imaging mapping and modelling continental lithosphere extension and breakup an introduction abstact this special publication is a direct outcome of a small but dedicated group of researchers who met in pontresina switzerland to review and define the fundamental observations characterizing extensional systems and their application in guiding and constraining modelling efforts and results the various summaries of the keynote addresses give an objective overview of the state of the art in modelling lithospheric extensional systems both from the regional scale using dynamic models to individual basins using kinematic models with an emphasis on capturing the extensional history of the iberia and newfoundland margins at the heart of all of these efforts is a simple question exactly what mechanisms allow the continental lithosphere to be thinned to the point of rupture related questions are 1 do crustal and mantle faults play a major role in this thinning process if so what is their geometry and does their importance and geometry change with time 2 are there other mechanisms of lithospheric and crustal thinning that cannot be imaged on seismic sections 3 how is deformation accommodated in space and time 4 what role do inherited mechanical thermal and or chemical heterogeneities play in controlling strain distribution and localization 5 when how and to what degree does magma production affect the distribution and localization of extension and 6 what is the stratigraphic record of continental extension and how does it document the extension of the crust and thinning of the lithospheric mantle the aim of this special publication is to address many of these fundamental questions concerning the extreme extension and thinning of continental lithosphere

new models for evolution of magma poor rifted margins based on a review of data and concepts from west iberia and the alps direct observation and extensive sampling in ancient margins exposed in the alps combined with drill hole and geophysical data from the present day iberia margin result in new concepts for the strain evolution and near surface response to lithospheric rupturing at magma poor rifted margins this paper reviews data and tectonic concepts derived from these two margins and proposes that extension leading to thinning and final rupturing of the continental lithosphere is accommodated by three fault systems each of them characterized by a specific temporal and spatial evolution during rifting of the margin by its fault geometry and its surface response the data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting the place of final break up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres during final break up the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes

hydrothermal inputs at odp sites 836 837 838 and 839 in relation to eastern lau spreading centre propagation in the lau basin southwest pacific

control of rheological stratification on rifting geometry a symmetric model resolving the upper plate paradox numerical experiments reproduce the fundamental architecture of magma poor rifted margins such as the iberian or alpine margins if the lithosphere has a weak mid crustal channel on top of strong lower crust and a horizontal thermal weakness in the rift center during model extension the upper crust undergoes distributed collapse into the rift center where the thermally weakened portion of the model tears among the features reproduced by the modeling we observe 1 an array of tilted upper crustal blocks resting directly on exhumed mantle at the distal margin 2 consistently oceanward dipping normal faults 3 a mid crustal high strain zone at the base of the crustal blocks s reflector 4 new ocean floor up against a low angle normal fault at the tip of the continent 5 shear zones consistent with continentward dipping reflectors in the mantle lithosphere 6 the mismatch frequently observed between stretching values inferred from surface extension and bulk crustal thinning at distal margins upper plate paradox rifting in the experiment is symmetric at a lithospheric scale and the above features develop on both sides of the rift center we discuss three controversial points in more detail 1 weak versus strong lower crust 2 the deformation pattern in the mantle and 3 the significance of detachment faults during continental breakup we argue that the transition from wide rifting towards narrow rifting with a pronounced polarity towards the rift center is associated with the advective growth of a thermal perturbation in the mantle lithosphere

response to comment on major australian antarctic plate reorganization at hawaiian emperor bend time accurately locating boundaries between continental and oceanic crust is topical in view of locating offshore boundaries relevant to margin formation models plate kinematics and frontier resource exploration although we disagree with tikku and direen s interpretations the associated controversies reflect an absence of agreed upon geophysical criteria for distinguishing stretched continental from oceanic crust and a lack of samples from nonvolcanic margins

seismic stratigraphy and sediment distribution on the wilkes land and terre adelie margins east antarctica abstract seismic reflection data show the existence of two major sedimentary basins along the continental margin of wilkes land and terre adelie east antarctica that contain more than 5 s twt 9 km of sediments four seismic megasequences are identified ms4 to ms1 that are bounded by basement unconformities of interpreted turonian maastrichtian and early middle eocene age and the seafloor the 4 5 km thick rift and pre rift sediments are concentrated in a margin parallel basin sabrina basin on the basis of seismic correlation with the australian margin this basin is interpreted to be of late jurassic to mid cretaceous age the post rift sediments are generally thick along the margin and in the adjacent deep ocean basin but are particularly thick in a major depocentre off west wilkes land named here the budd coast basin bcb the bcb contains a maximum observed thickness of 5 s twt 9 km of post rift sediments and its location suggests that the sediments were largely derived from a sub glacial basin currently occupied by the totten glacier within the post rift sediments a major regional unconformity is identified that is interpreted to be of early middle eocene age 45 ma while some previous interpretations have suggested that this unconformity marks the onset of the glaciation of east antarctica the unconformity is interpreted here to be the product of the major global plate reorganisation in the middle eocene and the coincident sharp increase in the sea floor spreading rate between australia and antarctica a seismic unconformity that can be correlated with the onset of glaciation is not identified and the position of glacial onset within the sedimentary section can only be estimated through the extrapolation of sedimentation rates from the underlying section and from deep sea drilling project site 269 the post middle eocene section is much thicker than the underlying post rift section and this is interpreted to be largely due to the very high sediment fluxes associated with the period of polythermal glaciation from 34 9 ma sediment isochron and isopach maps derived from a regional seismic grid show that the depocentre of the bcb trends approximately north south beneath the continental slope but is deflected to the east beneath the deep ocean basin this distribution indicates that deep marine processes were important in lateral along margin sediment transport a west to east direction of movement is inferred by the sediment distribution this direction is opposite to the present day marginal currents along the continental slope and rise

central gulf of aden conjugate margins yemen somalia tectono sedimentary and magmatism evolution in hybrid type margins abstract magma rich and magma poor passive margins are generally studied separately yet the spatial evolution from one type to another is not well understood central gulf of aden margins are at the transition between these two types of margins based on new seismic data published wells and gravity data we determine seismic stratigraphy and structural patterns we map the distinct crustal domains continental ocean continent transition oct oceanic domains and propose a tectonic evolution of the yemeni and somalian conjugate margins in an oblique rifting context the most striking results are the significant segmentation narrowness and asymmetry of conjugate margins as well as the gradual eastward decrease in the intensity of magmatic activity of these hybrid type margins west of bosaso hami fracture zone bhfz the hami ahl medo segment presents magmatic type margin features in the distal yemeni margin related to the strong influence from the afar plume seaward dipping reflectors develop since the late syn rift stage 21 18 ma near the xiis al mukalla fracture zone conversely east of bhfz the bosaso sayhut margins segment is characterized by magma poor margins for which the exhumation stage is characterized by the unroofing of mantle along multiple detachment faults in the oct and shortly followed by diachronous volcanism 18 ma the central gulf of aden is affected by a major uplift during the final syn rift stage and up to oct formation this event is associated with the formation of erosional surfaces in the east and with gravitational deformation taking place on decoupling surfaces at the base of the syn rift sequence lower ghaydah fm that lead to the sliding of syn rift units on top of the exhumed mantle rocks in the oct

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

lithospheric extension from rifting to continental breakup at magma poor margins rheology serpentinisation and symmetry the symmetry or asymmetry of the process of continental breakup has been much debated over the last 20 years with various authors proposing asymmetric simple shear models others advocating more symmetric pure shear models and some combinations of the two the unroofing of vast expanses of sub continental mantle at non volcanic margins has led some authors to argue in favour of simple shear models but supporting evidence is lacking subsidence evidence from conjugate margin pairs is equivocal and the detailed crustal and lithospheric structure of such pairs not generally well enough known to draw firm conclusions in the porcupine basin where the final stages of break up are preserved the development of structural asymmetry is demonstrable and apparently related to late stage coupling of the crust to the mantle following the complete embrittlement of the crust this agrees with theoretical modelling results which predict that asymmetric models can develop only on a lithospheric scale when the crust and mantle are tightly coupled however whether such asymmetry is maintained during continued exhumation of the mantle is unclear

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

an investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel 1 conditions corresponding to the initiation of the erosion and deposition process 1 the present study investigates the flow physics and the role played by the main coherent structures in the scouring processes around a vertical spur dike in a straight channel at conditions corresponding to the start flat bed of the scouring process large eddy simulation les is performed at a relatively low channel reynolds number re 18 000 in the range where most flume studies with clear water scour conditions are conducted similar to these studies the incoming flow is fully turbulent and contains realistic turbulence fluctuations visualization experiments are conducted to better understand the nature of the interactions between the dominant coherent structures playing a role in the erosion process it is found that the structure of the horseshoe vortex hv system at the base of the spur dike changes considerably in time and in vertical sections perpendicular to the trajectory defined by the axis of the main necklace vortex however its intensity is the largest at vertical sections situated around the tip of the spur dike it is in this region that the core of the main necklace vortex oscillates aperiodically between two preferred modes in one of them zero flow mode the necklace vortex is closer to the spur dike and more compact and the near bed jet flow beneath it is weak in the other one back flow mode a strong near bed jet flow convects the primary necklace vortex away from the spur dike and its core is more elongated and less compact this explains the large amplification by about 1 order of magnitude compared to the surrounding turbulent flow of the turbulent kinetic energy and pressure fluctuations inside the hv system in the region situated around the tip of the spur dike and the double peak distribution of the turbulent kinetic energy past the spur dike in the legs of the necklace vortex the intensity of the bimodal oscillations decreases such that they are not observed in spanwise sections situated at more than one channel depth behind the spur dike it is found that the legs of the horseshoe vortices can interact at times with the vortex tubes shed in the detached shear layer dsl and with the tip of the spur dike these events typically result in a significant change in the coherence of the hv system the largest bed shear stress values in the mean flow are present in the strong acceleration region near the tip of the spur dike but high bed shear stress values are also observed beneath the upstream part of the dsl the bed shear stress fluctuations around the local mean values can be very high especially in the region situated beneath the upstream part of the dsl at random times some of the vortices shed in the dsl merge or interact with eddies from the recirculation region this leads to an increase in their strength and to a large increase of the bed shear stress along their path

on the fit of broken ridge and kerguelen plateau abstract we present revised finite rotation parameters for australia and east antarctica in the early tertiary incorporating new magnetic data off of wilkes land antarctica reconstructions based on these and recently published rotation parameters document a relatively stationary position of broken ridge abutted against central kerguelen plateau between chron 34 83 ma and chron 20 43 ma prior to the commencement of seafloor spreading between the two late cretaceous features these reconstructions are significant as they are the first which are consistent with geological and geophysical constraints in not producing an overlap of broken ridge and kerguelen plateau between chron 34 and chron 24 53 ma and not requiring significant extension or strike slip motion within kerguelen plateau we also find that the finite rotations for chrons 20 to 34 outline the area of rough topography inferred from the satellite derived free air gravity for the diamantina zone and labuan basin a consequence of these rotations is that there is considerable overlap of continental crust in the region of tasmania the south tasman rise and east antarctica in the late cretaceous solutions to this problem may imply either post santonian 79 ma extension or transform motion in the bass strait or late cretaceous or cenozoic extension in wilkes land the reconstructions also agree with prior estimates of roughly 150 km of motion between east and west antarctica since chron 20

extension discrepancy at north atlantic nonvolcanic rifted margins depth dependent stretching or unrecognized faulting rifted margins show an apparent discrepancy between the amount of extension measurable from faults and the amount of crustal or lithospheric thinning determined from wide angle data or subsidence this extension discrepancy has been interpreted in terms of depth dependent stretching in which the upper crust is extended and thinned far less than the rest of the lithosphere here i show that at those nonvolcanic margins where the velocity structure is well known the upper and lower crust appear to thin equally toward the margin implying that the discrepancy is not due to depth dependent stretching but more likely due to unrecognized polyphase and top basement faulting

australia s southern margin a product of oblique extension abstract recently developed detachment models of continental margin formation interpret the southern margin of australia to have formed when the lower plate australian margin was pulled out from beneath the upper plate antarctic margin data now available and summarised in this paper point very strongly to a generally nw se direction of initial continental extension for the southern margin in contrast to the widely held picture of simple n s rifting the evidence for this extension direction comes from the analysis of deep seismic data acquired by the bureau of mineral resources in 1986 in the central great australian bight gab the gravity field of the gab seismic and magnetic basement structures in the eyre sub basin polda trough ceduna depocentre and duntroon basin and from the analysis of the magnetic seafloor spreading anomalies produced during the slow first phase of drifting between australia and antarctica further it is now believed that the formation of the southern margin of australia can be described in terms of three phases of continental extension el to e3 and two phases of drifting d1 and d2 in summary these phases were as follows 1 e1 approximately 300 km of late jurassic or older to early cretaceous nw se oriented extension in the gab with strike slip motion in the nascent otway basin and along the tasmanian margin 2 e2 120 km of early cretaceous nne ssw oriented extension which formed the basins of southeastern australia otway bass gippsland and which probably produced a structural overprinting in the gab basin 3 e3 d1 minor continental extension and the first 500 km of slow drift between australia and antarctica on an azimuth of 165 wrenching on the tasmanian margin 4 d2 2600 km of fast drifting between australia and antarctica on a n s azimuth

blind orogen integrated appraisal of multiple episodes of mesoproterozoic deformation and reworking in the fowler domain western gawler craton australia abstract the fowler domain in the western gawler craton in southern australia is a poorly exposed region that can only presented be explored using a combination of information from drilling and potential field geophysics regional maps of the total magnetic intensity tmi field of the fowler domain highlight an anastomosing system of terrain scale shear zones that bound four crustal scale tectonic blocks from west to east the colona barton central and nundroo blocks integrated thermobarometry and electron microprobe chemical dating of metamorphic monazites from drillholes in the fowler domain suggest that the colona block in the west underwent two mid crustal amphibolite grade metamorphic events at ca 1643 and 1600 ma the younger age corresponds to the timing of regional high grade metamorphism in the barton block together the age data suggest that the western fowler domain underwent a major tectonothermal event at ca 1600 ma in contrast regional lower crustal metamorphism in the nundroo block which forms the eastern fowler domain occurred at ca 1545 ma in both the barton and nundroo blocks petrological relationships mineral zoning and pressure temperature p t modelling suggest the terrains cooled in the mid to lower crust rather than undergoing exhumation immediately following peak metamorphism age data from the geophysically defined shear zone systems that bound the blocks suggest that exhumation of these lower crustal domains occurred between ca 1470 and 1450 ma and was associated with transpressional reactivation of the terrain during the coorabie orogeny a key finding of this study is that the tectonic evolution of the crustal blocks in the fowler domain was not in concert until at least 1500 ma thus the evolution of the individual blocks is unlikely to be related to the macroscopic character of the terrain defined by the regional scale shear zone systems which are one of the youngest tectonic imprints on the fowler domain coincident gravity and magnetic forward modelling of significant bounding faults suggests the shear zones form a steeply dipping transpressional array consistent with the observed metamorphic field character of the different blocks this study provides a demonstration of the integration of geophysical and petrological approaches to investigate the time integrated tectonic evolution of poorly exposed basement terrains

effects of groyne layout on the flow in groyne fields laboratory experiments this research is aimed at finding efficient alternative designs in the physical economical and ecological sense for the standard groynes as they are found in the large rivers of europe in order to test the effects of various groyne shapes on the flow in a groyne field experiments were performed in a physical model of a schematized river reach geometrically scaled 1 40 four different types of schematized groynes were tested all arranged in an array of five identical groyne fields i e standard reference groynes groynes with a head having a gentle slope and extending into the main channel permeable groynes consisting of pile rows and hybrid groynes consisting of a lowered impermeable groyne with a pile row on top flow velocities were measured using particle tracking velocimetry the design of the experiment was such that the cross sectional area blocked by the groyne was the same in all cases depending on the groyne head shape and the extent of submergence variations in the intensity of vortex shedding and recirculation in the groyne field were observed the experimental data are used to understand the physical processes like vortex formation and detachment near the groyne head it is demonstrated that the turbulence properties near and downstream of the groyne can be manipulated by changing the permeability and slope of the groyne head it is also observed that for submerged conditions the flow becomes complex and locally dominated by three dimensional effects which will make it difficult to predict by applying depth average numerical models or by three dimensional models with a coarse resolution in the vertical direction

dominant symmetry of a conjugate southern australian and east antarctic magma poor rifted margin segment synthesis and modeling of published deep seismic and potential field data from the conjugate magma poor rifted margins of the great australian bight southern australia and central wilkes land east antarctica show that there is pronounced symmetry of structures in a 300 km wide zone straddling the axis of final breakup this symmetry is observed consistently for a distance of some hundreds of kilometers along strike from inboard to outboard both margins comprise a narrow zone of attenuation of the crystalline continental crust an approximately 4 km high basement ridge interpreted as unroofed peridotites at the location of maximum thinning of the continental crust and a 60 70 km wide continent ocean transition zone that contains a sedimentary basin that may be underlain by altered mantle and fragments of crystalline continental crust the marked breakup symmetry described here is in contrast to the asymmetry of the iberia newfoundland margin and is consistent with the operation of a symmetrical extensional detachment system deforming the whole crust in the center of the rift as envisaged by some numerical models for the continental rifting process

local scour associated with angled spur dikes a series of experiments were conducted in which the volume of the scour hole associated with model spur dikes was measured in a laboratory flume under clear water overtopping flows spur dike models were angled at 45 90 and 135 to the downstream channel sidewall with contraction ratios of 0 125 and 0 250 the main goals of the experiments were to evaluate the effect of the three angles on the volume of scour and potential aquatic habitat and on minimizing erosion adjacent to the streambanks the experiments showed that of the three angles tested the least erosion of the bed in the near bank region was associated with the spur dikes with 90 angles while the greatest volume of the scour hole was associated with the 135 spur dikes it was concluded that spur dikes with 135 angles showed the best potential for providing improved aquatic habitats while minimizing the potential for erosion of the channel bank

a numerical investigation of coherent structures and mass exchange processes in channel flow with two lateral submerged groynes 1 large eddy simulation is used to investigate the dynamics of the main coherent structures present in the flow around two vertical submerged groynes situated in a long flatbed open channel the mass exchange processes between the embayment region and the main channel are investigated by studying the ejection of a contaminant introduced instantaneously inside the embayment the instantaneous and mean structure of the horseshoe vortex system forming at the base of the upstream groyne and the bed shear distributions that determine the evolution of the scour in the groyne region are investigated it is found that the amplification of the bed shear stress in the accelerating region around the tip of the upstream groyne is around one order of magnitude larger relative to the mean bed shear stress in the incoming flow analysis of the instantaneous flow fields shows that the eddies that are shed inside the horizontal and vertical detached shear layers play an important role in controlling the mass exchange at the lateral and roof interfaces it is found that most of the pollutant leaves the embayment through the roof and bottom lateral sections the overall mass exchange process is qualitatively different and substantially accelerated compared to the case when the groynes are emerged however it is shown that similar to the emerged case the decay of the mass of contaminant within the embayment cannot be characterized by a unique value of the exchange coefficient used in simple dead zone theory models

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

seafloor spreading magnetic anomalies in the enderby basin east antarctica abstract the timing of the early separation of india from the contiguous antarctica australia is still an unresolved problem although it is well established that antarctica and india formed a single indo antarctic platform prior to the fragmentation of eastern gondwanaland inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of antarctica and india perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of india from antarctica analysis of magnetic and satellite derived gravity data in the enderby basin east antarctica reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones the observed magnetic anomalies can be interpreted as the younger sequence of mesozoic anomalies m11 m0 half spreading rates range from 6 5 to 2 8 cm yr and are comparable with those measured in the bay of bengal these similarities in the mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary a consistent plate reconstruction model can be derived from the identified conjugate patterns of mesozoic magnetic anomalies and fracture zones the occurrence of the oldest magnetic anomaly m11 close to the coasts in these two offshore basins unequivocally suggests that the break up of india from antarctica occurred before 134 ma

a marine geophysical study of the wilkes land rifted continental margin antarctica summary the wilkes land margin of east antarctica conjugate to the southern australian margin is a non volcanic rifted margin that formed during the late cretaceous during 2000 01 and 2001 02 geoscience australia acquired 10 000 line km of seismic reflection and refraction magnetic anomaly and gravity anomaly data over the margin we have used the seismic data to estimate the sediment thickness along the margin the data reveal a deep 11 km rift basin that contains over 9 km of sediments seaward of the totten glacier western wilkes land the limits of oceanic and continental crust that underlies the thick post rift and the significantly thinner pre and syn rift sediments are equivocal seismic reflection data suggest a 30 100 km wide continent ocean transition zone cotz along the margin the cotz extends over 400 km seaward of the shelf break off the eastern wilkes land terre ad elie sector

on the turbulent flow structure around an instream structure with realistic geometry we investigate the flow dynamics around a rock vane a widely used instream structure for stream restoration by conducting laboratory flume experiments and carrying out high resolution large eddy simulation les taking advantage of parallel computing the flume experiments are conducted under fixed and mobile bed conditions where the velocities and bed elevations are measured respectively the les is carried out for the fixed bed experiment by directly resolving the details of the rocks that constitute the vane and the individual roughness elements on the channel bed the les computed mean flow statistics show good agreement with the measurements and the analysis of the computed flow field reveals the existence of two counter rotating secondary flow cells downstream of the vane which originate from the plunging of the three dimensional streamlines onto a lower part of the sidewall downstream of the vane to further examine the role of the secondary flow cells under a mobile bed condition the les results are compared with the equilibrium bed elevation measured in the mobile bed experiment the mobile bed experiment reveals the existence of an oblique sand ridge downstream of the vane that is aligned with the line of flow convergence caused by the collision of the two secondary flow cells the results indicate that the two counter rotating cells downstream of the rock vane has a profound impact on the mean flow and bed shear stress as well as on the bed morphodynamics this article is protected by copyright all rights reserved

large woody debris and flow resistance in step pool channels cascade range washington abstract total flow resistance measured as darcy weisbach f in 20 step pool channels with large woody debris lwd in washington ranged from 5 to 380 during summer low flows step risers in the study streams consist of either 1 large and relatively immobile woody debris bedrock or roots that form fixed or forced steps or 2 smaller and relatively mobile wood or clasts or a mixture of both arranged across the channel by the stream flow resistance in step pool channels may be partitioned into grain form and spill resistance grain resistance is calculated as a function of particle size and form resistance is calculated as large woody debris drag combined grain and form resistance account for less than 10 of the total flow resistance we initially assumed that the substantial remaining portion is spill resistance attributable to steps however measured step characteristics could not explain between reach variations in flow resistance this suggests that other factors may be significant the coefficient of variation of the hydraulic radius explained 43 of the variation in friction factors between streams for example large woody debris generates form resistance on step treads and spill resistance at step risers because the form resistance of step pool channels is relatively minor compared to spill resistance and because wood in steps accentuates spill resistance by increasing step height we suggest that wood in step risers influences channel hydraulics more than wood elsewhere in the channel hence the distribution and function not just abundance of large woody debris is critical in steep step pool channels

insight into the nature of the ocean continent transition off west iberia from a deep multichannel seismic reflection profile a high quality multichannel seismic reflection profile across the west iberia margin confirms the existence of a 130 km wide transition zone between tilted continental fault blocks and unequivocal oceanic crust the zone is characterized by a 1 0 2 5 km thick seismically unreflective upper basement layer lying over reflective basement that contains major landward dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10 19 km below the basement surface and minor seaward dipping reflectors moho reflections are absent in this zone we suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation about 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread

architecture of proterozoic shear zones in the christie domain western gawler craton australia geophysical appraisal of a poorly exposed orogenic terrane abstract large scale mesoproterozoic crustal shear zones are partially exposed from beneath cover sequences in the christie domain of the northwestern gawler craton southern australia these structures are associated with gravity and magnetic anomalies that allow them to be mapped under cover and their three dimensional geometry and kinematics to be evaluated gravity and magnetic forward modelling indicate that these shear zones form an imbricate oblique thrust stack with combined top to the southeast left lateral transport the longest shear zones in the stack penetrate the crust to at least 15 km depth and dip to the northwest at 70 their inferred sense of motion are consistent with kinematic indicators from sparse outcrops from northwest to southeast the stack includes crustal slices bound by the karari fault zone tallacootra and blowout shear zones colona and coorabie fault zones and the muckanippie shear zone these structures separate discrete tectonometamorphic packages within the previously undifferentiated christie gneiss of the archean mulgathing complex and imply several generations of transport of material from the lower and middle crust three dimensional inversion of gravity data indicates that the muckanippie shear zone a shallowly rooted second order splay of the coorabie fault zone has an antithetic dip to the southeast and is part of a positive flower structure cross cutting relationships coupled with recent reconnaissance geochronology suggest that the western gawler craton part of the mawson continent was under a long lasting oblique slip deformational regime after ca 1590 ma this tectonic reworking is correlated with episodic pervasive metamorphism and deformation recorded elsewhere in the gawler craton between ca 1550 and 1450 coorabie orogeny pointing to a complex history of stabilization in proterozoic rodinia the tallacootra colona and coorabie structures are all cut at a high angle to their strikes by mesozoic rift faults of the southern rift system and may form piercing points in reconstructions of gondwana and earlier rodinia configurations

nature of the continent ocean transition zone along the southern australian continental margin a comparison of the naturaliste plateau sw australia and the central great australian bight sectors we document the interpretation of three crustal sections from coincident deep seismic reflection gravity and magnetic data acquired on australia s southern margin one section from the naturaliste plateau and the diamantina zone and two in the great australian bight gab interpretations are based on an integrated study of deep multichannel seismic gravity and magnetic data together with sparse sonobuoy and dredging information all interpreted sections of the margin show a transition from thinned continental crust through a wide continent ocean transition zone cotz in the gab the transition is to slow sea floor spreading oceanic crust that dates from breakup in the campanian c 83 ma in the naturaliste diamantina margin the earliest oceanic crust is undated the cotz on these margins is geologically and geophysically complex but interpretation of all data including dredge hauls is consistent with the presence of a mixture of modified continental lower crust breakup related volcanics and exhumed continental mantle serpentinized detachment faults are not well imaged but have been inferred from high amplitude magnetic signatures interpreted to arise from magnetite associated with the hydration of peridotites alternative models for the structure of the cotz involving either mafic underplating or aborted sea floor spreading have been explored but are considered unlikely on this margin similarity in the final architecture of these margins has major implications for the nature of rifting in the southern rift system and may point to the entire 4000 km long system being non volcanic in character second order differences in geometry and morphology of the two areas studied are unlikely to be a function of strain rate instead they probably reflect complexities owing to the multiple tectonic events that occurred during final gondwanide fragmentation the most dramatic of these is the impact of hotspot activity in the kerguelen plateau which commenced some 50 ma prior to final breakup in that sector

symmetric alternative to asymmetric rifting models we describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magma poor margins such as the galicia margin west of the iberian peninsula or the apulia margin in the alps 1 distal continental margins consist of fault bounded blocks separated by oceanward dipping normal faults 2 at the tip of the continent lower crust is scarcely preserved or absent and upper crust directly overlies exhumed mantle 3 the base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top to the ocean sense of shear in our model these structures do not reflect asymmetric rift geometry at a lithospheric scale instead they derive from upper crustal collapse over a mid crustal shear zone into the rift center and are present on both sides of the rift axis the model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center we hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well defined rift center

transition from continental to oceanic crust on the wilkes adelie margin of antarctica the wilkes adelie margin of east antarctica a passive margin rifted in the early cretaceous has an unusually reflective moho which can be traced seismically across the continent ocean transition velocity models and depth sections were constructed from a combined set of u s and french multichannel seismic reflection lines to investigate the transition from continental to oceanic crust these data show that the boundary between oldest oceanic crust and transitional continental crust is marked by a minimum in subsediment crustal thickness and in places by a shoaling of moho the moho reflection is continuous across the edge of oceanic crust and gradually deepens landward under the continental edge

bed surface adjustments to spatially variable flow in low relative submergence regimes in mountainous rivers large relatively immobile grains partly control the local and reach averaged flow hydraulics and sediment fluxes when the flow depth is similar to the size of these grains low relative submergence heterogeneous flow structures and plunging flow cause spatial distributions of bed surface elevations textures and sedimentation rates to explore how the bed surface responds to these flow variations we conducted a set of experiments in which we varied the relative submergence of staggered hemispheres simulated large boulders between runs all experiments had the same average sediment transport capacity upstream sediment supply and initial bed thickness and grain size distribution we combined our laboratory measurements with a 3d flow model to obtain the detailed flow structure around the hemispheres the local bed shear stress field displayed substantial variability and controlled the bedload transport rates and direction in which sediment moved the divergence in bed shear stress caused by the hemispheres promoted size selective bedload deposition which formed patches of coarse sediment upstream of the hemisphere sediment deposition caused a decrease in local bed shear stress which combined with the coarser grain size enhanced the stability of this patch the region downstream of the hemispheres was largely controlled by a recirculation zone and had little to no change in grain size bed elevation and bed shear stress the formation development and stability of sediment patches in mountain streams is controlled by the bed shear stress divergence and magnitude and direction of the local bed shear stress field

hyporheic exchange flow around constructed in channel structures and implications for restoration design in channel rock vane structures are widely used in stream restoration as a way to reduce stream channel erosion and create pool or riffle features when these structures change hydraulic gradients they may affect ecological stream functions such as hyporheic exchange flow hef patterns a study of constructed in channel structure controls on hef was conducted in the third order batavia kill new york using stream and hyporheic temperature amplitude analysis and computational fluid dynamics cfd hydraulic simulations temperature monitors were installed in the water column and channel bed at six locations around each of seven in channel restoration structures three cross vanes and four j hooks at baseflow in 2007 elevation surveys of the structures were then used to simulate hef using cfd the results indicate a pattern of pronounced upwelling in the run section just below the structure upwelling transitioning to downwelling within the pool and pronounced downwelling in the glide out of the pool this pattern is consistent with natural riffle pool sequences the direction of hef inferred from the temperature amplitude analysis agreed with the direction of flow simulated with cfd at 80 of the locations and the few disagreements were expected due to model limitations cfd simulation demonstrated that increasing stream flows result in changes in hef spatial patterns and magnitude at each structure this work illustrates how cfd simulations can guide design of in channel restoration structures for hef function copyright 2009 john wiley sons ltd

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

evolution of the newfoundland iberia conjugate rifted margins it is accepted that mildly extended sedimentary basins form by largely uniform thinning of continental lithosphere no such consensus exists for the formation of highly extended conjugate rifted continental margins instead a wide range of models which invoke differing degrees of depth dependent thinning have been proposed much of this debate has focussed on the well studied newfoundland iberia conjugate margins we have tackled the problem of depth dependency at this pair of margins in three steps first we have reconstructed water loaded subsidence histories by making simple assumptions about changes in water depth through time secondly we have used these reconstructed subsidence histories to determine the spatial and temporal variation of lithospheric strain rate an inversion algorithm minimizes the misfit between observed and predicted subsidence histories and crustal thicknesses by varying strain rate as a smooth function of distance across the margin depth through the lithosphere and geologic time depth dependent thinning is permitted but crucially our algorithm does not prescribe its existence or form given the absence of significant volumes of syn rift magmatism we have also applied a minimal melting constraint inverse modeling has yielded excellent fits to both reconstructed subsidence and crustal observations which suggest that rifting occurred from not vert similar 150 135 ma and at rates of up to 0 3 ma 1 strain rate distributions are depth dependent suggesting that lithospheric mantle thins over a wider region than the crust beneath highly extended parts of the margin crustal strain rates greatly exceed lithospheric mantle strain rates thirdly we have tested our strain rate histories by comparing the total horizontal extension with the amount of extension inferred from normal faulting patterns both values agree within error we freely acknowledge that there are important uncertainties in reconstructing the subsidence histories of deep water margins nevertheless stratigraphic records remain the only albeit imperfect means of determining how crust and lithospheric mantle thin through time and space

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

wide and narrow margins of the brazilian south atlantic two contrasting end members of passive continental margins here referred to as wide and narrow margins are present in the south atlantic although some margin segments are intermediate between these two styles narrow margins are characterized by a large bounding fault 4 km throw near the shelf edge a sharp transition zone 10 20 km wide from normal thickness to substantially thinned continental crust or oceanic crust where the total width of extended continental crust is usually less than 100 km wide continental margins are characterized by a broad continental shelf and a wide zone 50 600 km of thinned crust with an even distribution of faults wide margins usually have a more gentle continental shelf and slope narrow narrow wide wide and wide narrow conjugate margin pairs exist in the south atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting basement structure and strain rate appear to have exerted little control on the style of margin extension although basement architecture appears to control the trend of the initial rifts the positions of fracture zones which separate narrow and wide segments and the rate of lateral rift propagation

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

effects of bed forms and large protruding grains on near bed flow hydraulics in low relative submergence conditions in mountain rivers bedforms large relatively immobile grains and bed texture and topographic variability can significantly alter local and reach averaged flow characteristics the low relative submergence of large immobile grains causes highly three dimensional flow fields that may not be represented by traditional shear stress flow velocity and turbulence intensity equations to explore the influence of large protruding grains and bedforms on flow properties we conducted a set of experiments in which we varied the relative submergence while holding the sediment transport capacity and upstream sediment supply constant flow and bed measurements were conducted at the beginning and end of each experiment to account for the absence or presence of bedforms respectively detailed information on the flow was obtained by combining our measurements with a 3d numerical model commonly used velocity profile equations only performed well at the reach scale when shallow flow effects and the roughness length of the relatively mobile sediment were considered however at the local scale large deviations from these profiles were observed and simple methods to estimate the spatial distribution of near bed shear stresses are likely to be inaccurate zones of high turbulent kinetic energy occurred near the water surface and were largely controlled by the immobile grains and plunging flow the reach averaged shear stress did not correlate to depth or slope as commonly assumed but instead was controlled by the relative boulder submergence and degree of plunging flow for accurate flow predictions in mountain rivers the effects of bedforms and large boulders must be considered

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

comment on major australian antarctic plate reorganization at hawaiian emperor bend time whittaker et al reports 5 october 2007 p 83 presented reconstructions for australia and antarctica showing a change in relative plate motion 53 million years ago coincident with an inferred major global plate reorganization this comment addresses problematic areas in their assumptions and the geological consequences of their reconstructions

mean flow and turbulence around a laboratory spur dike the three dimensional turbulent flow field around a spur dike in a plane fixed bed laboratory open channel was studied experimentally using a microacoustic doppler velocimeter mean and turbulence characteristics in all three spatial directions were evaluated at upstream and downstream cross sections near the dike results showed that the primary flow separated in both lateral and vertical directions two counter rotating flow circulations consisting of the lateral and vertical velocity components originated at the dike section downstream of the dike the circulation in the flow separation zone is stronger than the one in the contracted primary flow zone the maximum bed shear stresses estimated using reynolds stresses is about three times as large as the mean bed shear stress of incoming flow

continental breakup and the onset of ultraslow seafloor spreading off flemish cap on the newfoundland rifted margin prestack depth migrated seismic reflection data collected off flemish cap on the newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system within continental crust there is no clear evidence for detachment surfaces analogous to the s reflection off the conjugate galicia bank margin demonstrating a first order asymmetry in final rift development anomalously thin 3 4 km magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized this indicates that initial accretion of the oceanic crust was in a magma limited setting similar to present day ultraslow spreading environments seaward oceanic crust thins to 1 3 km and exhibits an unusual highly reflective layering we propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep marine sheet flows to form this layering subsequent seafloor spreading formed normal 6 km thick oceanic crust this interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow spreading system was established

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

seismic stratigraphy of a large cretaceous shelf margin delta complex offshore southern australia seismic images along australia s southern continental margin reveal the internal geometry and depositional history of the hammerhead delta a late cretaceous shelf margin delta complex in the ceduna subbasin of the bight basin the hammerhead delta comprises the late santonian to maastrichtian hammerhead supersequence which is divisible into three third order sequence sets and their component sequences sequence set 1 late santonian to early campanian comprises three progradational sequences deposited after a major fall in sea level under a high sediment supply regime sequence set 2 early campanian to early maastrichtian comprises two sandy progradational sequences basinward shifts in facies caused by forced regression are apparent between each progradational sequence sequence set 3 early to late maastrichtian is a thick aggradational succession deposited when the rates of creation of accommodation space and sediment supply were balanced a transgressive episode at the top of sequence set 3 marks a rapid decrease in sediment supply and the end of deltaic sedimentation a long lived sand rich sediment supply most likely derived from erosion of the eastern australian highlands to the northeast was the major influence on delta formation rapid progradation and the formation of thick shelf margin clinoforms resulted in slope instability growth faulting and load induced collapse of the shelf margin during the campanian the hammerhead delta is characterized by sandy progradational clinoforms and lacks the thick coeval prodeltaic shales and shale tectonism that are common to many other large deltas the results of this study which included seismic facies mapping well correlations and comparisons to other large shale poor deltas suggest that the hammerhead delta has excellent reservoir potential

scale effects in flume experiments on flow around a spur dike in flatbed channel this paper presents the findings from a series of flume experiments conducted to determine scale effects in small scale models of flow around a single spur dike wing dam groyne or abutment placed in a channel whose bed is fixed and flat the flow features of primary interest are flow thalweg alignment line of maximum streamwise velocity around a dike and area extent of the flow separation region wake immediately downstream of the dike those features are of practical concern in the deployment of dikes for various channel control purposes the scale effects of concern herein are those attributable to use small length scales together with a bed shear stress parameter e g shields parameter as the primary criterion for dynamic similitude small scale models especially micromodels often are used for investigating channel control issues also the shear stress criterion is commonly used for models of flow in loose bed channels whereas froude number commonly is the primary similitude criterion for models of fixed bed open channel flows the experiments show that use of a shear stress parameter as the primary criterion for dynamic similitude influences the flow thalweg and flow separation region at a dike it does so by distorting pressure gradients around the model dike and by affecting turbulence generated by the dike it also is shown that for a range of small models thalweg alignment and extent of separation region do not scale with model length scales these findings are important for interpreting results from small hydraulic models especially micromodels of flow in loose bed channels

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

exhumed upper mantle within the ocean continent transition on the northern west iberia margin evidence from prestack depth migration and total tectonic subsidence analyses 1 the west iberia continental margin contains an enigmatic north south ridge of serpentinized peridotite located within the ocean continent transition we used prestack depth migration and total tectonic subsidence analysis to constrain the basement type adjacent to the ridge in order to learn more about its emplacement prestack depth migration was performed on segments of the nine east west lines from the iberia seismic experiment ise 97 and total tectonic subsidence analyses were performed on the depth sections the seismic data and subsidence analyses are consistent with exhumed upper mantle occurring immediately landward of the ridge suggesting that the ridge is located within a broad zone of exhumed mantle that has been serpentinized the subsidence analyses also provide minimum bounds on the thickness of the serpentinized layer which extends from the zone of exhumed mantle landward under thinned continental crust exhumed upper mantle has previously been identified in the wider ocean continent transition to the south in the southern iberia abyssal plain and we conclude that similar but narrower exposures occur to the north in the galicia bank area while upper mantle appears to have been tectonically exhumed along all the ise 97 lines it does not always form a distinct basement ridge there is no well defined ridge on three seismic profiles in the middle of the ise 97 survey where it is well developed the peridotite ridge parallels a deeply penetrating west dipping normal fault and the three middle lines appear to represent a transfer zone where this normal fault is not well developed