structural anatomy of the kushtagi schist belt dharwar craton south india an example of archaean transpression abstract the nw se trending kushtagi schist belt in the eastern dharwar craton records the tectonic consequences of a transpression regime it was formed due to late archaean oblique convergence in the dharwar craton this relatively unknown schist belt is characterized by two major episodes of deformation the dominant d 1 deformation forming tight to isoclinal non cylindrical folds and a regional axial planar schistosity is a consequence of transpression the transpression related sinistral shear regime is followed by development of a conjugate set of f 2 folds and s 2 cleavages that are superimposed on the d 1 structures the maximum shortening direction associated with the f 2 folds is nw se which suggests longitudinal compression subsequent to transpression riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate riedel shear fractures intrusive granite bodies within and outside the schist belt were emplaced in the late phase of d 1 transpression

the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

change in fabric of melange in the shimanto belt japan change in relative convergence systematic changes in fabrics of melanges have been detected in rocks of the shimanto belt japan several shear indicators including s c structures riedel shears and folds in the melanges indicate consistent sense of shear both in outcrop and microscopic scales the deformation mechanisms independent particulate flow and pressure solution suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening after lithification cataclastic deformation occurred locally but most of the earlier deformation features are preserved this process appears to have taken place along the decollement beneath the off scraped accretionary prism sinistral reverse sense of shear is observed in the campanian melange whereas a dextral reverse sense of shear is preserved in the early eocene melange in the shimanto belt this change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates three different plate models have been proposed for the late cretaceous to early tertiary western pacific margin 1 subduction of the kula pacific ridge along the continental margin 2 subduction of the pacific plate since 85 ma and 3 subduction of the aborted kula pacific ridge after 43 ma the observed change in fabric of melange is consistent only with the third model which predicts a change in relative convergence from sinistral to dextral at the appropriate time

mesoscale eddies in the south china sea observed with altimeter data 1 a composite time series 1993 2000 of sea surface height anomaly from several satellites is used to identify eddies in the south china sea scs the eddy lifetime radius strength and straight line travel distance are estimated altogether 58 anticyclonic eddies and 28 cyclonic eddies are identified for this period they are grouped into four geographical zones according to known eddy generation mechanisms and their statistics are discussed accordingly our geographical classification is a useful first step in gaining an overview of their generation

surface features of winter monsoon surges over south china abstract the surface features associated with two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns surface meteorological parameters over the region 15 50 n 90 130 e for the surges that occurred in the three winters october march from 1988 to 1991 are analyzed for the northerly surge the surface features found are 1 an abrupt temperature drop and wind direction turning from easterly to northerly which can be related to the passage of a cold front 2 an increase in the dewpoint depression and 3 a large north south pressure gradient on the other hand the easterly surge is found to be associated with strong easterly winds up to approximately 40 km h 1 little temperature or pressure change and a southeastward motion of a high pressure center from dahinggangling to the yellow sea together with a sharp pressure ridge along the east china coast furthermore an es and an ns are associated with different perturbations anomalies in

low frequency surface currents and generation of an island lee eddy in panay island philippines abstracthigh frequency doppler radar hfdr and acoustic doppler current profiler adcp time series observations during the philippine straits dynamics experiment philex were analyzed to describ

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

island wakes in deep water abstract density stratification and planetary rotation distinguish three dimensional island wakes significantly from a classical fluid dynamical flow around an obstacle a numerical model is used to study the formation and evolution of flow around an idealized island in deep water i e with vertical island sides and surface intensified stratification and upstream flow focusing on wake instability coherent vortex formation and mesoscale and submesoscale eddy activity in a baseline experiment with strong vorticity generation at the island three types of instability are evident centrifugal barotropic and baroclinic sensitivities are shown to three nondimensional parameters the reynolds number re rossby number ro and burger number bu the dependence on re is similar to the classical wake in its transition to turbulence but in contrast the island wake contains coherent eddies no matter how large the re value when re is large enough the shear layer at the island is so narrow that the ver

earthquake focal mechanisms and the intraplate setting of the bermuda rise a number of intraplate earthquakes occurring in the western north atlantic ocean are located near the perimeter of the bermuda rise focal mechanisms and depths of two earthquakes november 24 1976 mb 5 1 m0 2 96 1023 dyne cm and march 24 1978 mb 6 1 m0 3 58 1025 dyne cm were determined using rayleigh wave amplitude data in the period range 20 50 s the 1978 earthquake occurred approximately 380 km southwest of bermuda near magnetic anomaly m4 118 m y b p the focal mechanism for the 1978 event is of thrust type and has nodal planes striking 340 the depth of this event is 6 km below the seafloor near the local depth to mono the strike of the fault planes does not parallel the trends of either fracture zones 300 or magnetic lineations 035 in the area the fault planes do however parallel the strike of a magnetic gradient in the epicentral area the 1976 earthquake occurred approximately 300 km northeast of bermuda near muir seamount the depth of this event is 10 km below the seafloor the available data are suggestive of one nodal plane striking between 320 and 340 and nearly parallel to the trend of muir seamount and other volcanic features in the region in contrast to the 1978 event the 1976 earthquake appears to exhibit a significant component of strike slip motion p axes of both mechanisms are subparallel to the direction of absolute plate motion for north america we suggest however that strain release in the bermuda rise area is not occurring along major fracture zones or topography parallel to seafloor spreading anomalies but rather on smaller scale structures the stresses induced by variations of crustal thickness may be responsible for triggering intraplate seismicity in this region

dynamics of the polish and eastern slovakian parts of the carpathian accretionary wedge insights from palaeostress analyses the arcuate outer carpathian accretionary wedge formed in front of the east alpine carpathian pannonian alcapa megablock during the eocene sarmatian the wedge accreted sediments of the subducting remnants of the carpathian flysch basin a large oceanic tract left in front of the alpine orogen the palaeostress data for the orogenic hinterland particularly the data related to the early miocene extension that was expanding towards the ne combined with coeval subduction related volcanism that was expanding towards the ne indicate that the uneven roll back of the subduction zone was the main mechanism controlling the development of the northern west carpathian arc the palaeostress data for the tertiary accretionary wedge from the same time period are characterized by outward fanning 1 trajectories that changed gradually during the wedge development in contrast the palaeostress data for the hinterland are characterized by preferred directional stress events that changed abruptly during the wedge development these palaeostress results are in accordance with the behaviour of the wedge and the hinterland as the wedge behaved as a weak continuum and the hinterland behaved as a block mosaic with weak boundaries the fault traces of the northern west carpathian arc converge to both ends of the arc and suggest that the pre existing basin was the factor that controlled the arc location these fault trace patterns are asymmetric indicating a slightly oblique overall convergence in a ne sw direction in accordance with this convergence the palaeostress data for the accretionary wedge indicate that the western part of the wedge which is characterized by nw se oriented maximum principal compressional stress 1 was undergoing sinistral transpression meanwhile the eastern part which is characterized by ne sw oriented 1 was undergoing compression apparently the dynamics of the accretionary wedge was further influenced by the shape of an elongated ne sw trending alcapa megablock which was located behind the wedge and advanced in the direction of the general early miocene convergence during the most pronounced stages of the wedge development this megablock served as the local indenter as its strength surpassed that of the accretionary wedge located to its front further dynamic complexities were added because of the complex shape of the magura unit which was located in the most proximal portion of the wedge and was stronger than the units in front of it wedge outcrops indicate that the large scale shortening which is characterized by the development of detachments and ramps was preceded by an initial layer parallel shortening this is indicated by scaly fabrics and minor reverse faults that rotated into locked positions during the later accretion several outcrops with a wedge detachment fault indicate that there was a relatively low amount of friction during its development the decollement zone is several hundred metres thick and shows evidence of transient fluid flow that was driven by pressure gradients this is documented by frequent hydrofracturing sandstone dykes and fibrous veins that opened against the weight of the whole wedge all of which indicate cycles of higher pore fluid pressures that lowered the basal friction

interannual variability of winter eddy patterns in the eastern south china sea merged altimetry data collected over 21 years are analyzed to study the interannual variability of winter eddy activities in the eastern south china sea scs associated with the el nino southern oscillation composite analyses of eddy probability reveal that a luzon cold eddy lce northwest of the luzon island an anticyclonic eddy northwest of the mindoro island referred as mae and a cyclonic eddy west of the mindoro island referred as mce are common for winter further studies indicate that lce and mae weaken and mce strengthens in el nino winters whereas data show the opposite behavior for la nina winters in addition the geographical pattern of mae and mce in an el nino winter is different from that in a la nina winter the interannual wind curl anomalies west of the luzon and mindoro islands and the outflow from the scs via the mindoro strait may be responsible for the interannual eddy variability

monsoon surges trigger oceanic eddy formation and propagation in the lee of the philippine islands 1 two winter monsoon surge events northerly and easterly of january 2005 are captured in a one way coupled atmosphere 8 km resolution and ocean 3 km resolution simulation of the philippines region intensified wind jets and wakes in the lee of mindoro and luzon islands induce the generation and migration of a pair of counter rotating oceanic eddies in the model with propagation direction related to the orientation of the winds during each of the surges features shared by the eddies include size 100 200 km depth 300 m and propagation speed 0 1 0 15 m s 1 for cyclones mean wintertime model wind stress positive negative curl coincides with the climatological cyclone anticyclone distribution from a prior 8 year altimetry based census of eddies in the southeast quadrant of the south china sea during the winter monsoon moreover the simulation results agree with contemporaneous satellite and historical in situ data characterizing regional oceanic eddy and atmospheric surface jet properties

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

anatomy and evolution of a pull apart basin stellarton nova scotia the stellarton basin is a late paleozoic pull apart basin located close to the meguma avalon terrane boundary in the canadian appalachians at the stepover between the cobequid and hollow strike slip faults the basin contains 3 km of rapidly deposited pennsylvanian clastic sedimentary rocks representing lacustrine and deltaic environments extensively documented through coal related mining and drilling coal seams and oil shales allow stratigraphic correlation within the basin permitting reconstruction of basin subsidence and structural evolution coal seams represent approximately paleo horizontal surfaces thickness variations corrected for tilt and compaction in the most coal rich part of the basin fill show that the south basin margin subsided rapidly during deposition acting as a trap for coarse sediment and allowing coal forming mires to develop in the north abundant soft sediment deformation structures reflect synsedimentary tectonic activity north striking normal faults dissected the basin fill during and soon after deposition early in the diagenetic history contouring of mine plans allows fault heaves to be identified and also shows that both coal seams and faults were folded by east to northeast trending folds consistent with 1 an environment of deformation involving dextral strike slip motion and 2 clockwise rotation of fault blocks during progressive strain later north northeast striking contractional structures within the basin and a positive flower structure at the northwest margin probably represent a transition from transtension to transpression the basin illustrates the role of rotational progressive strain in the reorientation of structures generated during transtension it displays evidence for changes in tectonic style through time from transtension to transpression that may typify basins developed on major strike slip faults

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

paleomagnetism in the northeastern central massif france evidence for carboniferous rotations of the hercynian orogenic belt abstract a paleomagnetic study has been carried out on visean volcanics of northeastern central massif standard paleomagnetic techniques have been applied to a hundred of samples alternating field and thermal demagnetizations yield an unusual direction d 78 i 2 95 12 for the 9 best sites as the polarity is unknown two motions can explain this direction either a clockwise rotation of about 45 or an anticlockwise of 135 relative to the armorican massif in both cases the paleomagnetic data reveal important deformations of the hercynian orogenic belt probably due to strike slips in middle carboniferous times

a kinematic analysis incorporating incremental strain data for the frontal pennine zones of the western french alps abstract an incremental and finite strain study of the zones adjacent to the frontal pennine thrust fpt has been undertaken using syntectonic fibres taken from veins pyrite type pressure shadows and similar fringes from around pebbles in conglomeratic rocks the resultant strain data together with detailed mapping of local deformational geometries and regional trends has shown that the pennine zones show a more complex structural history than previously envisaged the structural analysis divides the pennine zones into two regions north and south of creve tete the southern termination of the valais basin the zones to the south of creve tete mont fut valbuche nielard and aiguille d arves show an essentially piggy back thrust sequence directed towards the foreland in which the elongation of the strain ellipse initially parallels the transport direction wnw but finally rotates to become sub parallel to the tectonic strike nne ssw the structures of the valais zone to the north of creve tete are more complex with both n and wnw verging folds thrusts and strike slip faults here the long axis of the strain ellipse appears initially to parallel the tectonic strike ne sw this is followed by a rotation into the late transport direction wnw this localised alternation between strike and transport parallelism is problematic specific examples of strike parallel extension may be explained in terms of radial transport buttressing against earlier extensional structures strike slip motion and or stretching over lateral culmination walls the major division in fibre pattern north and south of creve tete however is best explained as being a function of compartmentalization of deformation styles between adjacent areas models which invoke a dominant deformation mechanism e g thrust tectonics along the entirety of the alpine chain thus need to be rethought similarly the recognition of widespread strike parallel extension suggests that two dimensional balanced section techniques are simplistic giving only minimum shortening estimates in a single direction and thus failing to reflect the true situation

timing of exhumation of a high pressure mafic granulite terrane of the paleoproterozoic ubende belt west tanzania abstract the ubendian belt west of the archean tanzanian craton is known as a prominent crustal structural weakness the belt is composed of different terranes set in a nw se oriented lateral shear pattern sharply outlined by steeply inclined inter terrane shear fault zones its predominant mafic terranes ubende and mbozi display different and more complex evolutionary characteristics than their adjacent felsic terranes ufipa metamorphic studies reveal that mafic and ultramafic high pressure granulites and rare eclogites subsist as rigid blocks and lenses within the highly sheared amphibolite facies country rock p t studies suggest two succeeding decompressional stages for the uplift of the high pressure mafic granulite terranes the second stage being identified on high pressure blastomylonites included in the latter are amphibolites mafic tectonites for which barroisite constitutes a p t marker near the southwestern boundary of the nw se oriented mafic terranes these barroisite bearing tectonites display a pronounced extensional shear banding considering that this tectonite has been formed during the second stage of uplift of the ubende high pressure mafic granulites timing of that stage is pursued ar ar stepwise heating analyses on different barroisite separates from this mafic tectonite have yielded a weighted average cooling age of 1848 6 ma for the argon fractions released at intermediate temperatures this age falls in the time range 1950 1850 ma associated with the second phase of the evolution of the ubende belt characterized by amphibolite facies metamorphism the isotopic composition of argon released at higher temperature steps from compositionally different phases in the separates yields apparent ages tentatively considered only as indicative for a possible older paleoproterozoic event based on geochronological and structural data the uplift of the high pressure granulite ubende terrane is interpreted as related to a late paleoproterozoic dextral lateral shear deformation in ductile amphibolite facies conditions this appears to be associated with the emplacement of granites subsequent proterozoic strike slip reactivation of the paleoproterozoic ubende shear belt did not fundamentally contribute to the near surface exhumation of the high pressure mafic granulites

position of the lhasa block south tibet during the late cretaceous the latitude of the lhasa block xizang south tibet during the late cretaceous early tertiary is of prime importance for understanding the collision of india and eurasia and the mechanism of the formation of the qinghai xizang plateau during the first field expedition of the french chinese collaboration several geological formations have been sampled in southern tibet between lhasa 29 40 n 91 09 e and dingri 28 35 n 86 38 e we present here the first results obtained for middle cretaceous to palaeocene red beds from the lhasa block north of the yarlung zang bo suture zone recognized as the main suture between the indian and the eurasian plates they indicate a palaeolatitude of the lhasa block of 20 n during the late cretaceous while india was situated 30 s

islands as eddy transformation and generation hotspots cabo verde case study abstract deep ocean islands have long been associated with the generation of oceanic eddies in their wakes however their interaction with incoming eddies has seldom been considered this study focuses on the characterization of background and locally generated mesoscale eddies in the cabo verde archipelago between 2003 and 2014 special attention is given to the interaction of incoming eddies with the bathymetry of the islands along with their impacts on the local generation of eddies island induced wind shear effects are also considered in addition some examples of the biological response to background and locally generated eddies are discussed this is achieved by combining remote sensing satellite observations for wind sea surface height and chlorophyll a chla surface concentrations the results show that the interaction between incoming background eddies and the archipelago is a recurrent phenomenon which results in eddy deflection splitting merging intensification and termination sorted by highest to lowest number of occurrences local island induced disturbances are also significant mainly due to atmospheric effects such processes result in the generation of island induced eddies and in wind mediated eddy intensification and confinement more often observed in the leeward group nonetheless it is strongly suggested that many of the locally generated eddies are a direct product or by product of the interaction of background eddies with the islands with respect to the biological realm a locally generated cyclonic eddy is observed to originate a pronounced phytoplankton bloom in the vicinity of the tallest island nonetheless background eddies generated off the african coast are often associated with enhanced chla concentrations when they intersect the archipelago such observations challenge the idea that local biological productivity in deep oceanic islands is exclusively driven by island induced mechanisms

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

an oceanic cyclonic eddy on the lee side of lanai island hawai i 1 a young cold core cyclonic eddy displaying a significant increase in surface chlorophyll was observed offshore lanai island hawai i where the marine optical buoy moby is located during one of its deployments moby broke free from its mooring in the course of its 3 day free drifting period moby followed a cyclonic eddy which is manifested by satellite remote sensing data chlorophyll data from moderate resolution imaging spectroradiometer and sea surface temperature sst from a geostationary operational environmental satellite the time series of the sst show that the cold core eddy was in a formative stage it existed as a stand alone eddy for about 9 days before it merged with cold water south of oahu island a high resolution numerical model simulation reproduces similar eddies in terms of location size and intensity an eddy detection algorithm is described and applied to locate and track the modeled eddies the results demonstrate that mesoscale and submesoscale eddies are frequently generated on and pass through the lee side of lanai island and the statistical analysis quantifies the general features of eddies in the area

kinematics of austro alpine cover nappes changing translation path due to transpression upper and middle austro alpine nappes of the eastern alps the structural uppermost subunits of the alps suffered two major superposed deformations d1 d2 during the eo alpine cretaceous orogeny d1 is characterized by nw to w directed thrusting and nappe internal rock flow d2 by large scale folding and imbrication toward the north and northeast structures strain metamorphic history microfabrics and stretching trajectories indicate that the superposition of d2 over d1 structures can be interpreted as a progressive deformation it is concluded that the emplacement of the nappes follows a curved translation path a model which emphasizes non coaxial deformation structures explains the detachment of the nappes the configuration of the separating thrust planes and the distribution of intense deformation zones the overall kinematic history of the austro alpine nappes fits a model in which strike slip shearing horizontal shortening and vertical lengthening act together continuously over a finite deformation zone transpressional model this model emphasizing a gradually changing strain regime eliminates the space problems encountered in models which consider simple south over north imbrication during the alpine orogeny the study stresses two interpretations 1 1 stretching lineations indicate nappe transport directions 2 2 transpression may be a common feature of orogenic belts

u pb zircon evidence for a pan african granulite facies metamorphism in the central african republic a new interpretation of the high grade series of the northern border of the congo craton abstract the results of u pb zircon and sm nd whole rock analyses are reported on three meta igneous granulitic samples from the central african republic basement formerly ascribed to an archaean complex u pb data point to late proterozoic ages 833 66 ma 652 19 ma and 639 3 ma and sm nd model ages in the 1700 2200 ma range preclude an exclusive derivation from an archaean source whatever their precise geologic significance either first crystallization of zircons at the time of igneous emplacement or complete resetting during metamorphic recrystallization the u pb data clearly rule out an archaean age for the high grade event in contrast with previous opinion indeed the granulite facies metamorphism cannot be older than late proterozoic this is in good agreement with the recent recognition of a major collision type orogen oubanguides belt of pan african age in this area it is proposed that two main generations of granulitic rocks occur at the northern margin of the congo craton an archaean one in the foreland of the oubanguides belt e g ntem complex cameroon and a pan african one in the internal zones of this orogen

ductile thrusting within subduction complex rocks on signy island south orkney islands abstract a model involving repeated nnw directed ductile thrusting in subduction complex rocks on signy island south orkney islands accounts for the consistent nnw trending alignment of fold axes stretching and shearing directions in the absence of major discrete thrust planes stratigraphic control or younging criteria the evidence for consistent ductile thrusting is more cryptic and is found in folding stretching relations porphyroblast rotations and mylonite fabrics early fold axes parallel to stretching syn tectonic garnet rotations augen lineations and sheath folds all support nnw directed thrusting as do a common late shear band foliation and minor shear zones development of late open folds with axes parallel to stretching may be due to constrictional deformation a lateral ramp or differential movement during thrusting the structures described from signy island may characterize the type of deformation in deep level ductile thrust zones at the base of an accretionary wedge or in a zone of subcretion the thrusting direction on signy island is inferred to be parallel to the direction of subduction in the south orkney islands in the early jurassic plate reconstructions suggest that this direction was at a high angle to the arcuate trend of the antarctic peninsula continental margin

observations of exchange between the south china sea and the sulu sea 1 the velocity and transport with thermohaline properties are determined from simultaneous moored time series within mindoro tablas and panay straits that connect the south china sea scs and sibuyan sea to the sulu sea these passages provide the only pathway for the scs throughflow below thermocline depths and play a critical role in regulating the heat and freshwater balance within the philippine archipelago the upper layer in mindoro strait has a distinctly seasonal cycle with northward flow during the boreal summer southwest monsoon and southward flow during the winter northeast monsoon in contrast upper layer flow in panay and tablas straits is intraseasonal extraordinarily strong pulses that begin at intermediate depth in the fall transition and shoal toward the sub thermocline during the northeast monsoon are found in mindoro and panay straits these southward flows are strongly correlated to the scs large scale circulation and remote wind forcing off vietnam temperature and salinity in mindoro support the scs source of this subtropical water southward benthic flow in mindoro and panay act to ventilate the deeper sulu sea bottom temperatures suggest the cooler mindoro overflow mixes with the warmer benthic waters from tablas before exiting panay strait into the sulu sea the 2008 mean annual total depth transports were very small o 0 1 sv although individual transport estimates of 1 2 sv were observed in all depth layers over the year the mooring deployment coincided with a strong la nina when the pacific inflow into the scs is expected to be weaker and subsequently transports are lower

upper layer circulation in the south china sea abstract upper layer circulation is investigated by using all available historical temperature profiles combined with climatological temperature salinity relationships in the south china sea two cyclonic eddies are revealed one is located east of vietnam called the east vietnam eddy and the other is off northwest luzon called the west luzon eddy both local ekman pumping and remotely forced basin scale circulation are important mechanisms controlling these two eddies the luzon strait transport relative to 400 db is estimated to be of the order 3 0 sv 1 sv 1 106 m3 s 1 in the mean and has a seasonal cycle dominated by the annual signal with a maximum 5 3 sv in january february and a minimum 0 2 sv in june july pressure gradients are also examined to explore the dynamics of the intrusion of waters from the pacific into the south china sea through the luzon strait

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

transition from convergence to escape field evidence from the west carpathians abstract the tertiary deformation of the west carpathians occurred during northeast directed convergence of the european and apulian plates and was complicated by space constraints imposed by orogen advance palaeostress fields computed from fault slip data and structural interpretation of known gravimetric magnetic seismic palaeomagnetic lithostratigraphic sedimentological fission track and borehole data indicate that during deformation the orogen consisted of both frontal thrust stacking zones and internal extensional trans tensional zones synthesis of available data indicates that the episodes of tectonism in litho tectonic units were both time and plate position related and were caused by conditions which varied in different areas ranging from the foreland to back arc region in each time period discrete deformation phases record the transition from convergent to tectonic escape tectonisrn principal horizontal stress trajectories were approximately co linear with the plate convergence vectors prior to escape the onset of escape conditions is indicated by 1 clockwise rotation of palaeostress orientations during the time of known escape in the eastern alps and 2 the cessation of thrusting of the flysch belt nappes onto the neogene foredeep in the westernmost part of the west carpathians tectonic escape along strike slip faults subparallel to the collision suture zone began in early badenian time in the west carpathians various regions along the orogenic belt were affected by escape that became progressively younger towards the east this agrees with the observations of various authors that the final phase of thrusting towards the european platform became younger to the east along the west carpathian arc in addition palaeostress trajectory patterns for various periods are similar to those modelled by a rigid block indenting a viscous half space an attempt is made to complete the mosaic of information on the cenozoic evolution of the alpine carpathian pannonian area which in the models given by ratschbacher royden horvath bergerat and others is based on extrapolated data the object is to detail the timing and complexity of the local time and plate position related mechanisms in the context of the entire orogen in order to update previous rather simplified models

chaotic advection in an archipelago abstract techniques from dynamical systems theory have been applied to study horizontal stirring of fluid in the philippine archipelago the authors analysis is based on velocity fields produced by two high resolution 3 and 6 km numerical models particular attention is paid to identifying robust surface flow patterns and associating them with dominant lagrangian coherent structures lcss a recurrent wind driven dipole in the lee of the coastline is considered in detail the associated lcss form a template for stirring exchange and biological transport in and around the dipole chaotic advection is argued to provide a relevant framework for interpreting mesoscale horizontal stirring processes in an archipelago as a whole implications for the formation of filaments the production of tracer variance and the scale at which stirring leads to mixing are discussed in connection with an observed temperature record

mesoscopic structural fabric of early paleozoic rocks in the northern sierra nevada california and implications for late jurassic plate kinematics abstract analysis of the mesoscopic structure of the early paleozoic shoo fly complex northern sierra nevada california reveals three phases of deformation and folding the first phase of folding is pre late devonian and the second two are constrained by regional relations as due to the late jurassic nevadan orogeny main phase nevadan deformation produced penetrative slaty cleavage which is steep nnw trending and parallel to tectonostratigraphic terranes of the region cleavage is axial planar to ubiquitous isoclinal similar folds fold axes define a nnw trending girdle with a distinct near vertical maximum main phase nevadan folds have nearly ideal class 2 orthogonal thickness geometry although some class 1c forms exist in more competent units the overall geometry of main phase folds suggests formation by progressive deformation in a flattening regime with cleavage as the flattening plane and a steep extension axis defined by the fold axis maximum a steep extension axis direction for main phase nevadan deformation is supported by analysis of interference relations where folds of this generation deform pre late devonian folds late nevadan folds range from kink flexures to ideal class 2 similar folds with incipient axial planar cleavage the kinematic significance of late nevadan folds cannot be evaluated because of their varying style and orientation throughout the northern sierra nevada penetrative ductile deformation and near vertical extension during the nevadan orogeny was synchronous with accretion of oceanic and or island arc rocks against the western margin of the northern sierra nevada the kinematic framework of deformation defined for nevadan deformation is consistent with essentially orthogonal convergence of these exotic terranes with the sierran margin and argues against a transform transpressive regime

ductile thrusting in the himalayas shear sense criteria and stretching lineations the most useful criteria used for the determination of the sense of shear can be found in the huge himalayan ductile shear zone known as m c t they allow the determination of the sense of shear in relation with an unequivocal southward directed intracontinental thrust the various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution however most of the observed shear criteria are due to late nappe emplacement postdating the climax of the metamorphism the most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt it parallels everywhere the shear direction and thus indicates the transport direction of the nappes the observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence

observations of internal waves and parametric subharmonic instability in the philippines archipelago 1 internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing by wind and tide and interior diapycnal mixing we report here on measurements from mindoro strait in the philippines connecting the south china sea to the sulu sea of an internal wave field with a number of surprising properties that point to previously unrecognized processes at work in the region continuum spectral levels are very close to typical background values found in the open ocean but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated and higher at the northern mooring mp1 than the southern mp2 two particularly energetic depth ranges stand out at mp1 an upper layer centered near 300 m and one at the bottom of the water column near 1800 m the upper layer contains both internal tides and a near inertial band with upward and downward propagating waves and an apparent spring neap cycle the combination is suggestive of parametric subharmonic instability as the forcing for the near inertial band a conclusion supported by bicoherence estimates mixing estimated from density overturns is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide both diurnal and semidiurnal near inertial currents in this deep layer are dominantly rectilinear and not well correlated with the mixing bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography suggesting additional enhancement elsewhere in the archipelago

three long lived anticyclonic eddies in the northern south china sea 1 in august 2007 three long lived anticyclonic eddies ace1 ace2 and ace3 were detected by both satellite sea level anomaly sla map and in situ measurements in the northern south china sea scs ace3 had a two core ace3 1 and ace3 2 structure in situ stations along 18 degrees n almost cut through the centers of ace2 and ace3 2 near the centers of ace2 and ace3 2 mean temperature and sound velocity are similar to 0 65 degrees c and similar to 2 m s 1 larger than those in their surrounding areas respectively while mean salinity and density are similar to 0 02 psu and similar to 0 15 m 3 s 1 smaller than those in their surrounding areas due to downwelling near the eddy cores the depths of maximum and minimum salinity near the eddy cores are similar to 65 m and similar to 35 m larger than those in their surrounding areas the vertical depth with current speed larger than 0 05 m s 1 can reach similar to 900 m their detailed evolutionary processes were depicted based on the variation of geostrophic currents and the trajectories of five drifting buoys ace1 lasted 147 days while ace2 and ace3 lasted 168 days and 210 days respectively ace1 had a smaller mean sla 18 8 cm in its lifetime than ace2 21 8 cm and ace3 25 3 cm but had a larger negative mean vorticity 7 7 x 10 6 s 1 than ace2 7 0 x 10 6 s 1 and ace3 7 0 x 10 6 s 1 one short lived anticyclonic eddy that split from ace2 and another one that merged with ace3 both had a smaller sla negative vorticity and diameter than ace2 and ace3 respectively

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

upper level features associated with winter monsoon surges over south china abstract the upper level features associated with the two kinds of winter monsoon surges over south china are studied the easterly surge es and the northerly surge ns the study is similar to that used by wu and chan except that a broader region 0 60 n 70 160 e is considered the outbreak of an ns is associated with a breakdown of the siberia mongolia high the upper level features suggest that the evolution of the siberia mongolia high in an ns can be related to an eastward passage of a short wave trough and the polar jet based on the quasigeostrophic theory the intensification of the siberia mongolia high appears likely to be caused mainly by the adiabatic cooling due to rising motion associated with the trough and the polar jet passages after the passage of the trough and the jet the siberia mongolia high breaks down in response to the sinking motion upstream of the trough causing a surge of the northerly winds over the south china coast for the es the passage of an upper ridge is obse

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

palaeomagnetism of the ibero armorican arc and the hercynian orogeny in western europe van der voo1 has pointed out that further advances in the understanding of the hercynian orogeny require palaeomagnetic data from well studied orogenic areas we report here palaeomagnetic results of palaeozoic rocks taken in brittany portugal and spain in the ibero armorican arc iaa a large geological feature considered to be a key area for the hercynian orogeny in western europe they show evidence that the iaa formed in two stages extending results obtained in spain by ries et al 2 giving precise constraints for the former shape of the arc we can use pre hercynian palaeomagnetic directions found in spain and portugal to follow its evolution during palaeozoic times this megastructure linking brittany to iberia squeezed between gondwana and the northern continents during the carboniferous did break away from the former block during ordovician times

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

coupled ocean atmosphere nested modeling of the adriatic sea during winter and spring 2001 1 realistic simulations of the adriatic sea for over 125 days are conducted using the navy coastal ocean model with atmospheric forcing provided by the coupled ocean atmosphere mesoscale prediction system coamps is a registered trademark of the naval research laboratory coamps in two separate simulations of the adriatic a nested 2 km resolution ocean model is forced by the inner 4 km and outer 36 km nests of the atmospheric model two meteorological stations and two acoustic doppler current profiler observation sites are used to evaluate modeled atmosphere and ocean velocity fields for 28 january 4 june 2001 modeled observed correlations of atmospheric 10 m velocity are greater than 0 85 for both resolution models oceanic 5 and 25 m current fluctuations from both simulations generally match the magnitude and orientation of the observations the 4 km resolution atmospheric model is differentiated from the 36 km resolution model by its ability to resolve the small scale flow structures of the bora wind and by its better agreement with observed wind velocity statistics the ocean simulation forced by the 4 km resolution model is distinguished from the one forced by the 36 km resolution model by its ability to reproduce the expected double gyre circulation in the northern adriatic and by its ability to better capture the magnitude and shape of the observed depth dependent velocity correlation with wind at the deeper site though the 36 km forced ocean model agrees better with many observed velocity statistics the 4 km forced ocean model produces the highest correlations with observations exceeding 0 78 at subsurface depths that are most strongly correlated with winds

generation of mesoscale eddies in the lee of the hawaiian islands 1 the ocean west of the main hawaiian islands is characterized by enhanced eddy kinetic energy arising from the abundance of locally generated mesoscale eddies most frequently in the area west of the island of hawaii two mechanisms of eddy generation in the wake of an island are examined with numerical model experiments the first eddy generation and shedding by an oceanic flow around an oceanic barrier requires the existence of strong westward flows to the north and south of the island of hawaii model solutions show such westward flows and generation of eddies by these flows although the intensity of the eddies and the generation frequency are much lower than that derived from altimetry as a result these eddies contribute an insignificant amount of eddy kinetic energy in the region the second eddy generation and shedding by an atmospheric flow around an atmospheric barrier is based on oceanic upwelling and downwelling induced by surface wind shear effectively introducing sinks and sources to the ocean interior previous idealized modeling studies have shown that oceanic eddies can be generated by sufficiently strong forcing source or sink providing an explanation why eddy occurrences in the lee of the island of hawaii coincide with periods of strong trade winds eddy generation characteristics in the model experiments are consistent with this mechanism in terms of time of occurrence strength and the resulting eddy kinetic energy major discrepancies are in eddy propagation and therefore eddy distribution in the regional domain due to the complex nature of eddy eddy interactions

wave breaking induced surface wakes and jets observed during a bora event 1 an observational and modeling study of a bora event that occurred during the field phase of the mesoscale alpine programme is presented research aircraft in situ measurements and airborne remote sensing observations indicate the presence of strong low level wave breaking and alternating surface wakes and jets along the croatian coastline over the adriatic sea the observed features are well captured by a high resolution coamps simulation analysis of the observations and modeling results indicate that the long extending wakes above the boundary layer are induced by dissipation associated with the low level wave breaking which locally tends to accelerate the boundary layer flow beneath the breaking farther downstream of the high peaks a hydraulic jump occurs in the boundary layer which creates surface wakes downstream of lower terrain passes the boundary layer flow stays strong resembling supercritical flow

an observational study of oceanic eddy generation mechanisms by tall deep water islands gran canaria 1 oceanic eddy generation by tall deep water islands is common phenomenon it is recognized that these eddies may have a significant impact on the marine system and related biogeochemical fluxes hence it is important to establish favourable conditions for their generation with this objective we present an observational study on eddy generation mechanisms by tall deep water islands using as a case study the island of gran canaria observations show that the main generation mechanism is topographic forcing which leads to eddy generation when the incident oceanic flow is sufficiently intense wind shear at the island wake may acts only as an additional eddy generation trigger mechanism when the impinging oceanic flow is not sufficiently intense for the case of the island of gran canaria we have observed a mean of ten generated cyclonic eddies per year eddies are more frequently generated in summer coinciding with intense trade winds and canary current

the tectono metamorphic evolution of gneiss complexes in the middle urals russia a reappraisal abstract the middle urals are characterized by a major virgation in the linear trend of the urals orogen and represent the most highly contracted part of the late palaeozoic collisional belt this part of the orogen is dominated by metamorphic complexes and major fault and shear zones the main uralian fault zone muf the east dipping suture of the orogen containing low grade metamorphic rocks separates the sysert complex in the east from the ufaley complex in the west the sysert complex in the hanging wall of the muf consists of intensely deformed gneisses granitic intrusions and a metamorphosed melange zone tectonic and isotopic investigations suggest the following stages for the evolution of the sysert complex a pre orogenic rifting and magmatism during ordovician and silurian times b oceanic closure island arc formation related to convergence and subduction during devonian times c major ductile deformation under amphibolite facies conditions related to nw directed thrusting associated with crustal stacking during collision in carboniferous times d exhumation and contractional intracontinental tectonics during permian times and e closing of isotope systems related to cooling and the end of orogenic shortening through triassic times the ufaley complex in the footwall of the muf is interpreted as an east dipping crustal stack that records an amphibolite facies uralian metamorphism lithologically the complex can be divided into pre orogenic european basement west ufaley and intensely deformed palaeozoic metasediments and amphibolites east ufaley high pressure relics in the east ufaley complex are interpreted to be the result of subduction whereas intense ductile deformation is related to overthrusting onto west ufaley the west ufaley complex is composed of gneisses amphibolites migmatites and granitic intrusions and has been thrust onto devonian limestones along a major shear zone in both sysert and ufaley complexes nw trending stretching lineations and top to the nw kinematic indicators suggest an oblique plate convergence with a significant sinistral component the muf is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall

an archean fold thrust belt in the northwestern abitibi greenstone belt structural and seismic evidence an integration of structural field data and lithoprobe seismic reflection line 28 in the northwestern abitibi greenstone belt agb reveals a crustal scale south to southwest vergent thrusting event that developed in sequence above a shallowly 15 north dipping sole thrust at a mid crustal level seismic reflector geometry above this decollement suggests a mid crust 6 20 km depth dominated by low angle thrusts with smooth trajectory ramps and culmination folds or antiformal stacks similar to the structural style of neighbouring high grade plutonic gneissic opatica and sedimentary pontiac subprovinces in contrast low to high angle east west trending thrusts at the upper crust greenstone belt level 6 9 km depth are interpreted to be listric they occur in two fault systems the chicobi and taibi that resemble imbricate fan systems the contrasting structural geometry of the upper and mid crust is interpreted as variations in level through the thrust stack and resembles paleozoic mountain

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

microaftershock survey of the 1978 bermuda rise earthquake on march 24 1978 a magnitude 6 0 intraplate earthquake occurred 380 km southwest of bermuda near magnetic anomaly m4 118 m y b p a catalog of seismicity for the bermuda rise indicates that this is an area of significant intraplate seismicity in the western north atlantic ocean the fault plane solution for the 1978 event is of thrust type and strikes 340 in an intermediate direction to the trends of major fracture zones 300 and abyssal hill topography 035 in the area the p axis of this mechanism is nearly horizontal and trends 259 subparallel to the absolute plate motion vector for north america aftershock activity was detected teleseismically for approximately 8 months after march 24 and the entire sequence is best described as a prolonged mainshock aftershock series during june 18 28 1978 we conducted a microaftershock survey of the area using ocean bottom hydrophones and recorded 250 events 0 mb 2 these smaller events are located in a tight cluster 5 10 km northeast of the neis mainshock location sp p data from microaftershocks recorded during this survey indicate a relatively narrow band of activity about 10 km wide smaller than would be inferred based on neis aftershock locations alone many of the recorded microaftershocks exhibited prominent water wave phases which have been used to constrain further the epicentral locations the depths of these events while not well deter mined appear to be shallow i e less than 5 km below the seafloor estimates of the b value determined from two instruments for the 10 day recording period are 1 10 0 30 to 1 50 0 60 overall both the fault plane solution and subsequent aftershock locations of the 1978 event suggest that faulting did not occur along a major oceanic fracture zone but rather along a smaller scale feature within the m11 m4 spreading discontinuity the spatial clustering of seismicity near the perimeter of the rise suggests that variations of crustal thickness associated with the bermuda rise may be responsible for triggering intraplate seismicity in this region

structural constraints on the proterozoic reworking of archaean crust in the rayner complex macrobertson and kemp land coast east antarctica abstract the high grade metamorphic rocks exposed along the macrobertson and east kemp land coasts consist of in the west archaean gneisses reworked during a ductile mid proterozoic deformation and in the east foliated proterozoic charnockite gneiss containing large remnants of gneiss of probable archaean age evidence for three folding events is recognized in the reworked gneisses an early recumbent terrain is deformed by two upright events possessing near vertical axial surfaces the charnockite gneiss preserves evidence of only the final event these three events comprise the rayner structural episode which was a consequence of the ductile thrusting of archaean gneiss westward toward a stable archaean block the napier complex kinematic indicators are consistent with formation of the proterozoic recumbent terrain by dominantly shear deformation dissection of both the rayner and napier complexes by retrograde shear zones and mylonite pseudotachylite zones was a consequence of the northward thrusting of the two complexes during the cambrian

lee region of gran canaria the mountainous canary islancls present obstacles to the trade wincls ancl to the canary current flowing equatorwarc past them in situ observations of hydrographic properties ancl surface wincls south of gran canaria together with advancecl very high resolution racliometer ancl synthetic aperture racial images e uring 2 weeks in summer 1995 are analyzecl a cyclonic edcly shecl l rom the west of the islancl c1riftecl southwestwarcl at 5 s j while the southeast coast was approachecl by an upwelling filament originating off nw africa a wincllee region bounclecl by intense horizontal shear lines hacl a weak return islandwarcl wincl in its center the lee formee a triangulai c1iurnally varying war111 water 001 with two sea surface temperature maxima separatecl by lower temperatures below the return wincl shallow temperature stratification occurrecl behincl the islancl in contrast to the l1niform surface mixecllayer in exposecl regions upwelling ancl clownwelling of 10 20 111 were indicatecl on the cyclonic ancl anticyclonic sicles of the lee region in the sar la u lines of strong current shear along a temperature front between the cyclonic eclcly the upwelling filament were iclentifiable i iowever the racial images were clominatecl atmospheric phenomena including mountain lee wave paclcets winclrows and wincl lines estimation of the wincl field l rom the sar backscatter intensity revealed ex structure and intensification on the eclges of the warm lee

duplex geometry an example from the moine thrust belt abstract the geometry and microstructure of a small duplex formed in one bed from the moine thrust belt of northwest scotland is reported the structure is seen in oblique section within the cambrian pipe rock in an area of low strain a range of movement direction indicators are present in the structure an early grain shape fabric developed close to the roof thrust is taken as the best estimate of the overall movement direction towards 287 slickensides in the gouge developed on movement planes within the duplex show varied orientations on a given plane and are not considered useful indicators of thrust transport direction branch lines exposed converge and diverge suggesting little lateral continuity of the exposed structure the microstructures present within the structure indicate an increase in localised deformation and in cataclastic behavior as the duplex evolved early layer parallel shear is ubiquitous giving rise to an elongate grain shape fabric close to bedding surfaces in early formed horses a layer parallel oblate grain shape fabric which shows localised slip zones is overprinted by gouge formation later formed horses show only fracturing and gouge development this sequence is attributed to stick slip behavior in the propagation or displacement of the original fault now the floor thrust

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

early stage soliton observations in the sulu sea abstractobservations of early stage large amplitude nonlinear internal waves in the sulu sea are presented water column displacement and velocity profile time series show the passage of two solitary like waves close to their generation site additional observations of the same waves are made as they propagate through the sulu sea basin these waves of depression have an estimated maximum amplitude of 44 m observed wave amplitude and background stratification are used to estimate parameters for both a korteweg de vries k dv and a joseph wave solution these analytic model solutions are compared with a fully nonlinear model as well model wave half widths bracket the observed wave with the joseph model narrower than the k dv model the modal structure of the waves change as they transit northward though the sulu sea with higher mode features present in the southern sulu sea which dissipate by the time the waves reach the north observed and modeled energies are roughly comparable with observed pot

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

tertiary extension development and extension compression interplay in the west carpathian mountain belt abstract this paper describes palaeostresses calculated from fault striae data the inferred palaeostrain patterns and determines the inter relation of the compression and extension during the tertiary development of the west carpathians the calculated stress and inferred strain patterns for the palaeogene burdigalian indicate that the ancestral west carpathians formed a more or less straight orogenic belt this belt underwent contraction and uplift in its narrow frontal zone stretching along its strike during the paleocene chattian and regional contraction and uplift during the chattian burdigalian the strain stress pattern is similar to the collision related pattern known from the eastern alps for the paleocene burdigalian during the burdigalian tortonian the calculated and inferred west carpathian stress and strain patterns indicate narrow frontal contractional and lateral sinistral transpressional zones in the orogenic front and broad extensional and dextral transtensional zones in the orogenic interior the stress strain pattern is similar to the subduction related pattern known from areas such as the hellenic or sunda banda arcs

global observations of large oceanic eddies 1 ten years of sea surface height ssh fields constructed from the merged topex poseidon t p and ers 1 2 altimeter datasets are analyzed to investigate mesoscale variability in the global ocean the higher resolution of the merged dataset reveals that more than 50 of the variability over much of the world ocean is accounted for by eddies with amplitudes of 5 25 cm and diameters of 100 200 km these eddies propagate nearly due west at approximately the phase speed of nondispersive baroclinic rossby waves with preferences for slight poleward and equatorward deflection of cyclonic and anticyclonic eddies respectively the vast majority of the eddies are found to be nonlinear

the metamorphic architecture of a transpressional orogen the kaoko belt namibia s 64 61 goscombe b hand m 2001b metamorphic model for the inland branch of the damara orogen a paired metamorphic mountain belt pmmb geological society of australia specialist group in tectonics and structural geology conference ulverstone

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

thermoelastic stress in oceanic lithosphere due to hotspot reheating we investigate the effect of hotspot reheating on the intraplate stress field by modeling the three dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere a thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements the stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series wherein each term is a source or an image modified from a closed form half space solution we apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities our results predict a stress field with a maximum deviatoric stress on the order of 100 mpa 1 kbar covering a broad area around the hotspot plume the predicted principal stress orientations show a complicated geographical pattern with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere although stress data near oceanic swells are limited the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model these results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere

modeling south china sea circulation response to seasonal forcing regimes 1 a three dimensional ocean model has been utilized to study circulation and its seasonal variation in the south china sea scs in response to the forcing of the asian monsoon and the kuroshio intrusion the scs ocean model has a resolution of approximately 10 km horizontal spacing and 30 vertical levels with a realistic bottom topography the model is forced with time dependent wind stress and heat flux from national center for environmental prediction reanalysis data as well as with lateral fluxes from a pacific ocean model of 40 km horizontal resolution this study reports on the analysis of the mean seasonal circulation and dynamic processes in response to monsoonal wind stress the kuroshio intrusion and other intrinsic forcing processes it is found that the seasonal circulation in the scs is mainly driven by the monsoonal wind stress and greatly influenced by the inflow from the kuroshio intrusion strong currents along the continental margin of the scs form mean basin wide cyclonic and anticyclonic circulations in the winter and summer respectively multiscale eddies are embedded in the general circulation across the basin while mainstream of the kuroshio passes through the luzon strait without intruding into the scs partial intrusion occurs in the upper 200 m near the shelf margin southwest of taiwan at times when winter dynamic conditions prevail in the north scs the intrusion of the kuroshio into the scs also occurs at depths in all seasons mainly along the continental slope the coastal current separation to the east off southern vietnam and the associated eddy formations characterize the circulation in the south scs the simulated results compare well with the corresponding observed fields dynamical processes involved in the forced flow fields are investigated by examination of the momentum balances the analyses reveal that the circulation in the scs is generally dominated by the geostrophic currents north of the luzon strait positive nonlinearity in the zonal direction is locally intensified which leads to the formation of centripetal acceleration for the mainstream of the kuroshio to turn eastward the kuroshio intrusion at depths is governed by the ageostrophic flows and highly associated with the net westward pressure gradient force coastal jet separation to the east off vietnam is mainly associated with the local wind stress field and with the shelf topography in the summer and winter respectively sensitivity study reveals that the weakening of the kuroshio markedly enhances kuroshio s intrusion and forms an anticyclonic eddy west of the luzon strait

geo tectonic framework of the himalaya of n pakistan in the karakorum range there is a structurally complicated cretaceous are comprising the kohistan sequence on its northern side the northern suture consists of a mega melange and is bounded to the s by tightly folded pillow bearing volcanics and sediments to the s the kohistan plutonic belt consists of southwards a early foliated and late post tectonic tonalites and diorites b aplites and pegmatites up to 30 of rock volume c basic dykes up to 10 m thick d the chilas complex a stratiform cumulate body over 300 km long and 8 km thick chromite layered dunites gabbros and norites with a low pressure granulite facies mineral fabric of tectonic origin e an amphibolite belt with a complex mixture of other rocks and f the jijal complex a 200 km 2 tectonic wedge of high pressure granulites and chromite layered dunites cumulate graded units in the chilas complex show that it is folded by an isoclinal anticline f 1 the mid upper crust of the are is folded by a 50 km half wavelength f 2 syncline the whole kohistan sequence with its two phases of isoclinal folds was tilted during himalayan collision so that the structures are now subvertical the southern suture main mantle thrust has a wedge of glaucophane schists the indian plate contains a basement of psammites and schists intruded by cambrian granites and overlain by isoclinally folded and metamorphosed carbonates and shales

kuroshio intrusion into the south china sea a review the kuroshio carrying the northwestern pacific water intrudes into the south china sea scs through the luzon strait significantly affecting the temperature salinity circulation and eddy generation in the scs thus the kuroshio intrusion makes important contributions to the momentum heat and salt budgets in the scs in the past decades much work has been done on the kuroshio intrusion this paper reviews past efforts and summarizes our current understanding of the kuroshio intruding processes from observational evidence laboratory results theoretical analyses and a range of numerical model simulations in addition discrepancies between results simulated by models as well as those between simulations and observations are presented specifically this paper addresses the following topics 1 different types of the kuroshio intrusion into the scs and their identification 2 vertical structure of the kuroshio in the luzon strait 3 an overview of the luzon strait transport resulting from observations and numerical model simulations 4 seasonal and interannual variations of the kuroshio intrusion as well as eddy generation due to the kuroshio path variation and 5 dynamical mechanisms e g wind forcing interbasin pressure gradient beta effect and hysteresis potential vorticity eddy activity controlling the kuroshio intrusion into the scs finally several future research topics for gaining a better understanding of the kuroshio intruding processes are suggested c 2014 the authors published by elsevier ltd

progressive evolution from uplift to orogen parallel transport in a late acadian upper amphibolite to granulite facies shear zone south central massachusetts fabrics within a 1 km wide shear zone along the eastern margin of the bronson hill anticlinorium in central massachusetts provide important information on the regional tectonic development during the devonian acadian orogeny two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates both lineations are associated with shearing and development of mylonitic fabrics the character of the two lineations and related fabrics differs mainly in orientation and in the relationship of each lineation to the local orientation of quartz c axis fabrics the interpreted earlier lineation has a steep west plunge and the later lineation has a shallow plunge and north south to southwest trend parallel to the regional trend of the orogen extensive dynamic recrystallization of feldspar the possible activity of prism slip in quartz and the mineral assemblages in apparent equilibrium during deformation suggest that both lineations probably formed under upper amphibolite facies conditions just after the thermal peak of acadian metamorphism a progressive change between the two deformation fabrics is suggested by evidence of overlap between the two by their striking similarity in style and metamorphic conditions of formation and by the similarity in orientation of their bulk strain axes with a change only in the transport direction between them kinematic indicators associated with each phase of lineation development are consistent throughout the shear zone the earlier phase has a west side up sense of shear and the later phase indicates west side north movement these phases are correlated with the regional backfold and dome stages of acadian deformation the kinematics and relationships between them suggest a revised tectonic model for the later part of the acadian orogeny and for development of orogen parallel lineations in this region in this model uplift and thickening of basement gneisses in the bronson hill anticlinorium during the backfold stage was impeded by gravitational instability in a thermally weakened crust this led to a change from subvertical to subhorizontal extension east west acadian shortening was still in progress constraining this extension to a north south direction parallel to the trend of the orogen

combined thrusting and wrenching in the ibero armorican arc a corner effect during continental collision this paper is an attempt to reconcile strain pattern and deformation history in an overall arcuate structure with continental collision processes geological data are first presented for the early history 400 ma of the variscan orogenic belt in france principal stretch trajectories are on a large scale more or less parallel with the trend of the belt they became strongly oblique to this trend near the basal surfaces of the nappes a kinematic model combining two orthogonal shear components is proposed to explain such a strain pattern it is concluded that the observed strain trajectories result from a combination of a southward thrusting and a sinistral wrenching this conclusion is in good agreement with previous conclusions gained by several local microtectonic studies the geodynamic significance of combined thrusting and wrenching at the scale of the ibero armorican arc is discussed and a progressive deformation model is proposed

the bermuda earthquake of march 24 1978 a significant oceanic intraplate event the bermuda earthquake m 6 occurred near the westerly extension of the kane fracture zone roughly 370 km southwest of the island of bermuda it is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of north america because of its size and location it has provided an excellent set of wwssn body waves they can be used to infer its depth and faulting parameters by waveform modeling techniques the results indicate a north northwest striking thrust mechanism strike n20 w dip 42 ne rake 90 with the hypocenter located at a depth of 11 km which for an oceanic crust places it predominantly in the mantle the event had a seismic moment of 3 4 1025 dyne cm and its time history was modeled with a symmetric trapezoidal time function 3 s in duration the north northwest strike of the event is in good agreement with the bathymetry of the area the epicenter being close to the southwestern edge of the bermuda rise the strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region the presence of such fracture zones is indicative of local weak zones in the lithosphere the bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the north american plate in the direction of its absolute motion this is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of north america

experimental modeling of viscous inclusions in a circular high strain shear rig implications for the interpretation of shape fabrics and deformed enclaves 1 deformation experiments with initially spherical and prolate viscous inclusions suspended in a viscous newtonian matrix in a circular high strain annular shear rig provide insights on the shape development of inclusions in high strain shear zones during progressive deformation inclusions with a specific viscosity ratio with respect to the matrix material show distinct types of three dimensional shape development for instance at a high viscosity ratio between matrix and inclusion a pulsating ellipsoid develops which both continuously rotates and changes its shape from a sphere to an ellipsoid and back to a sphere the experiments show that the shape of an inclusion that has a viscosity different to the matrix material cannot be taken as a reliable indicator for the magnitude or type of finite strain in naturally formed shear zones in which strain rates are heterogeneous from boundary to center viscosity contrast and power law flow will result in a large number of different shape developments trends for inclusions such as mafic and microgranitic enclaves pebbles or mineral aggregates

seafloor spreading history of the western north atlantic basin derived from the keathley sequence and computer graphics abstract an analysis of sea surface magnetic profiles and an aeromagnetic contour chart west of bermuda yields a new tectonic framework for the keathley m series seafloor spreading magnetic anomaly sequence changes in spreading direction derived from the western keathley sequence occur at times corresponding to m24 m21 and m11 of which m11 is a time of continental breakup the models facilitate new identifications of the anomalies between m16 and m4 obviating the need for anomalously slow spreading between m11 and m4 previously suggested by other workers and by us and placing the location of the well defined rough smooth basement boundary at m11 the models show that spreading rates contemporaneously vary among profiles collected between flowlines as determined by a recently presented kinematic history this strongly suggests that individual ridge axis segments act independently of another in detail our reconstruction technique records in map form where the ridge crest has made significant departures from perfect symmetry and a predictable spreading rate during its evolutionary history among eight profiles collected parallel to seafloor spreading flowlines in the western north atlantic encompassing only 2700 km of normal oceanic crust we can identify more than fourteen ridge jump events there is evidence for a small 30 km ridge jump to the east of many ridge axis segments at m24 this is supported in the eastern north atlantic conjugate crust of the canary basin where m25 and m24 are poorly represented a series of four ridge jumps to the east along one paleoridge axis segment now located at 36 n 65 w is related to the passage of the mesozoic mid atlantic ridge crest over the verde hotspot at 137 m y b p the project magnet magnetic anomaly contour chart for the area between 29 n and 40 n has been used to determine stage poles of motion for processes occurring at the ridge crest related to its segmentation which have a component oblique to spreading between chrons m21 and m11 these trends may relate to the motion of the ridge crest over the mantle

metamorphic response in orogens of different obliquity scale and geometry abstract investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets to illustrate the problems in documenting flow vectors we present integrated structural metamorphic datasets from two transpressional systems the kaoko belt in namibia and the kalinjala shear zone in south australia these orogens experienced widely differing metamorphic responses to transpressional deformation integration of kinematic and metamorphic datasets from the kaoko belt indicate shallow up plunging extrusion trajectories in the orogen core and show that the maximum stretching direction pattern matches the inferred flow vectors high grade domains 800 840 c and 7 0 8 0 kb in the orogen core developed low angle upward verging maximum stretching direction trajectories whereas a low grade domain 575 600 c and 5 0 5 5 kb in the orogen core has downward verging lineation trajectories the barometric differential between these high grade and low grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high grade domains that were extruded obliquely for the amount of lateral shear estimated for the orogen core the kalinjala shear zone in south australia contrasts strongly with the kaoko belt in this example the high grade and high strain shear zone core of the orogen experienced high t high p metamorphism with low thermal gradients of 21 26 c km and steep decompressive p t paths the lower grade external domains experienced lower t lower p metamorphism with high thermal gradients of 35 37 c km sub horizontal maximum stretching directions do not match the vertical extrusional flow in the high grade core that is indicated by the metamorphic data this comparison shows that in general and on a gross scale maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis in some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow consequently information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record these two datasets must be used in concert to obtain realistic constraints on first order material flow trajectories at orogenic scales the horizontal component of flow is typically best recorded by structural fabrics maximum stretching direction and sense of shear whereas the vertical component is typically best recorded by metamorphic information such as p t paths temperature over depth ratio g and metamorphic field gradients i e t p and g across the orogen

the role of strike slip fault systems at plate boundaries a new analysis shows that most 59 plate boundaries have a relative velocity vector that is markedly oblique greater than 22 to the boundary normal a significant proportion 14 have vectors that are nearly 22 parallel to the boundary accommodation of the oblique motion usually involves strike slip faulting but the kinematic role of these faults differs at divergent and convergent boundaries four main types of plate boundary related strike slip faults are distinguished ridge transforms boundary transforms trench linked strike slip faults and indent linked strike slip faults discrimination of the four types should be possible in ancient orogenic belts but is complicated by the common reactivation of the strike slip zones in other roles plate boundary related strike slip faults form major lineaments at the present day ridge transforms have a low preservation potential in continents boundary transforms and indent linked faults often re use old lineaments but trench linked strike slip faulting is an effective method of forming new lineaments in continental crust strike slip faulting in general is less commonly recognized in ancient orogenic belts than its abundance in present plate boundary orogens requires this under recognition results both from poor understanding of strike slip kinematics and from deeper prejudices about the way in which orogenic belts form

intraplate seismicity in the atlantic ocean basin a teleseismic catalog a comprehensive compilation and reassessment of intraplate seismicity in the atlantic ocean basin is presented for the years 1918 through 1990 of the 846 earthquakes whose original bulletin epicenters were located in atlantic ocean crust and were at least 2 from active plate boundaries 289 were found to be reliably intraplate a significant number 237 of the original epicenters were actually mislocated interplate earthquakes that located to active plate boundaries upon relocation or whose 95 confidence locations determined through monte carlo simulations intersected seismically active boundaries many of these erroneous locations particularly in regions surrounding the mid atlantic ridge frequently appear in global seismicity maps the resulting reliable catalog of intraplate seismicity is not randomly distributed but shows significant clustering in space and time much of the seismicity is associated with the locations of hot spots such as the bermuda canary and cape verde islands seismicity is also clustered around other tectonic features such as the diffuse plate boundary between north america and south america and the fossil spreading ridge between greenland and north america while we did not examine intraplate earthquakes within 2 of the mid atlantic ridge and some of our unreliable near ridge earthquakes may be truly intraplate we do not find an indication within our reliable data set of an increase in intraplate seismicity with increased proximity to the mid atlantic ridge beyond our 2 limit

seismicity at intersections of spreading centers and transform faults ocean bottom seismographs were used in a microearthquake monitoring experiment at the eastern junction of the oceanographer transform with the mid atlantic ridge at 35 n microearthquake activity at the junction occurred over a broad area 7 km these microearthquakes cut across the corner between the transform and median valley and are associated with fault scarps that form the inner walls on the west and north sides of the median and transform valleys at intersections of other major fracture zones 100 km offset and slow to moderate spreading centers microearthquake activity is also diffuse and cuts across the corner between the spreading center and transform fault the narrow zone of decoupling 1 km observed between spreading center and transform boundaries by detailed geological studies at the tamayo east pacific rise and vema mid atlantic ridge intersections suggest that the diffuse seismicity 20 to 30 km in width does not reflect a diffuse plate boundary at the transition from rift to transform valley instead the faulting probably reflects internal deformation of the corner by secondary faults off of the plate boundary

extrusional tectonics in the core of a transpressional orogen the kaoko belt namibia the pan african kaoko belt of nw namibia provides a wellexposed example of material flow in the evolving middle and lower crust during sinistral transpressional orogenesis the kaoko belt is a composite metamorphic belt with shear zone bounded zones of contrasting metamorphic style that were metamorphosed at approximately the same time 575 550ma it is divided into a barrovian style escape zone with inclined nappes and thrusts that verge outward towards the foreland a steep high grade orogen core that experienced intense wrench style deformation and in the hinterland an older 650ma exotic coastal terrane that was reworked at moderate t low p conditions during transpressional orogenesis the kinematic lineation array defined by stretching lineations and shear sense indicators shows variation in degree and polarity of lineation obliquity to the grain of the belt lineation plunge and different polarity of vergence along the plunge with respect to the overall sinistral shear sense the gross regional lineation pattern defines an arcuate array from near orogen parallel in the orogen core to higher angle obliquity across the escape zone reflecting lateral escape towards the orogen margin two highgrade lobes within the orogen core are coincident with obliqueinclined upward flow trajectories based on the kinematic lineation array and represent oblique extrusion of lower crustal material into middle crustal levels an intervening low grade metamorphic trough within the orogen core and the coastal terrane coincides with acute oblique downward and outboard directed apparent flow trajectories these kinematic patterns are consistent with barometric constraints for peak metamorphism of 5 2kbar and 4 5kbar in the low grade trough and coastal terrane and 8 0kbar in the two high grade lobes integration of the kinematic and metamorphic datasets shows that oblique shallowly inclined extrusion trajectories occurred within the orogen core of this classic transpressional orogen and that stretching lineations approximately reflect the particle paths experienced across orogen metamorphic gradients indicate a marked contrast between the escape zone and orogen core the escape zone experienced an inverted barrovian style high p moderate t metamorphism and low thermal gradients of 20 c km the orogen core experienced moderate p high t metamorphism and high thermal gradients of 30 40 c km clockwise p t paths were experienced in all parts of the kaoko belt tight and shallow conduction dominated p t paths were experienced in all orogen core domains involving heating dominated prograde paths followed by decompression consistent with extrusional tectonics steeper and more open advection dominated p t paths were experienced in the crustal over thrust escape zone which experienced low thermal gradients