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

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

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

nontransform offsets of the pacific cocos plate boundary and their traces on the rise flank a multi beam bathymetric reconnaissance of the fast spreading crest of the east pacific rise between 7 n and 2 6 n found the axial ridge to be interrupted by four 4 13 km non transform offsets they have probably persisted since initiation by a small change in spreading direction 3 5 ma at each offset the volcanic rift zones veer 15 toward each other and overlap for 5 25 km around deep basins intervening lateral displacements of the rift zones are too short 2 5 km to break the linear axial volcanoes but cause local bends in their plan shapes some examples have a morphology that mimics the larger intervolcano offsets interpretation of a thorough bathymetric and magnetic survey around the 9 km 5 5 n offset indicates that the mainly one sided spreading from the overlapped sections of rift zone occurs by a combination of highly asymmetric dike accretion and episodic small scale jumps of the eruptive zone the northern rift zone is elongating propagating at the expense of the southern one the resulting along axis migration of this and other examples at speeds comparable to spreading rates causes the rise flank scars of the nontransform offsets equivalent to the fracture zones of transform faults to be oblique to the spreading direction these traces are recognizable within the highly lineated fault block terrain of the rise flanks because of the distinctive morphology of the 5 25 km wide bands that accreted at overlapped sections of the rift zones here lineation is less marked and oblique to the spreading direction and volcanic relief is more important than relief caused by faulting the west flank trace of the 5 5 n offset is occupied by a line of volcanoes and shallow volcanic ridges several other volcanic chains previously identified as hot spot traces seem to have grown in similar tectonic settings

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

geochemical attributes pore structures and fractal characteristics of barakar shale deposits of mand raigarh basin india abstract pore structures in the shale matrix are an essential factor affecting the storage capacity of gas of shale beds as well as production performance twenty organic rich shale samples at different depths were collected from barakar formation of mand raigarh basin to examine the pore structure and their fractal characteristic fractal dimension was calculated by following fhh theory to investigate the complexity of pore surfaces storage mechanism and the pore network system the complex pore system influenced by thermal cracking of organic matter pore dissolution during geochemical weathering and tectonics of the basins having surface area 5 56 23 94 m2 g avg 14 21 m2 g whereas the pore volume determined using the bjh model varying from 0 035 to 0 076 cc g avg 0 053 cc g ftir spectrum demonstrated that significant presence of aliphatic side chains and aromatic carbon structures existing in studied shale influencing the pore volume it is summarized that pores and matrix of shale are built by complex mixtures of organic and inorganic content further the pores are categorized into organic pores evolved during thermal cracking inter granular intra granular and inter crystalline due to the large content of altered clays and minerals showing fissile nature the sem edx derived facies indicating the involvement of different chemical constituents following the trend of alteration as well as carbon enrichment the d1 and d2 values obtained through fhh model range from 1 17 to 2 45 and 2 54 2 70 the fractal values particularly d2 are 3 demonstrating the pore surfaces and structure of studied shales beds are complex and heterogeneous this study helps to enhance the exploration and advancement in the shale gas resources from ib river of mand raigarh basin india

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

numerical modeling of propagation and overlap of thrust faults with application to the thrust fold belt of central alberta abstract we use two computer programs to analyze the propagation of multiple thrust faults and their influence on the geometry of a thrust belt they use algorithms based on current equations of fault propagation to generate graphical simulations the simulations are used to demonstrate a model of thrust propagation and thrust belt development that fits current knowledge of fault propagation an alternative to the thrust transfer zone model is proposed called thrust overlap zones the computer simulations provide useful information about the generation and behavior of multiple faults the number of faults the positions of their nucleation points and hence their density the rock properties and rate of fault propagation govern the uniformity of the shortening along the thrust belt thus shortening of a thrust belt can be distributed more evenly by means of propagation of a large number of small faults than by a few large faults the final geometry of a large thrust fault on a geological map provides few clues about its origin i e whether it was produced by the coalescence of a series of en echelon thrust faults or from only a single fault a simple kinematic model based on map length and position of known thrust faults imitates the curved fault map patterns of a segment of the thrust fold belt of central alberta

constraints on archaean crustal composition and structure provided by deep seismic sounding in the yilgarn block abstract new seismic reflection data from the eastern goldfields and southern cross provinces coupled with results from earlier seismic refraction profiles recorded in the western southern cross province indicate that with the exception of the greenstone belts that occupy much of the surface of the eastern goldfields province the crust in the yilgarn block is composed predominantly of felsic material with only minor mafic material within the lowest part of the crust the greenstone belts themselves are shallow features and not deep rooted features as often proposed the transition from the crust to the mantle is sharp occurs over only a few kilometres and marks a significant change in composition and density the middle and lower crust along the section have no marked lateral change in reflection character between the greenstones terranes and the adjacent granite dominated terranes except that both the mid crustal and crust mantle boundaries have been bowed downwards by approximately 6 km under the greenstones the greenstone belts of the eastern goldfields province are 6 9 km thick they are superimposed on an otherwise uniform crust their base is planar and subhorizontal and appears to be block faulted in places the ida fault which defines the western boundary of the greenstone belts is planar dips to the east at 25 30 and can be traced to depths of approximately 25 km 8 s

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

an examination of along axis variation of magma chamber width and crustal structure on the east pacific rise between 13 30 n and 12 20 n we investigate the along axis variations of magma chamber width and crustal structure along the east pacific rise epr from 13 30 n to 12 20 n through reprocessed common depth point cdp reflection profiles the magma lens is predominantly a continuous feature in the study area with an average width of 500 m as determined from migrated cross axis cdp profiles this value is similar to widths estimated elsewhere along the epr suggesting that the axial magma chamber amc width is not spreading rate dependent once the threshold for a steady state magma chamber is reached the axial morphology of the 13 n area is generally not a good predictor of magma lens width or continuity a fairly continuous melt lens is imaged where the triangular axial topography might suggest waning magma supply in fact between 13 05 n and 13 01 n a shallow melt lens has been imaged which may be indicative of recent or impending eruptive activity this shoaling is similar to that observed near the 17 26 s region of the epr where the rise axis summit is domed and highly inflated generally the thickness of seismic layer 2a beneath the ridge crest is uniform and comparable to that estimated for 9 n 14 s and 17 s on the epr suggesting that the axial extrusive layer is invariant along fast spreading ridges uniformity of layer 2a thickness along axis implies that variations in magma chamber depth are directly attributed to changes in thickness of the sheeted dike complex seismic layer 2b contrary to expectations of decreasing melt sill depth with increasing spreading rate the average thickness of seismic layer 2b is slightly less 165 m at 13 n than at the faster spreading more robust 9 n area finally geochemical petrologic boundaries which may delineate separate melt supply regions occurring at the 13 20 n and 12 46 n devals deviation in axial linearity are observed to coincide with subtle changes in amc and layer 2a reflection characteristics

maximum link distance between strike slip faults observations and constraints the maximum fault link distance lateral separation between two interacting strike slip faults is studied in relation to fault length combined length of the two interacting faults data collected for laboratory generated strike slip faults indicate that fault linkage takes place when the lateral separation between two strike slip faults is less than one tenth of the combined length of the two faults strike slip faults developed in clay models require a smaller link distance than those developed in gouge materials about 98 of the data collected for natural strike slip faults and 93 of earthquake rupture data derived from knuepfer 1989 follow the rule these observations support a simple scaling relationship for strike slip faults i e w 0 1 l where wis lateral link distance and l the combined fault length an and sammis 1996a two possible explanations are discussed assuming a fault continues to be dominantly strike slip after various fault coalescences the fault link distance for both compressional coalescence leading to restraining bend and extensional coalescence leading to releasing bend can be constrained within 0 134 l assuming fault coalescence is caused by interaction between breakdown zones at fault tips then the scaling relationship b 0 1 l between the size of a breakdown zone b and the individual fault length l scholz et al 1993 leads to the scaling relationship w max 0 1 l where w max is the maximum link distance the above observed relationship between w max and l may not hold if faults coalesce through preexisting faults if a fault has a mixed mode other than strike slip or if a fault bend is caused by cross faulting rather than fault coalescence limited exposure of natural fault traces may also lead to errors in link distance measurements

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

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

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

ocean ridge discontinuities in iceland abstract the tjornes fracture zone in north iceland and the south iceland seismic zone are ocean ridge discontinuities that are partly exposed on land these are zones of high seismicity but contrasting style of crustal deformation detailed field studies and numerical modelling suggest that the tjornes fracture zone has all the characteristics of an oceanic transform fault is 7 9 ma old and developed along the zone of maximum shear stress between the nearby tips of the rift zones of the kolbeinsey ridge and north iceland conversely the results indicate that the south iceland seismic zone developed within an area of shear stress concentration between the overlapping rift zones in south iceland the development of the south iceland seismic zone is a consequence of the propagation of the eastern rift zone to the southwest during the past 3 ma

extensional hard linkages eastern gulf of suez egypt the araba abu durba area on the eastern margin of the gulf of suez exhibits two superb outcrop examples of extensional hard linkages in a rift basin here three large domino style basement cored northeast dipping fault blocks are formed by a series of major northwest trending normal faults these are offset by two north northeast trending sinistral oblique slip transfer faults that terminate in horsetail normal fault splays the transfer faults do not extend across the entire rift basin detailed mapping and structural analysis show that they developed by breakage of initial low strain relay ramps along reactivated north northeast trending basement fabrics between overlapping northwest trending normal fault segments paleostrain analysis of fault slip indicators shows that both the normal and the sinistral oblique slip transfer faults were formed synchronously in response to northeast southwest extension perpendicular to the main northwest rift trend

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

sand box modelling of basement controlled transfer zones in extensional domains the interaction modalities of transfer zones connecting rift segments may be influenced by several factors amongst these the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies sand box models were made to investigate how the orientation geometry and kinematics of transfer zones depend upon pre existing basement anisotropies analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults strike slip faults are present inside the transfer zone provided that angle between the extension direction and the axis of the transfer zone is less than 50 narrower transfer zones striking oblique to the extension direction occur for angle between the direction perpendicular to the extension direction and the direction of the basement anisotropy less than 90 wider transfer zones subparallel to the extensional direction occur for 90 increasing the overstep induces narrower transfer zones striking subparallel to the extension direction similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts the comparison between experimental and natural data shows how the geometry kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies transfer zones along inherited basement structures set at lower angles to the extension direction and display lower aspect ratios than transfer zones where inherited anisotropies are absent

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

extensional transform zones and oblique spreading centers extensional transform zones etzs are plate boundary segments of order 100 km long that strike at angles between 15 and 45 to the extension direction they are characterized by neovolcanic tectonic zones comprising overlapping en echelon volcanic systems and or faults that trend 30 75 to the extension direction sometimes accompanied by a riedel shear below these surficial en echelon structures the deformation is aseismic and ductile and the plate boundary is probably continuous etzs occur in fast and slow spreading and rifting environments and may persist in a stable configuration for several million years etzs link oblique spreading segments to transform faults in the manus and probably the lau backarc basins the reykjanes peninsula and tjornes fracture zone in iceland and the mak arrasou in afar are etzs that link subaerial to submarine spreading or rifting segments the brawley and cerro prieto seismic zones appear to be etzs in the imperial and mexicali valleys that link the san andreas imperial and cerro prieto transform faults experimental and analytical models of transtensional deformation in brittle ductile systems match many of the observed characteristics of etzs and oblique spreading centers including variably sigmoidal to straight en echelon faults that are not parallel to the extension direction the contrasting fault patterns reflect the rheology of the models and lithosphere they are more sigmoidal when the strain in the lower ductile layer is more focused causing the axial faults to curve as they propagate toward parallelism with the less ductile rift margins the angle o between the faults and the extension direction decreases with the angle between the strike of the zone and the extension direction etzs occur in the range 15 45 whereas oblique spreading centers have 45 90 and transform relay zones have 0 15 oblique fast spreading segments exhibit ridge parallel faults and volcanic systems o presumably reflecting locally rotated stress fields whereas at oblique slow spreading centers o is closer to orthogonal o 90

changes in fault displacement populations correlated to linkage between faults in carboniferous strata exposed at the western margin of the southern appalachian u s a fold thrust belt mesoscopic faults accommodated strains as the cumberland plateau thrust sheet moved 1 km wnw on a sub horizontal detachment these faults are confined to a layer 75 m thick above the detachment in the upper part of this 75 m thick layer a laterally persistent coal seam separates two stratigraphic units subjected to different amounts of sub horizontal elongation distinct sets of mesoscopic faults that cut bedding at high angles and have normal offsets accommodate the strains in each unit below the coal seam a sample line 500 m long intersects 504 faults a plot of log n size rank vs log d dip separation in mm for these faults is linear suggesting that a scaling law n ad c holds for this population least squares regression gives c 0 8 and log10 a 3 1 above the coal seam a sample line 250 m long intersects 745 faults a plot of log n vs log d for these faults has two distinct linear trends faults with separations 1 25 m yield a scaling law n ad c with c 0 5 and log10 a 2 9 seventeen faults with separations d 1 25 m define a second linear segment n a d c with c 1 8 and log10 a 6 9 the break in slope at d 1 25 m corresponds to faults with heights of 30 m roughly equal to the thickness of the faulted strata faults here with d 1 25 m and d 1 25 m which exhibit different scaling characteristics are respectively analogous to faults within and those that extend across the brittle crust in regional studies comparing the two displacement populations described here small offset faults make a proportionally smaller contribution to the total strain in the larger strain setting this pattern which is observed elsewhere suggests that fault systems within individual layers exhibit different scaling characteristics at different stages in their history this contribution argues that fault linkage is the key to such changes in fault scaling behavior it is a mesoscopic to macroscopic structural change that alters the subsequent development of faults fault linkage like fault size or sampling dimension should be considered in examinations of fault scaling laws

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

overlapping rift zones at the 5 5 s offset of the east pacific rise a seabeam and magnetometer survey of the pacific nazca plate boundary around 5 5 s mapped a nontransform offset whose geology and kinematics seem typical of a whole class of structures formed where fast spreading rises are laterally offset for distances less than the width of a subaxial magma chamber though the spreading center is abruptly displaced by 15 km right laterally there is no trace of strike slip faulting instead the ends of two 100 km long axial volcanic ridges veer 15 toward each other and overlap for more than 20 km in this overlapping region between 5 22 s and 5 32 s accretion of the upper oceanic crust by dike injection and eruption is partitioned between two adjacent and parallel rift zones with oblique and one sided spreading a continuous but dog legged magma chamber is inferred to underlie both rift zones and an intervening 9 km wide volcano studded basin a strong magnetic signature from the crust which accreted around this oblique chamber identifies the trace of the offset on the adjacent rise flank in the past 0 5 106 years the southern axial ridge has propagated north at 40 mm yr 55 of the spreading half rate

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

lithospheric stresses associated with nontransform offsets of the mid atlantic ridge implications from a finite element analysis nontransform offsets are a fundamental aspect of the offset geometry exhibited along the mid oceanic ridge system independent of spreading rate along the slow intermediate opening 40 mm y full rate mid atlantic ridge these offsets of the ridge axis range in length from less than 10 km to approximately 30 km and vary in age offset from 0 5 to 2 0 m y the variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike slip tectonism and that the geometries are unstable in time in many cases there appears to be an evolutionary relationship between transform fault boundaries and nontransform offsets as the result of prolonged differential asymmetric spreading between adjoining ridge segments the finite element method is used to study the complex stress field associated with these small offset discontinuities of ridges with slow 30 mm y and fast 100 mm y total opening rates a plane stress plate model examines the variation in the horizontal tectonic stress field produced by offsets with different lengths and changes in the ratio of a ridge normal tensile stress resisting plate separation to a shear stress resisting relative plate motion along the discontinuity the predicted fault patterns based on the calculated stress field are compared with seafloor observations in terms of the morphotectonic patterns and evolution of nontransform offsets for a slow spreading rate the analysis shows that all structural geometries observed can be modeled by a range of offset lengths 5 10 20 30 and 40 km and by a ridge normal stress 3 to 5 times greater than the discontinuity shear stress these findings suggest that nontransform offsets are zones of mechanical weakness relative to the surrounding lithosphere an offset length between 10 and 20 km is predicted to be the threshold length for maintaining a transform fault geometry as inferred from ridge axis morphology there seems to be a strong link between the magnitude of the stress ratio and the time varying magmatic activity along and between ridge segments while our models are consistent with a weak discontinuity shear stress relative to the ridge normal stress to explain the geometries of nontransform offsets of slow spreading centers a weaker ridge normal stress to discontinuity shear stress most closely models the development of an overlapping spreading center geometry the distinctive geometry of nontransform offsets of spreading centers opening at fast rates this difference is attributed to magma supply along axis relatively continuous for fast spreading centers and intermittent for slow spreading centers and a preexisting zone of mechanical weakness linked to the evolution of nontransform offsets from transform faults on slow spreading centers

rift segment interaction and its relation to hydrocarbon exploration in continental rift systems 1 in both the early stages of continental rifting and the onset of sea floor spreading in the more advanced rifting longitudinally continuous rifting is accomplished by the propagation and linkage of isolated pockets of extension in the early continental rift phase these isolated pockets of deformation are defined as rift segments rift segments initially are distributed in a loose association of in line and en echelon geometry their propagation interaction and linkage into a continuous rift system creates a variety of rift geometries including rift splays gaps offsets passes and large scale transfer zones an understanding of these geometries and their associated fault patterns is critical for effective hydrocarbon exploration in these zones

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

development of fault discontinuities in shear experiments abstract shear experiments using clay and fault gouge cakes were performed to study the origin of discontinuities along strike slip faults simple continuous strike slip faults initially nucleated in the samples and they were separated from each other discontinuities began to develop when these faults interacted each other two processes were especially important for the development of fault discontinuities cross faulting which caused segmentation and offsetting of originally continuous faults and fault interaction which changed fault propagation paths created various types of bends abandoned some old fault segments and caused coplanar faults to avoid each other cross faulting created type i fault discontinuities in which all fault segments in a fault stepped in one direction either all left or all right and fault interaction created type ii discontinuities in which fault segments in a fault stepped in both directions the experimental results are compared with some natural fault patterns to demonstrate that this type of experiments can be used to investigate the origin of discontinuities along natural strike slip faults

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

are large differential stresses required for straight fracture propagation paths abstract fracture growth geometries in linear elastic materials are sensitive to the ratio between the remote differential stress or t stress and the fracture driving stress two mechanically interacting fractures follow straighter paths for greater values of this ratio we demonstrate that the degree of curvature that develops between two mechanically interacting fractures is also controlled by such factors as sub critical fracture growth and fracture surface roughness sub critical fracture growth limits the development of fracture curvature by decreasing the fracture driving stress needed for propagation fracture surface roughness can affect the development of fracture curvature by limiting the magnitude of the shear displacement discontinuity that can be accommodated along the fracture thus propagation paths for two mechanically interacting fractures can be quite straight even in the presence of small to moderate differential stresses consequently the absence of fracture curvature does not necessarily imply a high differential stress state

slip distributions on intersecting normal faults abstract slip distributions on normal faults often are asymmetric and display multiple local maxima numerically computed slip distributions from elastic models indicate that such irregular slip distributions can be caused by mechanical interaction between intersecting faults that produce local perturbations of the stress field resolved on the faults a three dimensional boundary element model based on the displacement discontinuity method has been used to analyze the mechanical interaction of faults that form y or t shaped intersections slip distributions are asymmetric with a steeper slip gradient toward the line of intersection multiple slip maxima occur depending on the angle between the faults but generally they are not located along the intersections nor at the fault centers examples of intersecting normal faults taken from oil reservoir seismic surveys and sandbox experiments have been used to confirm the application of the theoretical results despite considerable simplifications in terms of geometry and boundary conditions the computed slip distributions capture the characteristics of the observed slip distributions

transtensional faulting patterns ranging from pull apart basins to transform continental margins an experimental investigation abstract seventeen small scale experiments were performed to study the deformation induced in brittle ductile models by a releasing stepover between two transform faults and by intersection between transform and divergent plate boundaries in both cases faulting depends on the rheological layering presence and strength of a ductile layer at depth and the width of the basal stepover successive types of pull apart basins are observed in the first set of experiments which are compared with natural examples firstly a lazy z shaped basin appears which is bounded by y faults above transform boundaries and r faults propagating from each corner of the stepover then r faults replace r faults leading to a rhomb shaped graben at every stage the basin length to width ratio remains between 2 2 and 3 8 suggesting that scale independence of pull apart basins is related to the geometrical shape of bounding faults in a second set of experiments deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin within the horsetail splay block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin a feature that can be compared with marginal ridges at transform margins

development of strike slip faults shear experiments in granular materials and clay using a new technique abstract a new experimental technique has been developed to study fault development in layers of moist granular materials clay and fault gouge in shear faults nucleated on pre existing pores and on low displacement protofaults in flaw free areas only a small number of the protofaults developed significant displacement forming conjugate simple fault sets after nucleation simple faults propagated in plane as these simple faults grew in length and new simple fault sets nucleated they began to interact and coalesce simple faults linked up to form compound faults and compound faults linked up to form even larger through going strike slip faults the fault patterns produced in the shear experiments were integrated fault networks consisting of several sets of conjugate shears and tensile structures compound faults exhibited both releasing and restraining steps formed during fault coalescence displacement along such a compound strike slip fault caused a mismatch of the two walls a few points became resistant barriers while the remaining segments became pull apart basins both releasing and restraining steps led to the development of pull apart basins fault displacement and propagation rate were linear functions of fault length the difference between the experiment described here and traditional riedel experiments is that the new experiments do not have a pre existing fault in the experimental setup therefore they are more suitable to study fault nucleation and evolution in a broad shear zone

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

geomorphology and structural segmentation of the crest of the southern pacific antarctic east pacific rise geomorphology of the boundary between pacific and antarctic plates was mapped with a sea beam multibeam echosounder and a seamarc ii bathymetric side scan sonar from the southern end of juan fernandez microplate at 35 s to heezen transform at 56 s there are six spreading center systems separated by two large left stepping nontransform offsets at 36 5 s and 41 5 s two right stepping transform fault systems and a left stepping hybrid structure with equally long strike slip and nontransform offsets axial rift zones spreading axes are generally within 1 of normal to the relative motion predicted by current plate rotation models most axial rift zones crop out along the crests of typical east pacific rise epr axial ridges which commonly have flat rather than humped long profiles about 2 of this fast spreading 84 100 mm yr rise crest has an axial rift valley instead of an axial ridge the longest rift valley segment is midway between menard and vacquier transforms near 51 s where an anomalously deep rise crest may mark a zone of below average mantle upwelling nontransform offsets structurally similar to those on the tropical epr but mostly formed by differential asymmetric spreading subdivide the four longest spreading center systems on the northern transform free two thirds of the rise net rift propagation at migrating offsets has been into the faster pacific plate this causes right steps to migrate north left steps to migrate south and crust to be transferred to the east flank this pattern has prevailed for several million years judging from the oblique fracture zones pseudofaults mapped on the rise flanks with older marine data and geosat altimetry on this rise crest the difference in stress on two plates with very different velocities may control the direction of offset migration

fractal characteristics of active normal faults an example of the eastern gulf of corinth greece abstract we present a model proposing the evolution of segmented normal faults in a currently extended area allowing an explanation of strong earthquakes hosted by short faults active normal faults accommodating the current extension in the gulf of corinth result from the linkage of smaller component faults well exposed fault zones in the sofiko range are arranged as pairs of graben bounding faults and thus our structural analysis has implications for the evolution of active normal faults as well as graben evolution a basal strip of fresh bedrock at the base of fault scarps suggests continuous seismic or aseismic creep thus the structural analysis of component faults that show a power law length distribution describes an evolved fault zone of particular interest is that faulting in this region is self similar between limits distribution is characterized by a fractal dimension of d 1 and the limits in our study are between 10 m mapping resolution and double the maximum displacement of each fault zone component segments of any compartmentalized fault show a length to maximum displacement ratio of two basin architecture and structural analysis suggest that the nucleation points of faults are detectable over the entire evolution of the fault zone all analyzed faults evolved by an independent nucleation model morphotectonic and kinematic data as well as fractal dimension of trace length population plots all suggest that fault zones defining north and south dipping conjugates are active at least over the late stages of basin evolution although this study was focused on a relatively small area this pattern of fault growth appears to be applicable to the entire fault population in the gulf of corinth and is thus an important model for normal faults

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

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

seismotectonics of the central part of the south iceland seismic zone abstract the largest earthquakes in iceland are associated with strike slip faults in the south iceland seismic zone sisz major destructive earthquake sequences occur in this zone at an average interval of 80 years the last two sequences being in 1784 and 1896 the last major single earthquake in the zone was an earthquake of magnitude 7 m in 1912 the seismicity is mostly associated with a conjugate system of nne trending mainly right lateral faults and ene trending mainly left lateral faults but in addition there are many nw trending faults in the sisz particular attention is paid to the comparison between present day fault activity earthquake mechanisms and quaternary faulting fault slip data this comparison reveals a general similarity despite the temporal difference between these subsets independently collected the fault populations are well exposed in the hyaloclastite mountain vordufell near the central part of the south iceland seismic zone where considerable fault slip occurred during the earthquake sequences of 1784 and 1896 the cumulative displacement on most of these faults is 1 6 m in addition to the large scale faults we measured 79 minor faults displacements of the order of centimetres in the vordufell mountain most of the minor faults are left lateral analysis of these minor faults recording palaeostresses in rocks ranging in age from 3 1 to 0 7 ma as well as 50 focal mechanisms from a swarm of present day microearthquakes occurring beneath the vordufell mountain gave consistent results in terms of seismotectonic stress orientations of stress axes and ratios of principal stress differences this shows that the tectonic regime remained unchanged through recent times the results indicate however that there are two contrasting stress regimes in this part of the sisz the primary subset indicates nw se tension and ne sw compression whereas the secondary subset indicates nw se compression and ne sw tension we propose that the primary subset is consistent with the dominating time averaged regional stress field in the sisz this dominating stress field can be largely explained as a consequence of simple plate pull parallel with the spreading vector in south iceland the secondary subset and the associated stress field however may be partly related to stress release and rebound and partly to dike injections in the nearby segments of the rift zone in south iceland

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

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

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model