spreading evolution of the troodos ophiolite cyprus some recent developments particularly the recognition of transform tectonized and nontransform tectonized fabrics close to the southern troodos transform fault provide a framework for a new evolutionary model of the troodos ophiolite this model proposes that most of the ophiolite formed at the solea axis against a dextrally slipping southern troodos transform spreading was largely accommodated by magmatic accretion the ridge then jumped to the east a last phase of amagmatic stretching driving normal faulting above a detachment horizon in the solea graben a ridge jump boundary separates the old crust formed at the solea axis from the new crust generated at the outside corner of the new ridge with a dextrally slipping continuation of the southern troodos transform the final episode involved extension of the crust generated at the new ridge to form the larnaca graben in the eastern part of the ophiolite

bookshelf tectonics rotated crustal blocks within the sovanco fracture zone the 15 kilometer wide sovanco fracture zone is a dextral slip transform fault linking the juan de fuca and explorer ridges in the n e pacific seabeam bathymetry shows that the sovanco comprises several rhomb shaped elevated blocks of oceanic crust bounded by northeast and northwest trending lineaments we hypothesize that the northeast trending lineaments are sinistral strike slip faults that are reactivated ridge parallel normal faults both the sinistral block bounding faults and the blocks themselves have rotated clockwise about 30 degrees in response to dextral shear within the sovanco zone

strike slip faulting and block rotation in the lake mead fault system strike slip faults in the basin and range province have often been considered passive boundaries between differentially extended domains of tilted normal faults and are thus considered secondary in accommodating regional horizontal deformation paleomagnotic investigation of late miocene age volcanic rocks displaced by the left lateral fault system of lake mead nevada shows 1 that these rocks have not been affected by significant structural tilt the difference between observed and expected inclinations being only 0 6 14 9 and 2 a significant horizontal counterclockwise rotation of 29 4 8 5 about a vertical axis this rotation was accommodated by slip on northwest trending right lateral strike slip faults this implies significant west northwest elongation results of the investigation indicate that strike slip faulting is the primary process accommodating crustal deformation along the lake mead fault system and that tilting in response to normal faulting is secondary

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

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

distortions rotations and crustal thinning at ridge transform intersections when an oceanic ridge intersects a transform fault the newly formed oceanic crust may be severely distorted frequently exhibiting major rotations of fault blocks1 4 such effects typically extend over a 10 km wide strip parallel to the transform by contrast studies of contemporary oceanic transforms suggest that deformation is restricted to a single fault trace within the transform domain5 9 this apparent inconsistency can be reconciled if the rotations occurred at the ridge transform intersection during accretion of the crust here i develop a kinematic model that considers the effect of an ephemeral period of stretching on a region that broadens as the transform is approached along the ridge the strains calculated from this model indicate extension at the ridge simple shear at the transform and composite strains in the region of distributed deformation the model predicts very rapid rotation rates 100 in 104 105 yr the implications for the topography and structure of ridge transform intersections are considered and the alternating periods of extension by stretching and by intrusion are illustrated by reference to contemporary transforms on the mid atlantic ridge the model may also explain the common occurrence of cross cutting dykes near transform faults in ophiolites

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

superimposed tectonics of the cyprus ophiolitic massifs fault planes and slickenslides have been systematically studied in the troodos and kellaki ophiolitic sequences and in their overlying uppermost cretaceous to neogene sedimentary cover exposed in southern cyprus a relative chronology of three main deformational events has been recognized the most recent event occurring during the late neogene is characterized by a n s striking extension from late cretaceous to late miocene the deformation became compressional with a n160 strike an older tectonic event contemporaneous with the development of the cretaceous ophiolitic suite is characterized by a ese wnw extension or extensional strike slip

structural and volcanic expression of a fast slipping ridge transform ridge plate boundary sea marc i and photographic surveys at the clipperton transform fault a high resolution near bottom survey has been conducted of the clipperton transform fault and adjoining segments of the east pacific rise epr using the sea marc i side looking sonar system and the lamont doherty geological observatory olympus based camera system the transform fault zone tfz is a narrow well defined belt of transform parallel lineaments which varies along strike from a single sharp edged notch to a complex band of subparallel lineaments up to 1 km wide the tfz is set within a 5 km wide band of unusually fine grained side scan texture which could indicate nonbasaltic seafloor and or pervasively sheared and mass wasted basaltic crust the fine grained swath is surrounded by constructional volcanic terrain with no hint of strike slip motion this observation puts an upper limit of 5 km on the extent of lateral migration of the tfz in the last 1 5 m y both ridge transform intersections rtis are dominated by bathymetric highs located on the old plate opposite the spreading center a mantling of fresh looking constructional volcanic terrain on side scan images suggests that the highs are built in part by recent extrusive and intrusive volcanism thermal expansion may also play a part the epr south of clipperton has recently experienced extrusion of high effusion rate basalts burial of faults and fissures by lava flows and development of vigorous hydrothermal circulation on the epr north of clipperton the axial zone of faults and fissures tapers toward the transform fault this may reflect a change in the shape or size of the underlying shallow level magma feeders as a function of distance from the site of magma upwelling or distance toward the transform fault

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

shear zones as a result of intraplate tectonics in oceanic crust the example of the basal complex of fuerteventura canary islands ductile shear zones affecting igneous rocks pyroxenites syenites gabbros and carbonatites belonging to the basal complex of the island of fuerteventura are described here they develop typical mylonites with their cortege of structures and crystallographic fabrics an evolution from brittle to ductile conditions has been recognized and interpreted as being due to the ascent of alkaline magmas and related fluids and subsequent emplacement of weak rocks carbonatites syenites along the shear zones local metamorphic changes favour the ductile behaviour via a reaction enhanced softening mechanism the shear zones are arranged in a nearly orthorhombic pattern kinematic criteria suggest an extensional tectonic scenario with a bulk irrotational nonplane deformation the long axis of the finite deformation ellipsoid is e w and horizontal the available data allow us to establish this deformation as being from the late oligocene to the early miocene this is tentatively related to the plate tectonics evolution and mantle anomalies in the northwestern corner of the african plate

off ridge alkaline magmatism and seamount volcanoes in the masirah island ophiolite oman abstract the masirah ophiolite offers an unique opportunity to study well preserved small seamount structures obducted seamounts have not been described up to now and from the present day ocean floor they are almost exclusively known from bathymetric studies the thin oceanic crust of the masirah ophiolite was formed at a ridge transform intersect in upper jurassic time it was overprinted and reworked by a major intra oceanic tectono magmatic event at mid cretaceous time that has been well dated owing to the presence of interstratified sedimentary rocks late hauterivian to early barremian c 130 125 ma this mid cretaceous magmatism produced alkaline volcanic rocks ranging in chemistry from alkalibasalts to rhyolites volcanism occurred in a nw se extensional regime small elongate submarine volcano structures seamounts developed within widespread alkalibasaltic pillow lava and pillow breccia deposits which are interfingered with deep marine pelagic sediments the volcanoes reached a maximum of a few kilometres in diameter and a few hundred metres in height the seamounts are built up of basic to acid subvolcanic stock or sheet like intrusions several generations of dikes vent agglomerates and pyro to epiclastic deposits the latter range from coarse breccias to finely stratified lapilli and record explosive volcanism in a deep marine environment in the magma chambers under the volcanoes local differentiations to trachytic and rhyolitic members took place the alkaline rocks show a pronounced ocean island basalt oib character indicating the considerable contribution of a mantle plume source hotspot as cause of the volcanism we propose a combination of original transform setting followed by drift past the marion hotspot during the major plate tectonic reorganization between greater india madagascar and africa starting in mid cretaceous time

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

grain size distribution and rheology of the upper mantle abstract the physical processes that govern the grain size of rocks in the upper mantle are examined the analysis is based on the experimental data on creep recrystallization and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere the grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate above this depth the microstructures record the thermomechanical history of rocks rather than the in situ stress in the case of the oceanic lithosphere where the thermomechanical history is best known the following features of grain size distribution are found at the uppermost mantle where the amount of grain growth is limited the grain size is determined by the initial value and the growth rate and where the effect of grain growth dominates it increases with depth when the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform the grain size is then determined by the applied stress this grain size is however frozen when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization thus at the intermediate depth region the grain size records the fossil frozen stress at which the microstructures of rock have been frozen since the frozen stress increases with age the grain size in this depth interval decreases with depth finally the grain size below this level reflects the in situ stress and increases with depth its extent being dependent on the nature of return flow in the deep mantle thus the grain size versus depth relation may show a sigmoid curve the qualitative features of this curve may be similar also in the case of the continental lithosphere if a similar thermal event i e the intrusion of hot material and subsequent cooling occurs the results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules ave lallemant et al 1980 the present model suggests that the depth of minimum grain size 65 and 150 km at the continental rift zone and the shield region respectively corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels 1 mpa and in the geological time scale this result leads to a new definition of the thickness of lithosphere in terms of its rheological properties this thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness the model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy

an historical look at the binghamton geomorphology symposium abstract the binghamton geomorphology symposium bgs is an annual meeting held since 1970 usually in the u s a on timely topics in the field of geomorphology a special issue of this journal presents the papers from each meeting and provides an opportunity to disseminate to the international community research relevant to that year s theme written by key people in that field in this paper we review the history of the bgs examine the academic disciplines of each year s organizers compile a list of the researchers who have made multiple contributions to bgs note the citation impact of papers published in the annual proceedings map the spatial distribution of the meeting locations and categorize the gender distribution of the contributors to the symposium contributions from female authors to the bgs have steadily increased since the 1970s however from 2003 to 2013 females still only accounted for 23 6 of the proceeding s papers an increase of 16 4 from the 1970s these numbers are not surprising when compared to the gender distribution of the top positions in specialty groups in the u s between 1989 and 2010 over 7000 citations were from articles published in bgs special issues indicating their contributions to the geomorphology field when normalized by the number of years the articles have been available papers from bgs whose themes combined geomorphology with other fields i e natural hazards and geomorphology rated higher numbers of citations the dispersion of the location of bgs meetings illustrates how the series is maturing by moving away from its origin in the u s northeast

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

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

paleomagnetic constraints on the formation of the solea graben troodos ophiolite cyprus abstract the solea graben on the northern flank of the troodos ophiolite cyprus is a 15 20 km wide asymmetric structure primarily defined by dike attitudes in the sheeted dike complex the wider western portion of the graben consists mainly of gently dipping dikes while the eastern portion is narrow and consists of blocks of dikes with widely varying attitudes using a method which restores the sampled dikes to vertical and the primary remanent magnetizations to a previously known stable remanent magnetization direction for the troodos complex we have calculated rotation axes and amounts of rotation for the dikes the rotation axes for dikes in the western portion of the solea graben are subhorizontal and subparallel to the original strikes of the dikes in the eastern portion the rotation axes have varying attitudes including some which are very steep our results are consistent with field observations that suggest that the western solea graben formed by listric normal faulting near the axis of a spreading center with rotation of the dikes above a detachment surface across the graben axis on the eastern portion more intense intrusive activity and minor normal faulting resulted in less rotation about subhorizontal axes and some rotations about vertical axes

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

palaeomagnetic evidence for clockwise rotations related to dextral shear along the southern troodos transform fault cyprus abstract the southern troodos arakapas fault represents a late cretaceous oceanic fracture zone which was locally disrupted during late cretaceous eocene palaeorotation of the troodos microplate opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation palaeomagnetic studies of turonian zeolite facies lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the arakapas fault belt the western margin of the limassol forest complex and the eastern flank of the troodos ophiolite at seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes one fault block at the western end of the arakapas fault belt has experienced a net anticlockwise rotation while at six other sites only simple tilting about sub horizontal axes is indicated the overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right lateral slip along the transform cross cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post spreading disruption of the fracture zone sites located further south in the eastern limassol forest complex show no relative rotation with respect to the main troodos ophiolite to the north and have experienced only simple tilting however if our results are considered in conjunction with existing palaeomagnetic data from the area it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform some areas were rotated by over 100 about steeply inclined axes whereas others experienced only simple tilting the whole area was later subjected to a bulk 90 anticlockwise rotation along with the troodos microplate data obtained from the umbers and radiolarites perapedhi fm exposed on the eastern flank of the limassol forest block indicate that at least 30 and possibly up to 45 of this 90 rotation took place over a maximum of 15 ma

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

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

fault zone reactivation kinematics and mechanisms the kinematics and mechanisms of fault zone reactivation are reviewed reactivation is dependent upon fault zone orientation and the existence of weak mylonites along these zones the recognition of reactivation within mylonite zones and the softening processes that first concentrate deformation into these and secondly provide a weak medium for reactivation are discussed attention is given to the darling mobile zone western australia and the redbank zone see obee white this symposium as examples of reactivated zones

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

structure of a fossil ridge transform intersection in the troodos ophiolite recent palaeomagnetic studies of sheeted dykes adjacent to the east west trending southern troodos transform fault of cyprus have demonstrated significant clockwise block rotations which are most easily interpreted as being due to drag induced by dextral strike slip faulting along the transform1 6 we show here that such rotations have not occurred along the entire length of the transform instead to the west of the areas previously studied the sheeted dykes have been subjected to simple tilting only indicating normal faulting but no strike slip the changeover between these two domains of differing structural styles near the village of mandria is approximately in line with the centre of the solea graben a north south trending structure within the troodos massif which has previously been interpreted as a fossil ridge axis7 we suggest that the mandria area represents a fossilized ridge transform intersection and present the results of a detailed field investigation of the area together with preliminary palaeomagnetic data to determine the relationship between tectonism and magmatism at such plate boundary zones we use our findings to test a recent theoretical model8 of deformation processes at ridge transform intersections and show that distributed deformation due to block rotation is largely confined to the ridge transform corner itself

palaeorotation of the troodos microplate cyprus the troodos ophiolite represents one of the best preserved fragments of ocean floor crust that is exposed on land as the extrusive series of the ophiolite retains a stable magnetization that is directed westward the ocean crust is considered to have rotated anticlockwise through 90 since its formation in the late cretaceous1 to determine the timing of the rotation event over 3 000 orientated samples have been collected from the ophiolite and from its in situ turonian to recent sedimentary cover here we report that pelagic chalks immediately overlying the highest extrusive lava units of the ophiolite complex give reliable magnetic vector directions indicating that at least 60 of rotation occurred before the lower eocene and that rotation was completed by the end of the lower eocene rotation cannot have occurred within a single late miocene event as reported previously2 regional geological considerations support the rotation of only a small fragment of oceanic crust that was stranded adjacent to an active continental margin in this tectonic setting the oblique consumption of oceanic crust beneath troodos crust could provide the necessary driving force for tectonic rotation

microstructure of serpentinite mylonites from the josephine ophiolite and serpentinization in retrogressive shear zones california the josephine peridotite has experienced deformational episodes over a wide range of temperatures deformation within the stability field of a serpentine group mineral appears invariably accompanied by retrogressive metamorphism serpentinization two general types of deformed serpentinites have been identified 1 incohesive serpentinites most common which appear to have experienced low temperature predominantly brittle deformation and 2 serpentinite mylonites which have experienced plastic flow near the upper limit of antigorite stability the distinction between these two is based on textural and mineralogical criteria serpentinite mylonites are cohesive homogeneously foliated and typically strongly lineated rocks composed primarily of antigorite and magnetite they occur in planar bounded zones enclosed within the josephine peridotite that were the loci of both noncoaxial deformation and serpentinization we discuss the development of two important shear sense indicators porphyroclast systems and oblique shear band foliations as observed in these rocks porphyroclast systems in serpentinite mylonites are often comprised of cores and mantles of different mineralogy cores are usually composed of primary pyrogenetic minerals whereas mantles including tails are composed of products of the serpentinization reaction these systems are therefore termed porphyroctasts with neocrystallization tails oblique shear bands are common micro structures in serpentinite mylonites shear bands which offset the mylonitic foliation along narrow discrete fractures are termed f type whereas those which offset the mylonitic foliation along narrow ductile micro shear zones are termed d type the latter are considered more reliable for shear sense determinations strains accomplished by movement on shear bands are probably small strain resulting from a volume increase dur ing serpentinization while holding the shear zone width constant is sufficient to produce the shear bands therefore we suggest that oblique shear bands may develop to accom modate volume increases associated with deformation

on meteorologically induced flushing in three u s east coast estuaries abstract meteorologically induced exchange with the coastal ocean is examined for three u s east coast estuaries using time series of subtidal volume flux over a common five month period a volume of water equal to that of the estuary was discharged every 86 4 135 3 and 108 8 days for narragansett delaware and chesapeake bays respectively however a large fraction of the water discharged to the shelf is recirculated ocean water for chesapeake bay outflowing estuarine water can be considered as a mixture of 2 4 3 0 water of mean estuarine salinity and the remainder water of oceanic salinity the degree of meteorological flushing is strongly influenced by mean depth as a drop in sea level of a given amplitude will force a greater fraction of water from a shallow estuary than from a deep system the relative importance of the meteorologically forced circulation and the gravitational circulation is frequency dependent as illustrated by salinity variance at the entrance to chesapeake bay seasonal salinity variance must ultimately be controlled by seasonal changes in runoff through the mechanism of gravitational circulation however the high coherence of wind induced changes in bay volume and bay entrance salinity at periods of 2 20 days suggests that meteorologically forced advection is primarily responsible for salinity variance at these time scales

western limassol forest complex cyprus part of an upper cretaceous leaky transform fault field petrographic and geochemical studies suggest that the limassol forest complex formed in a leaky left lateral oceanic transform fault multiple mafic and ultramafic intrusions into tectonized harzburgite were synchronous with ductile and later brittle tectonism a history of increasingly chilled intrusive contacts a transition from ductile to brittle deformation and a decrease in metamorphic grade all indicate continuous cooling and uplift of the harzburgite mantle host while it was part of an active transform domain the occurrence of multiple 040 n trending mafic and picrite dikes with extremely depleted high field strength element abundance suggests that strike slip movement along the transform fault was accompanied by extension that led to magma emplacement and eruption

morphotectonic fabric of the orozco transform fault results from a sea beam investigation we have used data collected with the sea beam system to produce a detailed regional bathymetric map of the medium to fast slipping 90 mm yr orozco transform fault the morphotectonic fabric of the 90 50 km transform domain is spatially heterogeneous and is composed of relatively shallow areas 3200 m the plate boundary geometry is clearly identified morphologically and is characterized by a series of linked basins and troughs that approximate a zed pattern in plan view east west trending basins define the northern and southern boundaries of the transform domain and these troughs are thought to mark the location of strike slip tectonism a central north south trending ridge with an axial valley called here the mid orozco spreading center is thought to represent a short 30 km ridge segment that links two transform faults morphotectonic relationships developed along the southeastern flank of the survey area suggest that this plate boundary geometry has recently become unstable and is presently undergoing a transformation as the tip of the east pacific rise axis propagates northward across the transform domain bypassing the mid orozco spreading center if the southern ridge axis continues to propagate northward it will link with the aseismic extension of the northern strike slip strand and the present day plate boundary will evolve into a simple ridge transform ridge r t r geometry with an offset of approximately 70 km

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

multiple closely spaced transform faults in fast slipping fracture zones the 400 km long offset of the east pacific rise spreading axis between the quebrada fracture zone lat 3 5 s and the gofar fracture zone lat 4 6 s has been surveyed using the long range sidescan sonar gloria the offset is accomplished in three distinct fracture zones each of which contains up to four actively slipping transform faults separated by short spreading centers transform spacings range from 5 to 16 km each transform is associated with its own transverse valley but at quebrada fracture zone and to some extent at the other two these valleys merge into one large slightly oblique fracture zone valley the morphological trends of fracture zone valleys are therefore not necessarily reliable for estimating directions of relative plate movement multitransform fracture zones appear to be much commoner on the fast spreading east pacific rise than on slower spreading ridges the tectonic spreading fabric which comprises closely spaced normal faults formed near the spreading axes and striking subparallel to them is modified to a sigmoidal pattern between the closely spaced transforms even midway between transforms these faults remain oblique to the spreading direction the degree of obliquity increases as the transform separation decreases showing progressive rotation of the stress ellipse as transforms are approached

a sea beam investigation of the clipperton transform fault the morphotectonic expression of a fast slipping transform boundary a regional scale bathymetric map of the fast slipping 11 0 cm yr clipperton transform boundary and adjoining east pacific rise segments has been constructed using the high resolution capabilities of sea beam the transform domain is 85 km long varies in width from 10 to 20 km and is defined by an assemblage of transform parallel ridges and troughs the largest of which is a 40 km long median ridge that stands up to 700 m above the surrounding seafloor these major topographic elements of the transform domain are bisected by a very narrow 1 km swath of strongly lineated terrain composed of elongate depressions narrow lozenge shaped ridges and gullies that integrate spatially to create a narrow zone of disturbed topography this zone trends 082 and cuts like a knife across the grain of the clipperton transform domain linking one truncated ridge tip with the other we interpret this swath of disturbed terrain to be the product of strike slip tectonism thereby fixing the location of the present day plate boundary the present day structural geometry of the clipperton transform appears relatively simple and characterized by a narrow strike slip plate boundary linking two offset ridge axes

kinematic history of a reactivated oceanic suture the mamonia complex suture zone sw cyprus the mamonia complex suture zone in sw cyprus marks an arcuate tectonic suture between an upper cretaceous oceanic microplate of supra subduction zone origin troodos complex and a deformed triassic passive margin sequence mamonia complex mapping of the serpentinite filled fault zones that define the suture zone combined with microstructural studies of its constituent fault rocks has revealed evidence of three main tectonic events of upper cretaceous age 1 high temperature c 600 c transform related dextral strike slip and localized metamorphism of mamonia complex units c 90 83 ma 2 retrograde hydration c 83 73 ma 3 regional contractional reactivation at low temperatures which ceased by the latest maastrichtian c 65 ma the present day structural geometries are interpreted as the result of maastrichtian contractional reactivation of a pre existing steep and irregular structural architecture that formed during transtension the resultant orientation of finite strain axes and thrust displacements display a radial distribution documenting near fault normal contraction along the whole length of the arcuate suture zone this pattern of displacements suggests a model whereby the troodos microplate impinged on the mamonia complex during westerly to southwesterly directed plate motion contractionally reactivating the mechanically weak suture zone

volcanic activity and crust mantle exposure in the ultrafast garrett transform fault near 13 28 s in the pacific the 130 km long garrett transform fault which offsets two ultrafast 14 5 cm yr epr spreading segments is the site of volcanic and tectonic activity evidence for recent volcanic events was observed during several dives on oblique ridges 3100 3500 m high trending 040 alpha beta and gamma and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500 5000 m the constructional features show a fissured terrain suggesting a component of lithospheric opening the strike slip motion of the transform is inferred at several locations between the two epr transform intersection zones from the presence of abundant sediment free talus with crushed rock debris fresh scarps and well developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia the width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western epr transform intersection zones along the e w trending median ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south facing wall of the transform domain first escarpment the presence of recent volcanic activity implies that the garrett transform behaves as a leaky transform fault the most common volcanics consisting of plagioclase and olivine phyric lava erupted in the transform are compositionally more depleted k ti 0 08 and less evolved mg 60 70 than the basalts encountered on the southern epr segments mg 40 66 and k ti 0 05 0 25 this suggests that different magmatic conditions are operating in the transform domain the transform strike slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the median ridge at the contact with outcrops of metamorphosed basaltic breccia the ultramafics consist of residual harzburgites often impregnated with irregular lenses and dykes of basaltic liquids plagioclase wehrlite and gabbro and are found associated with troctolites dunite and gabbros the emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the median ridge in the central transform basin

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

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma