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

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

growth of surface breaching normal faults as a three dimensional fracturing process structures along the hopena normal fault of the koae fault system kfs on kilauea volcano hawaii provide a record of fault propagation in three dimensions this fault displays 1 a breached monocline along the scarp 2 a belt of discontinuous echelon fractures along the scarp and past its end 3 a belt of discontinuous fractures on the footwall 4 buckles at the base of the scarp and 5 a belt of discontinuous fractures on the hangingwall that converges towards the end of the fault trace solid mechanics analyses show that this ensemble can be accounted for by the tipline of a normal fault propagating up towards the surface accompanied by antithetic fracturing then intersecting it and propagating laterally normal fault propagation down from the surface cannot account for the observed structures lateral slip occurs along some fractures as a result of local stress changes associated with fault propagation discontinuous fractures at the surface form early above a blind normal fault and continue to develop as a normal fault propagates laterally linkage of the fractures to the fault produces a fault with an irregular trace the discontinuous irregular character of normal fault traces over a broad range of scale is an inevitable consequence of three dimensional fault growth

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

faulting and fault scaling on the median valley floor of the trans atlantic geotraverse tag segment 26 n on the mid atlantic ridge a quantitative assessment of faulting on the median valley floor of a slow spreading ridge is accomplished through the analysis of high resolution dsl 120 sidescan sonar and coregistered bathymetric data from the tag segment near 26 n on the mid atlantic ridge at this location faulting is exposed within a 3 5 km wide ridge parallel zone lying asymmetrically on the eastern half of the median valley floor mapped faults have a normal sense of displacement are 2 km in length and accommodate 1 5 brittle extension evidence of fault linkage within the fault population includes kinked and bent fault traces in map view the development of overlapping fault segments or relay ramps and the presence of multiple local maxima in the displacement distance profiles of some faults faults have a slight tendency to dip to the east or outward relative to the valley axis and exhibit little symmetry of fault dip about the axis of the faulted zone or any other ridge parallel line faults exhibit a roughly linear relationship between maximum fault throw and fault length with a mean ratio of 0 030 for the population regression of length frequency data indicates a power law distribution with an exponent of 1 64 1 96 demonstrating that fractal populations can exist in the mid ocean ridge environment the fractal nature of this length frequency distribution and the ratio of maximum fault throw to fault length differ significantly from those described previously for populations of larger abyssal hill faults in the fast spreading environment where the distribution is exponential and the throw to length ratio is 5 times lower these results suggest that the scaling of fault populations in the midocean ridge setting may vary as a function of spreading rate and or fault size

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

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

transport of soluble salts in a large semiarid basin river murray australia median annual concentrations of cl so4 hco3 na k mg and ca increase greatly downstream in the river murray along its 2500 km route to the sea chloride increases from less than 2 mg l 1 near the headwaters to about 170 mg l 1 near the downstream end of the main channel water near the head of the river has hco3 balanced by similar equivalents of na ca and mg while higher salinity water in the downstream half of the river is dominated by na and cl sources of these dissolved ions include a sea derived airborne salts contributing essentially all of the cl br and 80 of the na and b rock derived ions from weathering of sedimentary rock and soil minerals contributing most of the observed hco3 80 of the ca and 20 25 of the mg and so4 much of the change in chemistry along the axis of the murray is due to influx of saline groundwater which has a major ion composition similar to sea water inflows of irrigation drainage water also enhance downstream transport of salts especially hco3 the stable isotope composition of the river murray 2h and 18o begins with relatively depleted heavy isotope abundances 47 8 in the uplands of the southeastern margin of the murray darling basin increasing steadily by 30 and 6 respectively due to evaporative enrichment from river surfaces and influx of drainage returned from irrigation diversions despite the similarity of groundwater major ion chemistry to that of sea water stable isotope compositions of groundwaters clearly demonstrate that water molecules from past marine incursions have been completely flushed from the system similarly the cl br ratios of groundwaters are not appreciably different from seawater values indicating that dissolution of fossil evaporites does not contribute significant salt to the groundwaters or the river

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

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

surface water groundwater interactions in an alluvial plain chemical and isotopic systematics our work on the loire river forms part of a french national research program dedicated to wetlands and aims to better understand the global functioning of the system from the hydrological geochemical ecological and sociological aspects the present study using a coupled hydrological and geochemical stable and sr isotopes approach focuses on the soulangy site with its secondary anastomosing channels just below the confluence of the loire and allier rivers and also on the dorna nt site with two unconnected oxbow lakes 50 km upstream of the confluence the stable isotopes of water 18o 2h show that the alluvial or riverbank aquifer feeds the loire river during the summer but is not recharged by the river during flood periods in the winter the alluvial groundwater thus has a purely local origin from precipitation the major elements reveal an anthropogenic input of cl and more importantly of no3 especially near farms the 87sr 86sr isotopes identify different groundwater layers in the alluvium i e an upper and a lower alluvial aquifer and a perched aquifer at dornant that have relatively complex relationships with the surface water the two main rivers loire and allier present distinct geochemical characteristics reflecting the different lithologies that they drain upstream in addition the secondary channels lying parallel to the loire main stream at the soulangy site give different geochemical signatures which shows that they are not fed by the same overflows of the loire they are more or less well connected to the upper level of the alluvial plain and a longitudinal study of one of these channels has revealed a loire river influence progressively replaced by a water contribution from the upper alluvial aquifer similarly the two oxbow lakes at the dornant site are not supplied by the same water during the summer months a conceptual scheme of the loire hydrosystem based on 18o and 87sr 86sr suggests that the isotopic variations of the loire river can be related to a massif central surface water supply for the loire and allier main streams and to a groundwater supply from the alluvial plains

structures in segment boundary zones of the lost river and lemhi faults east central idaho geologic mapping and structural analysis of the lost river and lemhi normal faults in east central idaho suggest that many rupture segment boundaries previously inferred from geomorphic data coincide with two classes of structural complexities 1 zones of distributed faults and steps or jogs in the range front faults and 2 intersections with preexisting eocene to oligocene cross faults at 10 15 km depth segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5 6 km wide in plan view in contrast simple fault traces characterize the range front faults in the interiors of segments thus segment boundaries of normal faults are broad zones of intersecting faults not discrete points the structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the lost river and lemhi faults downdip projections of preexisting eocene to oligocene normal faults in the footwall of the lost river and lemhi faults may coincide with four or five segment boundary zones at about 15 km depth the depth at which large earthquakes nucleate in the basin and range province structural analyses also show that some cross faults which coincide with segment boundary zones at the surface are distant from these zones at depth conversely some potentially significant structures present at depth are far removed from the segment boundary zone at the surface thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones where cross faults define segment boundaries the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults

influence of an inhomogeneous stress field and fault zone thickness on the displacements and stresses induced by normal faulting we used the finite element method fem to simulate the temporal evolution of an inhomogeneous stress field with the aim of investigating the displacement and stress fields induced by slip upon a normal fault zone with a given thickness the modeling results reveal 1 the site of maximum slip is along the lower part of the fault not near the surface and the locations upon the fault of maximum shear stress drop and maximum slip do not coincide 2 the maximum horizontal and vertical displacements at the ground surface are located at some distance from the fault trace and 3 slip of the fault may result in two failure regions near the ground surface one close to the surface trace of the fault and the other within the hanging wall at some distance from the fault it is in the regions that a succession of normal faults is typically observed in geological examples these results differ from those obtained using seismic dislocation theory in semi infinite space where the effects of the inhomogeneous stress field and thickness of the fault zone are not considered

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

fault growth model and the universal fault length distribution we propose and solve a simple fault growth model describing the powerlaw dependence of the fault length distribution observed both in nature and in laboratory models of the lithosphere starting from an arbitrary initial fault length distribution and assuming that the rate of growth of a fault is proportional to a power of its size we show that the fault length distribution evolves towards a universal powerlaw curve comparison between this theory and experimental results is encouraging

understanding parafluvial exchange and degassing to better quantify groundwater inflows using 222rn the king river southeast australia 222rn is an important tracer for quantifying groundwater inflows to rivers especially where groundwater and surface water have similar major ion and stable isotope geochemistry uncertainties in the 222rn mass balance arise however from not accurately estimating the degree of degassing of 222rn to the atmosphere and the extent to which interaction within the parafluvial zone provides an additional source of 222rn this study estimates both 222rn production in the parafluvial zone and degassing along a 75km stretch of the king river australia in order to more precisely quantify groundwater inflows the contribution of 222rn from the parafluvial zone fp was estimated using 222rn emanation rates from near river sediments and assessment of the residence time of water in the parafluvial zone from 222rn activities and cl concentrations in water the alluvial sediments values of fp range from 40 400bq m day in the upper king to 8500bq m day in the lower king corresponding to differences in the mineralogy and the volume of the parafluvial zone the gas transfer coefficient k was estimated by matching groundwater inflows to observed increases in river discharge during a period of low discharge k decreases from 25day 1 in the upper king to 3day 1 in the lower king these k values are higher than those from most empirical formulations probably due to the extensive pool and riffle sections that promote degassing the king river is gaining in its upper and middle sections but several of the lower reaches are losing groundwater inflows on a reach scale are as high as 10m3 m day and cumulative inflows along the 75km stretch are up to 19 000m3 day groundwater inflows increase proportional to total flow reflecting the response of both groundwater and surface water systems to rainfall uncertainties in the calculated groundwater inflows are reduced by independently estimating k from the discharge data however calculated inflows are up to 40 higher if parafluvial flow were not taken into account predicted rn activities from emanation measurements agree with those measured in groundwater contribution of rn from parafluvial zone is assessed rn degassing is independently assessed by comparison with flow gauge data rn degassing is higher than predicted from empirical formulations due to pool and riffle sections king river transitions from gaining in upper sections to losing in lower sections

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

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

scaling relationships in intraplate fracture systems related to red sea rifting abstract the geometry of fracture systems was analysed at various scales in the western arabian sedimentary cover rocks investigations covered a wide range of scales from regional surveys to outcrops in well exposed areas and were based on the field mapping of fractures and the interpretation and digitizing of fracture patterns on aerial and satellite images as a first step a ground structural analysis was carried out which provided a better understanding of the relationships of joint and fault systems related to red sea rifting secondly fracture datasets were subjected to statistical and geostatistical analyses of direction size spacing and density and the variability in these quantities with space and scale the main result of this combined approach is that fracturing is not a self similar process from the centimetre scale up to the 1000 km scale in this case the spatial organization of joints and faults appears to be highly controlled by both the red sea extensional regime and the thickness of the different mechanical layers that constitute the crust at different scales i e sedimentary beds basins and the seismogenic crust

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

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

the estimation of fault slip from map data the separation pitch diagram although an essential feature of faults is that they produce a relative movement of two blocks the total or net slip is known only for a very small percentage of faults this note proposes a diagram for the graphical determination of net slip from the strike separation of markers recorded on geological maps with or without knowledge of the net slip direction the strike separation of one marker allows the drawing of a solutions line on the diagram depicting the range of slip vectors compatible with the measured separation the net slip vector can be obtained by either the intersection of two or more solutions lines or from a single solutions line if the direction of slip is independently known the diagram gives a visual appreciation of the effects of data uncertainty on the estimate of net slip and is a useful device for the representation of the kinematic characteristics of a collection of faults the use of the diagram is illustrated with examples from the south wales coalfield and the moine thrust zone

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

fissure swarms and fracture systems within the western volcanic zone iceland effects of spreading rates abstract the western volcanic zone wvz in iceland is 120 km long and 40 km wide it offers an opportunity to study rift zones in a local ultra slow spreading area close to a hotspot fractures were mapped from aerial photographs and digital elevation models most surface fractures are located in the southern part of the wvz the majority of the fractures have a north northeasterly orientation some deviations occur from this especially in the north part of the wvz fracture orientations are therefore quite uniform in the southern faster spreading part of the wvz but more irregular in the slower spreading northern part this suggests different stress fields in the north part which could be due to the influence of the hreppar microplate and possibly also due to stress fields induced by crustal deformation because of changes in glacial load in the area such glacially induced stress fields may have similar or even more influence than crustal spreading in the slower spreading northern part of the wvz lower fracture density towards the north within the wvz suggests lower frequency of rifting events in the north part in accordance with less spreading in the north as measured by gps geodetic measurements

solute sources and water mixing in a flashy mountainous stream pahsimeroi river u s rocky mountains implications on chemical weathering rate and groundwater surface water interaction abstract identifying solute sources and mixing processes between various water types is challenging in geologically diverse fractured rock settings where various minerals contribute to solute loads and mixing between groundwater and surface water can occur at difficult to delineate point locations in such regions chemical indicators that allow constraining characteristic mineral fingerprints of drainage lithology or aquifer end members are critical this study assesses solute sources and water mixing in the pahsimeroi river a small stream with highly variable discharge draining two heavily deformed mountain ranges of the u s rocky mountains solute inputs to the main stream were estimated using an end member mixing model with 87 sr 86 sr and na sr ratios as tracers considering that these depict the compositional variability pertaining to the various carbonate and silicate lithologies of the basin our results show that the mean solute input from carbonate dominated terrains decreases from 95 9 in the headwaters to about 36 4 in the lowlands while the corresponding inputs from volcanic and metamorphic sources increase from 3 63 to 51 0 and 0 45 to 12 6 respectively data further indicate highly variable chemical weathering rates with highest values observed in the steeper uplands calculated co 2 consumption rates mean value 0 14 mmol km 2 a 1 are lower than the reported continental average 0 21 mmol km 2 a 1 possibly due to lower than average streamflow at the time of sampling and significant input of carbonate solute enriched baseflow in the downstream sections where the basin shallows and decreases in width the herein delineated gaining stream segments are consistent with those deduced from downstream seepage runs which suggests that groundwater sustains perennial flow in the agriculturally developed lowlands

northern latitude chemical weathering rates clues from the mackenzie river basin canada abstract the main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate more than 110 river water samples from the mackenzie river system northern canada have been sampled and analyzed for major and trace elements and sr isotopic ratios in the dissolved phase the three main morphological units are reflected in water chemistry rivers from the canadian shield are very dilute dominated by silicate weathering millot et al 2002 whereas the rivers of the rocky and mackenzie mountains as well as the rivers of the sedimentary interior platform are dominated by carbonate weathering and are so 4 rich compared to the rivers of the mackenzie and rocky mountains the rivers of the interior plains are organic silica and na rich and constitute the dominant input term to the mackenzie river mainstream rivers of the canadian shield area do not significantly contribute to the mackenzie river system using elemental ratios and sr isotopic ratios a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering carbonate weathering rates are mostly controlled by runoff which is higher in the mountainous part of the mackenzie basin these rates are comparable to the carbonate weathering rates of warmer areas of the world it is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering but its extent remains difficult to assess conversely to what was stated by edmond and huh 1997 overall silicate weathering rates in the mackenzie basin are low ranging from 0 13 to 4 3 tons km 2 yr na k ca mg and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates in contrast to what has been observed in other large river systems such as the amazon and ganges rivers silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters this contradicts the raymo hypothesis raymo and ruddiman 1992 isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains e g by glaciers and deposited in the plains rather the relatively high chemical denudation rates in the plains are attributed to lithology uncompacted shales high mechanical denudation and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation the three to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by moulton et al 2000 between unvegetated and vegetated watershed this study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors such as organic matter associated with northern watersheds are able to counteract the effect of temperature this acceleration by a factor of 4 in the plains is equivalent to a 6 c increase in temperature

strontium isotopes as tracers of groundwater induced floods the somme case study france major ions trace elements and o h and sr isotopes have been determined for surface water and groundwater in the somme catchment northern france the present study using a coupled hydrological and geochemical stable and sr isotopes approach focuses on the interactions between ground and surface water stable water isotopes 18o 2h show that all waters have a purely local origin from precipitation without significant evaporation or water rock interaction as all points plot close to the general meteoric water line water chemistry in the different catchments shows large variations in major element contents plotting na vs cl contents and mg2 no3 k so42 sr2 concentrations reveals a slight variations in groundwater composition b agricultural input into surface waters and c some similarities between ground and surface waters suggesting the former to be responsible for the chemical signature of the dissolved river load strontium isotope ratios measured in groundwater range from 0 70769 to 0 70808 clearly higher than that of chalk matrix water 0 707337 variations of sr isotope signatures in groundwater from the chalk aquifer are significant and help identifying different groundwater bodies the different signatures in groundwater are better explained by matrix heterogeneity which can present lateral facies variations than by a strong anthropogenic influence the sr isotope compositional range in surface waters is very wide varying from 0 70775 to 0 70816 and defining a trend towards a more radiogenic sr isotope signature with high sr contents and high mg sr ratios the relationship between 87sr 86sr and the mg sr ratios allows different end members to be defined the rivers are influenced by anthropogenic disturbance even during flood periods and river water has geochemical characteristics very similar to those of groundwater indicating a strong influence from groundwater feeding the rivers

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

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

fault and magmatic interaction within iceland s western rift over the last 9 kyr we present high resolution chirp sub bottom profiler data from thingvallavatn a lake in iceland s western rift zone these data are combined with stratigraphic constraints from sediment cores to show that movement on normal faults since 9 ka are temporally correlated with magmatic events indicating that movements were controlled by episodic dyke intrusion sediment depo centres and the focus of subsidence migrated westwards over 3 4 kyr towards the locus of subsequent brittle failure we interpret this subsidence as related to dyke intrusion a few km along strike originating from the hengill volcanic system which occurred prior to major dyking faulting and subsidence within the lake at 1 9 ka

sr fluxes and isotopic compositions in the headwaters of the yangtze river tongtian river and jinsha river originating from the qinghai tibet plateau abstract water and sediment samples were collected from the headwaters the tuotuo river and chumaer river of the yangtze river tongtian river and jinsha river htjr originating from the qinghai tibet plateau qtp sr and 87 sr 86 sr have been measured to analyze temporal and spatial variation characteristics and determine relative contributions from atmospheric inputs anthropogenic activities and rock weathering these results show that sr in the htjr waters ranges from 1 37 to 11 07 mol l 1 and 87 sr 86 sr from 0 70948 to 0 71409 the headwaters tuotuo river and chumaer river receive 95 of their sr from evaporite dissolution the influence of evaporite decreases gradually downwards from source area as increase in discharge silicate weathering contributes 4 9 28 2 of dissolved sr to the jinsha river mainstream and 11 5 42 4 to the tributaries 87 sr 86 sr ranges from 0 71034 to 0 71177 in the jinsha river mainstream which is higher than those in other reaches of the yangtze river in the jinsha river suture belt granitoids and metamorphic rocks with relatively high 87 sr 86 sr are exposed moreover carbonate weathering is another important source of sr in the jinsha river waters it is estimated that 15 9 46 8 of sr in the mainstream and 37 64 5 of sr in the tributaries are derived from carbonate weathering contribution from atmospheric precipitation to the sr in the htjr is from 0 75 to 6 1 and influence of anthropogenic activities is slight the 87 sr ex flux forcing changes in seawater sr isotopic composition is 1 13 10 6 mol a 1 for the htjr at yibin being far lower than those in the ganges and brahmaputra and merely accounting for 1 36 of the total carried by the global rivers therefore impact of the htjr on steady increase of seawater 87 sr 86 sr ratio since 40 ma may be very small

origin damping and pattern of development of faults in granite observations of brittle fractures in the granite of the massif de la borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults it seems that all faults originate by shear along a preexisting plane which can be an important fracture joint former fault or a microflaw pore grain boundary the shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault we also found an arrangement of horsetail fractures damping the movements on the faults we then propose a pattern for the development of the faults they originate on preexisting plane and propagate by initiating en echelon cracks the movements are damped by horsetails which permit by faults connection in relays the creation of larger faults

a reactive transport model for examining tectonic and climatic controls on chemical weathering and atmospheric co2 consumption in granitic regolith we developed a 1d reactive transport model for examining how tectonic and climatic parameters namely uplift rate water seepage velocity and temperature control chemical weathering and atmospheric co2 consumption rates during regolith development the model consists of mass conservation equations describing how mineral and solute concentrations change temporally and spatially vertically during weathering of granite containing 30 plagioclase an20 and up to 3 accessory calcite the equations are coupled by volumetric weathering rates which depend on mineral abundances intrinsic dissolution rate constants and surface areas as well as the departure of pore water from thermodynamic equilibrium we numerically solve a non steady state non dimensional version of the model by eliminating the need to prescribe regolith thickness non dimensionalization introduces two key scaling parameters that drive variations in the model results the rock residence time r the time for rock to travel upward through the model domain and the water residence time w the time for water to travel downward through the model domain we use the model to examine fundamental properties of chemical weathering especially reaction front propagation with the primary aim of characterizing weathering regimes and identifying factors that maximize the dissolution of plagioclase because only silicate weathering regulates atmospheric co2 levels over geological timescales weathering regimes are commonly classified as transport or supply limited versus weathering or kinetically limited but as outgrowths of geomorphology these terms primarily refer to the role of r in regulating regolith thickness our findings suggest that the paradigm should be expanded to include kinetic controls determined by w both regimes can experience far from and near equilibrium mineral dissolution however transport limited regimes generally have lower w r ratios compared to kinetically limited regimes under transport limitation thick to thin reaction fronts propagate downward indefinitely yielding extensive mineral depletion deeply weathered regolith and high ratios of silicate to carbonate weathering under kinetic limitation thin to non existent reaction fronts evolve to steady state yielding minimal mineral depletion no regolith development and low ratios of silicate to carbonate weathering kinetically limited regimes typifying tectonically active mountain ranges have low long term atmospheric co2 consumption rates transport limited regimes typifying tectonically stable cratons also have low long term atmospheric co2 consumption rates but because much of the earth s surface is transport limited we infer that such environments have the greatest impact on the global atmospheric co2 consumption flux the model output leads us to hypothesize that the maximum contribution should occur for extensive regions of silicate bedrock consolidated in warm wet climates because the combination of tectonic stability high temperatures and rapid seepage velocities accelerates reaction front propagation facilitates calcite depletion and sustains deep regolith development over long timescales our findings point to the importance of internal climate feedbacks for stabilizing long term atmospheric co2 levels

effects of water sediment interaction and irrigation practices on iodine enrichment in shallow groundwater abstract high iodine concentrations in groundwater have caused serious health problems to the local residents in the datong basin northern china to determine the impact of water sediment interaction and irrigation practices on iodine mobilization in aquifers isotope 2 h 18 o and 87 sr 86 sr and hydrogeochemical studies were conducted the results show that groundwater iodine concentrations vary from 14 4 to 2180 g l and high iodine groundwater 150 g l mainly occurs in the central area of the datong basin sediment iodine content is between 87 sr 86 sr values and groundwater chemistry suggest that aluminosilicate hydrolysis is the dominant process controlling hydrochemical evolution along groundwater flowpath and the degradation of toc iodine rich sediment mediated by microbes potentially triggers the iodine release from the sediment into groundwater in the discharge area the vertical stratification of groundwater 18 o and 2 h isotope reflects the occurrence of a vertical mixing process driven by periodic surface irrigation the vertical mixing could change the redox potential of shallow groundwater from sub reducing to oxidizing condition thereby affecting the iodine mobilization in shallow groundwater it is postulated that the extra introduction of organic matter and o 2 no 3 so 4 could accelerate the microbial activity due to the supplement of high ranking electron acceptors and promote the iodine release from the sediment into shallow groundwater

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

major ion chemistry of the yarlung tsangpo brahmaputra river chemical weathering erosion and co2 consumption in the southern tibetan plateau and eastern syntaxis of the himalaya the yarlung tsangpo brahmaputra river drains a large portion of the himalaya and southern tibetan plateau including the eastern himalayan syntaxis one of the most tectonically active regions on the globe we measured the solute chemistry of 161 streams and major tributaries of the tsangpo brahmaputra to examine the effect of tectonic climatic and geologic factors on chemical weathering rates specifically we quantify chemical weathering fluxes and co2 consumption by silicate weathering in southern tibet and the eastern syntaxis of the himalaya examine the major chemical weathering reactions in the tributaries of the tsangpo brahmaputra and determine the total weathering flux from carbonate and silicate weathering processes in this region we show that high precipitation rapid tectonic uplift steep channel slopes and high stream power generate high rates of chemical weathering in the eastern syntaxis the total dissolved solids tds flux from the this area is greater than 520 tons km 2 yr 1 and the silicate cation flux more than 34 tons km 2 yr 1 in total chemical weathering in this area consumes 15 2 10 5 mol co2 km 2 yr 1 which is twice the brahmaputra average these data show that 15 20 of the total co2 consumption by silicate weathering in the brahmaputra catchment is derived from only 4 of the total land

stress estimates from the length width ratios of fractures the hypothesis is advanced that provided young s modulus and poisson s ratio of the rock are known the length width ratios of tension fractures can be used to estimate the tensile stress assumed constant along the length of each fracture at the time of fracture formation the hypothesis is tested on a fissure swarm in a 10 000 year old basaltic lava in iceland the length width ratios of the fissures give the average tensile stress as of the order of a few mpa

geochemical and 222rn constraints on baseflow to the murray river australia and timescales for the decay of low salinity groundwater lenses the distribution of 222rn activities and major ion concentrations show that during low river flows characteristic of the recent drought conditions the middle reaches of the murray river northern victoria were a predominantly gaining system at low river levels may 2009 may 2010 and were variably gaining and losing at higher river levels november 2009 the location of gaining and losing reaches are partially governed by the position of the river with respect to the edge of its floodplain gaining reaches occur where the floodplain narrows or where the river is adjacent to the steep slopes at the floodplain margin cumulative inflows along this 350km stretch of the river estimated from 222rn activities were 320 1500m 3 day in november 2009 and 560 4300m 3 day in may 2010 the relatively large uncertainty reflects the heterogeneous and poorly constrained groundwater 222rn activities major ion variations notably cl concentrations and na ca ratios correlate with 222rn activities and may be used as indicators of groundwater inflows baseflow contributed 1 of the total water to this part of the river and consequently has little impact on river water quality nevertheless the groundwater inflows remove water from low salinity groundwater lenses that underlie the river and during the drought conditions these lenses were degrading these reaches of the river murray are an example of a system that transitioned from being dominantly losing to being variably gaining due to diminishing surface water flows

metamorphic diogenite grosvenor mountains 95555 mineral chemistry of orthopyroxene and spinel and comparisons to the diogenite suite abstract grosvenor mountains gro 95555 is a relatively newly discovered and unique metamorphic diogenite it does not show the usual brecciated appearance of other diogenites or wide compositional variability of orthopyroxene or spinel electron microprobe analysis empa and secondary ion mass spectrometer sims analysis of orthopyroxene and empa of spinel show limited compositional variability compositions for orthopyroxene for fe fe mg atomic al zr y and yb fall in the middle of the compositional ranges of the diogenite suite apparently gro 95555 formed at sufficient depth or location to escape brecciation by meteorite bombardment and in a thermal regime that led to high grade metamorphism resulting in homogeneous mineral compositions the believed introduction howardite eucrite and diogenite hed lithologies are to have originated on asteroid 4 vesta during an extensive melting event at 4 6 ga consolmagno and drake

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

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

geodetic gps measurements in south iceland strain accumulation and partitioning in a propagating ridge system 1 gps observations in south iceland between 1994 and 2003 are compared with two dimensional elastic half space and viscoelastic coupling models for two parallel rift zones representing the western and eastern volcanic zones wvz evz gps data from the hreppar block between the wvz and evz fit a rigid block model within uncertainties spreading rates across the wvz increase from 2 6 0 9 mm yr in the northeast to 7 0 0 4 mm yr in the southwest conversely spreading rates in the evz decrease from 19 0 2 0 mm yr in the northeast to 11 0 0 8 mm yr in the southwest the direction of ridge propagation summed extension rates across the two rift zones are approximately constant and equal to the total plate rate 18 20 mm yr consistent with a simple propagating ridge model whereby the wvz is deactivating in the direction of evz propagation the coupling model confirms results from the simple elastic half space model including relatively shallow locking depths 5 km beneath the rift zones and allows for an estimate of mean viscosity 1019 1020 pa s beneath the elastic layer the location of maximum surface velocity gradient in the evz presumably the locus of subsurface magma accumulation and future rifting does not coincide with the 1783 1784 lakagigar fissure eruption but is 20 km to the west on the bar abunga veidivotn fissure swarm this had a small volume eruption in 1862 1864 but a major eruption in 1477 a d

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

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

chemical mixing model of streamflow generation at la selva biological station costa rica abstract la selva biological station occupies an area of lowland tropical rainforest in central costa rica sodium and chloride data were used to quantify the mixing proportions of local runoff and geothermal groundwater at several sites throughout the stream channel system in late april at the end of the dry season the fraction of streamflow accounted for by geothermal groundwater varied spatially between 0 and 0 85 indicating a significant contribution to streamflow and to stream solute loads from geothermal groundwater at some sites in general higher inputs of geothermal groundwater were found at lower elevations over half the flow from one basin the salto was due to geothermal groundwater suggesting a minimum annual runoff of about 0 7 m of geothermal groundwater from this basin a plot of na cl vs fraction of geothermal groundwater revealed watershed scale chemical differences between the two major drainage systems the sura and the salto differences that were not apparent from a traditional two solute plot of cl vs na concentration a small 21 mm storm produced relatively little change in mixing proportions as most throughfall was apparently retained in the relatively dry soils

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

aggregate properties of fracture populations abstract empirical studies indicate that the individual attributes of both faults and extension fractures follow power law scaling aggregate properties of fracture populations are important in a variety of problems and can be specified in terms of the scaling parameters of individual fracture attributes development of an expression for an aggregate property requires consideration of a number of independent factors including the topologic dimension of the aggregate property the topologic dimension of sampling and the possibility of scaling changes for fractures that span some mechanically significant layer the riemann zeta function provides an alternative to integration for the analytical and numerical solution of aggregate problems previous work regarding aggregate properties of fracture populations has focused on the strain due to faulting new expressions are developed here for other aggregate properties of interest fracture surface area fracture porosity fracture permeability and shear wave anisotropy a general characteristic of these aggregate properties is that for most values of scaling exponents the aggregate properties are dependent on the size of the sampling domain this implies that the aggregate properties are scale dependent additionally it appears that fracture surface area is concentrated in the smallest fractures of many populations fracture porosity is concentrated in the largest fractures of most populations but not as strongly as fracture permeability which probably derives almost entirely from the largest fractures in populations

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

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

geochemistry of surface waters and weathering effects in the upper catchment of the ctalamochita river cordoba s sierras pampeanas central argentina the chemical compositions of waters from the upper basin of the ctalamochita river draining the cordoba pampean ranges were analyzed to determine the chemical signature related to different forcings geochemical processes and solutes sources the catchment has four sub basins santa rosa river grande river quillinzo river and de los sauces river most of the riverine chemical compositions are dominated by the hco3 ca2 type to ca2 na mg2 the specific conductivity is higher in the santa rosa and de los sauces rivers 347 59 and 409 00 s cm respectively than in the grande and quillinzo rivers 125 56 and 87 54 s cm respectively mineral hydrolysis of andesine and oligoclase and the dissolution of calco magnesian limestone are the main dissolved solids contributors to the river waters higher contribution of bicarbonate ions is associated to those sub basin having quarry mining activity saturation indexes show that the santa rosa and de los sauces rivers are saturated with calcite dolomite and talc

controls on the δ13cdic and alkalinity budget of a flashy subtropical stream manoa river hawaii abstract hawaiian streams are flashy in nature because watersheds are small and steep and receive intense and unevenly distributed rainfall as a result stream chemistry is characterized by considerable spatiotemporal variability to examine how rainfall and streamflow affect the solute content of the manoa river in hawaii time series geochemical data collected during 17 sampling campaigns in spring fall of 2010 were evaluated in a coupled 13 c dic major ion inversion model spatially the stream is characterized by a distinct shift from a low hco 3 43 mg l low pco 2 3760 ppmv and heavy 13 c dic 6 5 fingerprint in the upper reaches to a high hco 3 91 mg l high pco 2 8961 ppmv and light 13 c dic 11 7 signature in the lowlands these trends are attributed to 1 progressive weathering of exposed aluminosilicates and 2 downstream enrichment in co 2 from organic matter decay in the soil zone solute i e nitrate yields from nitric acid weathering are generally low 13 c dic and rainfall rates in the mid stream section of the river which is consistent with an atmospheric co 2 dilution effect during high rainfall events this dilution effect needs to be accounted for to reliably describe the role of volcanic island river systems in global assessments of silicate weathering and co 2 degassing

atmospheric co2 consumption by chemical weathering in north america abstract co 2 consumption by chemical weathering is an integral part of the boundless carbon cycle whose spatial patterns and controlling factors on continental scale are still not fully understood a dataset of 338 river catchments throughout north america was used to empirically identify predictors of bicarbonate fluxes by chemical weathering and interpret the underlying controlling factors detailed analysis of major ion ratios enables distinction of the contributions of silicate and carbonate weathering and thus quantifying co 2 consumption extrapolation of the identified empirical model equations to north america allows the analysis of the spatial patterns of the co 2 consumption by chemical weathering runoff lithology and land cover were identified as the major predictors of the riverine bicarbonate fluxes and the associated co 2 consumption other influence factors e g temperature could not be established in the models of the distinguished land cover classes artificial surfaces dominated by urban areas increase bicarbonate fluxes most followed by shrubs grasslands managed lands and forests the extrapolation results in an average specific bicarbonate flux of 0 3 mmol km 2 a 1 by chemical weathering in north america of which 64 originates from atmospheric co 2 and 36 from carbonate mineral dissolution chemical weathering in north america thus consumes 50 mt atmospheric co 2 c per year about half of that originates from 10 of the area of north america the estimated strength of individual predictors differs from previous studies this highlights the need for a globally representative set of regionally calibrated models of co 2 consumption by chemical weathering which apply very detailed spatial data to resolve the heterogeneity of earth surface processes

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

magnetotelluric survey across the active spreading zone in southwest iceland abstract the results of 10 magnetotelluric soundings performed along a 110 km long profile crossing the constructive plate boundary in southwest iceland are presented apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross section along the profile the crust mantle interface contains a well conductive layer the depth to the good conductor increases with age of the crust and the distance from the axial zone this layer is interpreted as partially molten basalt at a temperature about 1100 c and a volume fraction of the melt phase in the range 10 20 the high conductivity layer probably disappears west of the borgarnes anticlinal axis which separates the older to the west and younger to the east flood basalts in western iceland indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber which agrees with the results of the magnetotelluric soundings

co2 evasion from the greenland ice sheet a new carbon climate feedback abstract decay of the greenland ice sheet gis figures prominently in climate change predictions but direct carbon cycle feedbacks are poorly constrained here we show that melting of the gis yields a previously unknown flux of co 2 to the atmosphere and we use a reactive transport model to examine if how and to what extent this flux will change as the ice sheet decays in a warmer world water emerges from beneath the isunnguata and russell glaciers in west greenland with co 2 partial pressures pco 2 3 10 supersaturated with respect to atmospheric equilibrium during downstream transport in the akuliarusiarsuup kuua river mineral weathering sequesters 75 of the excess co 2 as hco 3 a carbon sink on human timescales and the remaining 25 evades to the atmosphere scaled to all rivers draining the gis the evasion flux of 0 11 0 03 tg c year compares to fluxes reported for other rivers draining precambrian shield crystalline rocks and having similar dissolved carbonate systematics this flux is insufficient to influence modern day atmospheric co 2 levels and we find that higher meltwater discharge alone will cause only moderate future increases however more substantial increases could occur if meltwaters intersect basal ice known to have elevated pco 2 values worst case model scenarios yield evasion fluxes of 100 20 170 40 tg c year by 2100 these atmospheric co 2 inputs surpass those for arctic lakes and would augment by up to 25 those predicted for permafrost thaw our findings suggest that positive feedbacks linking greenhouse gas emissions arctic climate change and global warming may be stronger than previously realized

the importance of silicate weathering of a sedimentary aquifer in arid central australia indicated by very high 87sr 86sr ratios with the exception of brine systems and crystalline rock aquifers groundwater 87sr 86sr ratios rarely differ by more than 0 01 from the phanerozoic seawater value of 0 70923 groundwater 87sr 86sr ratios from central australia 0 72562 0 76248 are the highest ever recorded for a low salinity sedimentary aquifer we propose a model for the evolution of these 87sr 86sr ratios and demonstrate how the lowest 87sr 86sr ratios reflect water rock interactions with predominantly carbonate minerals while the highest 87sr 86sr ratios are ultimately controlled by weathering of old primary silicate minerals detailed analysis of aquifer core material identified albite and microcline as the most prevalent primary silicates and where these minerals were abundant relatively high rb concentrations were measured high rb combined with long time scales since deposition has produced very high 87sr 86sr ratios in the rocks and subsequently the groundwater therefore strontium isotopes can provide further constraints for chemical models of arid zone groundwater systems particularly when silicate hydrolysis is believed to significantly contribute to dissolved solute compositions

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

extension direction re orientation in the oceanic rift of iceland and comparison with continental rifts abstract in this study we report the results of a structural analysis carried out on fault systems that form the active rift segments of two areas of iceland namely 1 the northern part of the eastern rift segment northern volcanic zone nvz and 2 the western rift segment that include the western volcanic zone thingvellir area and the reykjanes peninsula in the n s trending nvz mainly krafla volcanic area the obtained extension directions trend around ese at thingvellir the extension directions trend ca nw se and are perpendicular to the main ne sw trending faults in the ene wsw trending tectono magmatic segments of the reykjanes peninsula our data show a marked difference with the gps data and spreading directions the extension directions trend roughly orthogonal to the strike of the boundary faults in all the investigated sectors extension shows an overall southward increase in divergence from the regional directions indicated by gps vectors this behavior has been compared with the continental main ethiopian rift of east africa where structural studies show a re orientation of the extension direction between the rift margins and the internal sectors resulting in a divergence between local and regional extension directions aimed at investigating the causes of these differences we performed a new series of analogue models by varying different parameters such as rift obliquity and width our findings suggest that rift width exerts a primary control on the boundary axis variation of the extension direction in particular significant differences in fault orientation and local direction of extension between the margins and the rift axis are observed in wider rift valleys e g continental rift of ethiopia conversely fault orientation and local extension direction are much more homogeneous in narrower rift valleys e g rifts of iceland where a variations in the direction of extension from the margins to the rift axis are not observed

strontium 87 strontium 86 as a tracer of mineral weathering reactions and calcium sources in an alpine subalpine watershed loch vale colorado sr isotopic ratios of atmospheric deposition surface and subsurface water and geologic materials were measured in an alpine subalpine watershed to characterize weathering reactions and identify sources of dissolved ca in stream water previous studies have noted an excess of ca in stream water above that expected from stoichiometric weathering of the dominant bedrock minerals mixing calculations based on 87 sr 86 sr indicate that on an annual basis 26 67 of ca export in streams is atmospherically derived 23 61 is from weathering of plagioclase and the remainder is from weathering of calcite present in trace amounts in the bedrock a potential source of error when applying sr isotopes in catchment studies is determination of the 87 sr 86 sr of sr released by mineral weathering which is complicated by the wide range of mineral isotopic compositions particularly in older rocks and the variable rates at which the minerals weather in this study base flow stream chemistry was used to represent the 87 sr 86 sr of sr derived from mineral weathering because it effectively integrates the potentially variable isotopic composition of sr released by weathering in the alpine environment

climatic and lithologic controls on the temporal and spatial variability of co2 consumption via chemical weathering an example from the australian victorian alps abstract a detailed geochemical study on river waters of the australian victorian alps was carried out to determine i the relative significance of silicate carbonate evaporite and sulfide weathering in controlling the major ion composition and ii the factors regulating seasonal and spatial variations of co2 consumption via silicate weathering in the catchments major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies the input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite the dissolution of vein calcite accounts for up to 67 of the total dissolved cations generating up to 90 of dissolved ca and 97 of mg dissolved sulfate has 34s values of 16 to 20 cdt indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of fes2 this militates against sulphuric acid weathering in victorian rivers ratios of si vs the atmospheric corrected na and k concentrations range from 1 1 to 4 3 suggesting incongruent weathering from plagioclase to smectite kaolinite and gibbsite estimated long term average co2 fluxes from silicate weathering range from 0 012 106 to 0 039 106 mol km2 yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks this is about one order of magnitude below the global average which is due to low relief and the arid climate in that region time series measurements show that exposure to lithology high physical erosion and long water rock contact times dominate co2 consumption fluxes via silicate weathering while variations in water temperature are not overriding parameters controlling chemical weathering because the atmospheric corrected concentrations of na k and mg act non conservative in victorian rivers the parameterizations of weathering processes and net co2 consumption rates in particular based on major ion abundances should be treated with skepticism

mechanics of tidally driven fractures in europa s ice shell abstract a fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden it is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on europa such penetration is found to be very unlikely in the absence of an ocean a cycloidal crack would then form as a sequence of near instantaneous discrete failures each extending roughly the brittle layer thickness in range linked with a much lower apparent propagation speed set by the moving tidal stress field the implications of this porous ice fracture model for ice penetrating radar scattering loss and seismic activity are quantified

proximal lava drainage controls on basaltic fissure eruption dynamics hawaiian basaltic eruptions commonly initiate as a fissure producing fountains spattering and clastogenic lava flows most fissures rapidly localize to form a small number of eruptive vents the location of which may influence the subsequent distribution of lava flows and associated hazards we present results from a detailed field investigation of the proximal deposits of episode 1 of the 1969 fissure eruption of mauna ulu k lauea hawai i exceptional preservation of the deposits allows us to reconstruct vent proximal lava drainage patterns and to assess the role that drainage played in constraining vent localization through detailed field mapping including measurements of the height and internal depth of lava tree moulds we reconstruct high resolution topographic maps of the pre eruption ground surface the lava high stand surface and the post eruption ground surface we calculate the difference in elevation between pairs of maps to estimate the lava inundation depth and lava drainage depth over the field area and along different segments of fissure aerial photographs collected during episode 1 of the eruption allow us to locate those parts of the fissure that are no longer exposed at the surface by comparing with the inundation and drainage maps we find that fissure segments that were inundated with lava to greater depths typically 1 6 m during the eruption later became foci of lava drainage back into the fissure internal drain back we infer that in these areas lava ponding over the fissure suppressed discharge of magma thereby favouring drain back and stagnation by contrast segments with relatively shallow inundation typically less than 1 m such as where the fissure intersects pre eruptive topographic highs or where flow away from the vent outflow was efficient are often associated with sub circular vent geometries in the post eruption ground surface we infer that these parts of the fissure became localization points for ongoing magma ascent and discharge we conclude that lava inundation and drainage processes in basaltic fissure eruptions can play an important role in controlling their localization and longevity

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

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

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

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

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

analogue modeling of tension fracture pattern in relation to mid ocean ridge propagation the face of the earth is constantly renewed by continental rifts that open and allow ridges to spread oceanic floor both types of these linear extensional structures are morphologically and structurally segmented this thesis aims to provide insights into the interaction of such segments as they propagate and mature it is based on the results of analogue models of evolving rifts and ridges together with field studies of populations of dilational normal faults in the active rift zone of iceland linkage of initially separate segments along rifts and ridges leads to formation of through going systems operating on a planetary scale it is argued here that the processes of segment initiation and growth are effectively scale independent and essentially the same in wide and narrow modes of extension and for shear and dilational failure three distinct types of segment coalescence are recognized these involve tip to tip one tip to sidewall and two tips to sidewalls segment linkage the overall structural pattern particularly the width of the extending domain and the range of displacements and orientations of smaller scale internal extensional structures is influenced by both the geometry of extension and the mechanical properties of the extending material types of segment linkage appear to be largely independent of localization of dilation or intensity of magmatic accretion instead they are controlled mainly by the relative distribution and orientation of interacting segments a few orders of segmentation along global rift ridge systems and arrays of dilational normal faults indicate that linear continuous structures accumulate extension in successive increments during which the segments change their geometries in semi continuous interaction and coalescence

propagation of blind normal faults to the surface in basaltic sequences insights from 2d discrete element modelling abstract a discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre existing blind normal fault as it propagates to the surface the cover materials representing basalt are homogeneous strong and display elastic brittle material behaviour cover deformation is seen to evolve through a series of distinct stages initial displacement on the underlying fault produces a very gentle monoclinal flexure with continued displacement open fractures develop at the monocline surface and propagate downwards whilst the deeper fault propagates upwards simultaneously a series of fractures in the future hanging wall of the main fault develop in the upper part of the cover the monoclinal flexure is then cut by these structures producing a surficial fault and fracture bounded wedge finally a prominent surface fracture and the upward propagating fault link cutting the entire cover sequence this fault is dilatant in the upper c 100 m of the cover has a significant surface aperture and forms a prominent fault scarp many of the key model results are strikingly similar to those seen in natural settings and emphasise that the occurrence of dilatant faults open fractures and cavities caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth therefore the results have implications for permeability and fluid flow in such settings comparison is also made with a weak cover experiment using granular materials with no cohesion or tensile strength similar to the dry sand used in many analogue modelling studies

quantifying vein attributes in massive mudstones triassic sw england implications for progressive evolution of opening mode fracture networks abstract attributes of gypsum vein arrays in the triassic massive mudstones of sw england were analyzed to infer the process of vein network evolution field data demonstrate that there is no clear correlation between vein orientation and thickness however sub horizontal veins have a higher average thickness than the inclined veins vein thickness distribution is best characterized by a log normal distribution the fractal dimensions d of vein networks analyzed range from 1 44 to 1 71 vein networks which contain abundant bridging veins between neighbouring sub horizontal veins to form an anastomosing pattern exhibit a higher d value the fraction of vein volume derived from image analysis ranges from 6 75 to 17 94 a positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein intersection points suggesting that an anti clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins the vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation hindering the nucleation of new veins as a consequence accommodation of the bulk rock strain could be facilitated mainly by expansion of the large backbone veins which follow a positive feedback mechanism of growth this study reveals a strong link between the geometric parameters of opening mode fractures with the history of their formation propagation and intersection

coupling volcanism and tectonics along divergent plate boundaries collapsed rifts from central afar ethiopia magma along divergent plate boundaries is erupted from fissures or vents from central volcanoes with limited impact on rift architecture here i summarize the geological and structural features accompanying the eruption of part of a km thick volcanic sequenc e stratoids along the red sea divergent boundary in central afar in the area of tendaho and dobi grabens more than 4700 km 3 ma per 100 km of rift length of magma have been produced by repeated fissure eruptions from within tendaho graben the graben sides show distinctive structural features as steep topographic gradients coinciding with inward tilted blocks forming dominoes coeval to the emplacement of the km thick volcanic sequence similar features are observed also in the dobi graben this allows proposing an original mechanism where the distinctive structure of the grabens results from the collapse at the surface induced by magma withdrawal during the emplacement of the volcanic sequence this portion of afar shows how rift architecture is shaped by voluminous fissure eruptions forming collapsed rifts these occur in continental domains during the breakup stage central afar and in oceanic domains where rifts narrow east pacific rise collapsed rifts represent an end member type of volcano tectonic activity where the width of the erupting reservoir balances that of the active rift zone along divergent boundaries the width of the reservoir influences the style of surface deformation a progressively higher ratio of the width of the reservoir emptied re to that of the active rift zone ri generates in sequence axial grabens calderas and collapsed rifts

quantification and regionalization of groundwater surface water interaction along an alluvial stream in the present study groundwater seepage to an alluvial stream and two tributary streams was examined at nine field sites using hydrological geophysical and geomorphological observations the data indicate that seepage enters the streams in the following ways i directly through the streambed ii as nearly superficial flow from diffuse discharge areas on the flood plains or iii as a combination of i and ii at about 40 of the sites more than 50 of seepage flows through the streambed moreover it was found that the ratio c defined as the width of the wet zone of the flood plain divided by the effective width of the stream can be used as an indicator of the percentage of water entering the stream directly through the streambed when c is small streambed seepage is large while when c is large streambed seepage is small and ground water enters the stream mainly as nearly superficial or over bank flow from the wet zone

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

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

extension across a divergent plate boundary the eastern volcanic rift zone south iceland 1967 1994 observed with gps and electronic distance measurements the average spreading rate in south iceland 19 mm yr is distributed over two parallel rift zones we measured a global positioning system gps network of 42 stations in the eastern zone in 1994 this network consists of stations measured with gps in 1986 and 1992 stations in a 60 km long distance profile measured several times since 1967 and a few new stations the 1994 gps data were processed using the bernese software version 3 5 and the average position uncertainties are about 3 mm in horizontal components and about 8 mm in the vertical component for baselines up to 100 km comparison with results of former gps campaigns gives a uniform extension steady in time perpendicular to the spreading axis a strain rate of 0 12 0 01 strain yr is observed across the 100 km wide network or an extension of about 12 mm yr minor deformation is observed in direction parallel to the spreading axis observations along the distance profile which lies across the rift zone gave a significant contraction during the period 1967 1977 but gave an extension during 1977 1994 this extension is about 138 47 mm and is mainly accommodated in the western part of the profile the observed extension across the rift zone during 1986 1994 can be simulated with a simple model of infinitely long dike intrusions into an elastic layer overlying a viscous layer this model is not able to simulate observed contraction along the distance profile 1967 1977 the observed irregularities along the western part of the distance profile coincide in space and time with volcanic and tectonic unrest near the hekla volcano the disturbances are probably caused by some common underlying process leading to crustal deformation eruptions and earthquakes

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