oceanic plateau formation by seafloor spreading implied by tamu massif magnetic anomalies tamu massif is an immense mesozoic submarine volcano the main edifice of the shatsky rise oceanic plateau it is located at a spreading ridge triple junction but considered to be a shield volcano formed by effusive volcanism from an emerging mantle plume however it is unclear how tamu massif eruptions interacted with the spreading ridges which are enormous linear volcanoes themselves here we create a magnetic anomaly map for tamu massif which can provide clues about crustal formation for tamu massif we find dominantly linear magnetic field anomalies caused by crustal blocks of opposite magnetic polarity this pattern suggests that tamu massif is not a shield volcano but was emplaced by voluminous focused ridge volcanism if the magma source at the shatsky rise was a plume it was closely connected to and controlled by seafloor spreading by implication even the largest oceanic plateau edifices can be formed by seafloor spreading we suggest that the widely accepted analogy between continental flood basalts and oceanic plateaus requires reconsideration the shatsky rise oceanic plateau formed by spreading ridge volcanism according to analyses of linear magnetic anomalies over the tamu massif submarine volcano

glacial rebound and plate spreading results from the first countrywide gps observations in iceland iceland is one of the few places on earth where a divergent plate boundary can be observed on land direct observations of crustal deformation for the whole country are available for the first t

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

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

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

lithosphere age depth and structural complications resulting from migrating transform faults if oceanic transform faults migrate back and forth across a finite width of the lithosphere as a consequence of minor ridge propagations and recessions many structural complications can arise patterns of lithosphere age and depth as well as internal structure may be determined by the frequency and magnitude of such migrations such processes are expected to be most likely at transforms bounded by relatively young thin weak lithosphere due to either short ridge offsets or fast spreading rates

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

origin of a rhyolite that intruded a geothermal well while drilling at the krafla volcano iceland magma fl owed into an exploratory geothermal well at 2 1 km depth being drilled in the krafl a central volcano in iceland creating a unique opportunity to study rhyolite magma in situ in a basaltic environment the quenched magma is a partly vesicular sparsely phyric glass containing 1 8 of dissolved volatiles based on calculated h 2 o co 2 saturation pressures it degassed at a pressure intermediate between hydrostatic and lithostatic and geothermometry indicates that the crystals in the melt formed at 900 c the glass shows no signs of hydrothermal alteration but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle derived magmas indicating that this rhyolite originated by anhydrous mantle derived magma assimilating partially melted hydrothermally altered basalts

edge driven microplate kinematics it is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate s motion relative to the bounding major plates are frequently located close to its margins with those plates close to the tips of propagating rifts we propose a class of edge driven block models to illustrate how slip across the microplate margins block rotation and propagation of rifting may be related to the relative motion of the plates on either side an important feature of these edge driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates according to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time pseudofault geometries and matching rotations of the easter microplate show that for most of its 5 m y history block rotation could be driven by the drag of the nazca and pacific plates on the microplate s edges rather than by a shear flow of mantle underneath

propagating rift model for the v shaped ridges south of iceland 1 we present new marine geophysical data which constrain the seafloor spreading history of the reykjanes ridge near iceland and the origin of its flanking v shaped topographic and gravity ridges contrary to the geometry assumed in pulsing plume models the v shaped ridges are not symmetric about the reykjanes ridge axis and seafloor spreading has not been symmetric about a stable axis thus existing models must at least be modified to include an additional asymmetry producing mechanism the best understood and documented such mechanism is rift propagation one possibility is that plume pulses drive the propagators however rift propagation also produces v shaped wakes with crustal thickness variations suggesting the possibility that a pulsing iceland plume might not be necessary to explain the reykjanes v shaped ridges scarps and troughs

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

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

the morphology of propagating spreading centers new and old recent seabeam investigations over the east pacific rise in the northeast pacific have shown the existence of a propagating spreading center at 18 n the propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault similar orthogonal lineations occur at 16 n 107 30 w on the western side of the trace of a 5 m y old propagating spreading center the amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere analogous to fracture zone topography of similar origin traces of former propagating spreading centers are thus a combination of two sets of landforms those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness

crustal control of ridge segmentation inferred from observations of the reykjanes ridge large amplitude variations in topography and inferred crustal thickness along the axes of mid ocean ridges often referred to as segmentation1 are mainly observed at slow spreading ridges2 4 this observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are low5 6 here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow thermal models predict that thick crust at a slow spreading ridge may be as hot as normal thickness crust along fast spreading ridges we accordingly test our hypothesis at a slow spreading ridge characterized by thick crust the reykjanes ridge topography and gravity data along the reykjanes ridge axis indeed show an absence of segmentation suggesting that the thermal state of the crust rather than any mantle process controls the development of this structure

displacement transfer at thrust terminations the saltville thrust and sinking creek anticline virginia u s a abstract the saltville thrust terminates in the core of the sinking creek anticline changes in the amplitude of the anticline may preserve profile shortening as the thrust displacement decreases towards the fault termination consequently thrust displacement would be transferred into the fold we combine newly published maps and new strain and mesostructural data from the tuscarora sandstone to argue that the sinking creek anticline did perform this kinematic role we propose that the anticline developed as a fault propagation fold that experienced both decollement and anticlinal breakthrough by the saltville thrust thrust displacement was accommodated by the development of the modified fault propagation fold and by transfer to a roof flat from the decollement breakthrough new strain and mesostructural data indicate no fixed pin lines in either the hinge or backlimb throughout folding consequently layer slip occurred through the hinge during folding and the forelimb deformed internally during breakthrough towards the plungeout of the sinking creek anticline the displacement on the saltville thrust is transferred to a floor thrust we apply this interpretation to the termination of the st clair thrust which has a similar surface geometry where the thrust terminates into an anticline a testable implication of this proposed interpretation for the st clair thrust is that the appalachian plateau must absorb over 20 km of thrust displacement as horizontal shortening

the e greenland rifted continental margin an examination of the coastal flexure the structure of the e greenland coastal flexure a major structure developed during continental break up is interpreted on the basis of field observations an earlier model which involves normal inland dipping antithetic faulting and seaward rotation of fault blocks is confirmed and amplified this model requires that regional dips are very much less than dips measured on bedding planes thus leading to considerably reduced thicknesses for the tertiary volcanic succession in e greenland detailed mapping confirms the validity of these thickness estimates and leads to the conclusion that the volcanic extrusion rate was similar to that of present day iceland thus obviating the need for special mechanisms of magma generation comparable structures occur in other regions of continental break up and voluminous basalt discharge and it is suggested that the flexure developed by collapse of the attenuated crust subsequent to the massive extrusion of plateau basalts

magmatic processes at slow spreading ridges implications of the ramesses experiment at 57 45 n on the mid atlantic ridge summary this paper is the first in a series of three this issue which present the results of the ramesses study reykjanes axial melt experiment structural synthesis from electromagnetics and seismics ramesses was an integrated geophysical study which was carefully targeted on a magmatically active axial volcanic ridge avr segment of the reykjanes ridge centred on 57 45aen it consisted of three major components wide angle seismic profiles along and across the avr using ocean bottom seismometers together with coincident seismic reflection profiles controlled source electromagnetic sounding csem and magnetotelluric sounding mt supplementary data sets included swath bathymetry gravity and magnetics analyses of the major components of the experiment show clearly that the sub axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids the avr is underlain by a significant crustal magma body at a depth of 2 5 km below the sea surface the magma body is characterized by low seismic velocities constrained by the wide angle seismic data a seismic reflection from its upper surface and a region of anomalously low electrical resistivity constrained by the csem data it includes a thin ribbon like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone which flanks and underlies the melt lens ramesses is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge the result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion but are short lived rather than steady state features the magma body can exist for only a small proportion of a tectono magmatic cycle which controls crustal accretion and has a period of at least 20 000 years these findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle and of its delivery into the crust beneath slow spreading mid ocean ridges

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

tectonic evolution of ridge axis discontinuities by the meeting linking or self decapitation of neighboring ridge segments recent deep tow sea beam and seamarc ii studies indicate that ridge axis discontinuities along the east pacific rise evolve in several different ways where magmatic pulses along the spreading center happen to meet head on a low point or saddle point occurs along the axial depth profile of the spreading center where the magmatic pulses misalign overlapping spreading centers develop and two distinct evolutionary paths are possible one spreading center tip may cut through to and link with the adjacent en echelon ridge chopping off the opposing ridge tip as proposed by macdonald and fox 1983 in addition either spreading center may cut inside or outside of itself repeatedly decapitating its own ridge tip crack propagation studies show that the crack propagation force g drops significantly when the ratio of crack overlap to crack offset l w exceeds 3 applying this relation to spreading centers we suggest that propagation of the individual spreading center tips may stall when l w is greater than 3 if linkage has not yet occurred then the next magmatic pulse that propagates along the ridge may be deflected away from the path of the existing ridge tip decapitating the ridge tip in a process we call self decapitation subsequent magmatic pulses may be deflected or derailed from the existing path of the ridge because freezing of the axial magma chamber near ridge axis discontinuities creates a core of coherent unfaulted gabbroic rock along the spreading axis which is strong relative to the intensely faulted lithosphere on either side of the frozen magma chamber in addition the local stress field rotates near the discontinuity so that subsequent magma pulses and associated cracking fronts will tend to deflect away from the preexisting path of the ridge for propagating rifts which represent first order changes in plate boundary geometry l w remains 1 5 so that g is maintained near its maximum value and episodes of continued propagation in the same direction are enhanced

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

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

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

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

coal cleat domains and domain boundaries in the allegheny plateau of west virginia regional face cleats cutting pennsylvanian and permian coal seams in the allegheny plateau of west virginia can be divided into domains separated by boundaries that are sharply defined along most of their lengths domains and domain boundaries are established based on cleat trends number of regional cleat sets and relative chronology of cleat set development regional cleats of each set show a common trend and abutting relationships between multiple sets within a domain describe a progressive history of changing stress fields and cleat development boundaries dividing the in situ directions of horizontal principal stress are coincident with cleat domain boundaries suggesting a common and persistent controlling factor uniform trends of face cleats within domains and ab upt changes in cleat signature across domain boundaries can be spatially related to regional basement structures and a depositional hinge line within the coal bearing and underlying mississippian rocks in addition fold structures in coal and underlying sedimentary rocks across the allegheny plateau commonly terminate or change trend abruptly at joint domain boundaries in some cases regions of common fold trends in coal bearing rocks are contained within specific domains face cleat trends commonly differ from joint trends in rocks immediately bounding coal seams however one domain boundary in coal can be traced into mississippian rocks through the unconformity at the base of the pennsylvanian section in this case mississippian and pennsylvanian joint trends differ it follows tha joint domain boundaries can extend downward through rocks of different lithologies as well as coincide with conformable and unconformable stratigraphic boundaries

asymmetric plume ridge interaction around iceland the kolbeinsey ridge iceland seismic experiment we present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the kolbeinsey ridge and tjornes fracture zone and thus quantifies the influence of the iceland hot spot on melt flux at the spreading center north of iceland north of the iceland shelf crustal thickness is relatively constant over 75 km 9 4 0 2 km along the southern portion of the kolbeinsey ridge on the iceland shelf crustal thickness increases from 9 5 0 1 km to 12 1 0 4 km over 90 km gravity inversion indicates a residual crustal gravity anomaly that decreases by about 30 40 mgal toward iceland we infer that the variations in crustal thickness and gravity are accompanied by mantle temperature changes of 40 to 50 c at similar distances from the iceland hot spot crustal thickness along the kolbeinsey ridge is 2 2 5 km less than at the reykjanes ridge consistent with the asymmetry in plume ridge interaction that has been inferred from the axial depth and geochemistry of these ridges average lower crustal velocities are also higher along the kolbeinsey ridge consistent with a lower degree of active upwelling than along the reykjanes ridge topography and crustal thickness patterns at the spreading centers around iceland are consistent with isostatic support for normal crustal and mantle densities however we infer that the lower crust beneath central iceland is considerably denser than that beneath the adjacent ridges crustal thickness and geochemical patterns suggest that deep melting is spatially limited and asymmetric about iceland while shallow melting is enhanced over a broad region this asymmetry may be due to a mantle plume that is tilted from south to north in the upper mantle and preferentially melts deeper enriched material beneath the reykjanes ridge

styles of surface rupture accompanying the june 17 and 21 2000 earthquakes in the south iceland seismic zone two large ms 6 6 earthquakes occurred on june 17 and 21 2000 in the south iceland seismic zone the roughly e w trending zone is undergoing left lateral shear however most known surface rupture has been along north striking right lateral strike slip faults rupture associated with the june 2000 events follows a similar pattern although the two earthquakes had similar magnitude fault plane solutions and overall rupture lengths of 15 to 20 km the pattern of rupture from each was notably different in character the june 17 event ruptured along a series of nne trending left stepping segments giving the fault as a whole an almost due north trend at the largest scale rupture is relatively straight and continuous at the smallest scale rupture style seems to vary with small scale topography and ground texture rupture from the june 21 event is more complex and can be divided into five discrete segments to the north deformation is distributed across two zones of left stepping fractures along which widening and subsidence have occurred the central segment consists of right stepping fractures defining a 2 km long ene trending zone sense of shear is clearly left lateral strike slip the two southernmost segments define an nne trend in several places along the rupture zones of both earthquakes it can be verified that the ruptures occurred along preexisting faults the observed faulting structures are similar to those of earlier earthquakes in south iceland both with regard to style and spatial arrangement however our observations suggest that some of the historical earthquakes may have been larger

along axis structure and crustal construction processes of spreading segments in iceland implications for magmatic rifts magmatic rift systems are composed of discrete spreading segments defined by morphologic structural and volcanic features that vary systematically along strike in iceland structural features mapped in the glaciated and exhumed miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone integrating information from both the active rift zones and ancient crust provides a three dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments crustal exposures in the skagi region of northern iceland reveal significant along strike variations in geologic structure the upper crust at exhumed magmatic centers segment centers is characterized by a variety of intrusive rocks high temperature hydrothermal alteration and geologic evidence for kilometer scale subsidence in contrast the upper crust along segment limbs which extend along strike from magmatic centers is characterized by thick sections of gently dipping lava flows cut by varying proportions of subvertical dikes this structure implies relatively minor upper crustal subsidence and lateral dike intrusion the differing modes of subsidence beneath segment centers and segment limbs require along axis mass redistribution in the underlying upper middle and lower crust during crustal construction this along axis material transport is accomplished through lateral dike intrusion in the upper crust and by along axis flow of magmatic to high temperature solid state gabbroic material in the middle and lower crust these processes inferred from outcrop evidence in skagi are consistent with processes inferred to be important during active rifting in iceland and at analogous magmatic oceanic and continental rifts

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

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

geologic structure of the uppermost oceanic crust created at fast to intermediate rate spreading centers abstract geological investigations of major fault scarps tectonic windows and dsdp odp drill holes provide direct views of the uppermost oceanic crust generated at fast to intermediate rate spreading centers these areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing inward dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of outward dip widespread fracturing faulting and hydrothermal metamorphism accompanied magmatic construction these geological relationships can be interpreted in terms of dramatic asymmetrical subaxial subsidence of upper crustal rock units that diminishes across the very narrow few kilometers wide zone of lava accumulation and dike intrusion at the ridge axis this type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

scismotectonics of the northwest external dinarides abstract the region of the tectonic contact between the alps and the dinarides is studied comparing all the available geological geophysical and seismological data the peculiar crustal structures in particular the difference in thickness of the alps dinarides and pannonian basin can be correlated to the seismically active zones the highest seismicity is related to the adria microplate underthrusting the southern alps beneath the friulian piedmont arc this movement produces a composite lateral slip and an underthrust which is subparallel to the dinarides along the yugoslav coast the seismicity in western yugoslavia is more diffuse than in friuli although it is possible to pick out three narrow parallel bands with dextral shear subparallel to the periadriatic vardar fault system which separate the apulia from the pannonian area the interaction of this system with the sinistral strike slip faults of the western carpathians belt enhances the seismicity of the easternmost sector of the alps analogies and differences in the seismotectonic behaviour between central friuli and western yugoslavia are studied and a unique model with a vertical mechanical disengagement at the rear of the compressive fronts of the southern alps and external dinarides is proposed

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

temperature of the icelandic crust inferred from electrical conductivity temperature surface gradient and maximum depth of earthquakes abstract two different models of the structure of the icelandic crust have been presented one is the thin crust model with a 10 15 km thick crust beneath the axial rift zones with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up domed asthenosphere the thick crust model assumes a 40 km thick and relatively cold crust beneath central iceland the most important and crucial parameter to distinguish between these different models is the temperature distribution with depth three methods are used to estimate the temperature distribution with depth first the surface temperature gradient measured in shallow wells drilled outside geothermal areas second the thickness of the seismogenic zone which is associated with a 750 c isothermal surface third the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 c at the base of the crust combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust these results are strongly in favour of the thin crust model the scattered deep seismic reflectors interpreted as moho in the thick crust model could be caused by phase transitions or reflections from melt pockets in the mantle

a hydrological approach to estimating daily glacier ablation reduced meltwater discharge owing to glacier retreat can have severe impacts on downstream water users to assess these impacts it is essential to differentiate between water from glacier melt and other sources we propose a method for doing this on a daily time scale by applying a mixing law to electrical conductivity and proglacial discharge measurements daily ablation is then estimated by applying a recession law to the glacier melt component testing this method on summer hydrology and meteorology measurements from the baounet glacier france taken over six consecutive years 2008 2013 allowed us to reconstruct daily ablation during the ablation period mean ablation rates ranged from 20 to 30 mm day 1 air temperature measurements showed that periods of low ablation during the summer were linked to cooler days and snowfall periods comparisons for three consecutive summers showed that the ablation rates obtained by summing calculated daily ablation were statistically similar to the rates recorded

strike slip faulting during the 2014 barðarbunga holuhraun dike intrusion central iceland over a 13 day period magma propagated laterally from the subglacial bar arbunga volcano in the northern rift zone iceland it created 30 000 earthquakes at 5 7 km depth along a 48 km path before erupting on 29 august 2014 the seismicity which tracked the dike propagation advanced in short bursts at 0 3 4 7 km h separated by pauses of up to 81 h during each surge forward seismicity behind the dike tip dropped moment tensor solutions from the leading edge show exclusively left lateral strike slip faulting subparallel to the advancing dike tip releasing accumulated strain deficit in the brittle layer of the rift zone behind the leading edge both left and right lateral strike slip earthquakes are observed the lack of non double couple earthquakes implies that the dike opening was aseismic

volcano deformation at active plate boundaries deep magma accumulation at hekla volcano and plate boundary deformation in south iceland volcano deformation at active plate boundaries deep magma accumulation at hekla volcano and plate boundary deformation in south iceland

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

current plate movements across the mid atlantic ridge determined from 5 years of continuous gps measurements in iceland 1 we analyze data spanning up to 5 years from 18 continuous gps stations in iceland computing daily positions of the stations with three different high level geodetic processing software packages we observe large scale crustal deformation due to plate spreading across iceland the observed plate divergence between the north american and the eurasian plates is in general agreement with existing models of plate motion spreading is taken up within a 100 150 km wide plate boundary zone that runs through the island of the two parallel branches of the plate boundary in south iceland the eastern volcanic zone is currently taking up the majority of the spreading and little is left for the western volcanic zone the plate boundary deformation field has been locally and temporarily affected in south iceland by two mw 6 5 earthquakes in june 2000 inflation at katla volcano during 2000 to 2004 and an eruption of hekla volcano in february 2000 all stations with significant vertical velocities are moving up relative to the reference station reyk with the highest velocity exceeding 20 mm yr in the center of the island

a survey of multi stage diagenesis and dolomitization of jurassic limestones along a regional shelf to basin transect in the ziz valley central high atlas mountains morocco abstract petrographic and geochemical data from five localities in the ziz valley of morocco indicate that jurassic limestones have undergone early diagenesis that varied with location from shelf to basinal settings burial diagenesis that was most pronounced in basinal settings and late diagenesis caused by compression and uplift of the high atlas mountains marine cements occur at all five localities from shelf to basin center although cement types vary from peloidal microcrystalline cements updip on the shelf to equant calcite in basinal settings presence of moldic grains and or mg poor fe poor blocky cements suggest that meteoric waters influenced early diagenesis at all shelf localities and on an upturned fault block in the basinal region leaving only one locality unaffected by early meteoric processes 87 sr 86 sr ratios of 0 70810 0 70895 greater than 87 sr 86 sr of coeval limestones mg contents that decrease upward from 47 5 to 43 0 mol mgco 3 presence of dolomitized marine cements and dolomite cements that postdate marine cements but predate meteoric to burial cements suggest that dolomitization and dolomite cementation at two shelf localities took place in mixed meteoric and marine waters early in diagenesis however poorer preservation of depositional fabrics lower 18 o values and larger and more anhedral crystals suggest that dolostones downdip underwent later modification during burial whereas those updip did not compaction during diagenesis generated numerous concavo convex and sutured intergranular contacts at updip shelf downdip shelf and basinal localities where earlier meteoric cementation was not extensive compaction was insignificant in more extensively cemented mid shelf settings high sr 1200 3800 ppm and fe 1000 2300 ppm contents in brachiopod grains suggest that lmc components underwent some modification during burial in basinal settings in sr rich reducing waters fe contents of late intergranular cements increase from 2000 ppm at the basin s edge to as much as 6000 ppm in the basin s center bedding parallel stylolites occur at all localities the most negative 18 o values of sparry dolomites near the tizi n firest fault 6 2 vs pdb imply diagenetic temperatures of 65 85 c assuming water 18 o values of 0 0 2 0 vs smow those temperatures are much less than previous estimates of burial temperatures in the high atlas basin an isotopic gradient extrapolating to roughly 5 km in diagenetically modified dolostones likewise suggests a geothermal gradient less than gradients previously proposed for at least parts of the area comparison of morphologies of transverse stylolites which are found at all localities with morphologies of bedding parallel stylolites suggests that transverse stylolites formed due to compression during late diagenesis uplift accompanying that compression allowed influx of low mg waters that along with other factors caused calcitization of dolomites the fe concentration of calcite that fills late fractures increases from less than 2000 ppm at the basin center to values in excess of 3000 ppm at the basin edge opposite trends in earlier cements and reflecting uplift of the high atlas mountains and resultant changes in patterns of groundwater flow

structural chronology of the alleghanian orogeny in southeastern west virginia the trend change between central and southern appalachian structures is sharply defined in southeastern west virginia there n30 35 e trends coincident with major central appalachian folds such as the browns mountain anticline end abruptly at n60 e trending southern appalachian structures along the st clair fault analysis of local and regional folds cleavage bedding perpendicular stylolite seams and bedding and fault slickenlines reveals that layer parallel shortening directed n10 30 w occurred in nonfolded greenbrier group mississippian carbonates well into the present appalachian plateau area this structural event is early and is associated with the evolution of southern appalachian folds and faults south of the st clair fault central appalachian folds and mesoscopic structures were superimposed on this early layer parallel shortening fabric this structural chronology indicates that southern appalachian folds and faults predated the development of central appalachian structures in the region

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

kinematic models of plate boundary deformation in southwest iceland derived from gps observations 1 we use data from gps campaign and continuous measurements from 1992 to 2004 in sw iceland to map the surface velocity field from the reykjanes peninsula to the eastern volcanic zone we divide the time series into preseismic july 1992 to june 2000 and postseismic june 2000 to may 2004 time intervals and we estimate gps station velocities for each interval as well as coseismic offsets due to the june 2000 earthquake sequence in the south iceland seismic zone sisz in addition to the plate spreading the preseismic velocity field shows the effects of inflation at hengill and hekla volcanoes whereas the postseismic velocities show deformation following the june 2000 earthquakes we consider several kinematic models to explain the preseismic velocities our preferred model includes several dislocations and point sources in an elastic half space with left lateral slip along the plate boundary on the reykjanes peninsula and below the sisz opening across the reykjanes peninsula and the western and eastern rift zones the optimal model has a locking depth of about 8 km in the central and eastern part of the reykjanes peninsula with a deep slip rate of about 17 mm yr and an opening of about 9 mm yr this locking depth is in agreement with the thickness of the seismogenic crust on the reykjanes peninsula which appears to vary between 7 and 9 km for the sisz we obtain a deep slip rate of about 19 mm yr below 16 km depth which is considerably deeper than the earthquake hypocenter depths in the area

fringe cracks key structures for the interpretation of the progressive alleghanian deformation of the appalachian plateau vertical joints in devonian clastic sedimentary rocks of the finger lakes area of new york state are ornamented with arrays of fringe cracks that reveal the complex deformational history of the appalachian plateau detachment sheet during the alleghanian orogeny three types of fringe cracks were mapped gradual twist hackles abrupt twist hackles and kinks gradual twist hackles are curviplanar en echelon fringe cracks that propagate with an overall vertical direction within the bed hosting the parent crack and are found in all clastic lithologies of the detachment sheet abrupt twist hackles propagate as planar features in thick shale beds above or below the siltstone beds hosting parent joints kinks propagate horizontally as planar surfaces from the tips of parent joints in siltstone beds the breakdown of the parent joint into either gradual or abrupt twist hackles depends on the orientation and magnitude of the remote stress field internal fluid pressure and the elastic properties of the bed the twist angle of gradual twist hackles is larger in coarser clastic beds indicating that stress and internal pressure are more important parameters than elastic properties in controlling breakdown assuming that the vertical stress axis s v equals 78 mpa at 3 km burial depth the difference in twist angle between sandstone and shale beds is used to estimate the maximum horizontal stress difference in the shale beds as s h s h 2 5 mpa when s h s h 12 mpa in sandstone beds the twist angle of the fringe cracks and the abutting relationships of parent joints give an indication of the overall change in stress field orientation within the detachment sheet during alleghanian tectonics these parent joints indicate a regional clockwise stress rotation of alleghanian age concordant with the twist angle of fringe cracks throughout the western part of the study area a counterclockwise twist angle in the eastern portion indicates a local stress attributed to drag where no salt was available to detach the eastern edge of the plateau sheet the clockwise change in stress orientation is consistent with the rotation in stress orientation found in the anthracite belt of the pennsylvania valley and ridge but is opposite to the sense of rotation in the southwestern portion of the detachment sheet western pennsylvania and west virginia the two regional rotation domains are separated by the juniata culmination

strike slip faulting normal faulting and lateral dike injections along a single fault field example of the gljufura fault near a tertiary oblique rift transform zone borgarfjorður west iceland 1 the crustal block of borgarfjor ur in western iceland was caught between the propagating reykjanes langjokull rift zone rlrz and the receding snaefellsnes rift zone srz during a tertiary rift jump at 7 ma this tertiary rift jump configuration presents an analogy with the presently unstable plate boundary in south and southwest iceland field examination of the gljufura fault in the tertiary lavas of borgarfjor ur reveals a complicated history of movements and magmatic activity during its evolution since tertiary time this n s fault acted as follows 1 the fault acted as a possible dextral strike slip fault as judged from riedel fractures in the oldest associated fault breccia 2 then the fault acted as a normal fault resulting in vertical displacement new brecciation of part of the oldest breccia hydrothermal activity and mineralization dominantly in n s mode i veins 3 the fault acted again as a dextral strike slip fault when the fault was injected laterally by dikes magma was injected mainly into the n s strike slip fault and secondarily into adjacent nne normal faults evidence of this stage are the en echelon geometry of the northerly dikes cooling cracks oblique to dike edges flow lineation elongated vesicles and soft striations on dike edges as well as bending and displacement of preexisting dikes 4 after the emplacement and partial erosion of the northerly dikes the fault acted dominantly as a strike slip fault with possible reactivation as an open fissure the gljufura fault strikes obliquely to the nne trending rifting structures active during crustal formation in this area its activity was initially related to a transform zone or an oblique rift connected to the srz then a shift in the plate boundary occurred around 7 6 ma when the rlrz became active during this shift the gljufura fault probably played a role similar to the n s strike slip faults of the presently active plate boundary within the south iceland seismic zone and the oblique rift of the reykjanes peninsula these faults may similar to the gljufura fault change mode and be injected by dikes if they are sufficiently close to a magma source

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

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