0 05 m to 2 m there is a positive linear relation between breccia thickness and throw the linear correlation coefficient being 0 78 it is proposed that the normal faults in iceland form by two basic mechanisms one is the development of faults from sets of inclined joints in the lava pile the initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1 for every 150 170 m depth the inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints the other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures the great majority of fractures within the active rift zone are tension fractures but when they attain the length of several hundred meters they normally change into normal faults it is concluded that normal fault development in iceland commonly starts at depths of about 1 km in the crust

formation and mechanics of magma reservoirs in iceland summary provided magma flow in a partially molten layer follows darcy s law of flow in porous media it can be demonstrated that magma in the proposed magma layer beneath iceland flows towards regions of reduced crustal thickness in these regions magma accumulates to form partially molten magma reservoirs which are thus located in the magma layer these reservoirs either give rise to surface eruptions or feed totally molten shallow magma chambers located in the crust that may erupt it is argued that the melt fraction varies throughout a reservoir being highest near the top where the reservoir may be totally molten but that the average is 0 25 it is concluded that for individual eruptions the reservoirs behave on the whole as a poroelastic material it is suggested that formation of crustal magma chambers is facilitated by the occurrence of stress barriers that lead to the formation of thick sills while liquid such sills absorb the magma of all dykes that enter them and may evolve into magma chambers ideal sites for stress barriers and thus for magma chambers are formations where individual layers have different elastic properties i e where the crust behaves as a multilayer denoting the bulk volume of the reservoirs by vb the volume of the crustal chambers by v and the volume of lava and dyke material in individual eruptions by ve it is found that vb 5365ve and v 1850ve using the estimated volumes of exposed plutons as a basis it is found that provided the feeder reservoir does not participate in the eruption the volume of individual eruptions from elastic crustal magma chambers in iceland is less than 0 25 km3 usually less than 0 1 km3 and that typical central volcano eruptions should be about 0 02 km3 for eruptions of the order of 1 km3 or larger the reservoirs must supply magma during the eruption it is concluded that from the point of view of volume historical eruptions of typical size could have come from elastic crustal magma chambers

three dimensional estimation of electron density over japan using the geonet gps network combined with sac c data and ionosonde measurements 1 the work presented here proposes a local tomographic approach to obtain a three dimensional description of the ionospheric electron density over japan to do this a combination of gps and ionosonde data is used in order to take advantage of the strong points of each data type the gps data is composed of ground and sac c low earth orbiter observations the ground observations correspond to measurements of 99 receivers out of about 1000 from the dense japanese gps network geonet sac c gps limb sounding observations are processed to obtain vertical profiles of electron density for this purpose an improved approach of abel inversion that considers vertical total electron content data has been used to overcome the assumption of spherical symmetry the resulting profiles have been averaged to build a set of background profiles which are rescaled with the ionospheric nmf2 values sounded by the japanese network of four ionosondes and used by the tomographic algorithm to provide with vertical description of electron density for the validation of this tomographic approach the hourly values of nmf2 given by the four japanese ionosondes and the vertical profiles of electron density measured by the mu radar in shigaraki during the sounding campaign of november 2002 have been used this validation with independent data shows relative discrepancies about 25 and moreover it shows that the use of the profiles obtained from sac c data offers an improvement on the dawn dusk and nighttime bottomside and topside estimations of the electron density with respect to the case of using the iri model

evolution of vertical faults at an extensional plate boundary southwest iceland vertical faults having both opening and vertical displacements are common in southwest iceland and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward we address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest iceland individual fracture segments are commonly rotated out of the general trend of the fracture zone suggesting oblique motion along subsurface normal faults these faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side based on these field observations and the results of numerical models we propose that 60 758 dipping normal faults in the subsurface propagated to the surface from below vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m 25 50 of the fault length in response to stress concentrations along the tip line model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures which subsequently breach the monoclines along the upper hinge line forming vertical fault scarps and open fissures at the surface if vertical fractures utilize pre existing cooling joints in basalt to connect directly to the surface the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace q 2003 elsevier ltd all rights reserved

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

effects of overpressure variations on fracture apertures and fluid transport abstract for an isolated rock fracture in a homogeneous isotropic rock subject to constant internal fluid overpressure or constant external driving stress the ideal opening displacement aperture profile or variation is that of a flat ellipse for many mineral veins dykes tension fractures normal faults and other rock fractures however the opening displacements show irregular aperture thickness variations very different from that of a flat ellipse here we present field data on typical fracture aperture variations as well as new numerical and analytical models to explain these data we present the overpressure variation by fourier cosine series a very flexible method that can be used to model abrupt overpressure and driving stress variations in vertical and lateral sections for fractures of various sizes and types we calculate the opening displacements of typical hydrofractures and discuss the results with reference to mineral veins and dykes we also present numerical models showing that when a fracture dissects layer or parts of a single layer such as a lava flow with different stiffnesses young s moduli the opening displacement may show irregular variation even when the overpressure is constant from the cubic law the volumetric flow rates in the large aperture segments of the mineral veins and dykes discussed in the paper may have been as much as 3 5 times the flow rates in the small aperture segments we propose that differences in volumetric flow rates are related to irregular opening displacement variations in feeder dykes is one principal reason for the development of crater cones a universal feature of volcanic fissures

improving the abel inversion by adding ground gps data to leo radio occultations in ionospheric sounding gps radio occultations allow the sounding of the earth s atmosphere i e troposphere and ionosphere the basic observable of this technique is the additional delay due to the refractivity index of a radio signal when passing through the atmosphere this additional delay is proportional to the integrated refractivity in such a way that we can obtain an estimation of the vertical refractivity profiles using observations at different elevation angles by solving an inverse problem traditionally the solution of this inverse problem is obtained by using the abel inversion algorithm assuming a refractivity index that only depends on the altitude in this paper we present a modified abel inversion algorithm for ionospheric sounding that overcomes the spherical symmetry assumption of the traditional abel inversion algorithm processing a set of simulated data and 1 day of real data with this algorithm a clear improvement over the traditional one can be obtained when comparing the derived critical frequencies with the ionosonde measurements it is also shown that this improvement is sufficient to measure critical frequencies associated with the ionospheric e layer

off rift and rift zone palaeostresses in northwest iceland abstract the vestfirdir peninsula of northwest iceland consists mainly of gently dipping basaltic lava flows 8 14 ma old detailed studies of its principal tectonic elements basaltic dykes normal faults and minor faults and observations of the trends of the major fjords and valleys make it possible to reconstruct the evolution of the associated stress field the results show that a kink shaped curved rift zone existed in this area until about 5 ma ago in the northern part of the peninsula the rift zone trended nne but ene in the southern part it is proposed that the tectonic elements of the vestfirdir peninsula were generated in two stress fields one operating inside the rift zone the other operating outside it the rift zone stress field was responsible for the formation of most normal faults dykes and the main extension direction inferred from the minor faults conversely the off rift stress field was responsible for the formation of several holocene normal faults in the southern part of the peninsula the direction of the secondary extension inferred from the minor faults as well as for most major fjords and valleys in the area boundary element studies suggest that the off rift stress field generated on the concave side of a curved rift zone is different from that generated on its convex side absolute tensile stresses that encourage the formation of valleys and fjords concentrate very near to or at the convex margin but at a considerable distance from the concave margin applying these model results to iceland as a whole it is concluded that the valley formation in northwest iceland occurred at a large distance form the associated rift zone whereas in east iceland the valleys formed near to the margin of the rift zone furthermore the model predicts that zones of relative tension may propagate radially from the convex side of the rift zone and act as magma channels for volcanoes the location and orientation of major fjords and valleys in east and northwest iceland as well as the existence of several major off rift holocene volcanoes support the model predictions

the geology of the occator quadrangle of dwarf planet ceres floor fractured craters and other geomorphic evidence of cryomagmatism abstract the dawn science team is conducting a geological mapping campaign at ceres during the nominal mission including iterative mapping using data obtained during each orbital phase we are using geological mapping as a method to identify the geologic processes that have modified the surface of dwarf planet ceres we here present the geology of the ac 9 occator quadrangle located between 22 s 22 n and 216 288 e the ac 9 map area is within the topographically high region named hanami planum features of note within the profile include impact craters occator azacca lociyo nepen and kirnis four of these craters have fractured shallow floors morphometrically comparable to floor fractured craters ffcs on the moon similar to models for the formation of the lunar ffcs we suggest that these ceres ffcs are a result of cryomagmatic uplift under the crater floors a set of regional linear structures called the samhain catenae do not have any obvious relationship to impact craters many of the catenae are comprised of smaller structures that have linked together suggestive of en echelon fractures it has been suggested that these fractures formed due to the uplift of hanami planum due to cryromagmatic plumes

discrete element modeling of extensional fault related monocline formation abstract the interplay of faulting and folding has long been recognized in extensional systems and numerous investigations have documented the importance of mechanical layering on development of normal fault related monoclines in this study we use discrete element modeling to explore the impact of mechanical layering and fault geometry on normal fault related folding for a well exposed field example in southwestern iceland the model honors the mechanically stratified character of the deformed sequence and replicates the monocline geometry by simulating displacement and upward propagation of a buried fault a close match between model geometry and field observations was obtained using a refracted fault geometry and a cover sequence with alternating 40 m thick relatively strong and 10 m thick relatively weak layers initial deformation is accommodated by folding of mechanically weaker intervals and fracturing of mechanically stronger layers and maximum monocline width is developed early as overall fault displacement increases throughgoing fracture connections along the footwall side of the monocline develop leaving the monocline limb attached to the hanging wall consistent with field observations model results suggest that significant near surface and subsurface fracture porosity is developed in strong layers reproducing the complexity of the natural normal fault related monocline in iceland requires incorporating strongly contrasting mechanical properties within a layered cover sequence

the formation of valles marineris 1 tectonic architecture and the relative roles of extension and subsidence 1 the formation of the valles marineris troughs on mars is widely held to involve some combination of horizontal extension and vertical subsidence or collapse but the specific tectonic mechanism is poorly understood this study uses boundary element models of valles marineris formation to evaluate a range of fault dip angles for two end member models of trough formation simple extensional tectonism and extension together with vertical accommodation at the base of the lithosphere representing either viscous lower crustal flow or some other form of deep seated collapse the models are constrained by the lack of footwall uplift on the plateau surface outside the troughs and the lack of significant tectonic failure of the plateau in response to the stress changes induced by valles marineris formation the results demonstrate that trough formation by extensional tectonism alone generates large surface uplifts in the surrounding plateau and enormous stresses in the lithosphere in conflict with the observations the only mechanism that is compatible with both the observed topography and the limitations imposed by the finite strength of the lithosphere is trough formation through displacement along steeply dipping to subvertical border faults together with vertical accommodation at the base of the subsiding fault block fault dips are constrained to be greater than or equal to 85 leading to a ratio of vertical subsidence to horizontal extension exceeding 5 7 and a maximum extension of less than 4 6 km these results confirm earlier studies suggesting that trough formation involved a substantial component of vertical subsidence or collapse with only modest amounts of total extension

magmatic and tectonic history of iceland s western rift zone at lake thingvallavatn high rates of holocene sedimentation in a lake spanning the thingvellir rift zone of western iceland provide an unusual opportunity to study the interaction of tectonic and magmatic processes on timescales of thousands of years lake thingvallavatn is oriented sw ne parallel to the trend of normal faults and fissures which extend northeast from hengill a central volcano system thingvallavatn s lake sediments provide a continuous high fidelity record of tectono magmatic processes chirp sub bottom profiler and sidescan records together with sediment core information enabled the lake stratigraphy to be constrained well since the emplacement of a post glacial lava at 9 1 0 3 ka this lava together with three younger horizons enabled detailed study of the main tectonic and magmatic events a major hengill volcanic event which controlled the development of the present day lake morphology occurred at 1 9 ka with the eruption of a scoria cone within the lake sandey during this event the nesjahraun lava was erupted into the southern part of the lake the sandey scoria cone was formed and major faulting and subsidence occurred in the northern part of the lake resulting in the formation of an asymmetrical rift within the southern part of the lake a deformed sequence of sediments of age 2 9 1 5 ka between undeformed younger and older sediments indicate that liquefaction phenomena are associated with the emplacement of the nesjahraun lava analysis of fault displacement reveals that the total throw summed over all faults across the width of the rift zone is approximately constant 110 130 m along the long axis of the rift we estimate an extension rate on the faults of 3 3 8 2 mm yr 1 since 9 1 ka assuming fault dips of 60 75 which represents 17 43 of the total plate boundary extension estimated from global plate motion inversion we speculate that the remaining extension must either be taken up elsewhere in iceland for example in the eastern rift zone or along the south island seismic zone or that extension estimated over the last 9kyr underestimates the long term extension rate due to incomplete sampling of the episodic magmatic component

architecture of normal faults in the rift zone of central north iceland this work examines the architecture and propagation of normal faults that accommodate extension in the rift zone of central north iceland it is based on a structural study of portions of three grabens their normal faults and dilational fractures in basaltic lavas of the fremri namur and dyngjufjol volcanic systems we analyzed the shape throw and dilation along faults in order to infer their possible evolution the shape of throw profiles departs from elliptical varying along each fault with throw being lower at the fault and segment tips propagation of fault segments is accommodated by dilational fractures nucleating at their respective tips segments link by lateral propagation of one or both tips or by the development of intervening linking segments two orders of fault segmentation indicate that the present throw accumulated by many increments as the fault scarps changed their geometries faults have large dilational components weakly correlated with fault throw and length

a simultaneous study of ionospheric parameters derived from formosat 3 cosmic grace and champ missions over middle low and equatorial latitudes comparison with ionosonde data accurate ionospheric modeling efforts are partly restricted by lack of enough reliable ground based data and the inability to validate the existing space based data in this article we present a first time comprehensive reliability and validation check of ionospheric data derived using the gps radio occultation ro technique from three separate missions formosat 3 cosmic grace and champ by comparing ro data with ionosonde data for low latitude equatorial and midlatitude stations simultaneously this paper discusses two main objectives a determination of the appropriate spatial resolutions for effective ro and ionosonde data comparisons and b estimating the accuracy of the ionospheric parameters derived from ro missions with respect to ionosonde data within the african sector for the first time ionospheric parameters retrieved from ro data have been compared in details to ionosonde data over the african sector specifically for the south african midlatitude stations grahamstown gr13l 33 3 s 26 5 e and madimbo mu12k 22 4 s 30 9 e for the equatorial and low latitude regions data for fortaleza fza0m 3 8 s 38 w brazil and ascension islands as00q 7 9 s 14 4 w was analyzed a simple but important method to determine the latitudinal and longitudinal range to be used in comparison with ionosonde data has been established based on statistical analysis it is found that 4 5 4 5 3 3 and 4 4 are the approximate suitable spatial resolutions in both latitude and longitude spaces over an ionosonde station for effective comparisons for midlatitude low latitude and equatorial regions respectively appropriate spatial coverage for effective comparisons vary with region and therefore a constant assumption should not be applied on regional global basis especially if the studies investigations or modeling extends from middle to low equatorial latitude zones for the three latitude regions cosmic overestimates the maximum height of the f2 layer hmf2 and underestimates the maximum electron densities these results provide a step by step basis and motivation for the usage of ro data in ionospheric modeling especially for models which give average conditions and characterisation over regions that do not have sufficient ground based instruments data

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

a 3 d geological and structural synthesis of the leinster area of the agnew wiluna belt yilgarn craton western australia with special reference to the volcanological setting of komatiite associated nickel sulfide deposits the leinster area is situated toward the southern end of the agnew wiluna belt in the yilgarn craton of western australia and hosts the giant perseverance komatiite associated nickel sulfide deposit at in excess of 1 2 mt contained ni this is the biggest type 1 basal sulfide rich accumulation deposit in the world and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the perseverance fault on the eastern margin of the belt in broad terms the greenstone sequence comprises an eastern rift proximal komatiite dacite sequence and a western and more marginal basalt dominated sequence with minor komatiite the komatiites display a wide range of compositions and textures from highly magnesian olivine meso to adcumulate rocks composed of olivine with forsterite contents of up to 94 5 to sequences of thin spinifex textured komatiite flow units this compositional range indicates considerable variation in volcanic facies from rift proximal to distal loci of focused flow termed pathways indicated by thickened cord like bodies of more olivine rich rock occur at all scales and are intimately associated with the ni sulfide mineralization the pathways are linked by thinner sequences of lower mgo rocks either layered olivine orthocumulate rocks or sequences of thin spinifex textured flow units some of the lower mgo komatiite sequences appear to emanate from the tops of pathways in the manner of breakout flows in tube fed basaltic terrains variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks the overprinting effects of regional deformation and metamorphism spanning at least 80 m y are considerable at least eight local deformation events have been recognized by previous workers these have been used to constrain the 3 d model and can be linked to craton wide structural schemes some of the major faults are clearly accretionary and reflect reactivation of underlying basin forming structures early north over south thrusting was followed by e w directed basin closure and the development of nnw trending open regional folds extension accompanied batholithic granitoid intrusion and reactivated rift parallel nnw trending faults producing the major strike slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains the ensuing orogenic collapse resulted in overturning of the stratigraphy to the east subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation there is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the ni sulfide deposits in this ancient terrane three dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt allowed the identification of komatiite pathway positions and their plunges and provided quantitative data on the magnitude and direction of fault offsets of different generations the modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of ni sulfide deposits even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism this is illustrated with case studies of the perseverance venus rocky s reward harmony and sir lancelot deposits three dimensional modeling has the capacity to integrate geologic geochemical and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration the recent discovery of the venus deposit is in part a tribute to the power of this approach

estimates of the precision of gps radio occultations from the cosmic formosat 3 mission 1 the constellation observing system for meteorology ionosphere and climate cosmic formosa satellite 3 formosat 3 is a six satellite radio occultation mission that was launched in mid april 2006 the close proximity of the cosmic satellites provides a unique opportunity to estimate the precision of the radio occultation remote sensing technique from closely collocated occultations 10 km separation of tangent points the rms difference of refractivity between 10 and 20 km altitude is less than 0 2 which is approximately twice better than previous estimates obtained from champ and sac c collocated occultations apparently due to smaller separation of the occultation pairs and due to parallel occultation planes in the lower troposphere the maximal rms is 0 8 at 2 km altitude and decreases abruptly to 0 2 between 6 and 8 km altitude the rms difference of electron density in the ionosphere between 150 and 500 km altitude for collocated occultations is about 103 cm 3

september 2005 manda hararo dabbahu rifting event afar ethiopia constraints provided by geodetic data faults the volume of the 2005 dike 1 5 2 0 km 3 is not balanced by sufficient volume loss at dabbahu and gabho volcanoes 0 42 and 0 12 km 3 respectively taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment above the dike faults slipped by an average 3 m yielding an estimated geodetic moment of 3 5 10 19 nm one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm between dabbahu and ado ale volcanic complexes significant opening occurred on the western side of the dike the anomalous location of the dike at this latitude offset to the east of the axial depression may explain this phenomenon a two stage intrusion scenario is proposed whereby rifting in the northern manda hararo rift was triggered by magma upwelling in the dabbahu area at the northern extremity of the magmatic segment although vigorous dike injection occurred during the september 2005 event the tectonic stress deficit since the previous rifting episode was not fully released leading to further intrusions in 2006 2009

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

data assimilation retrieval of electron density profiles from radio occultation measurements 1 in this paper the kalman filter is used to retrieve the electron density profile along the tangent points by assimilating the slant total electron content data observed during a radio occultation ro event into an empirical background model the ro data observed by cosmic satellites on day of year 266 in 2009 are selected to do both the simulation work and the real data retrieval test the results show that the data assimilation technique can improve the electron density retrieval in comparison with the abel inversion it is less influenced by the ionospheric inhomogeneity than the abel method some pseudo large scale features made by the abel retrieval such as the plasma cave underneath the equatorial ionization anomaly region and the three peaks along the latitude direction in the e layer disappear in the data assimilation retrieval results independent validation by ground based ionosonde observations confirms the improvement of data assimilation retrieval below the f2 peak in addition some potential research on ro data assimilation is also discussed

age limit and age underestimation using different osl signals from lacustrine quartz and polymineral fine grains as part of a systematic palaeohydrological reconstruction of lake level fluctuations during the last glacial maximum a transect of cores from ancient lake xinias in centra greece has already been studied with respect to pollen sediment and mineral magnetic analyses the chronology was based on c 14 ams dating of terrestrial plant macrofossil remains from peat and clayey peat palaeogeog palaeoclimatol palaeoecol 158 2000 65 this site thus provides an opportunity for the comparison of fine grain optically stimulated luminescence osl ages of water lain sediments with an independent chronology we present here infrared ir post ir blue and blue osl characteristics of the fine grain sediments from lake xinias and a preliminary comparison with independent ages the equivalent doses based on the ir results are about 40 of those based on post ir blue stimulation which in turn are 10 15 below those based on quartz osl we discuss the ages derived from the 3 signals in terms of the independent chronology and draw conclusions about initial bleaching of the quartz and feldspar components and the reliability of the post ir blue signal as a chronometer c 2003 elsevier science ltd all rights reserved less

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

surface deformation and strike slip faulting controlled by dyking and host rock lithology a compendium from the krafla rift iceland abstract in the present work we have obtained a detailed complete view of surface deformation at a volcano tectonic structure belonging to the krafla fissure swarm ne iceland through the integration of field mapping and the aerial structure from motion technique asfm as well as through orthomosaics and digital surface models dsms the documented 2 5 km long deformation zone is marked by a range of features such as topographic bulging parallel extension fractures and narrow grabens affected by local floor uplift which we have explained as the effect of the northward shallow propagation of a dyke from the krafla magma chamber our lithostratigraphic field survey of the area at a 1 5000 scale indicates that major changes in the pattern of surface deformation take place across the contacts between deposits with different rheological properties the transition from very stiff lavas to softer hyaloclastites produces changes from extensional fracturing to normal faulting though the width of the deformation zone does not change we have also documented a number of ne sw left lateral and nnw sse right lateral strike slip fault zones that are rotated in clockwise and anticlockwise sense to the main nne ssw graben trend and extend outwards as much as about 17 m the graben zone terminates to the north by splitting into two strike slip fault zones these were likely generated at the front of the dyke tip during its propagation and were later on bypassed by the advancement of the dyke as the dyke entered the soft hyaloclastites propagation was slowed down and the dyke became thicker

fault and graben growth along active magmatic divergent plate boundaries in iceland and ethiopia recent studies highlight the importance of annual scale dike induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries mdpb however the longer term 102 105 years role of diking on the cumulative surface deformation and evolution of mdpb is not yet well understood to better understand the longer term normal faults and graben along the axis of mdpb we analyze fissure swarms in iceland and ethiopia we first focus on the simplest case of immature fissure swarms with single dike fed eruptive fissures these consist of a 1 km wide graben bordered by normal faults with displacement up to a few meters consistent with theoretical models and geodetic data a similar structural pattern is found with asymmetric and multiple graben within wider mature fissure swarms formed by several dike fed eruptive fissures we then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation most faults terminate as open fractures on flat surface suggesting downward fault propagation this is consistent with recent experiments showing dike induced normal faults propagating downward from the surface however some normal faults also terminate as open fractures on monoclines which resemble fault propagation folds this suggests upward propagation of reactivated buried faults promoted by diking these results suggest that fault growth and graben development as well as the longer term evolution of the axis of mdpb may be explained only through dike emplacement and that any amagmatic faulting is not necessary

dem simulation of normal faults in cohesive materials abstract we performed discrete element method dem simulations of the formation of normal faults in a brittle cohesive material above a basement fault focusing on the formation of open fractures and cavities in areas of low normal stress i e near the surface of the model the simulations produce a wide range of fundamental structures similar to those observed in nature and in analogue models comparisons with these sandbox experiments show that the fundamental structures generated are similar between the analogue and the numerical models in a parameter study of the material properties and the model geometry we have investigated the effect of these parameters far beyond what is possible in analogue models over a range of parameters we show that the evolving structural style is robust i e all the models develop similar structures in a similar sequence results show a systematic dependence of some structural features such as location an style of faults on the simulation parameters in particular we observe that the maximum depth of the open fractures varies linearly with the cohesion suggesting that we can predict the depth of open cavities formed by tensile failure in natural fault systems if the material properties are known

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

along rift propagation of pleistocene holocene faults from a central volcano abstract the mechanisms of rift propagation are still not fully understood especially at mid oceanic ridges owing to the inherent difficulty in collecting submarine data here we investigate the fault slip profiles of the 60 km long theistareykir rift northern iceland that may suggest the direction of along axis rift propagation this is one of the few places on earth where rifting processes and mid oceanic ridge formation can be studied directly moreover this rift hosts an active central volcano and this enables to fully understand the relations between rift propagation and magma systems we reconstructed the slip profiles of all the 281 main pleistocene holocene faults that compose this n s rift by merging measurements performed in the field collected by unmanned aerial vehicle surveys and derived from digital surface models results indicate that north of the volcano 75 of the asymmetric faults propagated northward the value increases to 82 if the cumulated fault length is considered south of the volcano 47 of the asymmetric faults propagated southward 54 if the cumulated fault length is considered these data point to a dominant mechanism of along axis propagation of the rift outward from the volcano suggesting a genetic link with the underlying magma chamber two possible processes are suggested which can also occur in combination i faults develop following lateral dyke propagation outward from the magma chamber and ii faults nucleate near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux the rift architecture is complicated by the presence of tectonic zones with different dominant fault dips separated by transversal accommodation zones the latter also play the role of barriers guiding local fault propagation

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

structures in tessera terrain venus issues and answers many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onionskin on venus because tesserae are exposed mostly within crustal plateaus which exhibit thickened crust issues of tessera distribution and the mechanism of crustal plateau formation e g mantle downwelling or upwelling are intimately related a review of magellan data indicates that tessera terrain does not form a global onionskin on venus although ribbon bearing tesserae reflect an ancient time of a globally thin lithosphere individual tracts of ribbon bearing tessera terrain formed diachronously punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600 2500 km diameter regions above hot mantle plumes plume related magmatic accretion led to crustal thickening at these locations resulting in crustal plateaus crustal plateau surfaces record widespread early extension ribbon structures and local minor perpendicular contraction of a thin competent layer above a ductile substrate within individual evolving crustal plateaus the thickness of the competent layer increased with time and broad gentle folds formed along plateau margins and short variably oriented folds formed in the interior late complex graben cut folds local lava flows accompanied all stages of surface deformation in contrast to these conclusions gilmore et al 1998 summarized post magellan arguments in favor of downwelling models for crustal plateau formation in light of this discrepancy we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models we show that gilmore et al 1998 did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid they disregarded shear fracture ribbons thus invalidating their criticism of ribbon models they misunderstood previous radargrammetric work that constrains ribbon geometry and they relied solely on geometrical relations to constrain timing violating kinematic analysis methodology their stratigraphic constraints on ribbon fold temporal relations are invalid because they 1 misinterpreted implications of map relations 2 did not isolate radar artifacts due to local radar slope effects from proposed material units 3 chose a region for analysis that clearly shows the effects of younger tectonism and volcanism and 4 presented map relations that cannot be reproduced their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre magellan and post magellan upwelling models these misconceptions about the upwelling model and processes responsible for global warming phillips and hansen 1998 lead to serious errors in gilmore et al s 1998 criticism furthermore we show that the data of gilmore et al 1998 are actually more consistent with upwelling than downwelling models consistent with arguments that tessera terrain is not global in spatial distribution

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

ionospheric and plasmaspheric electron contents inferred from radio occultations and global ionospheric maps we introduce a methodology to extract the separate contributions of the ionosphere and the plasmasphere to the vertical total electron content without relying on a fixed altitude to perform that separation the method combines two previously developed and tested techniques namely the retrieval of electron density profiles from radio occultations using an improved abel inversion technique and a two component model for the topside ionosphere plus protonosphere taking measurements of the total electron content from global ionospheric maps and radio occultations from the constellation observing system for meteorology ionosphere and climate formosat 3 constellation the ionospheric and plasmaspheric electron contents are calculated for a sample of observations covering 2007 a period of low solar and geomagnetic activity the results obtained are shown to be consistent with previous studies for the last solar minimum period and with model calculations confirming the reversal of the winter anomaly the hemispheric asymmetry of the semiannual anomaly and the existence in the plasmasphere of an annual anomaly in the south american sector of longitudes the analysis of the respective fractional contributions from the ionosphere and the plasmasphere to the total electron content shows quantitatively that during the night the plasmasphere makes the largest contribution peaking just before sunrise and during winter on the other hand the fractional contribution from the ionosphere reaches a maximum value around noon which is nearly independent of season and geomagnetic latitude

absolute and relative locations of similar events with application to microearthquakes in southern iceland it is well known that similar earthquakes i e earthquakes having almost identical waveforms allow extremely accurate relative timing of the seismic arrivals this has traditionally been used for

volcanic rifting at martian grabens 1 a large fraction of surface extension on mars occurred at segmented grabens having width length ratios akin to oceanic rifts on earth association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes a martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery high resolution digital elevation models dems scaled experimental modeling and three dimensional boundary element modeling of magmatic and tectonic processes and a comparison with terrestrial rifts the dems were obtained from mars observer laser altimeter viking orbiter stereo images or a combination of both comparison of terrestrial rifts included afar iceland and the east pacific rise the ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly resulting in emplacement of elongated magma reservoirs along the grabens injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of mpa and induces surface collapse each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces the geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width we conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic martian grabens on the basis one graben one dike rather every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the icelandic fissure zones index terms 5475 planetology solid surface planets tectonics 8149 5480 planetology solid surface planets volcanism 8450 6225 planetology solar system objects mars 8121 tectonophysics dynamics convection currents and mantle plumes 8010 structural geology fractures and faults keywords rifting graben pit crater magma chamber dyke

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

dike induced contraction along oceanic and continental divergent plate boundaries the axis of divergent plate boundaries shows extension fractures and normal faults at the surface here we present evidence of contraction along the axis of the oceanic ridge of iceland and the continental main ethiopian rift contraction is found at the base of the tilted hanging wall of dilational normal faults balancing part of their extension our experiments suggest that these structures result from dike emplacement multiple dike injection induces subsidence above and uplift to the sides of the dikes the transition in between is accommodated by reverse faults and subsequent peripheral inward dipping normal faults our results suggest that contraction is a direct product of magma emplacement along divergent plate boundaries at various scales marking a precise evolutionary stage and initiating part of the extensional structures extension fractures and normal faults

displacement and gravity anomaly produced by a shallow vertical dyke in a cohesionless medium summary a crack model is presented for modelling magma emplacement within a shallow vertical dyke in a half space which responds elastically in compression but has vanishing tensile strength realistic initial stress profiles in the solid rock are considered corresponding to lithostatic and sedimentary equilibria while the magma is assumed to possess higher density than the host rock and to conform to the hydrostatic pressure gradient equilibrium crack width and extension are computed for several sets of model parameters and from these uplift and gravity anomaly at the ground surface it is shown that within the model s assumptions narrow gravity anomalies and uplift can be generated reaching 50 gal and 1 m respectively provided that the dyke top is very shallow and its vertical extension is large enough 3 km dyke injection is accompanied by increasing compression in the host rock at depth but tensile contributions are generated at shallower depths around the magma filled upper portion of the crack which may play an important role in driving the flow of fluids permeating the upper crust if the dyke propagates to a shallow enough depth its uppermost part may remain empty of magma

magmatic fissures and their systems in iceland a tectonomagmatic model 1 volcanic systems in iceland comprise a central volcano linked to magmatic fissures whose subsurface architecture remains enigmatic the surface dimensions and trends of active fissures show their spatial relation with fault systems structure of the fremri namur and hengill volcanic systems was analyzed the linear vent arrays marking recent fissures are tightly bound by vertical segmented faults while normal faults accommodating extension are common on the flanks of the fissure swarm evidence of normal frictional slip on the fissure bounded faults suggests that failure occurred in response to subsidence with subsequent dilation this is supported by inward tilting of strata adjacent to faults compatible with a downsag phase we propose that magmatic fissures have vertical feeders with lateral offshoots extending along the rift zone their inflation deflation during an eruptive cycle causes subsidence such magmatically generated faults can be subsequently modified by tectonic extension

propagation of brittle failure triggered by magma in iceland abstract the architectures of normal faults at a divergent plate boundary in iceland are examined by combining surface fault observations with cross sectional studies at different structural levels to constrain a model of failure propagation the structures of holocene faults defining graben are analyzed to characterize the upper most parts of ruptures the shapes of faults resulting from growth and interaction of separate segments are used to better understand failure propagation inferred to occur in the intervening stages of displacement accumulation and lateral propagation pleistocene faults in volcanic sequences exhumed from 800 to 1000 m are analyzed to characterize deeper portions of failure that occurred beneath the central rift zone tertiary dikes exhumed from depths of 1300 1500 m are studied to infer how magma controls the failure initiation field studies in combination with a literature review indicate that the planar ruptures are likely to initiate at depth as magma filled vertical fractures and lengthen upward and laterally as failures propagate to higher crustal levels they are likely to change into inclined normal faults at near surface levels faults link with cooling joints and dilational fractures propagating downward from the surface it is suggested that the inferred stages of fault propagation are characteristic for normal faults developed at spreading ridges

contemporaneous phreatomagmatic and effusive activity along the hverfjall eruptive fissure north iceland eruption chronology and resulting deposits abstract the 2500 bp hverfjall eruption in northern iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure the southern half of the fissure was located in a shallow lake whereas the northern part of the fissure extended onto dry land this specific setting with overlapping periods of activity at the different vents resulted in various mingling features between fine grained phreatomagmatic deposits and lava flows here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits the eruption can be divided into three main phases depending on the main depositional characteristics and vent locations the initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting the second phases involved opening of two new vents on dry ground and deposition of scoria and lava flows the third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges these surges display features consistent with drying up with increasing distance from the vent suggesting that their dynamics changed during emplacement most wet surges were channelized within a preexisting graben structure close to the vent but some more dilute i e dry surges were able to flow over this obstacle and continued to flow for more than 5 km away from the vent and 100 m uphill before stopping

structural control of geothermal reservoirs in extensional tectonic settings an example from the upper rhine graben abstract in extensional tectonic settings major structural elements such as graben boundary faults are typically oriented subparallel to the maximum horizontal stress component shmax they are often structurally accompanied by transfer zones that trend subparallel to the extension direction in the upper rhine graben such transfer faults are typically characterized by strike slip or oblique slip kinematics a major re orientation of the regional stress field by up to 90 of the upper rhine graben in the early miocene led to the present day normal and strike slip faulting regimes in the north and south of the upper rhine graben respectively and a transition zone in between consequently conditions for fault frictional failure changed significantly moreover it has been observed during tracer and stimulation experiments that such transfer faults may be of major importance for the hydraulic field of geothermal reservoirs under the present stress condition especially when located between production and injection well in this context we have investigated slip and dilation tendencies ts and td of major structural elements at reservoir scale for two representative geothermal sites bruchsal germany and riehen switzerland located close to the eastern main boundary fault of the upper rhine graben we have evaluated the quality and uncertainty range of both tendencies with respect to potential variation in shmax orientation despite significant differences in orientation of the structures and the stress regimes the resulting variation of ts and td reveal major similarities concerning the reactivation potential of both the graben parallel structures and the transfer faults the conditions of criticality for tensile failure and non criticality for shear failure suggest that transfer faults are most likely naturally permeable structures with low stimulation potential this is in agreement with the absence of both immediate tracer recovery and seismicity in the studied geothermal sites

mechanical behaviour of the arnes and hestfjall faults of the june 2000 earthquakes in southern iceland inferences from surface traces and tectonic model abstract two major earthquakes m s 6 6 occurred on june 17th and 21st 2000 in southern iceland this paper presents mapping and measurements of representative segments along their surface fault traces the arnes fault june 17th and the hestfjall fault june 21st the rupture trace of the arnes fault at mykjunes shows a conjugate strike slip pattern with a n30 e trending right lateral fault and a n60 e trending left lateral fault each of these faults is formed by en echelon arrays of fractures and push ups the compression observed in the push ups is inferred to represent the lateral displacement across the fault which can be quantified by treating the push ups as a shortening problem thus the mapped push ups indicate an average strike slip offset of 0 22 m for both these faults the surface trace of the hestfjall fault near bitra reveals a n50 60 e trending left lateral strike slip fault including large open fractures n25 35 e trending en echelon fractures and some push ups indicating a left lateral offset of 0 50 0 60 m along the bitra fault segment two other fault segments on the hestfjall fault near eyvik and near torfagil show association of open fractures and normal faults the most characteristic feature of the june 2000 fault traces is the en echelon arrangement of fractures and push ups the 2000 rupture traces are compared with some historical seismic fault traces of the south iceland seismic zone in terms of scale hierarchy of fracture traces according to our observations at bitra and mykjunes the general fault map of the orkustofnun national energy authority in iceland is interpreted as a conjugate system of strike slip faults a deformation model is proposed aiming at reconciling the simple velocity boundary conditions imposed by plate kinematics along the left lateral e w trending transform zone and the brittle deformation inside the south iceland seismic zone the observations made along the arnes and hestfjall faults show that the actual earthquake related fault pattern is not restricted to the n s trending dextral strike slip revealed by focal mechanisms and n s alignment of aftershocks but includes conjugate systems consistent with the structural pattern of the south iceland seismic zone

global observation of the ionospheric electronic response to solar events using ground and leo gps data we present in this work the temporal evolution of the three dimensional electron density at global scale during two ionospheric storms october 18 19 1995 and january 10 1997 computed using only actual global positioning system data the tomographic model is solved by means of a kaiman filtering with a filter updating time of 1 hour in a sun fixed reference frame and with a resolution of 10 10 deg in latitude local time and 100 km in height including also a protonospheric component eight layers the data set contains the data from the international gps service igs with more than 100 ground gps stations worldwide distributed and the gps met low orbiting gps receiver both positive and negative elevation observations are used this means for each storm 1 000 000 of delays 400 occultations and 3000 unknowns per batch the international reference ionosphere and data coming from the ionosonde of the ebre observatory are used to show the reliability of the results

motions of the equatorial ionization anomaly crests imaged by formosat 3 cosmic 1 the equatorial ionization anomaly eia structures and evolutions are imaged using radio occultation observation of the newly launched formosat 3 cosmic f3 c satellite constellation three dimensional ionospheric images provide unprecedented detail of the eia structure globally this paper presents images of the eia structures during july august 2006 and discusses the development and subsidence of the eia clear seasonal asymmetries in both ionospheric electron density and layer height are observed two dimensional cross section maps at a meridian provide dynamic variations and motions of the northern and southern eia crests results suggest that in addition to the asymmetric neutral composition effect interactions between the summer to winter transequatorial neutral winds and strength of the equatorial plasma fountain effect play important roles in producing asymmetric development of the eia crests as imaged by the f3 c

surface deformation and shallow dike intrusion processes at inyo craters long valley california the inyo craters are the two largest of four phreatic craters that lie within a 2 5 km long 500 to 700 m wide n s trend of faults and fissures at the south end of the inyo volcanic chain in eastern california the alignment of these features with dike fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650 550 yr b p e w extension south of south inyo crater is ten to several tens of meters suggesting that the dike is at least that thick to understand how the faults and fissures developed we mapped and studied the fault and fissure pattern used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium and conducted physical model experiments of fault and fissure growth results of the field studies and experiments indicate that deformation develops according to the following sequence 1 extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike 2 fissures form along two parallel trends at the surface on opposite sides of the dike plane leaving a relatively unfractured region in between 3 dip slip movement on subsurface fractures and linkage of these fractures with surface fissures produces inward facing normal fault scarps that bound a nested graben above the dike experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium this fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface if the ratio between the depth to the dike top and the distance between the outermost surface fissures at inyo craters is within the range of ratios measured experimentally then the depth to the dike top is between 250 m and several times that a slant hole drilled in the summer of 1987 by the department of energy office of basic energy sciences intersected three breccia bodies of possibly intrusive origin directly below the center of south inyo crater between 550 m and 610 m below the crater rim no intact juvenile igneous rock was intersected suggesting that the hole passed above the magma filled dike top perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

quantitative study of the deformation at southern explorer ridge using high resolution bathymetric data abstract we present preliminary results of a morphological study of the summit of the southern explorer ridge ser the ser is an inflated intermediate rate spreading center located in the northeast pacific off the west coast of canada that hosts a large hydrothermal vent complex known as magic mountain a quantitative assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high resolution near bottom bathymetric data these data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures the ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer populations of small faults exhibit an exponential size frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain 3 7 to 18 4 accommodated by the normal faults we propose that the 500 to 600 m wide and 60 m deep asymmetric axial summit graben of the ser formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben we find a well defined geographic distribution in the types of lava flows which indicates a general decrease of the eruption rate through time we also find that the magic mountain hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile transition depth this fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature