analogue modeling of overlapping spreading centers insights into their propagation and coalescence abstract the propagation and segmentation of mid ocean ridges is studied using centrifuged analogue models built with non linear materials the deformation of the brittle ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force this linear upwelling laterally stretches the model mantle that in turn induces failure in the upper layer simulating the brittle crust arrays of fractures initiate within zones of high stress concentration above the diapir fractures propagate laterally in a direction perpendicular to the maximum tensile the minimum principal stress secondary tension cracks initiate in the vicinity of parent fracture tips through going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns the overlap overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence non overlapping sub parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture if overlapping fractures are parallel they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material fracture curvature in this case results from crack crack interaction and is similar to that of overlapping spreading centers oscs observed along mid ocean ridges overlapping non parallel fractures tend to coalesce by one of their tips propagating sub parallel to the spreading direction toward the other fracture such offsets can serve as models for the development of the orthogonal ridge transform fault patterns common along mid ocean ridges

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

lithosphere strength controls oceanic transform zone structure insights from analogue models summary oceanic transform zones have often been regarded as plate boundaries the origin of their structural variability is poorly constrained a simple observation indicates that the transform zone is narrow and linear when the offset is large while it is wide with a complex faulting pattern in the case of a small offset on the other hand for a given offset large structural differences exist between transform zones located on the fast spreading south east pacific rise and on the slowspreading mid atlantic ridge in general the transform zones in slow spreading environments are linear with a simple fault pattern while in fast spreading systems they are wide with a complex pattern of deformation we perform small scale analogue modelling to constrain the influence of lithospheric strength on the development of deformation above a transform boundary the models are made up of sand and silicone putty as analogues of the brittle layer and the viscous layer of the lithosphere respectively two plastic sheets coming from shifted gashes form a set up of two diverging discontinuities connected by a transform boundary the rheological layering and strength of the model were modified using different shapes of the viscous layer placed on the transform boundary above the divergent discontinuities the faulting pattern is always formed by parallel normal faults when no viscous layer is placed on the transform boundary strong discontinuity the deformed zone is narrow and has few linear faults by adding a narrow and thin viscous layer the deformed zone becomes wider with a complex faulting pattern formed by oblique slip faults on the limits and by pure strike slip faults in the inner part these latter strike slip faults trend oblique to the transform boundary when a viscous layer with a wide lateral extent overlays the transform discontinuity weak strength the faulting is dominated by obliquely normal faults extending over a wide zone and the strike slip is restricted to the inner part of the deformed zone therefore the mechanical strength of the small scale model controls the shape of the deformed zone and the deformation partitioning these results were applied to 24 oceanic transforms zones we point out that the spreading rate and the transform offset are the two dominant parameters controlling the deformation pattern these two factors directly control the lithospheric strength at the transform boundary however the distance to the nearest hotspot which may generate warmer thermal conditions even in slow spreading environments should modify this result

petrology magnetic fabric and emplacement in a strike slip regime of a zoned peraluminous granite the campanario la haba pluton spain

shear fracture patterns and connectivity at geometric complexities along strike slip faults shear fracture patterns and connectivity at geometric complexities such as echelon fault steps and fault bends along strike slip faults are investigated by simulating shear fracture propagation paths using a displacement discontinuity boundary element method together with a maximum distortional strain energy density criterion it is shown that shear fracture patterns and connectivity depend on the specific geometric configurations the applied stress orientations and the coefficient of friction for a given initial configuration of the major faults the subsequent patterns of the associated shear fractures and their linkage can be predicted if the remote state of stress and the coefficient of friction are known the analyses of fault geometries commonly encountered in nature have important implications for evaluating the effect of faults on fluid flow and earthquake hazards associated with the geometric complexities along strike slip faults in the upper crust

width of continental rifts and rheological layering of the lithosphere abstract a review of some well known continental rifts shows that rift features such as maximum basin depth are dependent on the amount of extension conversely the initial rift width appears to be independent of the amount of extension a comparison is made with laboratory models of rifts developed in a brittle ductile lithosphere experimental models show that normal faults which bound a graben initially meet at the brittle ductile interface a relation between initial rift width and the depth of the brittle ductile transition is thus suggested implications for natural examples are discussed

relations between surface faulting and granite intrusions in analogue models of strike slip deformation analogue models investigating the emplacement of granitic magmas at upper crustal levels during strike slip deformation are presented in this paper variations in parameters such as ratio between horizontal displacement dv and vertical magma upraising iv and the number of feeding points were considered during the experiments model results show that a strong interaction between surface faulting and granite intrusions develops during deformation on one side magma intrusion influences the sequence of structures development and the final fault pattern on the other side magma emplacement is strongly influenced by the syn intrusion lateral displacement dt which in turn reflects variations in dv and iv for high dt values i e high displacement velocity or low injection rate the importance of the horizontal displacement with respect to the vertical magma upraising results in asymmetric and drop shaped plutons with a sheared tail elongating in the sense of the strike slip displacement in this case the intrusion long axis tracks the long axis of the strain ellipsoid for low dt values i e low displacement velocity or high injection rate the strike slip displacement is strongly subordinated to the vertical magma upraising and pluton emplacement is controlled by development of tensional shears at the surface in these conditions the long axis of the magmatic bodies lies in the quadrants of compression of the strike slip deformation and intrusions are apparently dragged in an opposite sense to that imposed by the transcurrent deformation

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

analogue models of the effect of long term basement fault movement on volcanic edifices long term fault movement under volcanoes can control the edifice structure and can generate collapse events to study faulting effects we explore a wide range of fault geometries and motions from normal through vertical to reverse and dip slip to strike slip using simple analogue models we explore the effect of cumulative sub volcanic fault motions and find that there is a strong influence on the structural evolution and potential instability of volcanoes the variety of fault types and geometries are tested with realistically scaled displacements demonstrating a general tendency to produce regions of instability parallel to fault strike whatever the fault motion where there is oblique slip faulting the instability is always on the downthrown side and usually in the volcano flank sector facing the strike slip sense of motion different positions of the fault beneath the volcano change the location type and magnitude of the instability produced for example the further the fault is from the central axis the larger the destabilised sector also with greater fault offset from the central axis larger unstable volumes are generated such failures are normal to fault strike using simple geometric dimensionless numbers such as the fault dip degree of oblique motion angle of obliquity and the fault position we graphically display the geometry of structures produced the models are applied to volcanoes with known underlying faults and we demonstrate the importance of these faults in determining volcanic structures and slope instability using the knowledge of fault patterns gained from these experiments geological mapping on volcanoes can locate fault influence and unstable zones and hence monitoring of unstable flanks could be carried out to determine the actual response to faulting in specific cases

holocene vertical deformation outside the active rift zone of north iceland in iceland the main deformation activity occurs not only in the active rift zone where volcanism and seismicity coexist in fissure swarms but also in the transfer zones north and south where the main part of the seismic activity is located many studies described extension and vertical displacement associated with rifting and magmatism inside this rift zone they have classically assumed that no deformation occurs outside the rift zone however few studies have already been conducted outside the rift zone this work focuses on an area west of the rift zone in the akureyri area which displays an anomalous topography we illustrate an ene tilting of 0 23 since approximately 17 000 years ago using ancient lacustrine shorelines mapped in detail with a differential gps survey and analysis of spot images this degree of tilting corresponds to a vertical displacement of 160 m i e a deformation rate of 1 cm year which is close to values measured in the active zone this long wavelength deformation is compatible both with i the topographic anomaly of the akureyri area and with ii the very diffuse microseismicity that affects northern iceland the origin of this tilting is discussed and the different hypotheses glacial loading response rift uplift transform zone effect thermal doming local magmatic intrusion are reviewed our results structural data and the topography are more consistent with a local magmatic intrusion inducing local doming this work is the first attempt at quantifying long term deformation outside the rift zone in iceland

the deep structure of the grenville front a new perspective from western quebec the grenville front is a major tectonic boundary exposed on the canadian shield the front is defined as the northwestern limit of grenvillian deformation on the basis of geochronological and metamorphic data this boundary is also evident in some geophysical data sets the lithoprobe abitibi grenville transect crosses the grenville front near lac temiscamingue in western quebec a new 114 km long deep seismic reflection line shows a crustal structure quite different from that seen on previous surveys across the grenville front the archean foreland pontiac subprovince has a pattern of reflectivity similar to that seen in most of the superior province this pattern continues for some 30 km south of the surface exposure of the grenville front there is no evidence for a band of dipping reflectors truncating the horizontal pontiac reflectors in fact the leading edge of the grenville province is difficult to identify on the seismic section the moho is well defined and reveals that the crust thins under

internal structure of the cadillac tectonic zone southeast of val d or abitibi greenstone belt quebec the kirkland lake larder lake cadillac break is a major fault zone of the abitibi greenstone belt well known for its spatially associated gold camps a detailed structural analysis of this fault zone in the val d or area has shown the presence of a 200 750 m wide zone of high strain defined here as the cadillac tectonic zone ctz which includes the narrower schist zone generally regarded as the cadillac break the boundaries of the ctz which coincide with major lithologic contacts strike approximately east west and dip 80 to the north the internal structure of the ctz is best analysed in terms of two increments of the same progressive deformation the d1 strain increment is characterized by an east west striking subvertical s1 foliation at low angle in strike to the bound aries of the ctz and containing subvertical elongation lineation primary lithological contacts have been transposed into s1 and folded by f1a folds f1b intrafolial folds of s1 ranging from noncylindrical subhorizontal f

formation and suppression of strike slip fault systems strike slip faults are a defining feature of plate tectonics yet many aspects of their development and evolution remain unresolved for intact materials and or regions a standard sequence of shear development is predicted from physical models and field studies commencing with the formation of riedel shears and culminating with the development of a throughgoing fault however for materials and or regions that contain crustal heterogeneities normal and or thrust faults joints etc that predate shear deformation kinematic evolution of strike slip faulting is poorly constrained we present a new plane stress finite strain physical analog model developed to investigate primary deformation zone evolution in simple shear pure strike slip fault systems in which faults or joints are present before shear initiation experimental results suggest that preexisting mechanical discontinuities faults and or joints have a marked effect on the geometry of such systems causing deflection lateral distribution and suppression of shears a lower limit is placed on shear offset necessary to produce a throughgoing fault in systems containing preexisting structures fault zone development observed in these experiments provides new insight for kinematic interpretation of structural data from strike slip fault zones on earth venus and other terrestrial bodies

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

conductivity discontinuities in the upper mantle beneath a stable craton we present evidence for approximate collocation of seismic and electrical transitions in the upper mantle more than two years of very long period magnetotelluric mt data were recorded at a lakebottom observatory in the central canadian shield after processing to contend with non stationary source effects and removal of galvanic distortion the underlying structure is 1d for periods of one hour to four days the response was extended to periods of 100 days by appending geomagnetic depth sounding data to the mt curves minimum structure linearised inversion nonlinear extremal inversion and a new genetic algorithm for nonlinear hypothesis testing reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities the jumps occur over limited depth ranges

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

temperature effects and their geological consequences at transform margins a thermal model of transform margin evolution including both shear heating and lateral conduction of heat from hot oceanic to colder lithosphere was developed to gain insight into transform margin crustal structure results indicate that over 2 km of crustal uplift may occur at the fault trace for a modelled transform fault 500 km in length with spreading half rates of 1 0 and 4 0 cm year this uplift decreases away from the fault over a distance of 60 80 km the viscosity of the lower continental crust and upper mantle adjacent to the transform margin is reduced by a factor of more than 100 in response to plate motion and asthenospheric upwelling at the spreading ridge flow of this thermally weakened material may also play a role in continental crustal thinning thermal model predictions are compared with geological observations and crustal structure across transform margins in particular we show that the geology of the southwest newfoundland transform margin eastern canada and the cape range frac

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

analogue experiments of propagation of oblique rifts abstract we present the results of analogue models of oblique rifting which were carried out using a isostatically compensated two layered small scale models the experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular rifted basins set en echelon along the axial zone of the experiment box in time the basins propagate lengthways to merge and form a unified composite rift however traces of the initial en echelon rifts expressed in deeper axial basins separated by shallower threshold zones are clearly discernible in the mature rifts both the basins and the threshold zones are located within the composite rift so that the rift margins are abrupt and steep along the basins and more diffuse along the threshold zones the thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor model experiments of nearly orthogonal extension showed a single unified rift which differed conspicuously from the composite oblique rifts the results of the experiments suggest that oblique extension forms en echelon rift segments the axes of which are perpendicular to the extension direction separated by threshold zones therefore there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

focused and diffuse extension in controls of ocean ridge segmentation in analogue models 1 this work aims to provide insights into the initiation and evolution of segments along ocean ridges as they propagate and mature it is based on the results of analogue modeling performed in a centrifuge with applied acceleration of gravity force the effect of the varied degree of extension allowed during propagation of analogue ridge results in segments being focused along the ridge axes or widely distributed within the entire axial zone three distinct types of segment interaction are documented controlled by the relative distribution and orientation of interacting segments and leading to the development of linear ridges overlapping spreading centers and oblique transform zones the importance of the initial distribution of segments in the ridge evolution inferred in the models suggests its significance for formation of natural ridges whose mature configurations must be largely inherited from that of spreading centers formed in the early stages of focused or diffuse extension

oblique rifting at oceanic ridges relationship between spreading and stretching directions from earthquake focal mechanisms the relationship between spreading and stretching directions is investigated at oblique spreading oceanic ridges using earthquake focal mechanisms the stretching direction at ridge axes corresponds to the direction of the greatest principal strain 31 taken as the mean trend of the seismic t axes of extensional earthquake focal mechanisms it is compared with the spreading direction provided by global plate motion models we find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend a result in line with analogue experiments of oblique rifting this result is satisfactorily accounted for with an analytical model of oblique rifting for which the direction of 31 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics for low stretching factors typical of incremental seismic deformations 31 obliquity is two times lower than rifting obliquity for higher stretching factors the stretching and spreading directions become parallel 2006 elsevier ltd all rights reserved

oblique rifting and segmentation of the ne gulf of aden passive margin 1 the gulf of aden is a young obliquely opening oceanic basin where tectonic structures can easily be followed and correlated from the passive margins to the active mid oceanic ridge it is an ideal laboratory for studies of continental lithosphere breakup from rifting to spreading the northeastern margin of the gulf of aden offers the opportunity to study on land the deformation associated with oblique rifting over a wide area encompassing two segments of the passive margin on either side of the socotra fracture zone exhibiting distinct morphologic stratigraphic and structural features the western segment is characterized by an elevated rift shoulder and large grabens filled with thick synrift series whereas the eastern segment exhibits low elevation and is devoid of major extensional structures and typical synrift deposits though the morphostructural features of the margin segments are different the stress field analysis provides coherent results all along the margin four directions of extension have been recognized and are considered to be representative of two tensional stress fields with permutations of the horizontal principal stresses 2 and 3 the two dominant directions of extension n150 e and n20 e are perpendicular to the mean trend of the gulf of aden n75 e and parallel to its opening direction n20 e n30 e respectively unlike another study in the western part of the gulf our data suggest that the n150 e extension stage is older than the n20 e extension stage these conflicting chronologies which are nowhere unambiguously established suggest that the two extensions coexisted during the rifting on land data are compared with offshore data and are interpreted with reference to oblique rifting the passive margin segmentation represents a local accommodation of the extensional deformation in a homogeneous regional stress field which reveals the asymmetry of the rifting process the first order segmentation of the sheba ridge is inherited from the prior segmentation of the passive margin

metamorphism of deep seated rocks from the garrett ultrafast transform east pacific rise near 13 25 s in spite of 10 years of extensive exploration of the east pacific rise the petrology of the fast slipping transforms remains poorly documented in 1980 metagabbros and serpentinites were dredged from the deepest trough 4616 4820 m in depth of the garrett ultrafast transform which intersects the east pacific rise near 13 25 s 16 cm yr total spreading rate previous works on their protolith concluded that the samples consist of serpentinized refractory upper mantle and lower crust gabbroic cumulates in addition several ultramafic rocks showing evidences of magmatic impregnation were believed to represent mantle crust transition zone as observed in ophiolite complexes the present work focuses on the postigneous evolution of the rocks affected by hydrothermal circulation an early stage of amphibolitization occurred at about 700 400 c and probably down to 300 c in the most evolved gabbros ti fe gabbros the amphibole compositional variations suggest evolution of the metasomatic solution and temperature through time from the amphibolite facies down to the lower greenshist facies high chlorine ferruginous hornblendes 3 5 wt cl found in veins of the ferrogabbros suggests that pore fluids in this type of material evolved in situ to a ferruginous brine the decreasing temperature 300 200 c and further fracturing favored more or less pervasive serpentinization of the olivine rich samples ultramafics and olivine gabbros the release of ca in the solution contributed to the alteration of the plagioclase bearing rocks troctolites and gabbros which were invaded by calc silicates hydrogrossular prehnite pumpellyite and epidote a process which is referred to as rodingitization the occurrence of monomineralic veins of al pumpellyite in some metagabbros suggests very low o2 conditions in the lower ocean crust ph2o 2 kbar low o2 also inferred from the composition of the other calc silicates and indicates low permeability and limited fluid circulation during retrograde metamorphism further uplift and emplacement of the rocks on the seafloor of the transform domain were accompanied by low temperature crystallization of zeolite and aragonite high mg and co2 in monomineralic veins and clay minerals in veins and in external alteration rims the garrett s samples are by many respects comparable to typical ocean floor metabasites from various settings the rocks recorded igneous and metamorphic transformations similar to those expected beneath spreading ridges unaffected by transform tectonics the metamorphism is a consequence of the dynamics of spreading propagation of the cracks and penetration of seawater deeper in the cooling crust however the extent of fissuring and the subsequent hydration probably increase drastically with proximity to the garrett transform most of the observed metasomatic reactions have a limited extent and occurred at depth without significant deformation of the rocks rapid uplift of the rocks initiated by vertical components of the transform tectonics was probably induced by serpentinization of the ultramafics buoyancy of serpentinite and intense mass wasting at the surface of seafloor however emplacement by diapirism is not completely ruled out if the dredged samples are relatively undeformed poorly altered rocks carried up by protusions of ductile serpentine which is now too fragile to be sampled by dredging

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

buckling of the oceanic lithosphere from geophysical data and experiments two major hypotheses have been advanced for the formation of the long wavelength 100 300 km undulations of oceanic basement and overlying sediments developed in the central indian ocean basin whole layer folding buckling and local thickening inverse boudinage using appropriately scaled two layer analogue models for the oceanic lithosphere comprising a brittle layer above a ductile layer we show that buckling of the entire brittle layer is likely to be the mode of deformation however the lithosphere asthenosphere boundary remains undisturbed we find a relationship between the thickness of the brittle layer and the wavelength of folding such that the wavelength is 7 times the brittle layer thickness

active segmentation assessment of the tectonically active south wagad fault in kachchh western peninsular india abstract in present study the results of integrated studies of landform development and geomorphic indices of drainage networks across the south wagad fault swf zone have been presented the swf is a structure that is morphogenic in nature creating a 40 m high e w trending escarpment that extends 80 km at the southern end of the wagad uplift the swf got reactivated during the bhuj earthquake of 2001 continued aftershock activity of m 4 levels were recorded in the epicentral zone north of the kachchh mainland fault and west of the swf strath terraces along the river valleys and the truncation of alluvial fans along the mountain front incision and offset of rivers suggests an active nature of the swf to assess the relative tectonic activity morphometric indices such as stream gradient sl index hypsometric integral hi basin asymmetry af mountain front sinuosity smf index sinuosity index si and basin shape bs have been analyzed the landform development along the river valley in the swf zone suggests reactivation of the swf between 12 and 3 ka the southern front of wagad is found to be uplifting at the rate of 2 8 mm y during the last 12 ka as estimated through osl dates the relative index of active tectonics riat by the study of different geomorphic parameters and field checks indicates that most of the wagad highland is active the compressional strain of 0 05 strain y inferred from the gps study in the swf area attests to the activeness

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

seismic and electrical anisotropies in the lithosphere across the grenville front canada the grenville front is a prominent tectonic feature of the canadian shield which has been the focus of a lithoprobe transect a strong electrical anisotropy has been detected in a frequency range of 0 1 0 01 hz 50 to 150 km the most conductive direction being oriented n80e in order to compare electrical and seismic anisotropies ten seismometers have been deployed across the grenville front to record teleseismic events s wave splitting measurements show an average delay of 1 54 0 21 s between the fast and the slow components the fast axis direction is approximately e w at all stations showing a strong coherence between the stations along the complete transect this azimuth correlates well with that of the electrical anisotropy the electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km the similarity of the magnetotelluric and seismic anisotropy directions across the grenville front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by silver and kaneshima 1993

obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle teleseismic shear wave splitting and magnetotelluric experiments across the grenville front between the archean craton and the proterozoic grenville province in the regions of the pontiac subprovince and northwestern grenville province canada show a consistent obliquity between the polarization direction of the fast split shear wave and the most electrically conductive direction mt in the upper mantle transcurrent shear zones at all well recorded stations is nearly n103 e and mt is approximately n80 e the obliquity may be considered a potential kinematic indicator because the seismic and electrical anisotropies are thought to be controlled by lattice preferred and shape preferred orientations of mantle minerals mainly olivine respectively the dextral movement sense of the transcurrent shear zones in the mantle inferred from the observed obliquity is consistent with that inferred from surface geology of the crustal shear zones this consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

experiments on strike slip faulting and block rotation analogue model experiments investigating strike slip faulting in zones of distributed shear deformation show that fault orientations and fault evolution indicate modifications of the stress field with increasing bulk shear strain dextral bulk shear deformation at low strain is accommodated dominantly by synthetic strike slip faults riedel shears at higher strain secondary antithetic and synthetic faults develop mostly between earlier formed major riedel shears closely spaced parallel antithetic faults cross faults delimit domains that rotate about vertical axes with continuing shear deformation rotation of fault bounded domains results in sigmoidal antithetic faults that have a dip slip component and a dip direction that changes along strike there is good agreement between models and natural examples of distributed shear zones where block rotations about vertical axes have been documented

periodic instabilities during compression of the lithosphere 2 analogue experiments we have modeled the behavior of the continental and oceanic lithospheres under compression using materials with analogous properties in laboratory experiments to study the development of lithospheric buckling periodic instabilities which are a major deformation process during the compression of the lithosphere have already been described by several authors using an analytical perturbation method at small strains analogue experiments corroborate most of the results obtained by the perturbation method 1 the deformation modes geometrical relationships of interfaces and related wavelengths are mainly dependent on the spatial distribution of the brittle layer s and 2 the amplitude of buckling is an exponential function of the horizontal strain some departure from the perturbation method occurs when there are two instabilities growing concurrently the breakdown of the exponential growth occurs for strains of about 5 and is concomitant with the appearance of thrust faults in experiments including one brittle layer which model the compression of the oceanic lithosphere faults are regularly located at the inflection points of the folds in experiments including two brittle layers which model continental lithosphere faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan shaped symmetrical thrust system in the upper brittle layer such fault geometries give some new highlights on typical compressive geological structures such as those encountered in central asia

calculating earth moon system parameters from sub yearly tidal deposit records an example from the carboniferous tradewater formation abstract the secular evolution of the earth moon system remains poorly constrained largely because there are few continuous tidal rhythmite or foreset bundle sequences that preserve deposition over more than several neap spring tidal cycles deposits recording less than one year of deposition do not facilitate direct calculation of past lunar distance directly from kepler s laws but may prove necessary to reconstruct lunar orbital evolution because they are far more common than longer records a method is demonstrated to make use of shorter tidal deposit sequences by utilizing conservation of angular momentum between the earth and moon and estimating the solar component of tidal deposition while assuming a constant moment of inertia for earth since the proterozoic the precision and accuracy of spectral estimates obtained from short records are considered as are the limitations of subsequent calculations of earth moon parameters the late carboniferous abbott sandstone of the tradewater formation in the illinois basin preserves just over 6 apparently continuous neap spring cycles in its semidiurnal deposits the quality of these data as assessed via sedimentological evidence and statistical time series properties produces spectral estimates that are likely within at least 5 of the actual underlying periodicity 90 accuracy to test the usefulness of such records we assessed the possible scenarios of 90 and 95 accuracy at 90 accuracy the error bounds on earth moon parameter estimates become rather large and render individual data sets to be of limited use at 95 accuracy very general inferences about the evolution of the earth moon system may be made calculated mean lunar orbital distance at 315 ma is 3 798 10 8 m with error bounds of 0 086 10 8 m and 0 046 10 8 m we conclude that short sequences of cyclic tidal deposits offer rather limited resolution of lunar distance estimates utilizing multiple sub yearly data of similar age in ensemble may prove necessary if further investigations of secular changes in earth moon parameters are to proceed

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

analogue modeling of faulting pattern ductile deformation and vertical motion in strike slip fault zones the relationship between faulting ductile deformation and vertical displacement in provinces of strike slip faults were analyzed using analogue modeling the described experiment considered a two layer small scale model for the lithosphere set on a low viscosity fluid that allowed free isostatic compensation the development of the faults and their orientation follow generally riedel s model but fault spacing depends on the thickness of the brittle layer and lateral displacement of the brittle layer was associated with series of deep elongated depressions that developed along the trace of the principal strike slip fault the depressions correspond to local thinning of the brittle layer and the amount of thinning is more than 60 in places the underlying ductile layer displayed two types of superimposed deformation namely a series of tight parallel folds occurring on top of elongated domes the folds are attributed to the compressive component of the strike slip displacement and the updoming to the extensional component the elongated domes are located beneath the superficial depressions and the deeper troughs underlie shallow uplifted structures in the deformed band significant vertical motion was observed along faults that are considered as perfectly strike slip faults according to the classical riedel model the thinning of the brittle layer and the deep deformation of the ductile layer are in good agreement with actual examples of pull apart basins and elongated swells in large strike slip zones a new structural pattern developed during strike slip shear is proposed relating the geometrical relationships between faults folds and elongated domes to the stress axes

vema fracture zone central atlantic tectonic and magmatic evolution of the median ridge and the eastern ridge transform intersection domain the eastern vema fracture zone intersection with the mid atlantic ridge mar axis was surveyed with the french submersible nautile during the vemanaute cruise in 1989 at this ridge transform intersection rti an elongated e w ridge more than 50 km long is present in the transform valley this median ridge rises up to 1000 m above the surrounding seafloor the crest of the median ridge is lower and presents an arcuate shape at the tip of the mar axis dive observations on both the southern and northern flanks of the median ridge as well as sample studies suggest that this morphological feature is not a serpentinized mantle protrusion or a recent volcanic constructional ridge but represents a sliver of uplifted oceanic lithosphere covered by a sedimentary breccia formation this detrital cover consists of polymictic sedimentary breccias sandstones and siltstones composed of basaltic doleritic and gabbroic clasts with less frequent serpentinite and spinel fragments which originated from the disaggregation of shallow to deep levels of tectonically uplifted oceanic crust and upper mantle most of these clasts have undergone greenschist facies metamorphism prior to their incorporation in the detrital formation disaggregation mass wasting and rapid emplacement of detrital formations on the valley floor by gravity flows are likely to be related to a major tectonic episode that affected one or both the fracture valley walls this event could be related to the uplift of the southern wall of the fracture zone the transverse ridge which took place probably between 10 and 3 ma ago since this uplift episode the transverse ridge which is now undergoing subsidence and the detritus covered transform valley floor separated by the transform fault zone have migrated westward and eastward respectively vertical tectonics of the median ridge at the approach of the rti can not be explained solely by the hypothesis of a diapiric intrusion of serpentinite as proposed by earlier authors a possible interpretation follows the suggestions that the anomalous crust of the fracture valley near the western rti is more than 1 km out of isostatic equilibrium recent tectonic and magmatic events including subsidence and lava emplacement which occurred at the tip of the mar axis have been recorded on the southern flank of the median ridge several stages in the very recent tectonic volcanic history of the eastern rti that is roughly during the last 300 000 years can thus be defined the lower elevation and narrow arcuate shape of the median ridge east of 40 57 w are inferred to have resulted from tectonic extension during the creation of the nodal basin

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

a 400 km scale strike slip zone near the boundary of thetis regio venus we present a structural analysis of an area of venus surface centred on 3 s 116 e the area is dominated by a wide zone of deformation striking 050 n distinguished from the terrain on either side of it by markedly different structural and morphological characteristics the deformation zone contains two principal fault sets defined by form position and orientation members of the first set are present along the length of the deformation zone they are discontinuous consistently right stepping and are interpreted as riedel shears in a sinistral strike slip regime members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional grabens these faults are most prominent in the central region of the deformation zone and coincide with a deflection in strike of both the first fault set and the boundaries of the deformation zone a detailed kinematic analysis of all fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory supporting the hypothesis that this sinistral strike slip zone contains an extensional jog this interpretation is further supported by analysis of the interaction between faults belonging to the two sets variation in intercept geometry kinematic linking of the two fault sets and rotation of fault strike between the two fault sets all strongly suggest simultaneous development of these structures and therefore a common tectonic origin a driving mechanism for such large scale horizontal deformation on venus has yet to be established