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

three dimensional geometry of concentric intrusive sheet swarms in the geitafell and the dyrfjoll volcanoes eastern iceland sheet intrusions inclined sheets and dykes in the deeply eroded volcanoes of geitafell and dyrfjoll eastern iceland were studied at the surface to identify the location depth and size of their magmatic source s for this purpose the measured orientations of inclined sheets were projected in three dimensions to produce models of sheet swarm geometries for the geitafell volcano the majority of sheets converge toward a common focal area with a diameter of at least 4 to 7 km the location of which coincides with several gabbro bodies exposed at the surface assuming that these gabbros represent part of the magma chamber feeding the inclined sheets a source depth of 2 to 4 km below the paleoland surface is derived a second younger swarm of steeply dipping sheets crosscuts this gabbro and members of the first swarm the source of this second swarm is estimated to be located to the se of the source of swarm 1 below the present day level of exposure and deeper than the source of the first swarm for the dyrfjoll volcano we show that the sheets can be divided into seven different subsets three of which can be interpreted as swarms the most prominent swarm the njardvik sheet swarm converges toward a several kilometers wide area in the njardvik valley at a depth of 1 5 to 4 km below the paleoland surface two additional magmatic sources are postulated to be located to the northeast and southwest of the main source crosscutting relationships indicate contemporaneous as well as successive activity of different magma chambers but without a resolvable spatial trend the dyrfjoll volcano is thus part of a complex volcanic cluster that extends far beyond the study area and can serve as fossil analog for nested volcanoes such as askja whereas in geitafell the sheet swarms seem to have originated from a single focus at one time thus defining a single central volcanic complex such as krafla volcano

stratigraphy and k ar ages across the west flank of the northeast iceland axial rift zone in relation to the 7 ma volcano tectonic reorganization of iceland lithostratigraphy magnetostratigraphy and new k ar ages from the region between eyjafjordur and skjalfandi north central iceland reflect tectonic reorganization of crustal accretion zone activity 7 ma two flood basalt piles are in unconformable contact along this western flank of the northeast iceland axial rift zone neiarz the older ranges in age from 9 5 to 13 ma and is largely composed of tholeiite flows in eastern dalsmynni the upper to middle portions of this older basaltic pile define a 15 35 se dipping monoclinal flexure developed 6 7 ma during early development of the present neiarz lavas of a flood basalt group younger than about 6 5 ma were deposited unconformably on the older flexured basalt pile these postflexure age flows mostly consist of compound doleritic basalts and olivine tholeiites possible tillites are found in the upper portion of this sequence the unconformity between the older and younger basalt piles represents a major structural temporal and lithological boundary in north central iceland zeolite zones appear subhorizontal through the dalsmynni flexure zone and have been largely reset subsequent to flexuring by westward onlap of the younger basalt sequence onto the older subsided and flexured flows radiometrie ages regional isochron patterns and timing constraints for tjornes fracture zone related shear deformation indicate the model of a singular 130 km eastward jump of the axial rift zone in north iceland 6 7 ma is too simple crustal accretion older than 12 ma along a proto neiarz is apparently required indicating that northern iceland has had multiple branched crustal accretion zones up until about 7 ma at which time a single spreading zone along the present neiarz developed an eastward shift of axial rift zone activity 6 7 ma in southwestern iceland is time correlative with reorganization of spreading activity in northern iceland our proposed 6 7 ma reorganization of axial rift zone activity on land is time correlative with an independently proposed increase in discharge from the iceland mantle plume

plate boundary deformation in north iceland during 1992 2009 revealed by insar time series analysis and gps abstract in north iceland extensional plate motion is accommodated by the northern volcanic zone a set of en echelon volcanic systems and the tjornes fracture zone a transform offset in the mid atlantic ridge consisting of two parallel transform lineaments the southern lineament the husavik flatey fault is a 100 km long right lateral strike slip fault that has not ruptured for more than 140 years and poses a significant seismic hazard to husavik a fishing town located by the fault and to other coastal communities we present results of insar time series analysis data spanning almost two decades 1992 2009 that show extensional and interseismic deformation within the northern volcanic zone and the on shore part of the tjornes fracture zone the results also exhibit transient inflation at theistareykir volcano deflation at krafla central volcano and a broad uplift north of krafla the current plate extension is not uniform across the northern volcanic zone but concentrated at the western fissures of the theistareykir volcanic system and the outermost fissures of the krafla fissure swarm we combine a back slip plate boundary model with a set of point pressure sources representing volcanic changes to describe the current extensional plate boundary deformation and update the previous estimations of the locking depth and slip rate of the husavik flatey fault that were based on gps data alone using different combinations of input data we find that the husavik flatey fault has a locking depth of 6 10 km and with a slip rate of 6 9 mm yr is accommodating about a third of the full transform motion we furthermore show that while the insar data provide important constraints on the volcanic deformation within the nvz they do not significantly improve the model parameter estimation for the hff as the dense gps network appears to better capture the deformation across the fault

brunhes matuyama paleomagnetism in three lava sections in iceland this paper reviews previous field research on the paleomagnetism of volcanics in iceland at localities whose age is presumed to span the brunhes matuyama chron boundary additional mapping and laboratory measurements are presented from three localities of late quaternary reverse to normal transitions at two tjornes in north iceland and eyjafjoll in south iceland the identity of the transition is confirmed by k ar dating at tjornes and at ingolfsfjall in southwest iceland short period variations of the paleomagnetic field are demonstrated to be useful in stratigraphic mapping

application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in holocene and late glacial shelf sediments off north iceland abstract the north icelandic shelf is partly distinguished by the tjornes fracture zone featuring numerous active basins in a mud dominated shelf environment late glacial and holocene high resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores the correlation is extended with lithological logs and with magnetic susceptibility records a detailed multidisciplinary study including biostratigraphy and tephrochronology of a late holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal yr of special interest is the exact timing of a marked drop in sea surface temperature in the area indicated by ice rafting debris concentration to about 50 years before the hekla 3 eruption which occurred at 2980 cal yr bp this appears to predate most records of a general cooling event in nw europe by a couple of centuries two different possible age models one based on 14c dates combined with tephra markers and one based on tephra markers alone are discussed in context with the problem of different marine reservoir ages of the water masses in the area

spreading structure of the troodos ophiolite cyprus some paleomagnetic constraints it has been suggested that grabens on the northern flank of the troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge jumping this paleomagnetic survey of the solea graben was carried out to constrain the structural interpretation an analysis was devised which restores both the dikes sampled to vertical and the primary remanent magnetizations to a previously derived structurally corrected remanent magnetization direction typically two solutions for the original dike orientation and the pole and angle of net rotation are obtained for each dike sampled these solutions can be separated if a locally consistent original dike strike is assumed reasons are discussed for the selection of the solution that gives an original northwesterly dike strike results are consistent with a graben rotations are up to 78 at the sites studied rotation vectors are subhorizontal and subparallel to the original dike strike suggestive of an axial process for the development of the structure however on the basis of field observations an alternative mode of origin is proposed invoking ephemeral extreme upper crustal extension by listric faulting on the west flank of an intermediate to fast spreading axis the grabens being formed by antithetic faulting

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

the kamikatsura event and the matuyama brunhes reversal recorded in lavas from tjornes peninsula northern iceland abstract we report paleomagnetic results from four overlapping stratigraphic sections 55 flows through the lava pile in tjornes peninsula north iceland the initial aim of our study was to check the existence of intermediate directions previously interpreted as belonging to the matuyama brunhes geomagnetic reversal mbr and complete this record with thellier thellier paleointensity determinations and 40ar 39ar radioisotopic dating the directional results corroborate the findings by kristjansson et al 1988 a sequence of reversed transitional normal and transitional polarity r t n t is found in each section the polarity change is characterized by a jump from reversed virtual geomagnetic poles vgps to a quasi cluster of transitional vgps located over china followed by a second jump to a cluster of normal polarity before moving again towards intermediate polarities reversely magnetized flows exhibit a magnetic mineralogy well suited for paleointensity determination of these 25 flows 20 yielded paleointensity estimates of good quality unfortunately the systematic presence of coarse multidomain md titanomagnetite in the transitional and normal polarity flows hampered paleointensity estimates for these flows the virtual dipole moments vdm calculated for the reversed polarity flows vary from 4 9 to 7 0 1022 am2 with an arithmetic mean value of 5 5 0 8 1022 am2 this value is identical to the mean vdm obtained for the 0 3 5 ma time window and thus strengthens the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0 3 ma 40ar 39ar isochron ages were obtained with the incremental heating technique on groundmass separates or phenocryst poor whole rock samples six of the transitionally magnetized lavas yielded isochron ages that are indistinguishable from one another at the 95 confidence level the weighted average age is 862 51 ka in the upper part of this volcanic sequence the normally magnetized flows yielded a weighted average age of 787 40 ka this finding supports the conclusion that a geomagnetic excursion within the matuyama chron is recorded in the lower part of the tjornes volcanic sequence whereas only the latest portion of the mbr is recorded by flows in the upper part of the lava pile this conclusion is supported via comparison of the vgps obtained from the lavas with those predicted by a geomagnetic field model

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

shifts of the plate boundary in iceland some aspects of tertiary volcanism this paper points out five anomalies of the icelandic upper crust that are not compatible with currently accepted construction models these anomalies are 1 assumption of a single time averaged plate boundary 2 irregular depth distribution to seismic layer 3 3 the assumption of a uniform vertical compositional zonation 4 the excess width of iceland and 5 abnormal variations in the duration and stratigraphic thickness of chron 5 time lavas in western northern and eastern iceland a model involving frequent shifts of the volcanic zones in iceland is outlined that suggests a way to account for the above structural anomalies the model also suggests that detailed studies of basalt lava facies types may be used effectively to trace small scale shifts of the tertiary plate boundary

structure and paleoearthquake records of active submarine faults cook strait new zealand implications for fault interactions stress loading and seismic hazard 1 a new interpretation of active faulting in central cook strait new zealand reveals tectonic structures associated with the spatial transition from subduction to continental transform faulting marine seismic reflection profiles and multibeam bathymetric data indicate that there are no throughgoing crustal faults connecting the north island dextral fault belt and the marlborough fault system in south island the major faults terminate offshore associated with 5 20 km wide step overs and a change in regional fault strike this structure implies that propagation of strike slip earthquake ruptures across the strait is not probable faulted sedimentary sequences in the wairau basin marlborough shelf correlated to glacioeustatic sea level cycles provide a stratigraphic framework for fault analysis a high resolution study of the postglacial 20 ka vertical displacement history of the cloudy and vernon faults reveals up to six and five paleoearthquakes since 18 ka respectively these long timescale records indicate variable recurrence intervals and possibly variable stress drop thus conforming to the variable slip model of earthquake behavior integration of these data with other submarine and terrestrial paleoearthquake records indicates the presence of clustered earthquake sequences involving multiple faults different sequences do not always involve the same faults it appears that earthquake clustering is driven by fault interactions that lead to specific loading conditions favoring the triggering of earthquakes on major faults in relatively short time intervals present day regional coulomb stress distribution has been calculated in two scenarios considered to represent minimum and maximum loading conditions the models incorporating secular tectonic loading and stress changes associated with major crustal earthquakes indicate high stress loading in a large part of central cook strait these conditions may favor the triggering of future damaging earthquakes in this region

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

global geodesy using gps without fiducial sites baseline lengths and geocentric radii have been determined from gps data without the use of fiducial sites data from the first gps experiment for the iers and geodynamics gig 91 have been analyzed with a no fiducial strategy a baseline length daily repeatability of 2 mm 4 parts per billion was obtained for baselines in the northern hemisphere comparison of baseline lengths from gps and the global vlbi solution glb659 caprette et al 1990 show rms agreement of 2 1 parts per billion the geocentric radius mean daily repeatability for all sites was 15 cm comparison of geocentric radii from gps and sv5 murray et al 1990 show rms agreement of 3 8 cm given n globally distributed stations the n n 1 2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well defined center of mass geodetic information can be obtained by examining the structure of the polyhedron and its change with time

itrf2000 a new release of the international terrestrial reference frame for earth science applications 1 for the first time in the history of the international terrestrial reference frame the itrf2000 combines unconstrained space geodesy solutions that are free from any tectonic plate motion model minimum constraints are applied to these solutions solely in order to define the underlying terrestrial reference frame trf the itrf2000 origin is defined by the earth center of mass sensed by satellite laser ranging slr and its scale by slr and very long baseline interferometry its orientation is aligned to the itrf97 at epoch 1997 0 and its orientation time evolution follows conventionally that of the no net rotation nnr nuvel 1a model the itrf2000 orientation and its rate are implemented using a consistent geodetic method anchored over a selection of itrf sites of high geodetic quality ensuring a datum definition at the 1 mm level this new frame is the most extensive and accurate one ever developed containing about 800 stations located at about 500 sites with better distribution over the globe compared to past itrf versions but still with more site concentration in western europe and north america about 50 of station positions are determined to better than 1 cm and about 100 sites have their velocity estimated to at or better than 1 mm yr level the itrf2000 velocity field was used to estimate relative rotation poles for six major tectonic plates that are independent of the trf orientation rate a comparison to relative rotation poles of the nuvel 1a plate motion model shows vector differences ranging between 0 03 and 0 08 m y equivalent to approximately 1 7 mm yr over the earth s surface itrf2000 angular velocities for four plates relative to the pacific plate appear to be faster than those predicted by the nuvel 1a model the two most populated plates in terms of space geodetic sites north america and eurasia exhibit a relative euler rotation pole of about 0 056 0 005 m y faster than the pole predicted by nuvel 1a and located about 10 n 7 e more to the northwest compared to that model

external validation of the grace ggm01c gravity field using gps and doris positioning results 1 we have used the new gravity field model ggm01c derived from grace data to reanalyze doris data from 1993 0 to 2003 2 using the gipsy oasis software and a free network approach we have estimated the position and velocity of each doris station in itrf2000 in order to test the accuracy of these results we have compared them to the positions and velocities of 43 collocated gps stations using local ties and covariance information doris results computed using the ggm01c gravity field instead of the egm96 gravity field show a significantly improved external agreement with gps position agreement of 12 26 mm was reduced to 10 13 mm and velocity agreement of 3 3 3 7 mm yr was reduced to 2 4 3 3 mm yr this can be interpreted as an external test of the accuracy of the new ggm01c gravity field

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

vertical axis rotations in the las vegas valley shear zone southern nevada paleomagnetic constraints on kinematics and dynamics of block rotations paleomagnetic study of vertical axis rotations at 23 localities along the right lateral las vegas valley shear zone lvvsz in southern nevada indicates that clockwise rotations generally increase with proximity to the lvvsz reaching 100 at the closest localities rates of rotation determined at four localities range from l m y to 12 m y the maximum characteristic size of rotating blocks is 2 4 km considerably smaller than the dimensions of the zone of deformation associated with the shear zone 100 km in length 15 km in half width thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone instead the deformation appears to be quasi continuous when considered on a scale of 10 km we suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material mechanically decoupled from deeper parts of the crust and that the across strike distribution of rotations is controlled by the rheological properties of the upper crust the length of the shear zone and the total amount of offset across the shear zone

flooding the topography wave equation datuming of land data with rugged acquisition topography wave equation datuming overcomes some of the problems that seismic data recorded on rugged surface topography present in routine image processing the main problems are that 1 standard optimized migration and processing algorithms assume data are recorded on a flat surface and that 2 the static correction applied routinely to compensate for topography is inaccurate for waves that do not propagate vertically wave based processes such as stacking dip moveout correction normal moveout correction velocity analysis and migration after static shift can be severely affected by the nonhyperbolic character of the reflections to alleviate these problems i apply wave equation datuming early in the processing flow to upward continue the data to a flat datum above the highest topography this is what i refer to as flooding the topography this approach does not require detailed a priori knowledge of the near surface velocity and it streamlines subsequent processing because the data are regridded onto a regularly sampled datum wave equation datuming unravels the distortions caused by rugged topography and unlike the static shift method it does not adversely effect subsequent wave based processing the image obtained after wave equation datuming exhibits better reflector continuity and more accurately represents the true structural image than the image obtained after static shift

prestack layer replacement we present a method of layer replacement based on the kirchhoff integral solution to the scalar wave equation the term layer replacement refers to replacing the overburden velocity with the velocity of the substratum thereby eliminating raypath bendings at the interface between the overburden and the substratum it is the raypath bendings that induce distortions and disruptions on reflections beneath a complex structure when implemented before stack layer replacement provides an opportunity to revise velocity estimates after correcting for the nonhyperbolic moveout on cmp data caused by a complex overburden as a result layer replacement also yields an improved unmigrated stack section imaging can then be completed by time migration after stack we demonstrate the layer replacement technique on field data with irregular water bottom topography

using vertical axis rotations to characterize off fault deformation across the san andreas fault system central california we use vertical axis rotations from new paleomagnetic data to constrain off fault deformation within the san andreas fault system in central california samples were collected from 177 sites in the miocene monterey formation adjacent to the rinconada fault reliable means from 57 sites have 3 prominent patterns 1 the largest clockwise rotations are close to the rinconada and nacimiento faults 2 no significant rotation is observed near the san andreas fault and 3 counterclockwise rotations are observed at several sites northwest of paso robles these paleomagnetic results are compared to two other measures of off fault deformation where rotation can be calculated results from a fold based kinematic model show increasing clockwise rotations toward the rinconada fault consistent with pattern 1 few folds are observed in rocks on salinian basement near the san andreas fault suggesting that little deformation has occurred and providing an explanation for the negligible paleomagnetic rotations in pattern 2 rotations calculated from the global positioning system velocity field predict counterclockwise rotations that coincide with those observed from paleomagnetic data in pattern 3 broad patterns in the velocity field appear to be controlled by the transition from creeping to locked behavior along the san andreas fault and the region of counterclockwise rotation is linked to this transition thus we suggest that the creeping segment has been aseismic over geologic time scales in order to produce the observed paleomagnetic rotations the integration of all three data sets demonstrates that the san andreas fault borderlands record an important portion of fault parallel plate motion over geologic and geodetic time scales

crustal deformation measured by gps in the south iceland seismic zone due to two large earthquakes in june 2000 two large earthquakes struck the south iceland seismic zone in june 2000 the first on june 17 mw 6 5 and the second on june 21 mw 6 4 these are the largest earthquakes in the area in the past 88 years a network of gps stations was remeasured followingthe earthquakes the whole network was last measured in 1995 and partly in 1999 we correct for the interseismic motion from 1995 to 2000 to obtain the coseismic displacements the largest co seismic motion we observe is about 0 55 m in the epicentral area of the june 17 event we model the surface deforma tion for the two earthquakes usingrectang ular dislocations in an elastic half space best fit uniform slip models indi cate that the events occurred on two parallel n s vertical faults with right lateral strike slip motion this is the same style of faultingbelieved to have occurred in larg e historical earthquake sequences in south iceland

paleomagnetism and the orocline hypothesis eldredge s bachtadse v and van der voo r 1985 paleomagnetism and the orocline hypothesis in n l carter and s uyeda editors collision tectonics deformation of continental lithosphere tectonophysics 119 153 179 oroclines were originally defined by carey as curved mountain belts which initially were straight or at feast straighter than they are today in the last few years the definition has been broadened to include any curved mountain belt regardless of its original shape since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt the arcuate belts investigated are the sicilian calabr n arc and the umbrian arc of italy the swiss portion of the jura mountains the central portion of the appalachian mountains from pennsylvania to virginia u s a the wyoming idaho overthrust belt of western north america and the hercynides of western and central europe the jura mountains and the pennsylvania virginia portion of the appalachians fail to show significant oroclinal bending the wyoming idaho belt shows a combination of rotated possibly oroclinai and unrotated thrust sheets in the sicilian calabrian arc significant oroclinat bending caused by the impingement of the calabria peloritani nappes in the late tertiary can be demonstrated while the umbrian arc of similar age in the northern apennines also shows oroclinal bending on a smaller scale hercynian europe the only belt included in which deformation of basement rocks can be demonstrated shows oroclinal bending at least so as well as a marked original curvature 70 in its western part common to all the orociines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny causing subsequent deformation and bending of the fold belt

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

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

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

late glacial and holocene palaeoceanography of the north icelandic shelf high resolution gravity cores and box cores from the north icelandic shelf have been studied for palaeoceanographic history based on lithological and biostratigraphical foramin iferal data results from two outer shelf cores covering the last 13 6 k 14 c yr bp are presented in this paper the sediments accumulated in north south trending basins on each side of the kolbeinsey ridge at water depths of ca 400 m sedimentation rates up to 1 5 m kyr 1 are observed during the late glacial and holocene the vedde and saksunarvatn tephras are present in the cores as well as the hekla 1104 a new tephra kol gs 2 has been identified and dated to 13 4 k 14 c yr bp and another tephra geochemically identical to the borrobol tephra has been found at the same level at present the oceanographic polar front is located on the north icelandic shelf which experiences sharp oceanographic surface boundaries between the cold east icelandic current and the warmer irminger current past changes in sedimentological and biological processes in the study area are assumed to be related to fluctuations of the polar front the area was deglaciated before ca 14 kyr bp but there is evidence of ice rafting up to the end of the gs 1 greenland stadial 1 younger dryas period increasing again towards the end of the holocene foraminiferal studies show a relatively strong gs 2 pre 13 kyr bp palaeo irminger current followed by severe cooling and then by unstable conditions during the remainder of the gi 1 greenland interstadial 1 bolling allerod and gs 1 younger dryas another cooling event occurred during the preboreal before the holocene current system was established at about 9 kyr bp after a climatic optimum between 9 and 6 kyr bp the climate began to deteriorate and fluctuate copyright 2000 john wiley sons ltd

vsp beyond time to depth vsp or vertical seismic profile was originally designed and is currently primarily used to give us time to depth for seismic well ties beyond time to depth a number of possibilities exist recently there has been considerable interest in vsp imaging ray et al 2003 paulsson et al 2004 hornby et al 2004 hornby et al 2005a with extensive surveys being acquired both on land and offshore modeling studies using full waveform finite difference method fdm payne et al 1994 van gestel et al 2003 show us what we can image for a particular acquisition geometry and geology with best image results seen with 3d vsp surveys incorporating a large vsp array in the well and a 2d source pattern acquired using a surface seismic shooting vessel traditionally vsp imaging has been implemented using surface seismic processing algorithms however the vsp geometry poses its own challenges and unique opportunities in this article we explore some imaging methods to attempt to take advantage of the vsp g

transform deformation of tertiary rocks along the tjornes fracture zone north central iceland significant fracture zone deformation has been imparted to tertiary lavas on the peninsula flateyjarskagi located between eyjafjordur and skjalfandi in north central iceland the region lies immediately south of the flatey fault one of a series of wnw trending left stepping faults comprising the right lateral oblique slip husavik flatey fault system hffs the hffs defines the present southern margin of the 70 to 80 km wide tjornes fracture zone tfz lava bedding dikes and faults in the western half of flateyjarskagi display a progressive 0 110 clockwise change of trend and steepening of lava dip angles to 15 45 over an 11 km wide zone nearest the flatey fault accompanying this structural curvature is an increase in fault and dike frequency and amygdule vein mineralization northeast striking faults develop predominant downthrows to the east approaching the tfz accommodating the steep lava dips via repetition of stratigraphic section and normal fault bounded block rotations although the possibility cannot yet be conclusively ruled out that structural trends formed in curved orientations the simplest interpretation suggests that the lava pile was tectonically rotated by heterogeneous simple shear maximum on land shear strain has been crudely estimated at 3 5 estimates of shear displacement suggest that the tfz was active for at least 2 m y sometime prior to 6 7 ma at a strain rate of approximately 5 5 10 14 s 1 antithetic faulting and associated flexural bending may have been the means by which a great part of the proposed simple shear rotations were accomplished wnw extension produced by rotational normal faulting is interpreted to have accommodated systematic variations in shear strain parallel to the direction of shear paleomagnetic and structural evidence suggest that rocks of tertiary age on tjornes peninsula immediately to the east also have been tectonically rotated implying an original shear zone width of at least 25 km

control of rift obliquity on the evolution and segmentation of the main ethiopian rift the main ethiopian rift has evolved in two stages with successive activation of differently oriented fault systems analogue modelling of the earth s lithosphere demonstrates that such a rift evolution requires neither magma weakening nor a change in plate kinematics the main ethiopian rift is an active rift in the break up stage and it marks the incipient boundary between the nubia and somalia plates1 rifting started with the activation of large boundary faults and diffuse volcanism followed by focused magmatism and faulting in the rift floor along step like segments oblique to the rift axis that now act as a protoridge for future seafloor spreading2 this concentration of volcano tectonic activity has been thought to be either magma assisted2 3 or controlled by a change in rift kinematics with a late oblique rifting phase that would have caused the development of the step like fault segments that focused magma upwelling4 5 geodetic6 7 seismic8 and stress field9 data confirm current oblique rifting kinematics but plate kinematics models do not predict a change in nubia somalia motion in the past 11 million years10 here i use lithospheric scale analogue models of oblique rifting to analyse the development of the main ethiopian rift i find that neither magma weakening nor a change in plate kinematics are required to simulate a two phase evolution with successive activation of differently oriented fault systems i conclude that rift evolution and segmentation are controlled by rift obliquity independent of magmatic processes

interferometric imaging of a salt flank using walkaway vsp data vsp has long been a tool for investigating the location of salt flanks traditionally a salt proximity survey has been used to estimate the location of salt boundaries relative to a wellbore however this technique depends on accurate knowledge of the shape of the salt and the local sediment velocities in addition this method does not work well with complicated 3d salt structure beyond the traditional salt proximity survey many researchers have developed various methods and strategies for improving the locating and or imaging of salt flanks o brien et al 2002 brandsberg dahl et al 2003 zhao et al 2006 with these methods the quality of the resultant image depends on a prior knowledge of the possibly anisotropic overburden velocity field for very long offsets and also is concerned with multi pathing through salt invalidating a 2d assumption in looking for a simpler approach to this problem a single well imaging concept may be attractive using downhole sources and receivers it is possi

holocene sedimentation in the skagerrak interpreted from chirp sonar and core data high resolution chirp sonar profiling in the northeastern skagerrak shows acoustically stratified sediments draping a rough surfaced substratum a 32 metre long sediment core retrieved from the survey area encompasses the entire holocene and latest pleistocene the uppermost seismo acoustic units in the chirp profiles represent holocene marine sediments the lowermost unit is interpreted as ice proximal glacial marine sediments rapidly deposited during the last deglaciation the end of ice proximal sedimentation is marked by a strong reflector interpreted to have been formed during latest pleistocene time as a consequence of rapid ice retreat and drastically lowered sedimentation rate the subsequent distal glacial marine sediments were deposited with initially high sedimentation rates caused by an isostatic rebound associated sea level fall based on correlation between the core and the chirp sonar profiles using measured sediment physical properties and ams 14 c dating we propose a revised position for the pleistocene holocene boundary in the seismo acoustic stratigraphy of the investigated area copyright 2005 john wiley sons ltd

the mechanics of the south iceland seismic zone the south iceland seismic zone appears to act as a transform fault between offset sections of the mid atlantic ridge however there is no morphological expression of a throughgoing transform between the ridges the anticipated throughgoing east west transform is represented by a series of north south strike slip faults we study the mechanics of the south iceland seismic zone using boundary element methods first we examine the implications of the differences between the geometry of an orthogonal ridge transform ridge system and the geometry of the structures observed in iceland then we consider the role of the north south faults in accommodating transform motion in an east west direction we conclude that the north south faults can accommodate deformation if they are longer or more spatially frequent than mapped the south iceland seismic zone is subject to sequences of large earthquakes every 45 112 years in comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models we confirm that the system of north south faults can act as a transform fault the seismic moment release derived from the earthquake magnitudes is 1 4 times greater than that predicted by our models if our application of the moment magnitude relation is correct the above could imply that the brittle layer extends to 14 km rather than the 10 km assumed in our models other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and very long baseline interferometry vlbi

submarine canyons velocity replacement by wave equation datuming before stack submarine canyons incised into the continental slope interfere with the quality of common midpoint cmp stacked seismic data obtainable from reflectors beneath the sea floor the interference problem is caused by rough topography in conjunction with the contrast between the acoustic velocity of sea water and the velocity of the exposed rock layers geophysicists have long recognized that part of the solution is to replace the traveltimes of raypaths through the water by their traveltimes through an identical thickness of rock however use of wave equation datuming to effect velocity replacement yields an additional correction for the change in raypath direction that occurs in crossing from rock to sea water the wave equation datuming implementation of velocity replacement is more comprehensive and complete the wave equation datuming method requires an accurate sea floor profile as part of the input along with values of replacement velocity it does not require knowledge of geology or velocities at de

fault architecture in the main ethiopian rift and comparison with experimental models implications for rift evolution and nubia somalia kinematics abstract the main ethiopian rift mer offers a complete record of the time space evolution of a continental rift we have characterized the brittle deformation in different rift sectors through the statistical analysis of a new database of faults obtained from the integration between satellite images and digital elevation models and implemented with field controls this analysis has been compared with the results of lithospheric scale analogue models reproducing the kinematical conditions of orthogonal and oblique rifting integration of these approaches suggests substantial differences in fault architecture in the different rift sectors that in turn reflect an along axis variation of the rift development and southward decrease in rift evolution the northernmost mer sector is in a mature stage of incipient continental rupture with deformation localised within the rift floor along discrete tectono magmatic segments and almost inactive boundary faults the central mer sector records a transitional stage in which migration of deformation from boundary faults to faults internal to the rift valley is in an incipient phase the southernmost mer sector is instead in an early continental stage with the largest part of deformation being accommodated by boundary faults and almost absent internal faults the mer thus records along its axis the typical evolution of continental rifting from fault dominated rift morphology in the early stages of extension toward magma dominated extension during break up the extrapolation of modelling results suggests that a variable rift obliquity contributes to the observed along axis variations in rift architecture and evolutionary stage being oblique rifting conditions controlling the mer evolution since its birth in the late miocene in relation to a constant post ca 11 ma n100 e nubia somalia motion

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

identification and definition of primary and reworked tephra in late glacial and holocene marine shelf sediments off north iceland nine tephra layers in marine sediment cores md99 2271 and md99 2275 from the north icelandic shelf spanning the late glacial and the holocene have been investigated to evaluate the effectiveness of methods to detect tephra layers in marine environments to pinpoint the stratigraphic level of the time signal the tephra layers provide and to discriminate between primary and reworked tephra layers in a marine environment these nine tephra layers are the borrobol like tephra vedde ash askja s tephra saksunarvatn ash and hekla 5 hekla 4 hekla 3 hekla 1104 and v1477 tephras the methods used were visual inspection magnetic susceptibility x ray photography mineralogical counts grain size and morphological measurements and microprobe analysis the results demonstrate that grain size measurements and mineralogical counts are the most effective methods to detect tephra layers in this environment revealing all nine tephra layers in question definition of the tephra layers revealed a 2 3 cm diffuse upper boundary in eight of the nine tephra layers and 2 3 cm diffuse lower boundary in two tephra layers using a multi parameter approach the stratigraphic position of a tephra layer was determined where the rate of change of the parameters tested was the greatest compared with background values below the tephra the first attempt to use grain morphology to distinguish between primary and reworked tephra in a marine environment suggests that this method can be effective in verifying whether a tephra layer is primary or reworked morphological measurements and microprobe analyses in combination with other methods can be used to identify primary tephra layers securely the study shows that there is a need to apply a combination of methods to detect define the time signal and discriminate between primary and reworked tephra in marine environments copyright 2011 john wiley sons ltd

neogene rotations and quasicontinuous deformation of the pacific northwest continental margin paleomagnetically determined rotations about vertical axes of 15 to 12 ma flows of the miocene columbia river basalt group of oregon and washington decrease smoothly with distance from the plate margin consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid the average rate of northward translation of the continental margin since 15 ma calculated from the rotations using this model is about 15 mm year which suggests that much of the tangential motion between the juan de fuca and north american plates since middle miocene time has been taken up by deformation of north america the fluid like character of the large scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere

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

postglacial tectonic activity within the skjalfandadjup basin tjornes fracture zone offshore northern iceland based on high resolution seismic stratigraphy abstract multibeam bathymetric and high resolution seismic reflection data chirp have been used to illuminate the structural framework and tectonic evolution of the 300 km2 skjalfandadjup basin one of three large rift basins within the tjornes fracture zone a complex transform zone offshore northern iceland the skjalfandadjup basin together with the nafir seamounts form the nascent nafir skjalfandadjup volcanic system of the oblique grimsey volcanic zone the skjalfandadjup basin is made up of normal faults with maximum displacement of 50 60 m and a dominant strike of n10 20 w correlation of chirp data with tephrochronology from sediment core md99 2275 provided constraints on tectonic movements along individual faults from late glacial time throughout holocene maximum vertical displacement is observed on three listric normal faults marking the eastern boundary of the main basin postglacial tectonic activity commenced as early as 14 15 kyr bp followed by three major rifting episodes prior to 10 kyr bp and three smaller rifting episodes during the last 4 2 kyr the 10 12 kyr period of enhanced tectonic activity was accompanied by eruptions within the tjornes fracture zone isostatic rebound following rapid deglaciation caused rifting with significant vertical displacement on normal faults offshore n iceland

anomalous oceanic lithosphere formed in a leaky transform fault evidence from the western limassol forest complex cyprus detailed studies in the western limassol forest complex cyprus have revealed plutonic and structural histories that differ markedly from the penrose type ophiolite of the troodos massif to the north a tectonized harzburgite of upper mantle origin acts as host to multiple mafic and ultramafic intrusions forming a plutonic core complex mafic and picrite dykes cut and are cut by e w trending shear zones demonstrating syn tectonic magmatism a retrogressive metamorphic and progressive cooling history of the intrusive suite indicates uplift of the host mantle harzburgite while in a sea floor setting it is suggested here that the geological evolution of the western limassol forest complex indicates formation of anomalous oceanic lithosphere in the active domain of an upper cretaceous leaky transform fault

evolution pattern and partitioning of deformation during oblique continental rifting inferences from lithospheric scale centrifuge models oblique rifting is investigated through centrifuge experiments that reproduce extension of a continental lithosphere containing a preexisting weakness zone during extension this weakness localizes deformation and different rift obliquity is obtained by varying its trend with respect to the stretching direction model results show that deformation is mostly controlled by the obliquity angle a defined as the angle between the orthogonal to the rift trend and the extension direction for low obliquity alpha 45 degrees no boundary faults form and the extensional deformation affects the rift depression since early stages of extension dominance of the strike slip motion over extension leads to the development of oblique slip and nearly pure strike slip faults oblique to both the rift trend and the orthogonal to the extension direction with no strain partitioning between the margins and the rift floor these results suggest that oblique reactivation of preexisting weaknesses plays a major role in controlling rift evolution architecture and strain partitioning findings that have a significant relevance for natural oblique rifts

fracture pattern of thjorsardalur central volcano with respect to rift jump and a migrating transform zone in south iceland an analysis of dykes mineral veins and tectonic joints in the matuyama age thjorsardalur volcano provides insights into the relative development of rift and transform zones at slow spreading centres rifting resulted in the formation of nne fractures transform faulting gave rise to shear fractures striking predominantly n s and ene parallel to the main earthquake fractures of the south iceland seismic zone sisz but also wnw e w and nnw all fractures were filled with the same geothermal fluid and magma the strikes motions and fracture cross cuttings show that the transform fault was active early in the history of the volcano coeval with the rift s the shear fractures are consistent with a riedel shear model of an e w transform zone and with a minimum principal stress 3 striking n135 e to n150 e these values are close to the direction of extension across the eastern rift zone erz supporting an interpretation that the e w shear zone has migrated from the latitude of thjorsardalur to the sisz with the southward propagation of the erz migration of synchronous transform and rift systems at a slow spreading centre yields a distinctive fracture pattern but lacks some of the elements found at similar migration sites for fast spreading centres

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

scaling and spatial complementarity of tectonic earthquake swarms abstract tectonic earthquake swarms tes often coincide with aseismic slip and sometimes precede damaging earthquakes in spite of recent progress in understanding the significance and properties of tes at plate boundaries their mechanics and scaling are still largely uncertain here we evaluate several tes that occurred during the past 20 years on a transform plate boundary in north iceland we show that the swarms complement each other spatially with later swarms discouraged from fault segments activated by earlier swarms which suggests efficient strain release and aseismic slip the fault area illuminated by earthquakes during swarms may be more representative of the total moment release than the cumulative moment of the swarm earthquakes we use these findings and other published results from a variety of tectonic settings to discuss general scaling properties for tes the results indicate that the importance of tes in releasing tectonic strain at plate boundaries may have been underestimated

evolution of migrating transform faults in anisotropic oceanic crust examples from iceland major transform fault zones link extensional segments of the north american eurasian plate boundary as it transects the iceland hotspot changes in plate boundary geometry involving ridge jumps

structure and origin of holocene cold events abstract the present interglacial the holocene spans the period of the last 11 700 years it has sustained the growth and development of modern society the millennial scale decreasing solar insolation in the northern hemisphere summer lead to northern hemisphere cooling a southern shift of the intertropical convergence zone itcz and a weakening of the northern hemisphere summer monsoon systems on the multidecadal to multicentury scale periods of more stable and warmer climate were interrupted by several cold relapses at least in the northern hemisphere extra tropical area based on carefully selected 10 000 year long time series of temperature and humidity precipitation as well as reconstructions of glacier advances the spatiotemporal pattern of six cold relapses during the last 10 000 years was analysed and presented in form of a holocene climate atlas hoclat see http www oeschger unibe ch research projects holocene atlas a clear cyclicity was not found and the spatiotemporal variability of temperature and humidity precipitation during the six specific cold events 8200 6300 4700 2700 1550 and 550 years bp was very high different dynamical processes such as meltwater flux into the north atlantic low solar activity explosive volcanic eruptions and fluctuations of the thermohaline circulation likely played a major role in addition internal dynamics in the north atlantic and pacific area including their complex interaction were likely involved

comparison between interferometric migration and reduced time migration of common depth point data one of the difficulties in seeing beneath salt is that the migration velocity in the salt and above it is not well known this can lead to defocusing of migration images beneath the salt in this paper we show that reduced time migration rtm and interferometric migration im can partly mitigate this problem rtm time shifts the traces with the time difference between the calculated arrival time sg ref and the natural arrival time sg ref of a reference reflection where s and g denote the source and receiver locations on the surface respectively we use the terms natural and calculated to represent respectively the arrival times that are velocity independent traveltimes directly extracted from the data without knowledge of the velocity model and velocity dependent traveltimes calculated by ray tracing through a given velocity model the benefit of rtm is a significant reduction of defocusing errors caused by errors in the migration velocity im on the other hand requires extrapolation of

wave equation traveltime inversion this paper presents a new traveltime inversion method based on the wave equation in this new method designated as wave equation traveltime inversion wt seismograms are computed by any full wave forward modeling method we use a finite difference method the velocity model is perturbed until the traveltimes from the synthetic seismograms are best fitted to the observed traveltimes in a least squares sense a gradient optimization method is used and the formula for the frechet derivative perturbation of traveltimes with respect to velocity is derived directly from the wave equation no traveltime picking or ray tracing is necessary and there are no high frequency assumptions about the data body wave diffraction reflection and head wave traveltimes can be incorporated into the inversion in the high frequency limit wt inversion reduces to ray based traveltime tomography it can also be shown that wt inversion is approximately equivalent to full wave inversion when the starting velocity model is

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

the gravity recovery and climate experiment mission overview and early results 1 the grace mission is designed to track changes in the earth s gravity field for a period of five years launched in march 2002 the two grace satellites have collected nearly two years of data a span of data available during the commissioning phase was used to obtain initial gravity models the gravity models developed with this data are more than an order of magnitude better at the long and mid wavelengths than previous models the error estimates indicate a 2 cm accuracy uniformly over the land and ocean regions a consequence of the highly accurate global and homogenous nature of the grace data these early results are a strong affirmation of the grace mission concept

magnetic properties of hematite in lava flows from iceland response to hydrothermal alteration the present study investigates changes in the magnetic properties of hematite more precisely low ti titanohematite in icelandic subaerial basalts during low temperature metamorphism 300 c the results have implications for detection of secondary magnetization directions in basalts and perhaps for seafloor magnetic anomaly modeling samples were collected both from outcrops and from the iceland research drilling project irdp core and represent 3 6 km of crust the natural remanent magnetization nrm unblocked below 590 c during thermal demagnetization was assigned to magnetite and that unblocked above 610 c to hematite unblocking temperatures of hematite gradually increased in the laumontite zeolite zone from the original low range of 610 650 c to 650 680 c we attribute this increase to the migration of a small amount of initial ti out of the hematite lattice titanium migration is consistent with microprobe analyses the intensity of the hematite magnetization correlated with that of magnetite until the top of the laumontite zeolite zone 1 8 km depth below this depth the intensity of the magnetite magnetization decreased and correlation became poor due to alteration of magnetite to nonmagnetic minerals both magnetite and hematite appeared to retain their primary directions of magnetization to the bottom of the laumontite zeolite zone even though ti had migrated out of the lattices if field controlled chemical remanent magnetization is not produced during ti migration then seafloor basalts undergoing the same processes may also faithfully retain primary directions of remanence below 3 1 km depth in the epidote metamorphic zone large secondary components in some samples could not be completely removed by thermal demagnetization to 550 c and were attributable to a thermo viscous remanent magnetization thermo vrm in magnetite the durability of the hematite magnetization suggests that the identification of hematite components in subaerial basalts may help reveal subtle secondary components contained in magnetite directions of subaerial basalts

a new age model for the pliocene pleistocene tjornes section on iceland its implication for the timing of north atlantic pacific palaeoceanographic pathways abstract the plio pleistocene outcrops of the tjornes peninsula in northern iceland present a unique section in which near shore and terrestrial deposits alternate with lava flows this section plays an important role in the correlation of climate signals from around the north atlantic but a solid age model has not been available for the entire sequence due to problematic k ar dating and incompletely recorded palaeomagnetic signal a palynological analysis with dinoflagellate cysts of 68 samples from the tjornes beds and twenty samples from the younger breidavik group was carried out in order to establish an age model for the tjornes section independent of the available k ar ages the dinoflagellate cyst record of the tjornes beds consists mainly of pliocene taxa and indicates most probably a post miocene age for the tapes and mactra zones of the tjornes beds both operculodinium tegillatum and batiacasphaera minuta which have a highest occurrence near the top of the zanclean at c 3 8 ma were present in the section up to the middle of the serripes zone reticulatosphaera actinocoronata with a highest occurrence around 4 4 ma was recorded in situ at the base of the serripes zone this suggests that the entire serripes zone was deposited between c 4 0 and 4 5 ma further the normal polarities below and above the skeifa lavas in the serripes zone were consequently linked to the nunivak and cochiti subchron respectively the entire tjornes beds were likely deposited in the early pliocene before c 4 0 ma the breidavik group is deposited much later in the quaternary from just before the olduvai subchron at c 2 2 ma the gilbert gauss transition is located between the tjornes beds and the hoskuldsvik lavas and the gauss matuyama transition between the furuvik and horgi formations the exact position of both reversals is not preserved because two appreciable hiatuses of c 600 kyr up to maximally 900 kyr occur at these locations according to the new age model the major invasion of pacific molluscs at the base of the serripes zone took place before 3 8 ma in combination with the palaeopolarity data this event can be placed in the nunivak subchron at c 4 5 ma the invasion is likely a result of the northward flow of pacific waters through the bering strait to the north atlantic which may have been caused by the shoaling of the central american seaway between 4 7 and 4 2 ma sarnthein et al 2009

volcanic ash layers from the last glacial termination in the ngrip ice core the tephrochronological record of the 1400 1640 m depth 10 000 16 000 calendar ice core years before present of the ngrip ice core has been established by particle screening of selected samples ash was identified in 20 samples correlation with ice marine and terrestrial records from volcanic source regions in the northern hemisphere positively identifies the saksunarvatn ash and the vedde ash ash zone 1 major element chemistry of the remaining identified ash layers mainly points towards an icelandic origin this tephrochronological record provides new important marker horizons for correlating the timing of the climatic changes associated with the last glacial termination within the north atlantic region as well as outlining more details concerning the frequency and composition of volcanic eruptions occurring at this deglaciation copyright 2005 john wiley sons ltd

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

local migration with extrapolated vsp green s functions we have developed a novel vertical seismic profile vsp imaging method that requires only a local velocity model around the well this method is denoted as a local migration where the model based forward propagation and backward propagation operators are computed using a local velocity model between the well and the target body the velocity model for the complex overburden and salt body is not needed and the source side statics are automatically accounted for in addition kinematic and dynamic effects including anisotropy absorption and other unknown undefined rock effects outside of this local velocity model are mostly accounted for numerical tests on an acoustic 2d sigsbee vsp data set an elastic salt model and offset 2d vsp data in the gulf of mexico partly validate the effectiveness of this method by accurately imaging the sediments next to the vertical well

locking depth and slip rate of the husavik flatey fault north iceland derived from continuous gps data 2006 2010 summary located at the northern shore of iceland the tjornes fracture zone tfz is a 120 km offset in the mid atlantic ridge that connects the offshore kolbeinsey ridge to the on land northern volcanic zone this transform zone is seismically one of the most active areas in iceland exposing the population to a significant risk however the kinematics of the mostly offshore area with its complex tectonics have not been adequately resolved and the seismic potential of the two main transform structures within the tfz the grimsey oblique rift gor and the husavik flatey fault hff in particular is not well known in summer 2006 we expanded the number of continuous gps cgps stations in the area from 4 to 14 the resulting gps velocities after four years of data collection show that the tfz accommodates the full plate motion as it is predicted by the morvel plate motion model in addition envisat interferograms reveal a transient uplift signal at the nearby theistareykir central volcano with a maximum line of sight uplift of 3 cm between summers of 2007 and 2008 we use a combination of an interseismic backslip and a mogi model in a homogeneous elastic half space to describe the kinematics within the tfz with a non linear optimization approach we fit the gps observations and estimate the key model parameters and their uncertainties which are among others the locking depth the partition of the transform motion between the two transform structures within the tfz and the slip rate on the hff we find a shallow locking depth of 6 3 1 7 1 2 km and transform motion that is accommodated 34 3 per cent by the hff and 66 3 per cent by the gor resulting in a slip velocity of 6 6 0 6 mm yr 1 for the hff assuming steady accumulation since the last two large m6 5 earthquakes in 1872 the seismic potential of the fault is equivalent to a mw6 8 0 1 event

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

seismic time lapse imaging using interferometric least squares migration case study seismic time lapse surveys are susceptible to repeatability errors due to varying environmental conditions to mitigate this problem we propose the use of interferometric least squares migration to estimate the migration images for the baseline and monitor surveys here a known reflector is used as the reference reflector for interferometric least squares migration and the data are approximately redatumed to this reference reflector before imaging this virtual redatuming mitigates the repeatability errors in the time lapse migration image results with synthetic and field data show that interferometric least squares migration can sometimes reduce or eliminate artifacts caused by non repeatability in time lapse surveys and provide a high resolution estimate of the time lapse change in the reservoir

influence of rift jump and excess loading on the structural evolution of northern iceland 1 new structural data combined with published structural and geochronological data allow reconstruction of the structural evolution that followed the last rift jump across northern iceland tertiary lava flows erupted along the skagafjordur paleo rift have been down bent under the weight of and in the direction of plio pleistocene lava flows emitted from the northern volcanic zone and the central part of iceland this down bending process involved development of local flexure zones and a flexural extension along the resulting monoclines this structural reorganization explains the existence of the hunafloi skagi synform without need for a paleo rift axis along it in agreement with previous radiometric dating the large amount of plio pleistocene lava flows erupted in central iceland may have been enhanced by ice cap loading

distributed deformation in an oceanic transform system applying statistical tools to structural and paleomagnetic data near the husavik flatey fault northern iceland

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

effects of atmospheric pressure loading and seven parameter transformations on estimates of geocenter motion and station heights from space geodetic observations 1 variations in fluid loads such as the oceans and the atmosphere deform the surface of the earth the accuracy of station coordinates in particular heights that can be estimated depends on how well one can separate these surface deformations from the associated translational motion between the center of mass of the solid earth and the total earth cm we applied simulated atmospheric pressure loading effects to the coordinates of sites in the cm frame to explore to what level of accuracy both geocenter motion and accurate station coordinates can actually be recovered from geodetic analyses we found that standard seven parameter transformations three rotations three translations scale generally recover about 80 of the geocenter motion however the inclusion of a scale factor permits the aliasing of surface loading deformation introducing scale errors of up to 0 3 ppb and daily height errors as large as 4 mm this limits the geophysical studies that can be performed accurately using the results of geodetic analyses where the magnitudes of the signals are small e g tectonic movement of tide gauges uplift rates for interpreting glacial isostatic adjustment the quality of the geodetic results is extremely sensitive to the number and distribution of sites used to estimate the transformations and becomes worse when regional rather than global sets of sites are used if the scale factor parameter is omitted then the amount of aliasing of surface loading effects is reduced considerably and more accurate site velocities and geocenter motion estimates are achieved

dikes minor faults and mineral veins associated with a transform fault in north iceland abstract the tjornes fracture zone is a transform fault connecting the axial rift zone in north iceland with that of the kolbeinsey ridge off the north coast of iceland on the north coast of the flatcyjarskagi peninsula there is a 3 5 km wide zone of intense deformation which we interpret as the main on land part of this transform fault this transform zone strikes n64 w and is characterized by unusually steeply dipping lavas 25 44 wnw extensive secondary mineralization numerous slickensided surfaces normal faults 1 20 m throw dikes that strike parallel to the zone and occasional 3 20 m wide subzones of crushed rocks striking parallel to the transform fault in a 17 km long continuous coastal profile south of the transform zone there is a 30 clockwise change from south to north in the lava strike the dike strike also changes clockwise north along this profile but only by 4 this indicates that the clockwise change in the lava strike is original and not due to subsequent tectonic rotation when the axial rift zones of north iceland and the kolbeinsey ridge are modelled as single offset mode i cracks finiteelement results show that the tjornes fracture zone developed between these mode i cracks in a region of maximum shear stress the underlap between these axial rift zones has apparently decreased during the past several million years the model predicts that as this underlap decreased the angle between the husavik flatey fault and that of the associated axial rift zones increased and the activity of the husavik flatey fault decreased both these predictions are borne out by the available data

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

the utility of statistical analysis in structural geology abstract recent advances in statistical methods for structural geology make it possible to treat nearly all types of structural geology field data these methods provide a way to objectively test hypotheses and to quantify uncertainty and their adoption into standard practice is important for future quantitative analysis in structural geology we outline an approach for structural geologists seeking to incorporate statistics into their workflow using examples of statistical analyses from two locations within the western idaho shear zone in the west mountain location we test the published interpretation that there is a bend in the shear zone at the kilometer scale directional statistics on foliations corroborate this interpretation while orientation statistics on foliation lineation pairs do not this discrepancy leads us to reconsider an assumption made in the earlier work in the orofino location we present results from a full statistical analysis of foliation lineation pairs including data visualization regressions and inference these results agree with thermochronological evidence that suggests that the orofino area comprises two distinct subparallel shear zones the r programming language scripts that were used for both statistical analyses can be downloaded to reproduce the statistical analyses of this paper

continuum calculations of continental deformation in transcurrent environments a thin viscous sheet approximation is used to investigate continental deformation near a strike slip boundary the vertically averaged velocity field is calculated for a medium characterized by a power law rheology with stress exponent n driving stresses include those applied along boundaries of the sheet and those arising from buoyancy forces related to lateral differences in crustal thickness calculations are performed with either velocity or stress prescribed on the transcurrent boundary exact and approximate analytic solutions for a region with a sinusoidal strike slip boundary condition are compared with solutions for more geologically relevant boundary conditions obtained using a finite element technique the across strike length scale of the deformation is approximately 1 4 n 1 2 times the dominant wavelength of the imposed strike slip boundary condition for both the analytic and the numerical solutions this result is consistent with length scales observed in continental regions of large scale transcurrent faulting an approximate linear relationship between displacement and rotation is found that depends only on the deformation length scale and the rheology calculated displacements finite rotations and distribution of crustal thicknesses are consistent with those observed in the region of the pacific north america plate boundary in california

overlapping rift zone segments and the evolution of the south iceland seismic zone the south iceland seismic zone is a 20 60 km wide north south and up to 70 km long zone of north and north northeast trending holocene arrays of en echelon tension fractures these fracture arrays are related to dextral strike slip faults buried by holocene lava flows in this zone major destructive earthquake sequences occur at intervals of 45 112 years the largest events reaching magnitude 7 ms we propose that this seismic zone is located between overlapping rift zone segments spreading centers where the eastern segment has been propagating to the south during the past 3 ma we made a finite element study of this configuration with the segments modeled as mode i cracks loaded in tension the results suggest that the south iceland seismic zone in general and the north and north northeast trending dextral faults in particular develop in response to the shear stresses generated between the rift zone segments

kinematic model of the south icelandic tectonic system summary due to the hot spot ridge interaction the tectonical situation in the southern part of iceland is quite complex within this investigation it is shown that the western volcanic zone is in spite of missing recent volcanic activity still an active spreading zone its annual widening of 8 1 1 0 mm yr 1 represents more than 45 per cent of the spreading of the southern part of iceland the full spreading rate of the investigated region amounted to 17 1 3 0 mm yr 1 these are the results of a constrained linear multiepoch deformation analysis of six gps measurement campaigns which have been carried out between 1986 and 1999 in the southwest of iceland the presented solution combines the motion information of 79 stations to yield a linear velocity field which is a snapshot of the medium term motion the used gps measurement campaigns had changing network geometries with a lack of point identities

mitigation of defocusing by statics and near surface velocity errors by interferometric least squares migration with a reference datum abstractimaging seismic data with an erroneous migration velocity can lead to defocused migration images to mitigate this problem we first choose a reference reflector whose topography is well known from the well logs for example reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms interferometric least squares migration ilsm is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ilsm we have also determined that ilsm can be used for 4d surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next the limitations of ilsm are that it requires prior knowledge of

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors