sea ice motion in response to geostrophic winds it is an old rule of thumb that sea ice moves with a speed of about 2 of the surface wind and about 45 to the right of the wind a similar relationship between the ice velocity and the geostrophic wind is examined here it is found that only about half of the long term several month average ice motion is directly related to the geostrophic wind the other half being due to the mean ocean circulation on shorter time scales and in all seasons more than 70 of the variance of the ice velocity in the central arctic ocean is explained by the geostrophic wind within about 400 km of the coasts the geostrophic wind is less successful in explaining the ice motion the spatial variations in ice velocity are also partly explained by the geostrophic wind about half of the variance in the large scale ice vorticity and shear are accounted for on the other hand none of the large scale ice divergence can be explained by the wind the long term average ocean current is estimated by subtracting the share of the ice motion caused by the wind from the total ice motion

coupled ice ocean dynamics in the marginal ice zones upwelling downwelling and eddy generation this study is aimed at modeling mesoscale processes such as upwelling downwelling and ice edge eddies in the marginal ice zones a two dimensional coupled ice ocean model is used for the study the ice model is coupled to the reduced gravity ocean model through interfacial stresses the parameters of the ocean model were chosen so that the dynamics would be nonlinear the model was tested by studying the dynamics of upwelling winds parallel to the ice edge with the ice on the right produce upwelling because the air ice momentum flux is much greater than air ocean momentum flux thus the ekman transport is greater under the ice than in the open water the stability of the upwelling and downwelling jets is discussed the downwelling jet is found to be far more unstable than the upwelling jet because the upwelling jet is stabilized by the divergence a constant wind field exerted on a varying ice cover will generate vorticity leading to enhanced upwelling downwelling regions i e wind forced vortices steepening and strengthening of vortices are provided by the nonlinear terms when forcing is time varying the advection terms will also redistribute the vorticity the wind reversals will separate the vortices from the ice edge so that the upwelling enhancements are pushed to the open ocean and the downwelling enhancements are pushed underneath the ice

anelastic response of the earth to a dip slip earthquake the deformation of the earth s surface immediately following an earthquake is well modeled by a dislocation in an elastic half space some time after the event however the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle the asthenosphere this relaxation causes further slow displacements observable at the earth s surface this paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30 dip thrust fault a variety of rheological models are studied these models encompass variations in asthenosphere thickness mesosphere viscosity fault depth and most important power law flow both newtonian n 1 and non newtonian n 3 maxwell rheologies are considered for the asthenosphere and mesosphere the results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault a comparable downwarp develops on the upthrown side these peripheral warps grow to a maximum height of about 5 of the total fault slip some time after the earthquake event and then collapse at much greater times the vertical motions for a 30 dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone horizontal displacements propagate asymmetrically away from the rupture zone the largest displacements occurring in the overthrust plate the principal result of the comparison of n 1 and n 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar the major difference is the time dependence of the development of the pattern in spite of the complex geometrical structure of the subduction zone the displacements ui 1 t for newtonian rheology are nearly the same as ui 3 t for the non newtonian rheology if t t3 thus although the qualitative pattern of the displacements is not diagnostic of power law flow the great difference in time dependence allows a clear discrimination of the two flow laws

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

lithology sedimentology and genesis of the wadena drumlin field minnesota u s a abstract studies on both the regional and small scales of the lithology sedimentology and stratigraphy of the wadena drumlin field in central minnesota u s a provide insights to the processes that operated within the wadena lobe during the drumlin forming phase horizontal changes in the down glacier direction in the nature of the drumlin till include an increase in the carbonate content decrease in grain size and magnetite content and variations in fine fraction mineralogy these changes also occur with depth at individual sites it is inferred that these patterns reflect progressive incorporation and mixing of the substrate which for much of the drumlin field is a till of northwestern origin with the till already in transit within the base of the wadena lobe derived from the northeast factors contributing to enhanced erosion within the wadena drumlin field include increased vertical strain rates over a major subglacial highland the presence of deformable bed material and a lowered ice profile which permitted elevated rates of bed deformation the patterns of mixing are generally consistent with substrate incorporation either by regelation or in the context of a deforming bed although neither of these processes adequately explain all of the observed features of the drumlin till where localized departures from the generalized pattern exist it can be shown that these are the result of either syn or post depositional processes primarily winnowing or the influence of a different substrate additional data are presented including macrofabric measurements and detailed site descriptions that are consistent with an accretionary progressive build up of the drumlin forms

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

fault kinematics and stress fields in the rwenzori mountains uganda the rwenzori mountains in western uganda form an active rift transfer zone in the western branch of the east african rift system here we quantify local stress fields in high resolution from field observations of fault structures to shed light on the complex polyphase tectonics expected in transfer zones we apply the multiple inverse method which is optimized for heterogeneous fault slip data to the northern and central rwenzori mountains observations from the northern rwenzori mountains show larger heterogeneity than data from the central rwenzori including unexpected compressional features thus the local stress field indicates polyphase transpressional tectonics we suggest that transpression here is linked to rotational and translational movements of the neighboring victoria block relative to the rwenzori block that includes strong overprinting relationships stress inversions of data from the central rwenzori mountains indicate two distinct local stress fields these results suggest that the rwenzori block consists of smaller blocks

the rift to break up evolution of the gulf of aden insights from 3d numerical lithospheric scale modelling abstract the gulf of aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore recent surveys showed that the spatio temporal evolution of the gulf of aden rift system is dominated by three fault orientations displacement orthogonal wsw rift parallel wnw and an intermediate e w trend the oldest parts of the rift that are exposed onshore feature displacement orthogonal and intermediate directions whereas the subsequently active necking zone involves mainly rift parallel faults the final rift phase recorded at the distal margin is characterised by displacement orthogonal and intermediate fault orientations we investigate the evolution of the gulf of aden from rift initiation to break up by means of 3d numerical experiments on lithospheric scale we apply the finite element model slim3d which includes realistic elasto visco plastic rheology and a free surface despite recent advances 3d numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved we address this issue by proposing a simple post processing method that uses the surface stress tensor to evaluate stress regime extensional strike slip compressional and preferred fault azimuth the described method is applicable to any geodynamic model and easy to introduce our model reproduces the observed fault pattern of the gulf of aden and illustrates how multiple fault directions arise from the interaction of local and far field tectonic stresses in an evolving rift system the numerical simulations robustly feature intermediate faults during the initial rift phase followed by rift parallel normal faulting at the rift flanks and strike slip faults in the central part of the rift system upon break up displacement orthogonal as well as intermediate faults occur this study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the gulf of aden and continental rifts in general

wind induced mesoscale features in a coupled ice ocean system a quasi geostrophic mesoscale eddy resolving model is coupled with free drifting sea ice to examine mesoscale oceanic structures which are induced by wind forcing on a nonuniform ice distribution and redistribution of sea ice initial concentration of sea ice has anomalies on the motionless ocean and the sea ice is suddenly forced by a wind stress differences in ekman transport between the high low concentration area and the surrounding area generate dipole eddylike features in the ocean with anticyclonic and cyclonic eddies on the right left and left right respectively looking in the downwind direction redistribution of ice by the ocean circulation continues after the wind ceases forming arch or hammer head shapes of ice anomalies interactions with the other anomalies can produce various shapes which could lead to misinterpretation of ocean flow field it is suggested that to detect ocean flow field in an ice covered area not near the ice edge trajectories should be used instead of shapes of ice cover

rift propagation in rotational versus orthogonal extension insights from 4d analogue models abstract in rift settings extension rates often vary along strike due to rotation about a vertical axis or euler pole yet tectonic modelers traditionally apply constant along strike deformation rates here we compare rift development and propagation under traditional orthogonal extension versus rotational extension conditions the set ups involve brittle viscous layering and localize deformation through structural weaknesses seeds our models provide first order insights into the differences in rift development between both boundary conditions orthogonal extension produces a rift basin with constant synchronous along strike features whereas rotational extension induces along strike structural gradients diachronous rift development causing rift propagation and the development of v shaped basins we observe important viscous flow associated with differential pressure gradients in rotational extension we also describe the important effects of strain partitioning between rift axis and model boundaries the quantifying of which is crucial to avoid incorrect model interpretations although our model results are first order only they are in good agreement with various natural examples and previous modeling studies and highlight the importance of considering the third dimension when studying tectonic systems

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

eastern indian ocean microcontinent formation driven by plate motion changes the roles of plate tectonic or mantle dynamic forces in rupturing continental lithosphere remain controversial particularly enigmatic is the rifting of microcontinents from mature continental rifted margins with plume driven thermal weakening commonly inferred to facilitate calving however a role for plate tectonic reorganisations has also been suggested here we show that a combination of plate tectonic reorganisation and plume driven thermal weakening were required to calve the batavia and gulden draak microcontinents in the cretaceous indian ocean we reconstruct the evolution of these two microcontinents using constraints from new paleontological samples 40ar 39ar ages and geophysical data calving from india occurred at 101 104 ma coinciding with the onset of a dramatic change in indian plate motion critically kerguelen plume volcanism does not appear to have directly triggered calving rather it is likely that plume related thermal weakening of the indian passive margin preconditioned it for microcontinent formation but calving was triggered by changes in plate tectonic boundary forces

three dimensional dynamical model of continental rift propagation and margin plateau formation we investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three dimensional finite element implementation of the yield stress envelope model in its initial configuration the three dimensional model represents a 1000 km by 1000 km area of continental lithosphere an initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates the model region beyond the weak trend is uniform model conditions are varied by changing the thickness and composition of the crust the geotherm and extension rate the strength of the initial weakness and the dimensions of the preweakened and uniform regions for each model breakup nucleates at a point on the weak trend in response to an applied constant extension rate the break propagates rapidly along the weak trend and then slows as it crosses the uniform region this produces a locked zone in which breakup is delayed resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus the total extension required to tear across the 1000 km length of the model is 210 km for 40 km thick crust 400 km for 45 km thick crust and 710 km for 50 km thick crust smaller increases in total extension occur for elevated geotherms and reduced extension rates this indicates that prior extension estimates based on subsidence which exceed 600 km for margin plateaus can be explained by rift propagation in three dimensions differential extension along the model margin is linearly related to the length of the locked zone the slope of the line which we call is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in from 0 2 to 1 3 we compare values computed from our model to those estimated from the subsidence of bonavista platform and rockall plateau conjugate margins in the north atlantic basin and georges bank and blake plateau and their conjugate margins in the central atlantic basin these results indicate that the differential extension associated with these locked zones which ranges from 170 km to 550 km can be predicted in terms of their initial crustal thickness and their length along the margins

interactions between propagating rotational rifts and linear rheological heterogeneities insights from three dimensional laboratory experiments the lateral propagation of rifts is a consequence of the relative divergence of lithospheric plates about a pole of rotation modern and ancient examples of rifts are known to overprint pre existing linear anisotropies in the crust and lithosphere such as lithospheric boundaries crustal sutures and thermal anomalies here we investigate how propagating rifts interact with pre existing structures by using three dimensional analogue experiments with rotational extensional boundary conditions and variably oriented linear weak zones in the lithospheric mantle when linear weaknesses are oriented at low angles to the rift axis early strain localization occurs in narrow domains which merge at later stages resulting in continental break up by unzipping strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis in these experiments early sub parallel v shaped basins propagate toward the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary the experimental results are characterized in terms of their evolution patterns of strain localization and surface topography as a function of the lithospheric heterogeneity obliquity angle comparison of the experiments to ancient and modern examples in nature may help to elucidate the common but still poorly understood process of propagating rift lithospheric heterogeneity interaction

the role of asthenospheric flow during rift propagation and breakup continental rifting precedes the breakup of continents leading to the formation of passive margins and oceanic lithosphere although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling or passive rifting caused by far field extensional stresses it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere three dimensional numerical experiments of rifting near an euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis parallel motion in the asthenosphere dome but also to promote along axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole we show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time dependent phases of compressional and transcurrent stress regimes despite an overall divergent plate setting our experiments predict overdeepened bathymetry at the tip of the propagating rift as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting we also explain the episodes of basin inversion documented in many rifted continental margins

continental break up of the south china sea stalled by far field compression the outcome of decades of two dimensional modelling of lithosphere deformation under extension is that mechanical coupling between the continental crust and the underlying mantle controls how a continent breaks apart to form a new ocean however geological observations unequivocally show that continental break up propagates in the third dimension at rates that do not scale with the rate of opening here we perform three dimensional numerical simulations and compare them with observations from the south china sea to show that tectonic loading in the direction of propagation exerts a first order control on these propagation rates the simulations show that in the absence of compression in that direction continental break up propagates fast forming narrow continental margins independently of the coupling when compression is applied propagation stagnates forming v shaped oceanic basins and wide margins changes in out of plane loading therefore explain the alternation of fast propagation and relative stagnation these new dynamic constraints suggest that the west to east topographic gradient across the indochinese peninsula prevented continental break up propagation through the 1 000 km wide continental rift of the central and west basin of the south china sea until the direction of stretching changed 23 million years ago resulting in bypassing and acceleration of continental break up propagation tectonic loading in the direction of propagation exerts an important control on the propagation of continental break up according to three dimensional simulations of the south china sea

subglacial sediment dispersal and till composition dispersal of subglacial sediment down ice from its bedrock source assumes the form of a negative exponential decay curve dispersal decay curves can be quantified by their decay constants half distances half lives and total transport distances mapped dispersal decay curves associated with pleistocene ice sheets in north america and europe identify several scales of dispersal ranging from local kilometers to continental 100 s to 1000 s km scales till composition reflects the various scales of dispersal local scales short transport distances result in tills that reflect underlying bedrock whereas continental scales long transport distances result in tills derived largely from sources far up ice a number of parameters proposed as influencing subglacial sediment dispersal and transport distances do not explain the range of scales observed although they may explain minor variations kilometers in transport distances topography and basal ice velocity sliding velocity are interpreted as

mesoscale eddies in the fram strait marginal ice zone during the 1983 and 1984 marginal ice zone experiments during the summer marginal ice zone experiment in fram strait in 1983 and 1984 fourteen mesoscale eddies in both deep and shallow water were studied between 78 o and 81on sampling combined satellite and aircraft remote sensing observations conductivity temperature depth observations drift of surface and subsurface floats and current meter measurements typical scales of these eddies were 20 40 km rotation was mainly cyclonic with a maximum speed in several cases subsurface of up to 40 cm s observations further suggest that the eddy lifetime was at least 20 to 30 days five generation sources are suggested for these eddies several of the eddies were topographically trapped while others primarily formed by combined baroclinic and barotropic instability moved as much as 10 15 km d with the mean current the vorticity balance in the nontrapped eddies is dominated by the stretching of isopycnals accompanied by a change in the radial shear in the most completely observed eddy south of 79on the available potential energy exceeded the kinetic energy by a factor of 2 quantitative estimates suggest that the abundance of these eddies enhances the ice edge melt up to 1 2 km d

fission track evidence on the initial rifting of the red sea two pulses no propagation fission track analyses indicate that the red sea initially opened simultaneously along its entire length two distinct pulses of uplift and erosion characterized the early stages of rifting in the red sea throughout egypt and in southwestern saudi arabia the first pulse began at 34 million years ago ma the second pulse began in the early miocene 21 to 25 ma and marked the start of the main phase of extension these data support a rigid plate model for continental extension these results also indicate that the initiation of rift flank uplift and therefore rifting and volcanism occurred nearly simultaneously this conflicts with classical models of active and passive extension that predict sequential development of these features

three dimensional modeling of pull apart basins implications for the tectonics of the dead sea basin we model the three dimensional 3 d crustal deformation in a deep pull apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the dead sea basin the brittle upper crust is modeled by a boundary element technique as an elastic block broken by two en echelon semi infinite vertical faults the deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress free shear zone in the vicinity of the fault zone the bottom displacement represents the regional relative plate motion results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin bounding faults as the width of the shear zone increases the depth of the basin decreases the rotation around a vertical axis near the fault tips decreases and the basin shape the distribution of subsidence normalized by the maximum subsidence becomes broader in contrast two dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone our models also predict full graben profiles within the overlapped region between bounding faults and half graben shapes elsewhere increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin we suggest that the observed structure of the dead sea basin can be described by a 3 d model having a large overlap more than 30 km that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

a process study of mesoscale meanders and eddies in the norwegian coastal current abstract a series of noaa satellite images shows that the norwegian coastal current ncc over the norwegian trench was disturbed by mesoscale meanders with 60 100 km wavelengths in the first several days the meanders grew and propagated northward some meanders were pinched off seaward forming anticyclonic eddies the flow pattern became very chaotic around 23 february and then on 25 27 february it changed into a systematic pattern again with three cyclonic vortices developing on the offshore side of the ncc axis accompanied by seaward meanders or separated anticyclonic eddies on their northern sides acoustic doppler current profiler measurement showed these vortices to have significant barotropic components and to move northward at 5 km day 1 a quasi geostrophic two layer model is employed first to show the basic behavior of the system model sensitivity is examined with various vertical profiles of the initial jet and various bottom topography baroclinic instability is an essential mechanism t

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

oppositely directed pairs of propagating rifts in back arc basins double saloon door seafloor spreading during subduction rollback 1 when a continent breaks up into two plates which then separate from each other about a rotation pole it can be shown that if initial movement is taken up by lithospheric extension asthenospheric breakthrough and oceanic accretion propagate toward the pole of rotation such a propagating rift model is then applied to an embryonic centrally located rift which evolves into two rifts propagating in opposite directions the resultant rhombic shape of the modeled basin initially underlain entirely by thinned continental crust is very similar to the oligocene to burdigalian back arc evolution of the valencia trough and the liguro provencal basin in the western mediterranean existing well and seismic stratigraphic data confirm that a rift did initiate in the gulf of lion and propagated southwest into the valencia trough similarly seismic refraction gravity and heat flow data demonstrate that maximum extension within the valencia trough liguro provencal basin occurred in an axial position close to the north balearic fracture zone the same model of oppositely propagating rifts when applied to the burdigalian langhian episode of back arc oceanic accretion within the liguro provencal and algerian basins predicts a number of features which are borne out by existing geological and geophysical particularly magnetic data these include the orientation of subparallel magnetic anomalies presumed to be seafloor spreading isochrons in both basins concave to the west fracture zones southwest of the north balearic fracture zone and concave to the east fracture zones to its northeast a spherical triangular area of nw oriented seafloor spreading isochrons southwest of sardinia the greater nw extension of the central youngest magnetic anomaly within this triangular area in agreement with the model predicted northwestward propagation of a rift in this zone successively more central younger magnetic anomalies abutting thinned continental crust nearer to the pole of rotation in the liguro provencal basin the latter feature demonstrates that a rift also propagated northeast in the liguro provencal basin at least in its oceanic accretion phase of development an adaptation of an existing model for subduction slab detachment occurring along the north african margin in the late burdigalian langhian proposes propagation in opposite directions of the slab tear the resultant rhombic slab detachment is closely associated in space and time with the rhombic form of the algerian liguro provencal basins suggesting a cause and effect relationship

viscous relaxation as a prerequisite for tectonic resurfacing on ganymede insights from numerical models of lithospheric extension abstract ganymede s bright terrain formed during a near global resurfacing event or events that produced both heavily tectonized and relatively smooth terrains the mechanism s by which resurfacing occurred on ganymede e g cryovolcanic or tectonic and the relationship between the older dark and the younger bright terrain are fundamental to understanding the geological evolution of the satellite using a two dimensional numerical model of lithospheric extension that has previously been used to successfully simulate surface deformation consistent with grooved terrain morphologies we investigate whether large amplitude preexisting topography can be resurfaced erased by extension i e tectonic resurfacing using synthetically produced initial topography we show that when the total relief of the initial topography is larger than 25 50 m periodic groove like structures fail to form instead extension is localized in a few individual isolated troughs these results pose a challenge to the tectonic resurfacing hypothesis we further investigate the effects of preexisting topography by performing suites of simulations initialized with topography derived from digital terrain models of ganymede s surface these include dark terrain fresh relatively deep impact craters smooth bright terrain and a viscously relaxed impact crater the simulations using dark terrain and fresh impact craters are consistent with our simulations using synthetic topography periodic groove like deformation fails to form in contrast when simulations were initialized with bright smooth terrain topography groove like deformation results from a wide variety of heat flow and surface temperature conditions similarly when a viscously relaxed impact crater was used groove like structures were able to form during extension these results suggest that tectonic resurfacing may require that the amplitude of the initial topography be reduced before extension begins we emphasize that viscous relaxation may be the key to enabling tectonic resurfacing as the heat fluxes associated with groove terrain formation are also capable of reducing crater topography through viscous relaxation for long wavelength topography large craters viscous relaxation is unavoidable we propose that the resurfacing of ganymede occurred through a combination of viscous relaxation tectonic resurfacing cryovolcanism and at least in a few cases band formation variations in heat flow and strain magnitudes across ganymede likely produced the complex variety of terrain types currently observed

factors controlling the style of continental rifting insights from numerical modelling passive continental margins similarly formed by lithospheric stretching and rifting can differ significantly in terms of width ranging from 50 to 400 km or subsidence history our interest lies in determining what properties of the lithosphere and what stress and strain boundary conditions control this regional variability and the siting of rupture continental breakup occurs probably as the result of a stretching instability which develops in a zone of material or geometrical heterogeneity to analyze the factors controlling these instabilities we use a numerical thermo mechanical model of lithosphere extension based on a finite element program the particular program was chosen because it combines constitutive models appropriate to our knowledge of lithosphere rheology elasticity plasticity and nonlinear temperature dependent creep with a kinematic formulation for large inelastic strain we choose to weaken the plate by locally increasing the crustal thickness by a very small amount and examine how this defect or perturbation acts to localize the deformation we compare the geometry and timing of necking for relatively wet versus dry rheology cold versus warm geotherm and fast versus slow strain rate we examine as well how the width and amplitude of the perturbation itself affects the deformation history since a velocity boundary condition is applied the required extensional forces are evaluated and compared to estimates of plate driving forces conditions that favor deformation by plasticity rather than creep cold geotherms dry rheologies lead to a more localized extension and rapid rupture but the forces involved 1013 n m are larger than typical plate driving forces increasing the amplitude of the defect i e crustal thickness contrasts accelerates necking as well cooling of the thinned area becomes significant at low extension rates depending on the initial rheology cooling simply delays the moment when deformation becomes unstable or conversely inhibits necking

paleogene initiation of the western branch of the east african rift the uplift history of the rwenzori mountains western uganda abstract the two branches of the east african rift system ears are believed to have initiated diachronously however a growing body of work continues to suggest the onset of rifting in the western branch occurred in the paleogene coeval to the eastern branch due to a lack of pre miocene stratigraphy attempts to resolve the geological history of the western branch must study the uplift and erosional histories of the modern rift topography in this study the rock uplift history of the rwenzori mountains western uganda is resolved to better our understanding of the tectonic history of the western branch of the ear through the application of low temperature thermochronology mapping and the modelling of river profiles we show that rock uplift of the rwenzori dates back to the oligocene with thermal history models suggesting uplift induced exhumation may date back as far as the eocene this provides tangible evidence that extension began in the region in the paleogene coeval with the eastern branch and not the late neogene these results have broad implications for the tectonic evolution of the entire east african rift system and suggest our current understanding of the region s rift history remains incomplete

propagation of rifting along the arabia somalia plate boundary the gulfs of aden and tadjoura the localization and propagation of rifting between arabia and somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern gulf of aden and the gulf of tadjoura using bathymetric magnetic seismological and structural evidence large scale southwestward propagation of the aden ridge markedly oblique to the arabia somalia relative motion vector began about 30 myr ago between the error and sharbithat ridges it was an episodic process with stages of rapid propagation mostly at rates 10 cm yr interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the arabia somalia plate the longest pause was at the shukra el sheik discontinuity 45 e where the ridge tip stalled for 13 myr between 17 and 4 ma west of that discontinuity rifting and spreading took place at an azimuth n25 10 e and rate 1 2 0 3 cm yr different from those of the global arabia somalia motion vector n39 1 73 cm yr implying an additional component of movement n65 10 e 0 7 0 2 cm yr due to rotation of the danakil microplate at shukra el sheik the typical oceanic ridge gives way to a narrow wsw trending axial trough resembling a large fissure across a shallow shelf this trough is composed of about eight rift segments which result from normal faulting and fissuring along n110 n130 e trends all the segments step to the left southwestward mostly through oblique transfer zones with en echelon normal faults only two segments show clear significant overlap there is one clear transform the maskali fault between the obock and tadjoura segments the latter segment which encroaches onland is composed of two parallel subrifts iboli ambabbo that propagated northwestward and formed in succession the most recent southwestern subrift ambabbo represents the current tip of the aden ridge we propose a mechanical model in which the large scale propagation of the ridge followed a wsw trending zone of maximum tensile stress while the small scale propagation of its nw trending segments was dictated by the orientation of that stress oblique propagation was a consequence of passive lithospheric necking of the arabia somalia plate along its narrow section in map view between socotra and the kink of the red sea ethiopian rift above the afar plume individual ridge segments oriented roughly perpendicular to plate motion like lithospheric cracks were forced to jump southward because of confinement within the necking zone self sustaining plate scale necking may explain why the aden ridge did not connect with the red sea through bab el mandeb but continued straight into afar

the laptev sea rift abstract in north eastern siberia the active mid ocean gakkel ridge interacts with the continental shelf of the laptev sea extension has affected the shelf since at least the early tertiary and has resulted in the formation of a complex horst and graben system we present new seismic data from the laptev sea including deep seismic soundings the most prominent rift basin is the ust lena rift with a minimum e w width of 300 km at latitude 75 n and a cenozoic infill up to 13 km in thickness the asymmetric shape of the basin and conclusive evidence for a detachment imply a simple shear geometry the suggested rift model combines a ramp and flat geometry for the detachment with ductile stretching beneath the detachment a major west dipping hingeline listric fault separates the ust lena rift from the laptev horst the 100 150 km wide laptev horst is subdivided into three units by narrow rift grabens another prominent rift graben is the anisin basin which is located in the northern shelf area though the laptev sea rift formed in interaction with an active mid oceanic ridge there are indications that the laptev sea rift is of the passive rift type the rift was developed east of a sw ne trending transfer zone which links the gakkel ridge to the laptev sea rift

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the effect of bimineralic composition on extensional processes at lithospheric scale we investigate how lithospheric scale compositional heterogeneities affect kilometric scale deformation processes to this end we perform numerical experiments of lithospheric extension in which we vary the moho temperature and the mineralic composition of the mantle and the crust in both the crust and the mantle we use an explicit bimineralic composition by randomly distributing two mineral phases in the materials comparison of our models to simulations using an implicit bimineralic composite one average viscous flow laws for a two phase aggregate crust and mantle demonstrates that an explicit bimineralic composition assimilated to heterogeneities succeeds in explaining observations related to the formation of rifted margins such a 1 the absence of a sharp deformation zone at the brittle ductile transition bdt 2 the initiation of the rifting process as a wide delocalized rift system with multiple normal faults dipping in both directions 3 the development of anastomosing shear zones in the middle lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing patterns observed in the field or in seismic data 4 the preservation of undeformed lenses of material leading to lithospheric scale boudinage structure and resulting in the formation of continental ribbons as observed along the iberian newfoundland margin

along rift propagation of pleistocene holocene faults from a central volcano abstract the mechanisms of rift propagation are still not fully understood especially at mid oceanic ridges owing to the inherent difficulty in collecting submarine data here we investigate the fault slip profiles of the 60 km long theistareykir rift northern iceland that may suggest the direction of along axis rift propagation this is one of the few places on earth where rifting processes and mid oceanic ridge formation can be studied directly moreover this rift hosts an active central volcano and this enables to fully understand the relations between rift propagation and magma systems we reconstructed the slip profiles of all the 281 main pleistocene holocene faults that compose this n s rift by merging measurements performed in the field collected by unmanned aerial vehicle surveys and derived from digital surface models results indicate that north of the volcano 75 of the asymmetric faults propagated northward the value increases to 82 if the cumulated fault length is considered south of the volcano 47 of the asymmetric faults propagated southward 54 if the cumulated fault length is considered these data point to a dominant mechanism of along axis propagation of the rift outward from the volcano suggesting a genetic link with the underlying magma chamber two possible processes are suggested which can also occur in combination i faults develop following lateral dyke propagation outward from the magma chamber and ii faults nucleate near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux the rift architecture is complicated by the presence of tectonic zones with different dominant fault dips separated by transversal accommodation zones the latter also play the role of barriers guiding local fault propagation

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

long term elasticity in the continental lithosphere modelling the aden ridge propagation and the anatolian extrusion process summary the evolution of the gulf of aden and the anatolian fault systems are modelled using the principles of elastic fracture mechanics usually applied to smaller scale cracks or faults the lithosphere is treated as a plate and simple boundary conditions are applied that correspond to the known plate boundary geometry and slip vectors the models provide a simple explanation for many observed geological features for the gulf of aden the model predicts why the ridge propagated from east to west from the owen fracture zone towards the afar and the overall form of its path the smaller en echelon offsets can be explained by upward propagation from the initially created mantle dyke while the larger ones may be attributed to the propagating rupture interacting with pre existing structures for anatolia the modelling suggests that the east anatolian fault was created before the north anatolian fault could form once both faults were formed however activity could switch between them the time scales over which this should take place are not known but evidence for switching can be found in the historical seismicity for aden and anatolia pre existing structures or inhomogeneous stress fields left from earlier orogenic events have modified the processes of propagation and without an understanding of the existence of such features the propagation processes cannot be fully understood furthermore a propagating fault can extend into an active region where it would not have initiated the north anatolian fault encountered slow but active extension when it entered the aegean about 5 ma and the stress field associated with the extending fault has progressively modified aegean extension in the central aegean activity has been reduced while to the north west on features such as the gulfs of evvia and corinth activity has been increased the field observation that major structures propagate and the success of simple elastic models suggest that the continental crust behaves in an elastic brittle or elastic plastic fashion even though laboratory tests may be interpreted to suggest viscous behaviour there are major problems in scaling from the behaviour of small homogeneous samples to the large heterogeneous mantle and large scale observations should be treated more seriously than extrapolations of the behaviour of laboratory experiments over many orders of magnitude in space and time the retention of long term elasticity and localised failure suggests a similar gross rheology for the oceanic and continental lithospheres even though it is incorrect to attribute differences in behaviour to the former being rigid i e elastic and the latter viscous oceanic and continental lithosphere behave in different ways unlike oceanic crust continental crust is buoyant and cannot be simply created or destroyed the process of thickening or thinning works against gravity preventing large displacements on extensional or contractional features in the upper mantle the equivalents of ridge or subduction systems are suppressed before they can accommodate large displacements and activity must shift elsewhere on the other hand strike slip boundaries and extrusion processes are favoured

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

basin migration caused by slow lithospheric extension sedimentary basin migration caused by low lithospheric extension rates is investigated using a two dimensional dynamic numerical model of the lithosphere we find that continental breakup will eventually occur when larger extension velocities are used the duration of rifting prior to continental breakup is dependent on the extension velocity stretching the lithosphere with lower velocities does not lead to breakup instead the locus of maximum extension migrates deformation localizes outside the first formed basin that is in turn uplifted this basin then becomes a cold spot in the area in this case syn rift cooling predominates the lithosphere regains its strength during stretching instead of becoming weaker and the lithosphere necking zone becomes stronger than surrounding regions the transition velocity is for the cases studied about 8 mm yr while the locus of maximum thinning migrates after about 50 60 myr a comparison with observations of the mid norwegian galicia and ancient south alpine margins shows a close resemblance of important features fl 2002 elsevier science b v all rights reserved

a mesoscale ocean feature study using synthetic aperture radar imagery in the labrador ice margin experiment 1989 four synthetic aperture radar sar images were collected during the labrador ice margin experiment limex in march 1989 to study mesoscale ocean features over the newfoundland shelf ice floe movement is detected by displacement in the first two images collected on consecutive days streamfunction maps of ocean currents are drawn using an objective method after direct wind driven components are removed the general flow pattern shows the inshore branch of the labrador current to be isolated from the seaward flow over the northern edge of the grand banks in agreement with the mean circulation detected in noaa satellite images for 3 years by the same method ice distribution in the sar images implies mesoscale features much richer than those in the objectively drawn flow field it is difficult to detect ice movement in the images with two day separation because no persistent ice feature is identified in the study area it is suggested during the ers 1 ice mission to take airborne sar images one day before or after a satellite pass for ocean current detection from ice floe movement

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

fluid flow and metasomatic fault weakening in the moresby seamount detachment woodlark basin offshore papua new guinea low angle normal faults play a prominent role in discussions about fault strength as they require significant weakening to remain active at low angles the submerged moresby seamount detachment msd is arguably the best exposed active low angle detachment worldwide we analyzed dredged msd protoliths cataclasites and mylonites to investigate deformation mechanisms and fault weakening processes deformation is accompanied by important syntectonic fluid induced mass transfer controlling the rheological behavior of the msd while the mafic protolith behaves brittlely at the onset of deformation the metasomatic mineralogical and chemical changes cause a transition to plastic flow as the rock is progressively exhumed immobile elements provide a reference frame for total material gains and losses si ca and k are syntectonically enriched while fe ti mg and al are depleted mass increase is about 10 in the cataclasites and about 48 in the mylonites main mechanism is syntectonic veining causing enrichment in calcite and quartz thus making the mylonites capable to flow plastically minimum time integrated fluid flux is calculated as 3 105 m3 m 2 indicating that the msd is an important fluid conduit the fluids have a deep crustal source a bottom water temperature and turbidity anomaly suggests that the hydrothermal system is still active syntectonic veining in fault rocks and recent seismic activity both suggest that the msd is intermittently brittle implying a brittle plastic transition at unusually high temperature and low differential stress we conclude that fault zone metasomatism is crucial in forming weak detachments at passive margins and may be a prerequisite for successful crustal breakup

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

palaeocene recent plate boundaries in the ne atlantic and the formation of the jan mayen microcontinent abstract breakup and sea floor spreading between greenland and eurasia established a series of new plate boundaries in the north atlantic region since the late palaeocene a conventional kinematic model from pre breakup to the present day assumes that eurasia and greenland moved apart as a two plate system however new regional geophysical datasets and quantitative kinematic parameters indicate that this system underwent several adjustments since its inception and suggest that additional short lived plate boundaries existed in the ne atlantic among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented jan mayen microcontinent and subsequent deformation of its margins and surrounding regions the major oligocene plate boundary reorganization and microcontinent formation might have been precluded by various ridge propagations and or short lived triple junctions ne and possibly sw of the jan mayen microcontinent from the inception of sea floor spreading 54 ma to c18 40 ma our model implies a series of failed ridges offshore the faeroe islands a northern propagation of the aegir ridge ne of the jan mayen microcontinent and a series of triple junctions and or propagators in the southern greenland basin

causes and rate limiting mechanisms of ridge propagation a fracture mechanics model all known propagating rifts pr s are growing away from shallow ridge axis seafloor we examine a model for rift propagation in which the ridge axis conduit like a crack in the lithosphere propagates in response to a stress concentration at its tip the stress concentration near the tip which causes progressive failure of the lithosphere is primarily due to excess gravity spreading stresses associated with anomalously shallow ridge axis along the growing ridge this stress concentration near the tip is characterized by a stress intensity factor k which must exceed a threshold value to cause failure of the plate a remotely applied tensile stress would make a positive contribution to k but would act equally on both ridge segments and not favor the propagation of either the pressure in the crack may depending on its distribution make either a positive or negative contribution to k for a high enough propagation rate viscous flow induced suction due to the growth of the ridge axis as well as the reduced vertical flow near the pr tip would make k 0 this simple model thus includes a mechanism to control the speed of propagation and is consistent with the axial morphology of the spreading segment associated with the well studied 95 5 w pr of the galapagos spreading center analytical solutions for the contribution to the stress intensity factor of viscous forces resisting ridge axis growth and causing the pr axial topography are consistent with the observed propagation rate for asthenosphere viscosities of about 1019 pa s 1020 p we also examine a more general model of a ridge transform ridge system on the basis of energy conservation in terms of energetics this more general model reduces to that of a single propagating ridge axis crack when both the work done by forces driving propagation and the viscous dissipation due to forces resisting propagation are generated along the growing ridge axis however the more general model also includes mechanisms that may stop ridge propagation propagation will stop either when the energy needed for transform migration is larger than that available for ridge axis growth or when the strain energy absorbed at the dying ridge axis is comparable to that released at the growing ridge axis i e if the stress intensity factors for both ridge axes are similar both of these possibilities may occur when the propagating rift completely propagates through a ridge segment as at 85 w in the galapagos finally in this model the slight systematic differences in ridge axis orientation of the growing and dying ridges are a consequence rather than a cause of ridge propagation

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

sar retrieved wind in polar regions comparison with in situ data and atmospheric model output european remote sensing ers satellites synthetic aperture radar sar wind retrievals using cmod ifr2 are for the first time retrieved in the marginal ice zone miz and in arctic coastal areas and compared with in situ observations from reseach vessels rvs and output from a high resolution atmospheric model the root mean squares rms of the comparisons were 1 6 m s sup 1 and 2 m s sup 1 respectively the spatial variation of the sar wind fields established a decrease in wind speed close to the ice edge for the late summer situations where the wind was along the ice edge with the ice to the left this decrease is believed to be due to changes in atmospheric stability possibly through development of an internal boundary layer caused by the cold ice cover and melt water lower wind speed near the ice edge is confirmed by the atmospheric model and the in situ observations furthermore good results are obtained from sar wind retrieval in leads when compared with model output during a cold air outbreak routine measurements in the miz are useful for estimating the wind stress and therefore sar may play an important role in this region finally the identification of a jet out from hinlopen strait in the svalbard region and low wind wakes along the coast in the sar retrieved wind field is confirmed by in situ observations as the rv moves through the region the jet is also confirmed by the atmospheric model which is able to reproduce the situation

the influence of local bedrock an important consideration in the interpretation of textural and mineralogic analyses of till abstract bedrock produces variations in texture and composition of tills not only on a regional scale but also on a local one bedrock influence is evident in wisconsinan tills in an area as small as a township in the allegheny plateau of northeastern ohio generally till sheets of the plateau fine upward with decreasing age within the mogadore and northampton tills samples from borings terminating in sandstone bedrock on the upland contain significantly more sand and have higher quartz feldspar ratios than those from the same tills overlying lacustrine silts and clays in the valley on the upland samples from the northampton till overlying sandstone contains slightly more sand than that from the same till overlying shale and siltstone however quartz feldspar ratios increase down ice from sandstone bedrock highs a few kilometers of transport may have been required for ice to crush plucked sandstone fragments which added more quartz to the fine sand fraction of the northampton till geologists mapping tills using soil associations should be aware of textural and lithological variations produced by local bedrock

transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic 1 we present a comparison between numerical and analogue models focusing on the role of inherited lithospheric structures in influencing the process of continental break up our results highlight that the presence of pre existing anisotropies localizes strain and favors continental break up and formation of a new ocean for a fixed strain rate the pre rift lithosphere configuration influences rift duration melt production and width and symmetry of the continental margin pair model results show a mainly two phase tectonic history from continental extension to oceanization in the first phase extension affects contemporaneously the whole rift structure while in the second phase asthenosphere upwelling occurs into punctiform regularly spaced spots sequentially propagating in an extension orthogonal direction index terms 8109 tectonophysics continental tectonics extensional 0905 8120 tectonophysics dynamics of lithosphere and mantle general 8150 tectonophysics plate boundary general 3040 citation corti g j van wijk m bonini d sokoutis s cloetingh f innocenti and p manetti transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic geophys res lett 30 12 1604 doi 10 1029 2003gl017374 2003

tectonics of rift propagation into a continental margin western woodlark basin papua new guinea oblique convergence along the irregular boundary between the pacific and indo australian plates in the sw pacific has resulted in a transpressional regime the drag of the overriding pacific plate has produced a breakup of the solomon microplate from the indo australian plate and has induced its rotation within the mega shear zone between the two major plates the microplate is being separated from the indo australian plate by passive rifting and seafloor spreading in the woodlark basin which is propagating westward along the continental margin of papua new guinea the present day pole of the microplate rotation with respect to indo australian plate is estimated to be near the owen stanley fault zone which represents the suture after paleocene eocene arc continent collision the average rate of the seafloor spreading propagation is estimated to be 150 mm yr 1 over the last 3 4 ma the transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault the style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks

an observational and numerical study of wind stress variations within marginal ice zones published studies of ocean mesoscale processes in marginal ice zones mizs using numerical coupled ice ocean models usually assume that the surface wind speed is constant over the model domain and that wind stress variations are simply proportional to surface roughness variations we show that this assumption is not realistic in most situations because the surface wind stress is also significantly affected by mesoscale pressure variations by changes in the surface wind vector and by changes in surface layer stability two numerical case studies utilizing detailed surface and atmospheric measurements examine the factors affecting small scale 5 km variations in wind stress within mizs these case studies and surveyed observational and modeling results demonstrate that wind stress fields are qualitatively different from the surface roughness fields a realistic wind stress scenario consists of a maximum just inside the ice edge and another maximum in the open ocean stress minima occur within the pack ice region away from the miz and over grease ice if present the effect of the rougher miz ice is counteracted when wind stresses over the open ocean are enhanced by large surface heat fluxes over the ocean by a strong low level inversion over the ice or by a sharp atmospheric front with surface winds paralleling the ice edge such situations are common in miz regions some simple methods for including first order atmospheric effects on wind stress variations which could be incorporated into current ice ocean mesoscale models of miz regions are suggested

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

mantle compensation of active metamorphic core complexes at woodlark rift in papua new guinea in many highly extended rifts on the earth tectonic removal of the upper crust exhumes mid crustal rocks producing metamorphic core complexes these structures allow the upper continental crust to accommodate tens of kilometres of extension but it is not clear how the lower crust and underlying mantle respond also despite removal of the upper crust such core complexes remain both topographically high and in isostatic equilibrium because many core complexes in the western united states are underlain by a flat moho discontinuity it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating these processes should decouple upper crust extension from that in the mantle in contrast here we present seismic observations of metamorphic core complexes of the western woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension these core complexes are actively being exhumed at a rate of 5 10 km myr 1 and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density as indicated by low seismic velocities we conclude that at least in this case the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon but must involve mantle extension

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

quantifying the thermo mechanical impact of plume arrival on continental break up abstract the arrival of a plume head at earth s continental lithosphere is often considered to be an important factor for continental break up however the impact of plume impingement on strength and duration of a rift remains unclear in this study we quantify the mechanical and thermal influence of a plume i e lithosphere erosion on continental break up to do that we apply the three dimensional numerical code slim3d that features realistic elasto visco plastic rheology we model the thermo mechanical response of a segment of earth s lithosphere that is affected both by extension as well as plume related lithosphere erosion in order to evaluate the influence on the overall force budget we find that lithosphere erosion leads to a moderate lithospheric strength reduction of several tn m in a force limited environment however this strength reduction may have strong influence on the timing of continental break up or it may even control whether continental break up takes place at all additional reduction of the lithospheric strength is likely due to the massive emplacement of dikes that follows intensive melting within the plume head

insights from the jan mayen system in the norwegian greenland sea i mapping of a microcontinent summary in this contribution we present a model of microcontinent architecture within a system that involves a complex rifted margin setting and different phases of deformation and continental breakup our case study is the jan mayen microcontinent jmmc located in the central part of the norwegian greenland sea we have revised its basement and sedimentary geometries using modern and vintage seismic reflection profiles and updated potential field data the northern part of the jmmc consists of a 10 15 km thick continental crust body flanked by two major sedimentary basins we propose that the microcontinent is divided into six distinct segments characterized by different basement and sedimentary architectures the shallow stratigraphy is well imaged and has been detailed three distinct sedimentary units have been defined together with pronounced reflectors ja red jo and f related to erosional magmatic thermal or uplift events the continent ocean boundary has been revised together with the mapping of the distinct volcanic events that characterize the evolution of the microcontinent a companion paper aims to further detail the interpretation of the jmmc structure presented here by constraining the conjugated mid norwegian and mid east greenland structures

rift segment interaction in orthogonal and rotational extension experiments implications for the large scale development of rift systems abstract during extension of the continental lithosphere rift basins develop these are often initially offset and must interact and connect in order to create a continuous rift system that may ultimately achieve break up when simulating extensional tectonics and rift interaction structures analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation resulting in rotational extension and associated rift propagation and structural gradients here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings our modelling efforts show that rotational extension and orthogonal extension produce significantly different large scale structures rotational extension can cause important variations in rift maturity between rift segments delay rift interaction zone development and make rift segments propagate in opposite directions still local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting furthermore we find that various degrees of rift underlap produce three basic modes of rift linkage structures low underlap distance high angle experiments develop rift pass structures with increasing underlap distance ca 40 transfer zone basins develop high degrees of underlap 30 tend to result in en echelon sub basins our results match with data from previous modelling efforts and natural examples we furthermore propose a large scale tectonic scenario for the east african rift system based on rotational extension and associated rift propagation these insights may also be applicable when studying other large scale rift systems

patterns of continental extension along the conjugate margins of the central and north atlantic oceans and labrador sea the anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading we estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and north atlantic oceans and labrador sea from the total tectonic subsidence of the margins in the central atlantic basin the integrated continental extension varies from 225 km for the break between the southern appalachian segment of the north american margin and africa to over 600 km for the break between the blake plateau region of north america and the cape verde terrace region of africa the original width of the extended zone varies from 240 to 770 km in the north atlantic and labrador basins the integrated extension varies from 60 km for the break between northern labrador and greenland to over 600 km for the break between newfoundland and europe the original width of the extended zones in these basins varies from 100 km to over 1460 km in general continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain as a result seafloor spreading tends to begin first along segments which follow the structural grain extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain this phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain we conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain

lateral variations in tectono magmatic style along the lofoten vesteralen volcanic margin off norway abstract the 400 km long passive continental margin west of the lofoten vesteralen archipelago off northern norway links the volcanic rifted voring margin and the sheared w barents sea margin multi channel seismic reflection profiles supplemented with crustal velocity gravity and magnetic anomaly data are used to outline the regional setting and main tectono magmatic features a well defined along strike margin segmentation comprising three segments characterized by distinct crustal properties structural and magmatic styles sediment thickness and post opening history of vertical motion is revealed the margin segments are governed by changes in fault polarity on late jurassic early cretaceous border faults and are separated by coeval cross margin transfer zones which acted as persistent barriers to rupture propagation and reflect the trend and character of older structural heterogeneities the transfer zones spatially correlate to small offset early opening oceanic fracture zones implying a structural inheritance from one rift episode to another culminating with lithospheric breakup at the paleocene eocene transition the pre seafloor spreading margin structural evolution is governed by the older predominantly late jurassic early cretaceous structural framework however the margin also provides evidence for mid and late cretaceous extension events that are poorly understood elsewhere off norway furthermore the lofoten vesteralen post breakup subsidence history contrasts with the adjacent margins reflecting breakup in thicker crust and a diminishing volume of high velocity lower crust emplaced during breakup

insights into the mode of the south georgia rift extension in eastern georgia usa abstract the south georgia rift sgr lies oblique to the east coast margin of north america and across the alleghenian suture between laurentia and africa in southern georgia regionally the sgr can be divided into a southwest compartment and a northeast compartment across the jacksonville structure that is located in the vicinity of that suture analytical and numerical models are used to characterize the mode of rifting in the northeast compartment borehole cocorp seismic and regional geophysical information from the compartment that were used previously to infer the geometry of the basin are reassessed with the use of those models to analyze the lithospheric conditions influencing triassic extension this approach led to the interpretation of core complex mode extension and to the proposal of a model of progressive rifting the model shows how the riddleville and main sgr basins are associated and how changes in structural style of those two basins resulted from changing lithospheric conditions during extension the core complex model also indicated that extension was influenced by distributed deformation of a younger warmer and less stable lithosphere adjacent to the permian suture whereas extension in other east coast rifts that lie subparallel to structural fabric was probably localized by preexisting zones of weakness

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin