structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

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

longitudinal strike slip faults in oceanic rifting a mesostructural study from western to southeastern iceland abstract mesostructural analysis carried out in several localities of western southern central and southeastern iceland shows an unexpectedly frequent occurrence of strike slip faults parallel to or lying at relatively small angles with the axis of rifting some faults can be interpreted in terms of shear parallel to the rifts whereas others form conjugate systems referable to rift parallel compression axial shear can be explained by the accommodation of differences in the directions of tectonic extension between different segments of the axial rift system and subordinately by lateral rift parallel displacements between the major lithospheric plates axial compression may be attributable to rift oblique transform shear or to mantle transport parallel to the spreading axis compensating local deficiencies in mantle upwelling the data regard 1974 fault surfaces mainly small scale

dike intrusion and displacement accumulation at the intersection of the okataina volcanic centre and paeroa fault zone taupo rift new zealand abstract we examine the relationship between dike emplacement and normal fault displacement along the south western margin of okataina volcanic centre new zealand at this location dikes including a basaltic dike up to 5 m wide at the ground surface intruded during the 1886 ad tarawera eruption formed over the last 22 kyr in the tarawera linear vent zone within a few kilometres of the paeroa fault zone displacement analysis and mapping of fault scarps indicate that fault and dike extension are complementary over time periods of tens of thousands of years extension that is primarily accommodated by faulting outside the volcanic complex is achieved by dike intrusion and faulting within it historical observations following the 1886 ad eruption and previously published active fault trenching collectively indicate however that dike intrusion generally does not induce slip within the paeroa fault zone close to the lateral dike tip we suggest that displacement in the paeroa fault zone outside the okataina volcanic centre is generally not triggered by eruptions and that this fault is principally tectonic in origin

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

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

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

normal fault growth in layered basaltic rocks the role of strain rate in fault evolution abstract conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step wise growth pattern involving upward propagation of subsurface faults and surface monocline formation which are breached by subvertical open faults immature discontinuous normal faults are considered representative of the early stages of mature linked faults that accommodate extensional strains we consider the evolution of surface breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones the koa e fault system hawai i and the krafla fissure swarm ne iceland field mapping highlights similarities to current predicted geometries but also prominent differences that are not reconciled by current models variable deformation styles record magma supply changes within the rift zones which drive local strain rate gradients building on existing studies we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions we propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns variations in surface strains may be indicative of subsurface magmatic system changes with important implications for our understanding of volcano tectonic coupling

chemical weathering of basalts southwest iceland effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean river ground and peat water from a basaltic catchment area laxa in kjos southwest iceland were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt the formation of secondary minerals and runoff on fluxes of dissolved elements to the ocean the proportions of the dissolved solids in the water samples derived from precipitation air and rock weathering were estimated the amounts of na mg ca k and s originating from precipitation were calculated from the respective solute cl marine ratios and the estimated aqueous cl originated from precipitation which was calculated using measured b content of the waters and b cl molal marine and rock ratios the rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters iron and al showed low mobility compared to na this was primarily due to consumption of these elements by ferrihydroxides allophane imogolite and clay minerals also si and to a lesser degree ca show apparent slightly lower mobility than na these results are in good agreement with these phases being dominant in the soils of the study area wada and others 1992 and the main secondary minerals of basaltic glass in iceland crovisier and others 1992 reaction progress calculations support these findings according to a model interaction of basaltic glass and meteoric water al fe and si are consumed by amorphous fe and al hydroxides and imogolite and allophane at low reaction progress with increasing reaction progress ca fe mg smectite became the primary secondary minerals limiting the mobility of ca fe mg si and al these findings suggest that the weathering in the laxa in the kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio in the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes temperature lithology other than rock crystallinity and rock age were similar in all of the catchment areas in this study silica ca f s al k mg and b had a very similar dependence on runoff on the other hand na fluxes were found to be less dependent on runoff and fe fluxes were found to be independent of runoff basaltic glass dissolves faster than fully crystallized basaltic rocks gislason and eugster 1987a therefore increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff however because some of the solutes particularly al and fe were highly influenced or even controlled by the formation of secondary minerals this increasing dissolution with increasing glass content was hidden on the other hand basaltic glass was observed to enhance the fluxes of more mobile elements like na si ca f and s by a factor of 2 to 5 at a constant runoff of 200 cm yr constant vegetative cover and 0 to 100 percent glass content of the rocks the k and mg fluxes were found to be independent of rock crystallinity however runoff alone cannot explain the fluxes of these elements and it seems that vegetative cover and seasonal variations in biomass activities influence the k and mg fluxes

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

multi stage zeolite facies mineralization in the hvalfjordur area iceland the hvalfjordur area 30 km north of iceland s capital reykjavik belongs to the sequence of late tertiary to early quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites the basalts are affected by progressive low temperature metamorphism caused by the burial of the lava succession and higher heat flow from nearby central volcanoes low grade zeolite facies metamorphism of basaltic lavas in the hvalfjordur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile stage ia represents syn eruptive near surface alteration in which celadonite and silica were precipitated along primary pores during regional burial metamorphism stage ib hydrolysis of olivine and glass led to the formation of mixed layer chlorite smectite clays the chlorite content of stage ib phyllosilicate vesicle rims increases with increasing burial depth and temperature stage ii occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the laxarvogur and hvalfjordur central volcanoes altogether 11 different zeolites were found in the hvalfjordur area analcime chabazite epistilbite heulandite laumontite levyne mesolite stilbite stellerite thomsonite and yugawaralite in total three separate depth and temperature controlled zeolite zones occur in the hvalfjordur area

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

tectonic significance of the south iceland seismic transform zone the subaerial expression of the mid atlantic ridge on iceland comprises two overlapping spreading axes referred to as the west volcanic zone wvz and the east volcanic zone evz respectively the way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes we found four different strike slip stress phases an early n s compression phase a preceded a ne sw compression phase b this phase was followed by a se nw compression phase c however we cannot date an e w compression phase d relative to the other phases numerical modeling based on the assumption that the wvz has been permanently active during the last 3 myr and that the evz is propagating southward confirms that stress directions have rotated clockwise by more than 140 these results fit perfectly with our field analysis and we propose that phase a corresponds to initial evz ridge tip propagation phase b to emplacement of the evz southern tip near torfajokull and phase c to an extreme southern location near the surtsey islands of the evz southern tip phase d could correspond to an intermediate stage between phases b and c we suggest that this sequence of tectonics recorded in a regional overlapping ridge tip setting is directly analogous to smaller scale and more common phenomena at second order ridge discontinuities throughout the global mid ocean ridge system

form and dimensions of dykes in eastern iceland abstract a study was made of about 700 dykes in eastern iceland the majority of these belong to three swarms about 73 dip within 10 of the vertical most strike between 10 and 40 ne the strike of the dykes within the southernmost swarm alftafjordur changes along its trace from almost n at the north end to nne ssw southward along the swarm the average thickness of the dykes is about 4 1 m and the thickness does not change notably along the alftafjordur swarm the thinner dykes tend to have smaller grains than the thicker dykes of five dykes followed laterally the longest is over 22 km the thickness of individual dykes changes irregularly along their length and the dyke is often offset where its thickness changes many dykes appear to be completely discontinuous but some parts are connected by veins where the dykes end in a vertical section most of them simply taper away only about 10 5 of the dykes occupy faults the mechanical and thermal effects of the dykes on the country rock are small many of the dykes appear to be non feeders i e dykes that never reached the surface to feed lava flows using the length width ratio the depth of origin of three dykes has been estimated the maximum depth of origin of these three dykes is 7 5 km 9 km and 10 km below the original surface

rift transform kinematics in south iceland deformation from global positioning system measurements 1986 to 1992 crustal deformation in the plate boundary regions in south iceland is estimated from repeated global positioning system gps geodetic measurements in the period 1986 1992 we compare coordinate solutions for the 1986 and 1989 surveys with the results from the most recent survey in 1992 horizontal position uncertainty is about 2 cm for the 1986 and 1989 coordinates and about 4 mm for the 1992 coordinates little internal deformation is observed in the area west of the western volcanic zone within the north american plate and at the southern tip of the eastern volcanic zone within the eurasian plate the observed relative velocity of these two areas is 2 1 0 4 cm yr in direction n117 11 e 1 uncertainties compatible with the 1 94 cm yr widening in direction n 104 e across south iceland predicted from the nuvel 1 global plate motion model left lateral shear strain is developing across the intervening transform zone the e w trending south iceland seismic zone sisz strain is concentrated within a 20 to 30 km wide zone that correlates with seismic activity in the sisz about 85 15 of the relative plate motion is accommodated by this zone in such a way that the area south of the sisz is moving toward the east with the eurasian plate and the area north of the sisz is moving toward the west with the north american plate accordingly north of the sisz the western rift zone can accommodate a maximum of 15 15 of the relative plate motion within the sisz the shear strain results in an anticlockwise rotation of lineaments oriented north south at a rate of 0 5 1 rad yr the shear strain accumulation can be accommodated by bookshelf faulting on mapped recent north south faults in the sisz if the deformation is accommodated by an array of n s faults spaced 1 5 km apart an average slip rate of about 0 5 5 mm yr is required on each fault the rate of geometric moment release due to earthquakes averaged over centuries m 0 expected from such an array is about the same as expected from a simple transform fault 2vad where 2v is the relative plate motion a is the total length of the seismic zone and d is the thickness of the brittle crust if 2v 0 85 0 15 1 94 cm yr d 10 15 km ana a 75 85 km then m 0 1 0 2 5 107 m3 yr comparable to the rate of geometric moment release in large historical earthquakes in the sisz for the last several centuries we conclude that historical seismicity in the sisz can be attributed to left lateral shear accumulation across the seismic zone at a similar rate throughout historical times as during the years 1986 1992

breaking new ground estimates of crack depth along the axial zone of the east pacific rise 9 12 54 n using simple fracture mechanics models the depths of fissures that were observed along the axial zone of the east pacific rise epr from 9 12 to 54 n with the deep towed argo z vehicle are estimated the widest cracks 4 8 m along this fast spreading portion of the epr are the deepest and are spatially correlated with the broadest youngest and most hydrothermally active portions of the ridge crest if the widest deepest cracks are not primarily eruptive they should be most abundant in areas of older lava flows having increased in width with time this is not observed some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at 9 45 52 n and may be deep enough to reach the sheeted dikes of layer 2b these cracks may have tapped melt during the eruption and facilitated the flux of vapor rich hydrothermal fluids through overlying lava flows the narrowest presumably shallowest cracks correspond to the narrowest oldest and least hydrothermally active portions of the ridge crest we interpret the wide deep cracks as primarily eruptive in origin and suggest that they may be associated with the inflation of an axial magma chamber whereas the narrow shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far field plate stresses

uav based surveying in volcano tectonics an example from the iceland rift abstract in the present work we applied the use of an unmanned aerial vehicle uav a quadcopter and the aerial structure from motion digital photogrammetry image processing technique asfm to study volcano tectonics and tectonic features in an active icelandic rift data have been collected in order to evaluate the holocene deformation in the northern volcanic zone of iceland we mapped 397 structures mainly related to extension fractures and subordinately normal faults in the theistareykir fissure swarm obtaining 1098 and 21 structural data respectively this allowed to reconstruct an overall spreading direction of n108 during holocene times and to calculate a stretch of 1 013 regarding 8 10 ka old lava units deformation in the area is related to both dyke intrusions and extensional tectonics furthermore detailed geological structural field and uav surveys were also performed in two test areas in order to determine data accuracy and the associated reliability of this approach in addition to the above different flight heights were tested suggesting that photo collection with a 12 4 mpx camera at 100 m is efficient to study fracture dilation and kinematics

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

the path to covis a review of acoustic imaging of hydrothermal flow regimes abstract acoustic imaging of hydrothermal flow regimes started with the incidental recognition of a plume on a routine sonar scan for obstacles in the path of the human occupied submersible alvin developments in sonar engineering acoustic data processing and scientific visualization have been combined to develop technology which can effectively capture the behavior of focused and diffuse hydrothermal discharge this paper traces the development of these acoustic imaging techniques for hydrothermal flow regimes from their conception through to the development of the cabled observatory vent imaging sonar covis covis has monitored such flow eight times a day for several years successful acoustic techniques for estimating plume entrainment bending vertical rise volume flux and heat flux are presented as is the state of the art in diffuse flow detection

structure petrochemistry and evolution of a sheet swarm in an icelandic central volcano strike dip and thickness were measured for 504 sheets inclined sheets and dykes in the 4 6 ma old hafnarfjall central volcano in southwest iceland the average dip of sheets is 65 80 are less than 1 2 m thick and the thickness tends to decrease with decreasing dip in 0 5 km long traverses perpendicular to the average strike of sheets the percentage of sheets ranges from about 6 to 11 of 140 chemically analysed sheets most are quartz tholeiites a few are intermediate or acid the sheets are chemically more evolved than the host rock and were generated by a shallow crustal magma chamber at a mature stage of the central volcano whereas the host rock was generated earlier before the chamber was established trace element results suggest that the sheet magmas evolved by low pressure fractional crystallization as well as by mixing of primitive magmas and crustal melts a model is proposed where most of the sheets are generated by a growing shallow magma chamber as the chamber grows its shape changes and so does the local stress field associated with it because the sheets follow the stress trajectories of the local stress field the potential pathways of the sheets change with the growth of the chamber which may explain the common occurrence of cross cutting sheets from the evolved chemistry of the sheets as well as from the pattern of the stress trajectories it is concluded that the bulk of the sheets were injected from the upper part of the shallow magma chamber

mechanisms of normal fault development at mid ocean ridges 1 slow spreading ridge segments are characterized not only by small closely spaced faults that develop near the segment center but also by large widely spaced faults that develop near the segment ends typically at the inside corner of a ridge offset intersection in this study we investigate the competing effects of stress accumulation in the lithosphere and the yield strength of the lithosphere in controlling the location of normal fault formation and direction of propagation seismic velocity models from the mid atlantic ridge in the oceanographer hayes region and 29 n and the east pacific rise at 9 n were used to estimate the along axis change in dynamic young s modulus corresponding thermal and rheologic models were calculated to estimate the along axis variation in yield strength we then develop a thin plate model to calculate the predicted location of fault initiation or reactivation and the subsequent propagation direction for different combinations of linear along axis gradients in young s modulus and yield strength on the basis of this model we define two modes of normal fault development at slow spreading segments mode c center faults which develop at the segment center and propagate outward and mode e end faults which develop at the segment ends and propagate inward mode c faults are predicted to form at ridges where the along axis variation in yield strength dominates the along axis accumulation of stress conversely mode e faults are predicted to develop at ridges where stress accumulation toward segment ends overcomes the high yield strength in these locations in addition to the accumulation of stress caused by along axis gradients in young s modulus we illustrate that shear stresses resisting relative plate motion along a transform fault will generate higher effective stress at inside corners possibly concentrating mode e faulting in these locations at fast spreading ridges where along axis gradients in stress and lithospheric strength are relatively small more uniform patterns of faulting are predicted the results of this study quantify how the interplay between the along axis variations in stress state and the mechanical properties of the lithosphere controls the style of fault development at mid ocean ridge segments

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

modelling of surface stresses and fracturing during dyke emplacement application to the 2009 episode at harrat lunayyir saudi arabia abstract correct interpretation of surface stresses and deformation or displacement during volcanotectonic episodes is of fundamental importance for hazard assessment and dyke path forecasting here we present new general numerical models on the local stresses induced by arrested dykes in the models the crustal segments hosting the dyke vary greatly in mechanical properties from uniform or non layered elastic half spaces to highly anisotropic layers with strong contrast in young s modulus the shallow parts of active volcanoes and volcanic zones are normally highly anisotropic and some with open contacts the numerical results show that for a given surface deformation non layered half space models underestimate the dyke overpressure thickness needed and overestimate the likely depth to the tip of the dyke also as the mechanical contrast between the layers increases so does the stress dissipation and associated reduction in surface stresses and associated fracturing in the absence of open contacts the distance between the two dyke induced tensile and shear stress peaks and fractures if any at the surface is roughly twice the depth to the tip of the dyke the width of a graben if it forms should therefore be roughly twice the depth to the tip of the associated arrested dyke when applied to the 2009 episode at harrat lunayyir the main results are as follows the entire 3 7 km wide fracture zone graben formed during the episode is far too wide to have been generated by induced stresses of a single arrested dyke the eastern part of the zone graben may have been generated by the inferred arrested dyke but the western zone primarily by regional extensional loading the dyke tip was arrested at only a few hundred metres below the surface the estimated thickness of the uppermost part of the dyke being between about 6 and 12 m for the inferred dyke length strike dimension of about 14 km this yields a dyke length thickness ratio between 2400 and 1200 similar to commonly measured ratios of regional dykes in the field

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

a new mode of inner volcano growth the flower intrusive structure abstract a multiple sill laccolith nested within a centrally dipping sheet swarm is a recently discovered sub volcanic structure resembling a flower in section view which we have found in some eroded volcanoes in iceland suggesting that this structure can have some general relevance in deforming rock successions in volcanic areas after a brief summary of the main characteristics of these examples with details on stardalur volcano we present an evolutionary model to explain the whole structure both sills and sheets are fed by multiple pulses of magma following radial paths from a shallow chamber the earlier vertical set of the radial sheets above the middle of the chamber are abruptly deflected into sills by the overlying lava succession a proto laccolith starts developing composed of different vertically stacked sills originating from the central vertical feeder zone each sill contributes to the deformation of the overburden room for intrusions is provided by the higher strain that can be attained by the widespread hyaloclastites and breccias which have a softer behaviour with a lower young s modulus than the stiffer higher young s modulus lava flows combined with the doming of the overlying lavas later dikes get deflected outwards along the contact with the proto laccolith and acquire a convex upward shape in cross section

crustal fissuring on the crest of the southern east pacific rise at 17 15 40 s s with the near bottom dsl 120 andargo ii imaging systems we observe that the youngest lava flows on a relative age scale aresparsely fissured and that there is a cumulative increase in fissure abundance with time thatproduces a strong positive correlation between fissure density and relative age of lava flows average fissure widths were used to estimate fissure depths in the 17 15

on the conditions of sheet injections and eruptions in stratovolcanoes nearly all eruptions in stratovolcanoes composite volcanoes central volcanoes are supplied with magma through fractures consequently a primary physical condition for an eruption to occur in a stratovolcano is that a magma driven fracture is able to propagate to the surface magma filled fractures frozen or fluid are referred to as sheet intrusions more specifically they are named dykes when subvertical and inclined or cone sheets when inclined field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths for this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions whether a sheet intrusion becomes arrested or alternatively propagates to the surface depends primarily on the stress field in the stratovolcano a stratovolcano normally consists of layers of contrasting mechanical properties such as soft low young s modulus pyroclastic units and stiff high young s modulus lava flows we present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest the models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised we propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress field homogenisation field studies show that the sheet intrusions in individual stratovolcanoes have various dips some are vertical dykes others inclined sheets and still others horizontal sills analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption this effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high we conclude that the shallower the dip of a sheet intrusion the less will be its volumetric magma transport to the surface of a stratovolcano

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

gravity changes and natural gas extraction in groningen gravity observations made over the groningen gasfield from 1978 onwards are available the existing gravity data were reanalysed and a new survey was carried out the observed gravity changes obtained during four surveys spanning an 18 year period were compared with the gravity effect due to natural gas extraction computed from the reservoir model and the production data the random error in the gravity values is small enough to detect the effect of gas extraction after a few years up to 2 gal year the trends obtained from observed and calculated gravity changes agree well within their expected error margins after statistical data snooping due to the inadequate measurement set up of the previous surveys systematic errors present in the gravity data hampered the use of this data for refinement of the reservoir and the production models however with improved gravity monitoring in particular a very systematic network set up and well calibrated instruments this should now be feasible by using the network analysis from the 1996 measurements for the planning of a network in space and time most of the current problems can be avoided

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

crustal extension from exposed sheet intrusions review and method proposal abstract sheet intrusions are probably the most widespread magmatic features on earth they create a space for themselves by dilating the host rock the exposed sheet intrusions allow an estimate of a minimum value of the extension to which the crustal layers and the volcanic edifices are subject the correct evaluation of the contribution of sheet intrusions to the crustal extension is of great importance in various geologic settings the current methods used to evaluate the extension due to exposed sheet intrusions are reviewed in this paper the effect of the dip of sheet intrusions on the evaluation of horizontal crustal extension is analysed if the dip effect is neglected and the intrusions are sub vertical the error due to the dip effect in the extension evaluation is small if the intrusions are shallow dipping the error may be great and varies as a function of the dyke dip for dykes with dip range 45 65 the error due to neglecting the dip effect is 30 to 15 increasing discrepancies of more than 25 are produced on the computed dilation when the intrusions progressively dip less than 50 a simple method based on structural field measurements of exposed sheet intrusions and on trigonometric considerations is presented as an improvement of pre existing methods to estimate the horizontal component of the crustal extension using this method field data of 1154 dykes from different geodynamic contexts are analysed the horizontal component of extension is computed using dyke data from the tertiary massifs of lanzarote and tenerife and from the active stratovolcanoes of monte somma vesuvius and etna since the analysed dykes are steeply dipping the cumulative difference between the extension computed neglecting and taking into account the dip effect remains less than 10 for all the data sets the percentage difference is nearly constant and cumulatively higher for monte somma where the mean dip of the intrusions is smaller in lanzarote 237 dykes emplaced between 15 5 and 3 7 ma give a peak extension of 276 m in direction n135 and a time averaged minimum extension rate of 1 9 10 2 to 2 3 10 2 mm a 1 in tenerife an extension of 562 m is computed in the direction n65 from 563 dykes emplaced from 7 4 to 3 3 ma the time averaged minimum rate of extension for the tertiary massifs of tenerife ranges from 0 14 to 0 25 mm a 1 the peak extension computed for monte somma is 81 7 m in the direction n90 based on 96 exposed sheets very likely most of monte somma sheets intruded within 12 ka giving a time averaged minimum extension rate of 7 mm a 1 from 184 exposed dykes of etna a minimum extension rate of 6 2 mm a 1 is computed peak extension is 212 m in direction n35 the poor resolution of age constraints may affect deeply the reliability of the extension rates however from the examples presented it appears that the extension rates of pre erosional successions from intraplate volcanic islands can be smaller than those of active stratovolcanoes this may suggest a more active dynamics in the stratovolcanoes

brittle deformation and fracture patterns on oceanic rift shoulders the esja peninsula sw iceland abstract this paper provides detailed structural data on the crustal deformation of the esja peninsula at the southwestern margin of the rift zone in sw iceland forty percent of the faults are strike slip with steep dips 35 are normal dip slip with dips ranging from 50 to 90 for normal faults e w ne sw and nne ssw are the three major trends the paleostress tensors determined using the fault data indicate three main states of stress two extensional regimes 3 n180 e 10 and n110 e 10 are related to normal slip faults the third is a compressive stress regime with 1 trending n10 e n30 e associations between strike slip and dip slip faults are often observed resulting from both the geometrical requirements of interactions between faulted blocks and from local regional permutations between 1 and 2 we note a constant obliquity of 10 20 between the trend perpendicular to the regional structures and the computed direction of extension this conclusion is also supported by the major trends of dykes and tensional fractures there are two mechanisms of volcanic injection local sheets and regional dykes resulting in power law cumulative thickness distributions with exponents of 1 5 and 3 0 respectively

magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics the volcanic rift zone in iceland is characterized by elongate volcanic systems consisting of tension fractures normal faults and volcanic fissures where most of the volcanotectonic activity takes place most volcanic systems develop central volcanoes the formation of which is still poorly understood there is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes in the central volcanoes eruptions are frequent typically one every several hundred years of small volume normally less than 0 1 km3 and fed mainly by thin average thickness of 0 5 m inclined sheets outside central volcanoes the eruptions are rare typically one every several thousand years but of large volume typically more than 1 km3 and fed mainly by thick average thickness of 4 5 m subvertical dikes using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi infinite plate these empirical relations as well as the formation of specific central volcanoes in the volcanic systems can be explained once a cavity like magma chamber has formed its existence in the rift zone concentrates tensile stress this stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system which partly explains the formation of central volcanoes and their high eruption frequencies this concentration also gives rise to a local stress field that encourages injection of small volume sheets in all directions and at various dips from their source magma chamber the magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma through inclined sheets toward a limited area at the surface where the central volcano gradually forms the inverse relationship between eruption frequency and eruption volume when the central volcano is compared with other parts of the volcanic system is also partly due to the trap like nature of the magma chamber the chamber is normally much smaller than its source mantle reservoir so that a single magma flow through a dike from the reservoir lasting perhaps many years may trigger tens of magma flows through sheets from the chamber many of which would reach the surface in the central volcano

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

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

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

the role of tephrachronology in recognizing synchronous caldera forming events at the quaternary volcanoes longonot and suswa south kenya rift

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

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

formation of crystal magma chambers in iceland formation of crustal magma chambers in iceland may be facilitated by the occurrence of stress barriers that lead to formation of thick sills such sills absorb the magma of all dikes that enter them and may evolve into magma chambers ideal sites for stress barriers and hence for magma chambers are rock formations where individual layers have different elastic properties the rocks formed during the pleistocene have notably different elastic properties and when buried in the volcanic zones they form more promising sites for magma chambers than the tertiary rocks this may explain why the number of magma chambers indicated by the number of corresponding central volcanoes during the late pleistocene i e during the past 0 7 m y appears to be proportionally greater than the number of chambers i e central volcanoes active during tertiary time

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

magmatic cycles pace tectonic and morphological expression of rifting afar depression ethiopia the existence of narrow axial volcanic zones of mid oceanic ridges testifies of the underlying concentration of both melt distribution and tectonic strain as a result of repeated diking and faulting axial volcanic zones therefore represent a spectacular topographic expression of plate divergence however the submarine location of oceanic ridges makes it difficult to constrain the interplay between tectonic and magmatic processes in time and space in this study we use the dabbahu manda hararo dmh magmatic rift segment afar ethiopia to provide quantitative constraints on the response of tectonic processes to variations in magma supply at divergent plate boundaries the dmh magmatic rift segment is considered an analogue of an oceanic ridge exhibiting a fault pattern extension rate and topographic relief comparable to intermediate to slow spreading ridges here we focus on the northern and central parts of dmh rift where we present quantitative slip rates for the past 40 kyr for major and minor normal fault scarps in the vicinity of a recent september 2005 dike intrusion the data obtained show that the axial valley topography has been created by enhanced slip rates that occurred during periods of limited volcanism suggestive of reduced magmatic activity probably in association with changes in strain distribution in the crust our results indicate that the development of the axial valley topography has been regulated by the lifetimes of the magma reservoirs and their spatial distribution along the segment and thus to the magmatic cycles of replenishment differentiation 100 kyr our findings are also consistent with magma induced deformation in magma rich rift segments the record of two tectonic events of metric vertical amplitude on the fault that accommodated the most part of surface displacement during the 2005 dike intrusion suggests that the latter type of intrusion occurs roughly every 10 kyr in the northern part of the dmh segment

extensional tectonics in the vicinity of a transform fault in north iceland at the junction between an oceanic transform fault and the adjacent ridge segments there is normally a curved fabric the trollaskagi peninsula is located just south of the junction between the kolbeinsey ridge and the tjornes fracture zone and offers a rare opportunity for studying the three dimensional tectonics of the associated curved fabric for the purpose of elucidating its infrastructure we measured the attitudes of tens of lava flows and several hundred dykes faults and mineral veins in 28 profiles in the northern half of the trollaskagi peninsula at sea level the general dip of the lava pile is 6 14 but in the northernmost part of the peninsula the dip is 10 36 with an average of 22 most dykes are basaltic and strike north or nne they are exceptional as regards thickness frequency of multiple dykes and crustal dilation the average thickness 5 9 m and a crustal dilation of 28 in a 4 5 km long profile are the highest figures reported from regional dyke swarms in iceland most faults are normal faults the curved fabric is limited to the northernmost part of the trollaskagi peninsula and consists of two main sets of fractures one set includes north and nne striking faults mineral veins and dykes the other set consists of nw striking oblique faults mineral veins and tilted lava flows a boundary element model indicates that the abnormal lava dip and the nw striking oblique extensional structures in the northernmost part of the peninsula are attributable to the stress field associated with the junction between the kolbeinsey ridge and the tjornes fracture zone

fracture analysis near the mid ocean plate boundary reykjavik hvalfjordur area iceland abstract the geometry and thermal history of fractures have been determined at 59 stations from reykjavik to hvalfjordur in southwestern iceland the data provide information on crustal stress regimes in the vicinity of mid ocean ridges two major generalized fracture orientations are present 1 1 a northeast system trend 010 030 except on akranes where the orientation is 040 060 2 2 a broad east west system containing one or more sets with strike between 070 130 thermal history of the host rock and fractures was determined from secondary minerals in vugs and fractures the thermal history indicates that the northeast fracture set opened while the area was within the relatively hot axial zone of active volcanism and rifting some of the east west trending fractures also opened at this time but many formed later after the area had begun to cool and drift from the active zone the northeast fracture set is essentially parallel to the trend of dikes and normal faults in southwestern iceland they have been interpreted as extension fractures resulting in about 0 4 maximum extension forming generally from the same stress field associated with normal faulting and dike injection in the active zone fracturing in an east west direction estimated 0 1 maximum extension mainly near the edge and outside the active zone indicates a reorientation of this stress field the dominant mechanism related to the origin of the east west fractures may be thermoelastic stresses arising from axial and basal accretion and cooling of lithospheric plates both fracture systems are inferred to have formed in the griffiths idealization under nearly biaxial effective compressive loading on the order of 200 bar the discrepancy between this value and the kilobar order strengths of short time laboratory tests reflects such factors as high temperature stress corrosion and fatigue fracture propagation is assumed to have been stable but governed primarily by lateral load diminishing mechanisms rather than by progressive loading these relaxation mechanisms may have been episodic northeast system fissure swarm activity or steady state thermoelastic contraction in time

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

seismic anisotropy in the crust at the mid atlantic plate boundary in south west iceland summary seismograms of microearthquakes recorded on stations of the sil network in south western iceland exhibit strong shear wave splitting and are consistent with being caused by aligned parallel cracks in the uppermost crust splitting times of 0 1 0 3 s are observed with the larger values 0 2 0 3 s occurring beneath stations in the highly heated western volcanic zone and the smaller values 0 1 s in the younger less fractured south iceland seismic zone five of the stations have a fast shear wave polarization azimuth of n30 e n40 e7 parallel to the axis of the western volcanic zone i e the mid atlantic ridge in iceland and in the direction of maximum horizontal compression defined by fault dyke and fissure strikes and microearthquake focal mechanisms station gyg in the north western part of the network has the significantly different azimuth of n70 e this is parallel to one of the local strike slip fault trends and is probably due to a heated rock fabric caused by those faults

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

the stress pattern of iceland moritz ziegler mojtaba rajabi oliver heidbach gylfi pall hersir kristjan agustsson sigurveig arnadottir arno zang

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones