faulting pattern along slow spreading ridge segments a consequence of along axis variation in lithospheric rheology abstract we present here results from an analogue model designed to test the relative influence of along axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow spreading ridge rheologically calculated layered models are employed using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere respectively two parameters have been analysed in detail 1 the thickness of the brittle layer and 2 the viscosity of the ductile layer this study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults a decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer these results are consistent with observations along mid atlantic ridge segments for segment ends the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero offset discontinuity and on outside corners for segments bordered by a lateral offset discontinuity our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer the influence of segment length offset length and temporal variation in thermal input could explain the more or less important along axis variation in the deformation pattern observed along segments of slow spreading ridges

patterns of fault displacement and strain at yucca mountain nevada abstract yucca mountain nevada is the sole candidate site for underground disposal of high level radioactive waste in the united states the mountain is composed of tertiary 12 8 11 6 ma volcanic tuff cut by west dipping normal faults that divide the mountain into north trending east dipping cuestas geologic characterization of yucca mountain by the u s department of energy doe has focused on mapping lithostratigraphic units faults including single plane small displacement surfaces of discontinuity and large displacement fault zones and fractures quasi planar zones that have experienced loss of cohesion including joints partially mineralized joints veins and small displacement faults faults and fractures are important to repository design because they affect seismic hazard rockfall and fluid transmissivity in the surrounding rock mass geologic maps and detailed studies of rock pavements and tunnel walls reveal that faults and fractures within yucca mountain are not uniform in orientation or intensity we investigate two aspects of distributed deformation arising from fault displacement patterns at yucca mountain first fault parallel strains elongation parallel to cutoff lines where stratigraphic horizons intersect fault planes develop as a result of lateral fault displacement gradients using existing data we analyze the likely state of strain in fault blocks at yucca mountain second fault strike perpendicular strains can develop where two normal faults propagate past each other a component of the total strain is distributed into the surrounding rock to produce synthetic layer dip or a network of smaller faults and fractures we find that small scale faulting and fracturing at yucca mountain is variable and is strongly controlled by larger scale fault system architecture

coarse sediment bands on the inner shelf of southern monterey bay california abstract bands of coarse sand that trend parallel to the shore unlike the approximately shore normal bands found in many inner shelf areas occur in southern monterey bay at water depths of 10 20 m less than 1 km from the shore the bands are 20 100 m wide and alternate with bands of fine sand that are of similar width the coarse sand bands are as much as 1 m lower than the adjacent fine sand bands which have margins inclined at angles of about 20 the mean grain sizes of the coarse and fine sand are in the range of 0 354 1 0 mm and 0 125 0 354 mm respectively wave ripples that average about 1 m in spacing always occur in the coarse sand bands over a period of 3 yrs the individual bands moved irregularly and changed in shape as demonstrated by repeated sidescan sonar surveys and by the monitoring of rods jetted into the sea floor however the overall pattern and distribution of the bands remained essentially unchanged cores 0 5 1 0 m long taken in coarse sand bands contain 0 2 0 5 m of coarse sand overlying fine sand or interbedded fine and coarse sand cores from fine sand bands have at least one thin coarse sand layer at about the depth of the adjacent coarse sand band none of the cores revealed a thick deposit of coarse sand the shore parallel bands are of unknown origin their origin is especially puzzling because approximately shore normal bands are present in parts of the study area and immediately to the north

seafloor geology of the monterey bay area continental shelf abstract acoustic swath mapping of the greater monterey bay area continental shelf from point ano nuevo to point sur reveals complex patterns of rock outcrops on the shelf and coarse sand bodies that occur in distinct depressions on the inner and mid shelves most of the rock outcrops are erosional cuestas of dipping tertiary rocks that make up the bedrock of the surrounding lands a mid shelf mud belt of holocene sediment buries the tertiary rocks in a continuous 6 km wide zone on the northern monterey bay shelf rock exposures occur on the inner shelf near tectonically uplifting highlands and on the outer shelf beyond the reach of the mud depositing on the mid shelf since the holocene sea level rise the sediment starved shelf off the monterey peninsula and south to point sur has a very thin cover of holocene sediment and bedrock outcrops occur across the whole shelf with salinian granite outcrops surrounding the monterey peninsula coarse sand deposits occur both bounded within low relief rippled scour depressions and in broad sheets in areas like the sur platform where fine sediment sources are limited the greatest concentrations of coarse sand deposits occur on the southern monterey bay shelf and the sur shelf

observations regarding coarse sediment classification based on multi beam echo sounder s backscatter strength and depth residuals in dutch rivers this contribution investigates the behavior of two important riverbed sediment classifiers derived from multi beam echo sounder mbes operating at 300 khz data in very coarse sediment environments these are the backscatter strength and the depth residuals four mbes data sets collected at different parts of rivers in the netherlands are employed from previous research the backscatter strength was found to increase for increasing mean grain sizes depth residuals however are often found to have lower values for coarser sediments investigation of the four data sets indicates that these statements are valid only for moderately coarse sediment such as sand for very coarse sediments e g coarse gravel the backscatter strength is found to decrease and the depth residuals increase for increasing mean grain sizes this is observed when the sediment mean grain size becomes significantly larger than the acoustic wavelength of the mbes 5 mm knowledge regarding this behavior is of high importance when using backscatter strength and depth residuals for sediment classification purposes as the reverse in behavior can induce ambiguity in the classification

cenozoic tectonism in the central basin and range magnitude rate and distribution of upper crustal strain a wealth of pre cenozoic geologic markers across the central basin and range province defined mainly by thrust faults developed within a miogeoclinal wedge makes it one of the best areas on earth to reconstruct large scale continental extension we use this 100 000 km 2 region bounded by the southern sierra nevada on the west and the colorado plateau on the east to illustrate the concepts of correlation and misalignment for regional strain markers and of strain compatibility and kinematic viability in palinspastic reconstructions the results indicate 250 to 300 km of west northwest motion of the sierra away from the plateau accommodated by both crustal thinning and north south shortening tertiary intermontane basin deposits and mineral cooling ages of deeply exhumed rocks constrain the overall kinematics of motion most of the westward motion occurred between 16 and 5 ma at rates near 2 cm yr slowing to 1 to 1 5 cm yr in the last 5 ma we have quantified the partitioning of strain between vertical crustal thinning via normal faults and map view plane strain via conjugate strike slip faults by placing a grid of 10 x 10 km elements on a retrodeformed map of the region and measuring the increase in area of grid elements between the undeformed and present day grids this analysis yields a maximum finite elongation of the basin and range at 36 n to 37 n of 3 4 oriented n73 w map view area balance shows that 20 percent of this elongation is compensated by north south shortening and 80 percent by crustal thinning this yields an average thinning factor for the upper crust of 2 7 between the sierra and plateau inconsistent with the hypothesis that neogene deformation in the central basin and range is predominantly dextral shear plane strain these results in concert with the observation that the southern sierra nevada has similar crustal thickness to the central basin and range support the hypothesis of large scale eastward flow of sierran deep crust during extension

a model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with gloria sidescan sonar where the seafloor consists of a highly backscattering surface covered with a drape of fine grained sediment and where the sediment does not contribute significantly to the backscattered signal the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer a first order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments data collected with geological long range inclined azdic gloria are used to develop and test this model at an acoustic frequency of 6 5 khz the range of sediment thickness that the sonar can potentially recover is 0 20 m a range that is often below the resolution of vertical profilers in the first example the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer the surface is a lava flow north of hawaii covered by 1 2 m of fine grained sediments clague et al 1990 and gloria data analyzed with the model suggest an attenuation coefficient of 0 2 0 4 db m a value consistent with results from laboratory and field measurements of sediment attenuation in the second example a rocky surface is buried by varying amounts of sediment which is analyzed at almost constant grazing angle the surface is the volcanic basement of the southeast indian ridge and gloria data reveal the thickening of the sediment cover with distance away from the spreading center this is used to derive an average sedimentation rate of 6 mm ka which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions furthermore the model is used to calculate from image amplitude distributions the sediment thickness distribution which represents the accumulation of sediments in ponds and the exposure of abyssal hills these sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age i explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes the first order model assumes a uniform thickness at the subfootprint scale

a nonlinear rheology model for mid ocean ridge axis topography this paper describes a mechanical model of the mid ocean ridge axis based on a nonlinear rheology the oceanic lithosphere is modeled as a strongly temperature dependent power law material with the byerlee s friction law describing the brittle behavior of the upper oceanic lithosphere we examined two simple models a mantle only model predicted an axial rift valley 1 2 km deep and 10 15 km wide at slow half rates of 5 25 mm yr which is in fair agreement with observations however the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller thus the mantle only model failed to explain the observed transition in axial topography with spreading rate by including a uniform thickness oceanic crust with theological properties different from the mantle our second model produced the first order variation of axial topography with spreading rate a pronounced rift valley with 1 2 km relief at slow rates a smaller relief 500 m rift valley at intermediate rates and no rift valley at fast rates there are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models first horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis i e the stresses are concentrated in a very narrow zone for the nonlinear rheology this conclusion is remarkably independent of the boundary conditions at the seafloor second the computed width of the axial plastic failure zone decreases with spreading rate a result very different from that of a uniform viscosity model this is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate beyond some distance from the ridge axis the brittle plate roughly the region above the 750 c isotherm deforms very little both the horizontal strain rate 11 and the maximum shear strain rate max are less than 10 17 s 1 which is in good agreement with the 10 18 s 1 average for oceanic brittle plate deformation inferred from seismic moment data within the ductile region area beneath the 750 c isotherm the strain rates are in the range of 10 14 10 16 s 1 except within a narrow band near the ridge axis where higher strain rates are located we found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern or the rheology but is very sensitive to the cooling by hydrothermal circulation near the ridge axis axial structure is also influenced by factors other than the spreading rate within the range of slow and intermediate rates our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature in other words a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley in the vicinity of hotspots the absence of an axial rift valley even at slow spreading rates such as the reykjanes ridge can be well explained by the anomalously thicker crust and higher mantle temperatures in contrast at the australian antarctic discordance a deeper than normal section of an intermediate spreading ridge has very rough topography not present on the adjacent sections of the ridge this can be explained by our model as due to an anomalously low mantle temperature as has been suggested or perhaps also due to a thinner oceanic crust

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

predictive mapping of seabed substrata using high resolution multibeam sonar data a case study from a shelf with complex geomorphology abstract this study explores the full potential of high resolution multibeam data for the automated and accurate mapping of complex seabed features under a predictive modelling framework for an area of seabed on the carnarvon shelf in western australia morphometric variables and textural measures were derived from multibeam bathymetry and backscatter data several feature extraction approaches were applied to backscatter angular response curves to obtain new features these derivatives and new features were used separately and in combination in the predictions despite the complex distribution of various hard substrata within the study area we achieved a nearly perfect prediction of hard vs soft seabed types with an auc area under curve close to 1 0 the predictions were also satisfactory for gravel sand and mud content with r 2 values that range from 0 55 to 0 73 this study demonstrates that using a full range of derivatives and new features from both multibeam bathymetry and backscatter data optimises the accuracy of seabed mapping from the modelled relationships between sediment properties and multibeam data we confirmed that coarser sediment generally generates stronger backscatter return importantly the results again highlight the advantages of applying proper feature extraction approaches over using original backscatter angular response curves

the seafloor geomorphology of the windmill islands wilkes land east antarctica evidence of law dome ice margin dynamics abstract a high resolution multibeam sonar dataset covering an area of ca 33 km 2 was collected in the vicinity of the windmill islands 67 s 110 e wilkes land east antarctica the new data permit visualisation of the nearshore seafloor morphology in unprecedented detail providing invaluable insight into the ice sheet history of the region a range of geomorphic features are evident including prominent parallel northwest trending linear fault sets affecting mesoproterozoic metamorphic basement which appear to control the regional coastal physiography the fault systems probably formed during fragmentation of eastern gondwana during the mesozoic networks of sub glacial meltwater channels preserved on bedrock platforms and ridges indicate grounding of a thick ice sheet over the continental shelf during previous glaciations west trending subtle glacial lineations and streamlined landforms record evidence of the westward expansion of the grounded law dome ice sheet margin probably during the late pleistocene the direction of these features coincides with glacial striae on onshore crystalline bedrock outcrops perhaps the most striking glacial geomorphological features are sets of arcuate ridges confined mostly within glacially excavated u shaped troughs formed by erosion of the northwest trending bedrock fault sets these ridge sets are interpreted as push moraines or grounding zone features formed during episodic retreat of highly channelised topographically controlled ice streams following ice surging of the law dome margin this event was possibly triggered in response to local environmental forcing during the mid late holocene minor post glacial marine sedimentation is preserved in several small 1 km 2 isolated basins with shallow seaward sills

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

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

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

a systematic analysis of the mid atlantic ridge morphology and gravity between 15 n and 40 n constraints of the thermal structure multibeam bathymetry data obtained along a 2400 km long section of the mid atlantic ridge mar from 15 n to the azores platform 40 n and satellite derived gravity data were used to calculate the mantle bouguer anomaly mba along this portion of the mar both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference a long wavelength influence of the azores hot spot is characterized by a gentle continuous slope of the average ridge axial depth and a general gradient in the along axis mba profile this thermal influence of the azores hot spot controls a systematic southward propagation of the spreading segments at least to 26 30 n south of 26 30 n the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments except on the azores platform the systematic along axis 11 90 km long wavelength segmentation is independent of the long wavelength influence of the azores at the segment center the axial morphology is linked to the thermal state of the segments between 1 hotter segments characterized by a smooth axial morphology a well defined shallow inner valley high mba and a long length 2 colder segments which present a rough axial morphology with a deep wide and well defined rift valley a low mba and a small length for hotter segments the formation of the abyssal hills is mainly due to a magmato tectonic cycle over periods of 0 3 to 1 myr whereas on colder segments the axial morphology is mainly controlled by a tectonic rift valley formation we propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years

the structure of oceanic core complexes controlled by the depth distribution of magma emplacement extension at mid ocean ridges can be accommodated by detachment faults forming oceanic core complexes that develop under low rates of magma intrusion modelling reveals that oceanic core complexes can also form under high rates of magma intrusion if the magma is injected into the lower ductile layer of the crust

relationship between axial morphology crustal thickness and mantle temperature along the juan de fuca and gorda ridges we analyze gravity data over the juan de fuca and northern gorda ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate 28 mm yr half rate the cleft segment at the south end of the juan de fuca ridge has an axial high morphology while the northern segment of the gorda ridge has a rift valley residual mantle bouguer anomalies rmba on the northern gorda ridge are elevated relative to the cleft segment by 10 20 mgal indicating thinner crust and or a colder mantle the minimum value 50 mgal of the rmba along the juan de fuca ridge is over axial seamount and gradually increases south toward the blanco transform the observed rmba are interpreted to result from along axis variations in crustal thickness and mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs we estimate that mantle temperatures are elevated by 30 40 c beneath axial seamount resulting in an excess crustal thickness of 1 7 km the cleft segment is associated with crust that is estimated to be only 300 700 m thicker and mantle temperatures are only 10 15 c higher than beneath the northern gorda ridge however even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology these results are consistent with the predicted sensitivity of recent thermo mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates we attribute the systematic differences in axial morphology crustal thickness mantle temperature and lithospheric strength along the juan de fuca gorda ridge system to the presence of the cobb thermal anomaly at axial seamount the hotter mantle beneath the juan de fuca ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner weaker lithosphere than along the gorda ridge

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

quaternary slip history of the bare mountain fault nevada from the morphology and distribution of alluvial fan deposits analyses of alluvial fan sedimentation around bare mountain nevada indicate differential slip along the bare mountain fault during the pleistocene and holocene analyses show that the ratios of fan area to sediment source area and the degree of holocene sedimentation at the fan heads can be correlated with dip and displacement of the fault the well developed bajada and holocene deposits near the fan toes along the southwestern flank of bare mountain contrast with the location of holocene deposits near the fan heads and significantly smaller individual fan lobes on the eastern flank of the mountain adjacent to the bare mountain fault in addition ratios of fan area to source area along the eastern flank of the mountain decrease from north to south which we interpret to be caused by increased quaternary slip from north to south along the fault with relatively little holocene throw at tarantula canyon and maximum throw near wildcat peak the dip of the fault also changes from north to south with relatively shallow dip angles 45 to 50 at tarantula canyon and steeper 70 dips near wildcat peak geometric constraints dictate that for any given increment of horizontal extension vertical offsets will be smaller on those segments of the fault that have shallower dip angles on the basis of these observations we conclude that the recent slip rate of about 0 02 mm yr derived from trenching studies in the tarantula canyon fan should be considered a minimum value that may not represent the slip rate of the bare mountain fault as a whole

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

process related classification of acoustic data from the hudson river estuary acoustic surveying provides valuable information on the distribution of sedimentary environments in subaqueous settings frequently differences in acoustic backscatter strength are used to distinguish sedimentary environments on the basis of their grain size composition here we are presenting acoustic backscatter data from the haverstraw bay section of the hudson river estuary that has been mapped using sidescan sub bottom profiling and multibeam bathymetry as part of the benthic mapping project of the hudson river estuary program funded by new york state in addition hundreds of gravity cores and grab samples provide ground truth for a classification of the river bottom into discrete substrate types analyses of sedimentary environments reveal patterns in backscatter strength beyond those that can be related to the sediment grain size distribution alone an integrated interpretation of sidescan sub bottom profiling and high resolution bathymetry data indicates that the backscatter pattern can be attributed to spatial variations in the modern depositional environments which cause differences in bottom roughness and sediment compaction based on an integrated interpretation of the acoustic and sample data sets we distinguished eight different sedimentary classes many of the classes can be linked to dynamic processes including contemporary deposition erosion and sediment migration in sand waves the results provide a better understanding of the dynamic processes of the hudson river estuary and improve the interpretation of the acoustic backscatter data from fine grained sedimentary environments

changes of sedimentological patterns and morphological features due to the disposal of dredge spoil and the regeneration after cessation of the disposal activities abstract the interaction of dumped sediments with the morphology and sedimentology of a coastal environment and the recovery of the seafloor after cessation of dumping activities is still poorly known especially from a longer term perspective this paper presents a quantitative impact study of the dumping of dredged material on two overlapping disposal sites situated on different morphological entities an old disposal site located on a sandy shoal was closed in 1999 afterwards a new disposal site was designated mainly located in a tidal gully the approach included the interpretation of chrono sequential single beam echosoundings high resolution multibeam bathymetrical and backscatter data ground truthed with boxcores dumping has caused the formation of a large scale dump mound on both the old and new disposal site on the new disposal site the dump mound is superposed by individual disposal mounds and depressions that are probably formed by scouring events on the old disposal site also small depressions occur which are likely associated with the former dumping process the disposal has led to the accumulation of sand both on the old and new disposal site regardless of the high mud content of the dredge spoil for the new disposal site dumping efficiency has been estimated as 30 40 which would lead to a yearly growth of 380 000 m 3 a different sedimentation pattern was revealed for the old and new disposal site which is ascribed to the difference in morphological setting and differences in dredge spoil nature after cessation of the disposal of dredged sediments on the old disposal site a morphodynamic equilibrium was fairly quickly restored bedforms were formed within less than one year and their distribution and surficial sediments are again representative for the belgian coastal zone the consistent ebb dominated asymmetry of the very large dunes their sw migration and sediment accretion extending beyond the sw limit of the new disposal site suggest an important bedload transport in the direction of the navigation channels these findings contrast to the overall flood dominated residual current transport direction

evaluation of seabed mapping methods for fine scale classification of extremely shallow benthic habitats application to the venice lagoon italy abstract recent technological developments of multibeam echosounder systems mbes allow mapping of benthic habitats with unprecedented detail mbes can now be employed in extremely shallow waters challenging data acquisition as these instruments were often designed for deeper waters and data interpretation honed on datasets with resolution sometimes orders of magnitude lower with extremely high resolution bathymetry and co located backscatter data it is now possible to map the spatial distribution of fine scale benthic habitats even identifying the acoustic signatures of single sponges in this context it is necessary to understand which of the commonly used segmentation methods is best suited to account for such level of detail at the same time new sampling protocols for precisely geo referenced ground truth data need to be developed to validate the benthic environmental classification this study focuses on a dataset collected in a shallow 2 10 m deep tidal channel of the lagoon of venice italy using 0 05 m and 0 2 m raster grids we compared a range of classifications both pixel based and object based approaches including manual maximum likelihood classifier jenks optimization clustering textural analysis and object based image analysis through a comprehensive and accurately geo referenced ground truth dataset we were able to identify five different classes of the substrate composition including sponges mixed submerged aquatic vegetation mixed detritic bottom fine and coarse and unconsolidated bare sediment we computed estimates of accuracy namely overall user producer accuracies and the kappa statistic by cross tabulating predicted and reference instances overall pixel based segmentations produced the highest accuracies and the accuracy assessment is strongly dependent on the number of classes chosen for the thematic output tidal channels in the venice lagoon are extremely important in terms of habitats and sediment distribution particularly within the context of the new tidal barrier being built however they had remained largely unexplored until now because of the surveying challenges the application of this remote sensing approach combined with targeted sampling opens a new perspective in the monitoring of benthic habitats in view of a knowledge based management of natural resources in shallow coastal areas

ground truthing 6 5 khz side scan sonographs what are we really imaging a 1000 km2 area on the distal lobe of monterey fan shows a digitate pattern of juxtaposed high and low backscatter on gloria side scan sonographs this area was investigated using stereo photography high resolution seismic profiles and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6 5 khz side scan sonar stereo photography and bottom video were used to determine that the sediment water interface typically has a bed roughness less than 10 cm over the entire ground truth area consequently bed roughness is not a significant contributor to the sonar backscatter vertical incidence 3 5 khz profiles reveal that high backscatter areas allow less penetration and have slightly more relief than low backscatter areas closely spaced measurements of p wave velocity density and grain size made on transponder navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model the model results demonstrate that the sediment water interface is in most cases acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface the variability of the dominant lithofacies sands with minor silty clays or silty clay with minor sands accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers but at the detailed scale of hundreds to a few square meters we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence surface lithology measured acoustic impedance or attenuation however there is a qualitative but unexpected correlation with lithostratigraphic variability in this area thick up to 50 cm sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand this suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter the ground truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel 70 000 m2 relative to the area of an individual core 0 006 m2 bottom photograph 5 m2 or seismic profile although 6 5 khz sonographs appear easy to interpret in a conventional and simplistic manner caution should be used when interpreting lithofacies from backscatter intensities

seabed characterization on the new jersey middle and outer shelf correlatability and spatial variability of seafloor sediment properties abstract nearly 100 collocated grab samples and in situ 65 khz acoustic measurements were collected on the new jersey middle and outer shelf within an area that had previously been mapped with multibeam backscatter and bathymetry data and more recently with chirp seismic reflection profiling eighteen short cores were also collected and probed for resistivity based porosity measurements the combined data set provides a basis for empirically exploring the relationship among the remotely sensed data such as backscatter and reflection coefficients and directly measured seabed properties such as grain size distribution velocity attenuation and porosity we also investigate the spatial variability of these properties through semi variogram analysis to facilitate acoustic modeling of natural environmental variability grain size distributions on the new jersey shelf are commonly multi modal leading us to separately characterize coarse 4 mm fine

episodic caldera volcanism in the miocene southwestern nevada volcanic field revised stratigraphic framework 40ar 39ar geochronology and implications for magmatism and extension the middle miocene southwestern nevada volcanic field swnvf is a classic example of a silicic multicaldera volcanic field in the great basin more than six major calderas formed between 15 and 7 5 ma the central swnvf caldera cluster consists of the overlapping silent canyon caldera complex the claim canyon caldera and the timber mountain caldera complex active from 14 to 11 5 ma and centered on topographic timber mountain locations of calderas older than the claim canyon caldera source of the tiva canyon tuff are uncertain except where verified by drilling younger peralkaline calderas black mountain and stonewall mountain formed northwest of the central swnvf caldera cluster we summarize major revisions of the swnvf stratigraphy that provide for correlation of lava flows and small volume tuffs with the widespread outflow sheets of the swnvf new laser fusion 40 ar 39 ar isotopic ages are used to refine and revise the timing of eruptive activity in the swnvf the use of high sensitivity mass spectrometry allowed analysis of submilligram sized samples with analytical uncertainties of 0 3 1 permitting resolution of age differences as small as 0 07 ma these results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the swnvf major caldera episodes belted range crater flat paintbrush timber mountain and thirsty canyon groups erupted widespread ash flow sheets within 100 300 k y time spans and pre and post caldera lavas erupted within 100 300 k y of the associated ash flows peak volcanism in the swnvf occurred during eruption of the paintbrush and timber mountain groups when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1 35 m y separated by a 750 k y magmatic gap peralkaline and metaluminous magmatism in the swnvf overlapped in time and space the peralkaline tub spring and grouse canyon tuffs erupted early and the peralkaline thirsty canyon group tuffs and stonewall flat tuff erupted late in the history of the swnvf flanking the central volumetrically dominant peak of metaluminous volcanism magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data particularly in the overlapping grouse canyon and area 20 calderas of the silent canyon caldera complex volcanism in the swnvf coincided with the miocene peak of extensional deformation in adjoining parts of the great basin although regional extension was concurrent with volcanism it was at a minimum in the central area of the swnvf where synvolcanic faulting was dominated by intra caldera deformation significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the swnvf between the paint brush and timber mountain caldera episodes larger magnitude detachment faulting in the bullfrog hills southwest of the central swnvf caldera cluster followed the climactic timber mountain caldera episode postvolcanic normal faulting was substantial to the north east and south of the central swnvf caldera cluster but the central area of peak volcanic activity remained relatively unextended in postvolcanic time volcanism and extension in the swnvf area were broadly concurrent but swnvf area were broadly concurrent but in detail they were episodic in time and not coincident in space

digital image processing techniques for enhancement and classification of seamarc ii side scan sonar imagery the recent growth in the production rate of digital side scan sonar images coupled with the rapid expansion of systematic seafloor exploration programs has created a need for fast and quantitative means of processing seafloor imagery computer aided analytical techniques fill this need a number of numerical techniques used to enhance and classify imagery produced by seamarc ii a long range combination side scan sonar and bathymetric seafloor mapping system are documented three categories of techniques are presented 1 preprocessing corrections radiometric and geometric 2 feature extraction and 3 image segmentation and classification an introduction to the concept of feature vectors is provided along with an explanation of the method of evaluation of a texture feature vector based upon gray level co occurrence matrices glcm an alternative to the a priori texel texture element subdivision of images is presented in the form of region growing and texture analysis regata this routine provides a texture map of spatial resolution superior to that obtainable with arbitrarily assigned texel boundaries and minimizes the possibility of mixed texture signals due to the combination of two or more textures in an arbitrarily assigned texel computer classification of these textural features extracted via the glcm technique results in transformation of images into maps of image texture these maps may either be interpreted in terms of the theoretical relationships shown between texture signatures and wavelength or converted to geologic maps by correlation of texture signatures with ground truth data these techniques are applied to seamarc ii side scan sonar imagery from a variety of geologic environments including lithified and nonlithified sedimentary formations volcanic and sedimentary debris flows and crystalline basaltic outcrops application of the above processing steps provided not only superior images for both subjective and quantitative analysis but also the critical ability to discriminate between outcrops with distinct lithologies but similar image intensity

progressive increase in ice loss from greenland 1 laser altimeter measurements over greenland show increasing thickening rates above 2000 m reflecting increasing snowfall in a warming climate but near coastal thinning rates have increased substantially since the mid 1990s and net mass loss more than doubled from an average of 4 50 gt yr 1 between 1993 4 and 1998 9 to 57 105 gt yr 1 between 1998 9 and 2004 this increasing trend is very similar to findings from independent mass budget studies but differs widely from ers radar altimeter results this may result from limitations associated with the large ers footprint over sloping and undulating surfaces that typify fast narrow glaciers where thinning is most pronounced

the little skull mountain nevada earthquake of 29 june 1992 aftershock focal mechanisms and tectonic stress field implications the ml 5 6 29 june 1992 little skull mountain earthquake occurred in the southern nevada test site nye county nevada within 20 km of a potential national nuclear waste repository at yucca mountain nevada while microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock foreshock activity ml 2 were also sufficiently well recorded to provide local network p wave focal mechanism solutions the mainshock and the majority of the aftershocks through september 1992 display predominantly normal slip focal mechanisms but unambiguous strike slip and oblique slip focal mechanism solutions are also present in the aftershock series the mainshock and immediate foreshock depths were 10 1 km below sea level and depths of focus of aftershocks range from about 6 to 12 km a large subset of the aftershock hypocenters define a southeast dipping structure probably the mainshock fault plane no variation of mechanism style with depth is apparent two mainshock and 84 aftershock focal mechanism solutions were input into gephart s 1990 stressfield inversion program and principal compressional stress directions j j 1 2 3 were determined as well as the stress difference ratio r 1 2 1 3 the results indicate a uniform stress field in which 3 minimum stress is oriented 116 5 azi plunge and 1 maximum stress is oriented 219 69 with r 0 35 the inversion s selection of preferred nodal planes invokes bott s 1959 criterion which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces for the predominantly normal slip little skull mountain earthquake mechanisms having northeast striking planes the southeast moderately to steeply dipping dip 55 nodal planes are selected however for the dip slip earthquake mechanisms exhibiting more northerly trending strikes shallowly to moderately west dipping 25 dip 65 nodal planes are selected when a coulomb mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio 1 3 the ratio of resolved shear stress to normal stress n on the southeast dipping nodal plane of the mainshock exhibited a near maximum level suggesting a high fault strength in contrast a wide range of fault strengths or apparent coefficients of friction was suggested by the n ratios for little skull mountain aftershock preferred nodal planes seismicity on misoriented faults i e those on which shear stress was relatively low may have been the result of a temporarily elevated fluid pore pressure following the mainshock

erosional decay of the yucca mountain crest nevada abstract a simple numerical landscape evolution model is used to investigate the rate of erosional decay of the yucca mountain crest in nevada usa a location proposed as a permanent repository for high level radioactive waste the model is based on a stream power approach in which we assume that the rate of erosion is proportional to the size of the catchment as a proxy for water flux and to the square of the topographic gradient the proportionality constants in the model are determined using the structural history of the region extensional tectonics has dissected the region into a series of well defined tilt blocks in the last 11 my and the ratio of fault displacement and gully incision during this time is used to scale the model forward predictions of our model into the future show that the crest will denude to the level of the proposed site between 500 000 years and 5 my this prediction is based on conservative estimates for all involved parameters erosion may be more rapid if other processes are involved for example our model does not consider continuing uplift or catastrophic surface processes as they have been recorded in the region we conclude that any total system performance analysis tspa as has been performed for the yucca mountain region to predict geological events inside the ridge must consider erosion as an integral part of its predictions

organized patches of bioherm growth where the strait of dardanelles enters the marmara sea turkey abstract high resolution multibeam mosaics show that the seafloor across the southwestern marmara sea is host to remarkably organized near circular bioherm mounds which commonly are arranged into large tightly packed clusters grab samples and gravity cores reveal that the bioherms are predominantly composed of very fine grained calcareous silty mud with abundant bioclasts including centimetre scale masses of coralline red algae and intact disarticulated mollusc shells mainly the genera modiolus and mytilus geometric analysis of multibeam images reveals that the average bioherm is 15 6 m in diameter occupies 190 m2 of seabed stands 113 cm above the adjacent seafloor and its crest is 20 6 m from the crests of neighbouring bioherms in regions of tightly packed bioherm clusters referred to as bioherm colonies the inter mound depressions are on average 4 4 m wide and 33 cm deep although each bioherm mound is nearly circular the surrounding inter mound channels form a more rectilinear mesh of linked pentagonal and hexagonal polygons suggesting densest possible spatial packing of the mounds near neighbour statistics of r 2 11 2 14 indicate an essentially uniform spacing between the bioherms which is the expected result for close packing on a plane and full utilization of the available space the bioherms occur at depths between 30 m and 60 m they are absent above and below these depths in this part of the southwestern marmara sea at the eastern exit of the strait of dardanelles water depth controls important water mass properties such as salinity nutrient supply and availability of light other inferred critical controls are the availability of a suitable hard substrate where holocene muds are thin or absent and nutrient supply potentially including a component from escaping methane rich pore fluids evidence here and elsewhere in the marmara sea provisionally dates the onset of bioherm growth and development to the latest pleistocene earliest holocene after global sealevel rose to the breach depth of the strait of dardanelles at 13 8 cal ka

the july 1996 flood deposit in the saguenay fjord quebec canada implications for sources of spatial and temporal backscatter variations in july 1996 a major rainstorm and flood took place in the saguenay fjord backscatter strength measurements made with a simrad em1000 multibeam echosounder in 1993 1997 and 1999 have shown spatial and temporal variations which are interpreted in relation to the occurrence of the flood after empirical calibration of the different maps the data show an overall diminution of 5 db in backscatter strength in 1997 1 yr after the flood took place while the data sets obtained in 1993 and 1999 show comparable levels of backscatter strength the different data sets show a similar pattern of low and high backscatter patches which represent variations of the backscatter strength of a few decibels several grain size and water content measurements were also carried out on sediment box core and grab samples from the fjord bottom in 1997 and 1999 these have shown that the areas with higher acoustic backscatter correspond to the finer sediments while the low backscatter patches correspond to the coarser material the data show little relation between water content and backscatter strength thus indicating a poor dependence between the impedance terms bulk density and sound speed and the backscatter having taken this into account the major contribution to backscatter strength is assumed to result from differences in surface and volume roughness of the sediment since finer grain sizes offer higher backscatter the grain size of the sediment is not considered as being a major contributor to roughness and hence backscatter strength the major factor that seems to control roughness generation in the saguenay fjord is considered to be bioturbation the areas where geological processes most physically disturb the bottom which coincide with the areas where also most accumulation takes place and the coarser grain sizes are deposited are more sparsely colonized by organisms this results in a lower degree of bioturbation lower roughness and thus backscatter strength this hypothesis helps explain the variation in backscatter strength observed between the different measurement years the several million tons of sediment deposited in the fjord bottom after the 1996 flood buried the benthic fauna which was still recovering 1 yr later the less bioturbated sediment at this time would present lower roughness with respect to the 1993 and 1999 data resulting in a lower backscatter strength

acoustic discrimination of relatively homogeneous fine sediments using bayesian classification on mbes data highlights acoustically meaningful clustering correlating with ground truth data seascape scale acoustic mapping through classification per beam footprint bayesian approach for discriminating sedimentary units with low heterogeneity geo acoustic resolution a new measure of sedimentary acoustic class separability modern seafloor mapping is based on high resolution mbes systems that provide detailed bathymetric and acoustic intensity backscatter information we examine and validate the performance of two unsupervised mbes classification techniques for discriminating acoustic classes of sedimentary units with small grain size variability the first technique based on a principal components analysis pca is commonly used in literature and has been applied for comparison with the more recent approach of bayesian statistics by applying these techniques to a mbes dataset from an estuarine area in the netherlands we tested their ability to discriminate fine grained sediments at least 70 silt holding small percentages of coarser material such as sand shell hash or shells we focus on the bayesian technique as it outputs acoustically significant classes related to backscatter values this technique utilizes backscatter values averaged over scatter pixels projected pulse lengths inside the footprint of each beam the originality of our application lies in the fact that the optimal number of classes is derived by utilizing a number of beams simultaneously it is assumed that the backscatter values per beam vary relatively to the varying seafloor types by treating the beams separately across track variation in the seafloor type can also be accounted for thereby the classification is guided by outer more discriminative beams additionally we control the optimal number of classes by employing the quantitative criterion of goodness of fit 2 the bayesian acoustic classes show correlation with grain size parameters such as coarse fraction 500 m percentage and mean of the grain size 500 m when analyzed with multiple linear regression in order to examine the relative scale of the acoustic classification results we compare the bayesian acoustic classes with underwater video interpretation our results reveal that the bayesian approach enhances the sedimentological interpretation of mbes high resolution data by providing classification on seascape scale here meters to tens of meters hence we suggest that backscatter processing techniques are more commonly applied to produce classes that discriminate sediments with low grain size contrast to describe this ability we introduce the term geoacoustic resolution we want to encourage the use of the bayesian technique also in deep sea applications based on auv data where sediments express low variability but sampling would be time consuming and costly the advantages of this method would favor mapping of macro habitats which appear at meter scale and require datasets of sufficient resolution in order to be quantitatively described

the sedimentary record of the 2005 hurricane season from the mississippi and alabama coastlines abstract the instrumental record of hurricane activity is too short to fully capture the occurrence of the rare but most destructive hurricanes therefore obtaining a record of present and past landfalling hurricanes and their extent of geological and ecological impacts is one means to assess future risk reveal the spatial and temporal variability of hurricane activity and decipher its relationship with global climatic changes we rapidly dispatched survey teams to collect the readily available but perishable data of hurricanes katrina and rita storm surge from two salt marshes in mississippi and one salt marsh in alabama in mississippi we recorded hurricane katrina storm surge heights greater than 7 5 m north american vertical datum 88 navd88 with inland extents in excess of 700 m whereas in alabama the maximum recorded storm surge was 3 43 m navd88 at one salt marsh in mississippi we recorded the maximum inland extent and elevation of maximum penetration of hurricane rita as 370 m and 3 43 m navd88 respectively we observed a three dimensional distribution of hurricane induced storm surge deposits that tapered landward overlying salt marsh sediment there was a sharp or erosional boundary between the pre storm surge and storm surge sedimentary units which was accompanied by a change in color and lithology the overlying storm surge sediment unit was coarser than the pre storm surge unit with a lower organic content the thickness of the hurricanes katrina and rita storm surge sediments ranged from 9 to 13 cm and approximately 7 cm respectively foraminiferal analyses revealed a virtual absence of tests within the storm surge sediments whereas abundant agglutinated foraminifera were found in the underlying salt marsh deposits

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

three dimensional gravity study of the mid atlantic ridge evolution of the segmentation between 28 and 29 n during the last 10 m y during the high resolution survey sara segmentation ancienne de la ride atlantique sea beam bathymetry magnetic gravity and seismic reflection data were collected on the flanks of the mid atlantic ridge between 28 and 29 n this survey was designed to provide off axis information up to approximately 10 m y and to complement a detailed on axis survey carried out in 1988 and 1989 sempere et al 1990 between the atlantis and kane fracture zones a previous gravity study had revealed the existence of bulls eye shaped gravity lows centered on the axial segments and gravity highs centered on the non transform discontinuities lin et al 1990 we carried out a three dimensional calculation of the mantle bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center the off axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities after correcting for the gravity effect of lithospheric cooling away from the ridge the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry off axis bathymetry is characterized by southward pointing v shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously however the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows northward on the western flank and southward on the eastern flank the sense of offset between the gravity and bathymetric traces appears to be related to the right stepping nature of the axial discontinuities

segmentation volcanism and deformation of oblique spreading centres a quantitative study of the reykjanes ridge abstract new multi beam swath bathymetry charts and high resolution side scan sonar imagery provide detailed evidence for volcanic and magmatic segmentation along the reykjanes ridge by combining these data sets we have quantified some relationships between different orders of segmentation various characteristics of volcanism and changes in tectonism along the ridge axis three orders of segmentation along the reykjanes ridge have been recognised 1 a long wavelength depth variation or swell that is related to the shoaling of the ridge towards iceland 2 intermediate wavelength rises of the order of 40 120 km and 3 a short wavelength volcanic segmentation of the order of 5 30 km the long wavelength depth variation which is probably controlled by the temperature of the underlying asthenosphere has a maximum positive deviation from a predicted depth curve that forms a break in slope at 59 60 n this feature migrated southward along the ridge at between 5 and 10 cm yr at least four times in the past 10 ma the intermediate wavelength rises which are also temporally and spatially unstable are probably related to local rayleigh taylor style asthenospheric instabilities the short wavelength segments have an evolutionary life cycle starting with eruption via fissure volcanoes and progressing by increasing magma flux to the formation of conical and then shield like volcanoes these segments are tectonically dismembered by extensional faulting following a rapid decrease in magma supply we propose a model for the construction of the volcanic layer of the reykjanes ridge which involves a cyclicity of magmatic and tectonic activity

magnetization of 0 29 ma ocean crust on the mid atlantic ridge 25 30 to 27 10 n a sea surface magnetic survey over the west flank of the mid atlantic ridge from 0 to 29 ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust we find that magnetization decays rapidly within the first few million years although the filtering effect of water depth on the sea surface data and the slow spreading rate 13 km m y preclude us from resolving this decay rate a distinctly asymmetric along axis pattern of crustal magnetization is rapidly attenuated off axis suggesting that magnetization dominated by extrusive lavas on axis is reduced off axis to a background value off axis we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust indicative of induced magnetization in thin inside corner ic crust in general we find that off axis segment ends show an induced magnetization component regardless of polarity and that ic segment ends tend to have slightly more induced component compared with outside corner oc segment ends possibly due to serpentinized uppermost mantle at ic ends we find that remanent magnetization is also reduced at segment ends but there is no correlation with inside or outside corner crust even though they have very different crustal thicknesses this indicates that remanent magnetization off axis is independent of crustal thickness bulk composition and the presence or absence of extrusives remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in oc crust and a combination of crustal thinning and alteration in ic crust from all these observations we infer that the remanent magnetization of extrusive crust is strongly attenuated off axis and that magnetization of the lower crust may be the dominant source for off axis magnetic anomalies

bottom currents interpreted from iceberg ploughmarks revealed by multibeam data at tromsoflaket barents sea abstract multibeam data have been acquired on eastern tromsoflaket a bank in the southwestern barents sea comprising a plateau detailed examination of the multibeam backscatter data however reveals a high variability of sediments especially in and around iceberg ploughmarks we observe coarser sediments on the levees either side of the ploughmarks than inside the furrows moreover we frequently note asymmetries in the distribution of fine sediments across these ploughmark features drawing inspiration from previous studies of bottom currents which use the distribution of sediments across bedforms e g sand waves we have interpreted the direction of bottom currents on tromsoflaket by examining the local distribution of fine sediments across iceberg ploughmarks which are the most prominent bedform in this area further integration of our results with existing grain size current speed models have allowed estimates of the bottom current speeds to be obtained such interpretations are particularly valuable in this area where availability of bottom current measurements sparse our approach makes valuable and novel use of multibeam data and results show good agreement with the limited set of available current data and models we can identify near bottom pathways for the norwegian atlantic current and norwegian coastal current the two major current systems in this region and the patterns of circulation help to explain the sedimentation regime and identify recent erosion and deposition areas

acoustic backscatter of the 1995 flood deposit on the eel shelf abstract acoustic swath mapping and sediment box coring conducted on the continental shelf near the mouth of the eel river revealed regional variations in acoustic backscatter that can be related to the shelf sedimentology the acoustic backscatter variations observed on the shelf were unusually narrow compared to the response of similar sediment types documented in other areas however the acoustic data revealed four principal bottom types on the shelf that can be related to sedimentologic differences observed in cores the four areas are 1 low acoustic backscatter associated with the nearshore sand facies and the prodelta terraces of the eel and mad rivers composed of fine sands and coarse silts with low porosity 2 high acoustic backscatter associated with fine silts characterized by high porosity and deposited by the 1995 flood of the eel river 3 intermediate acoustic backscatter in the outer shelf muds where clayey silts are accumulating and the 1995 flood apparently had limited direct effect and 4 intermediate acoustic backscatter near the fringes of the 1995 flood deposits and in areas where the flood sediments were more disrupted by post depositional processes the highest acoustic backscatter was identified in areas where the 1995 flood sediments remained relatively intact and near the shelf surface into the summer of 1995 cores collected from these areas contained wavy or lenticular bedding the rapid deposition of the high porosity muddy layers results in better preservation of incorporated ripple forms than in areas less directly impacted by the flood deposit the high porosity muddy layers allow acoustic penetration into the sediments and result in greater acoustic backscatter from incorporated roughness elements

hybrid failure field evidence and influence on fault refraction abstract the occurrence of hybrid failure the transition from tensile to shear failure has been controversial production of hybrid failure in laboratory experiments in recent years has strengthened the case for hybrid failure but field recognition has remained elusive in this paper we present data from a small displacement fault in the hidden valley fault zone at canyon lake gorge texas which is refracted through mechanically layered carbonates and exhibits surface characteristics expected for hybrid failure on steep fault segments we discuss field criteria for identifying hybrid fractures and the deformation conditions under which hybrid failure is to be expected documented field examples of hybrid fractures are rare yet presence of hybrid fractures can be useful indicators of stress conditions at the time of failure specifically hybrid fractures are likely indicators of tensile minimum effective stress and low differential stress at failure

calibration techniques and sampling resolution requirements for groundtruthing multibeam acoustic backscatter em3000 and qtc view classification technology abstract both acoustic and sediment surveys were carried out in the broughton archipelago british columbia in order to map a former aquaculture site and calibrate acoustic surveys with georeferenced sediment properties the acoustic surveys included em3000 multibeam including backscatter and qtc view series iv technologies while the geotechnical survey entailed van veen grab sampling of surface sediments and associated analyses the two acoustic technologies were consistent in their ability to identify distinct regions of seafloor characterized by rock outcrops consolidated substrates or gel mud depositional fields both multibeam backscatter data and qtc view number coded classifications were extracted across a range of circular areas located at each georeferenced sampling station radii 2 3 4 5 8 12 16 20 m statistical correlations were observed between backscatter and certain geotechnical properties such as sediment porosity sediment grain size fractions

the southeast indian ridge between 88 e and 118 e gravity anomalies and crustal accretion at intermediate spreading rates although slow spreading ridges characterized by a deep axial valley and fast spreading ridges characterized by an axial bathymetric high have been extensively studied the transition between these two modes of axial morphology is not well understood we conducted a geophysical survey of the intermediate spreading rate southeast indian ridge between 88 e and 118 e a 2300 km long section of the ridge located between the amsterdam hot spot and the australian antarctic discordance where satellite gravity data suggest that the southeast indian ridge seir undergoes a change from an axial high in the west to an axial valley in the east a basic change in axial morphology is found near 103 30 e in the shipboard data the axis to the west is marked by an axial high while a valley is found to the east although a well developed axial high characteristic of the east pacific rise epr is occasionally present the more common observation is a rifted high that is lower and pervasively faulted sometimes with significant 50 m throw faults within a kilometer of the axis a shallow axial valley 1200 m deep valley across a transform at 114 e the changes in axial morphology along the seir are accompanied by a 500 m increase in near axis ridge flank depth from 2800 m near 88 e to 3300 m near 114 e and by a 50 mgal increase in the regional level of mantle bouguer gravity anomalies over the same distance the regional changes in depth and mantle bouguer anomaly mba gravity can be both explained by a 1 7 2 4 km change in crustal thickness or by a mantle temperature change of 50 c 90 c in reality melt supply crustal thickness and mantle temperature are linked so that changes in both may occur simultaneously and these estimates serve as upper bounds the along axis mba gradient is not uniform pronounced steps in the regional level of the mba gravity occur at 103 30 e 104 e and at 114 e 116 e and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other but rather represent distinctly different forms of axial structure and tectonics with a sharp transition between them the change from an axial high to an axial valley requires a threshold effect in which the strength of the lithosphere changes quickly the presence or absence of a quasi steady state magma chamber may provide such a mechanism the different forms of axial morphology are also associated with different intrasegment mba gravity patterns segments with an axial high have an mba low located at a depth minimum near the center of the segment at epr like segments the mba low is about 10 mgal with along axis gradients of 0 15 0 25 mgal km similar to those observed at the epr rifted highs have a shallower low and lower gradients suggesting an attenuated composite magma chamber and a reduced and perhaps episodic melt supply segments with a shallow axial valley have very flat along axis mba profiles with little correspondence between axial depth and axial mba gravity

rippled scour depressions on the inner continental shelf off central california abstract side scan sonar records taken during the recent coastal ocean dynamics experiment code show elongate shore normal rippled depressions of low relief on the inner continental shelf off central california between bodega bay and point arena these features extend up to 2 km from the coast into water depths of up to 65 m the proposed mechanism for their generation is storm generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine sand sediment and expose the coarser sand substrate in the depressions the zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features the large straight crested ripples within the depressions heights to 40 cm wavelengths to 1 7 m are probably formed by large amplitude long period surface waves generated by winter storms

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

sycamore knoll a wave planed pop up structure in a sinistral oblique thrust system southern california continental borderland at the boundary between the western transverse ranges province and inner continental borderland of southern california strain is partitioned across the sinistral oblique anacapa dume fault system sycamore knoll a pop up structure 20 km west of point dume in the hanging wall of the north dipping anacapa dume fault stands out as an anomalous bathymetric high along the southern california coast we synthesize new and existing geophysical datasets including multibeam bathymetry backscatter and single multichannel seismic to understand the morphology and tectonic implications of sycamore knoll we identify a complete eustatic low stand submerged marine terrace package encircling sycamore knoll platform that corresponds to the last glacial maximum lgm the marine terrace deposits and wave planed surface of sycamore knoll require a late quaternary uplift rate of 0 55 0 5 0 3 mm yr relative to sea level by combining our uplift rate with previous estimates of basin subsidence and a revised structural model of the anacapa dume fault we determine a maximum slip rate in latest quaternary time of 1 50 0 5 0 3 mm yr and hence a post lgm rate of convergence across the anacapa dume fault of 0 60 0 2 0 1 mm yr

characterisation of seafloor substrate using advanced processing of multibeam bathymetry backscatter and sidescan sonar in table bay south africa abstract a method to map seafloor substrates using machine learning based on geophysical data including multibeam bathymetry backscatter and side scan sonar is currently being developed results from a case study in table bay southwestern south africa are presented here showing a method of physical seafloor classification that uses a number of statistical algorithms and software programs in the first step of the process a customised tool was created within arcgis using python scripting language to classify seafloor bathymetry which can be applied to areas beyond south africa the tool created by the authors was based on pioneering work done by the national oceanic and atmospheric administration on a benthic terrain modelling toolbox in step two multibeam bathymetry backscatter and side scan sonar data processed using qimera fledermaus geocoder toolbox and navlog processing software were classified using machine learning techniques including decision trees random forests and k means clustering computer algorithms the results from these algorithms were compared to classify the seafloor substrate distribution our results have allowed a comparison of advantages and disadvantages of each machine learning technique and we found that the k means clustering techniques were the simplest to implement and understand and worked best based on their seafloor segmentation capabilities in table bay with all three data sets multibeam bathymetry backscatter and side scan sonar in future research ground truthing methods for example underwater video and grab samples will be used to validate the interpreted data to create accurate seafloor substrate maps this work provides the initial steps to develop a holistic predictive tool that classifies geophysical data into substrate maps using machine learning techniques the maps can be used to model biological communities and to produce benthic habitat maps for use in marine science and management

detection of an unusually large hydrothermal event plume above the slow spreading carlsberg ridge nw indian ocean about 90 of earth s volcanism occurs along the global mid ocean ridge system here sporadic volcanic and tectonic activity is thought to cause cataclysmic release of hydrothermal fluids forming event plumes each plume often contains as much hydrothermal effluent and heat as chronic hydrothermal venting from a typical vent site discharges during a year to date only a few event plumes have been detected and only above intermediate rate spreading ridges in the pacific here we report the first evidence for an unusually large event plume that originated from the slow spreading 3 cm yr full rate carlsberg ridge in the nw indian ocean at 70 km long up to 4540 km3 in volume and with up to 24 1016 j of excess heat this event plume was substantially larger than previous ones and demonstrates that dispersion of hydrothermal heat and biological products from slow spreading ridges may be more significant and effective than hitherto imagined

mechanisms for subcritical penetration into a sandy bottom experimental and modeling results this paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments and to quantify the results for use in sonar performance prediction for the detection of buried objects in situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured a parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface data covering grazing angles both above and below the nominal critical angle and in the frequency range 2 15 khz were acquired and processed the results are compared to two models that account for scattering of sound at the rough water sediment interface into the sediment although all possible mechanisms for subcritical penetration are not modeled the levels predicted by both models are consistent with the levels observed in the experime

the spreading rate dependence of three dimensional mid ocean ridge gravity structure we analyze over 1300 km of high resolution along axis gravity profiles at ridges with half spreading rates ranging from 1 2 to 5 5 cm yr the results show consistently higher along axis gradients of mantle bouguer anomaly at the slow spreading mid atlantic ridge mar 0 3 1 2 mgal km than at the intermediate to fast spreading cocos nazca ridge and east pacific rise epr 0 1 0 2 mgal km the regional peak to trough amplitude of mantle bouguer anomaly is also greater along the mar 30 60 mgal than the cocos nazca ridge and the epr 10 20 mgal with increasing spreading rate the regional peak to trough amplitude of axial seafloor depth decreases from 1000 1700 m to 200 700 m 3 d numerical experiments suggest that mantle contributions to the gravity can be significant only near large offset transforms at the more commonly observed non transform offsets gravity anomalies will reflect crustal thickness variations the along axis gravity data thus indicate that the amplitude of along axis crustal thickness variation decreases with increasing spreading rate we propose that this spreading rate dependent crustal accretion style may originate in the mantle finite amplitude mantle upwelling is intrinsically plume like 3 d beneath a slow spreading ridge but more sheet like 2 d beneath a fast spreading ridge such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate small amplitude 3 d upwellings may occur at a fast spreading ridge but their effects on crustal thickness variations will be significantly reduced by along axis melt flows along a persistent low viscosity crustal magma chamber in contrast the large crustal thickness variations due to 3 d mantle upwellings will be maintained at a slow spreading ridge because less along axis melt flows can occur in the colder and more rigid crust there

the effects of fine scale surface roughness and grain size on 300 khz multibeam backscatter intensity in sandy marine sedimentary environments abstract high frequency acoustic backscatter intensity that is returned from sediment is affected by a number of seabed characteristics including sediment grain size distribution the nature and magnitude of seabed surface roughness and volume scattering by subsurface scatterers in order to develop quantitative remote sediment classification techniques it is important to determine the relative and combined effects of the dominant variables to which the acoustic signal is sensitive data used in this study were collected at two sandy sites off the south shore of long island new york and include 300 khz multibeam simrad em 3000 backscatter intensity data as well as grab samples and stereo bottom photographs multivariate statistical analyses were used to investigate the relationships between grain size properties seabed roughness and backscatter intensity at a number of sample locations at each study site encompassing a moderate range of backscatter intensities from 32 5 to 24 db the results of this study demonstrate that the relative importance of variables that affect backscatter intensity varies at different sites suggesting that backscatter intensity alone cannot be used to quantitatively predict seabed characteristics however our study indicates that in sandy sediments the median grain size the standardized magnitude of seabed roughness and the sorting of sediment can together be used to predict 300 khz backscatter intensity with high significance and that the relative importance of these variables varies from site to site we present an empirical model derived from a combined dataset that includes two sites of similar sedimentary and hydrodynamic conditions to predict the magnitude of seabed roughness based on backscatter intensity and sediment grain size characteristics the spatially coherent patterns in predicted seabed surface roughness revealed through the use of this kind of model greatly improve our understanding of variations in backscatter intensity in sandy marine sedimentary environments as a result the multivariate statistical approach that we employ improves our overall ability to remotely assess the details of the sedimentary environment that are significant with respect to benthic habitats and physical processes and are applicable to other geophysical datasets including sidescan sonar data

model based acoustic remote sensing of seafloor characteristics the characterization of seafloor sediments by acoustic means is carried out using normal incidence dual frequency single beam echo sounder backscatter data at 20 locations in the western continental shelf of india a temporal backscatter model for a finite duration of transmitted signal is used to estimate the values of four sediment parameters through inversion mean grain size roughness spectrum strength roughness spectrum exponent and sediment volume scattering parameter ground truth information on sediment mean grain sizes at these locations is obtained from the laboratory analysis of grab samples applicability of the temporal backscatter model over different sediment compositions at 33 and 210 khz is investigated here for characterizing the seafloor through inversion moreover a combined two frequency inversion approach has been attempted for improved characterization by estimating a single set of values for the aforementioned four sediment parameters which is applicable to the backscatter data at both of the acoustic frequencies a linear regression analysis demonstrates that the estimated values of sediment mean grain size obtained from these three inversion cases statistically agree with the ground truth data in addition correlations of the estimated values of seafloor roughness spectrum parameters with the values of sediment mean grain size are compared with published information available in the literature

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

carlsberg ridge and mid atlantic ridge comparison of slow spreading centre analogues eighty per cent of all mid ocean spreading centres are slow using a mixture of global bathymetry data and ship board multibeam echosounder data we explore the morphology of global mid ocean ridges and compare two slow spreading analogues the carlsberg ridge in the north west indian ocean between 57 e and 60 e and the kane to atlantis super segment of the mid atlantic ridge between 21 n and 31 n at a global scale mid ocean spreading centres show an inverse correlation between segment length and spreading rate with segmentation frequency within this context both the mid atlantic ridge super segment and carlsberg ridge are similar spreading at 22 and 26 mm yr full rates respectively being devoid of major transform faults and being segmented by dextral non transform second order discontinuities for these and other slow spreading ridges we show that segmentation frequency varies inversely with flank height and ridge axis depth segments on both the mid atlantic ridge super segment and carlsberg ridge range in aspect ratio ridge flank height axis width depth and symmetry segments with high aspect ratios and deeper axial floors often have asymmetric rift flanks and are associated with indicators of lower degrees of melt flux segments with low aspect ratios have shallower axial floors symmetric rift flanks and evidence of robust melt supply the relationship between segmentation spreading rate ridge depth and morphology at both a global and local scale is evidence that rates of melting of the underlying mantle and melt delivery to the crust play a significant role in determining the structure and morphology of slow spreading mid ocean ridges

riverbed sediment classification using multi beam echo sounder backscatter data a method has recently been developed that employs multi beam echo sounder backscatter data to both obtain the number of sediment classes and discriminate between them by applying the bayes decision rule to multiple hypotheses simons and snellen appl acoust 70 1258 1268 2009 in deep water the number of scatter pixels within the beam footprint is large enough to ensure gaussian distributions for the backscatter strengths and to increase the discriminative power between acoustic classes in very shallow water 10 m however this number is too small this paper presents an extension of this high frequency methodology for these environments together with a demonstration of its performance using backscatter data from the river waal the netherlands the objective of this work is threefold i increasing the discriminating power of the classification method high resolution bathymetry data allow precise bottom slope corrections for obtaining the true incident angle and the high resolution backscatter data reduce the statistical fluctuations via an averaging procedure ii performing a correlation analysis the dependence of acoustic backscatter classification on sediment physical properties is verified by observing a significant correlation of 0 75 and a disattenuated correlation of 0 90 between the classification results and sediment mean grain size iii enhancing the statistical description of the backscatter intensities angular evolution of the k distribution shape parameter indicates that the riverbed is a rough surface in agreement with the results of the core analysis

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

the influence of geomorphology and sedimentary processes on benthic habitat distribution and littoral sediment dynamics geraldton western australia abstract understanding of the processes regulating sediment transport accumulation and erosion requires an appropriate mapping of coastal geomorphology seabed sediments and benthic habitat distribution to allow management issues to be identified understood and addressed in this study multibeam echo sounder data were used to map shallow water geomorphological features and the spatial distribution of benthic habitats with the support of underwater imagery for ground truthing the acoustic data at geraldton sediment analyses have revealed a dominant biogenic nature with modern carbonate sedimentation linked to the seagrass and macroalgal carbonate factories colonising these shallow

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

the high frequency backscattering angular response of gassy sediments model data comparison from the eel river margin california a model for the high frequency backscatter angular response of gassy sediments is proposed for the interface backscatter contribution we adopted the model developed by jackson et al j acoust soc am 79 1410 1422 1986 but added modifications to accommodate gas bubbles the model parameters that are affected by gas content are the density ratio the sound speed ratio and the loss parameter for the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment we treat the bubbles as individual discrete scatterers that sum to the total bubble contribution this total bubble contribution is then added to the volume contribution of other scatters the presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth the backscatter response of fine grained gassy sediments is dominated by the volume contribution while that of coarser grained gassy sediments is affected by both volume and interface contributions in deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected in shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased multibeam data acquired offshore northern california at 95 khz provides raw measurements for the backscatter as a function of grazing angle these raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model the analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that when compared to the model show general agreement

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

pitfalls of using entrenched fracture relationships fractures in bedded carbonates of the hidden valley fault zone canyon lake gorge comal county texas characterizing natural fracture systems involves understanding fracture types faults joints and veins patterns orientations sets and spacing within sets size distributions penetration across layering aperture and trace length and timing relationships traditionally observation based relationships to lithology mechanical stratigraphy bed thickness structural position failure mode and stress history have been proposed for predicting fracture spacing along with the relative abundance of opening mode fracture versus faults in fractured rocks developing a conceptual fracture model from these relationships can be a useful process to help predict deformation in a fractured reservoir or other fractured rock systems a major pitfall when developing these models is using assumptions based on general relationships that are often site specific rather than universal in this paper we examine a mixed carbonate shale sequence that is cut by a seismic scale normal fault where fracture attributes do not follow commonly reported fracture relationships specifically we find 1 no clear relationship between frequency or spacing of opening mode fractures joints and veins and proximity to the main fault trace and 2 no detectable relationship between fracture spacing and bed thickness however we did find that 1 the frequency of small displacement faults is strongly and positively correlated with proximity to the main fault trace 2 fracture networks change pattern and failure mode extension versus shear fracture from pavement to pavement through the mechanically layered stratigraphic section and 3 faults are more abundant than opening mode fractures in many areas within the fracture network we interpret that the major fracturing initiated near maximum burial under relatively high differential stress conditions where shear failure dominated and that mode 1 extension fracturing occurred later under lower differential stress conditions filling in between earlier formed shear fractures we conclude that whenever possible site specific observations need to be carefully analyzed prior to developing fracture models and perhaps a different set of fracture network rules apply in rocks where shear failure dominates and mechanical stratigraphy influences deformation

estimates of suspended sediment flux and bedform activity on the inner portion of the eel continental shelf energetic waves strong bottom currents and relatively high rates of sediment discharge from the eel river combined to produce large amounts of suspended sediment transport on the inner continental shelf near the eel river during the winter of 1995 1996 bottom boundary layer bbl measurements at a depth of 50 m using the geoprobe tripod showed that the strongest near bottom flows combined wave and current speeds of over 1 m s and highest sediment concentrations exceeding 2 g l at 1 2 m above the bed occurred during two storms one in december 1995 and the other in february 1996 discharge from the eel river during these storms was estimated at between 2 and 4 103 m3 s suspended sediment flux ssf was measured 1 2 m above the bed and calculated throughout the bbl by applying the tripod data to a shelf sediment transport model these results showed initially northward along shelf ssf during the storms followed by abrupt and persistent southward reversals along shelf flux was more pronounced during and after the december storm than in february across shelf ssf over the entire measurement period was decidedly seaward this seaward transport could be responsible for surficial deposits of recent sediment on the outer shelf and upper continental slope in this region sediment ripples and larger bedforms were observed in the very fine to fine sand at 50 m depth using a sector scanning sonar mounted on the tripod ripple wavelengths estimated from the sonar images were about 9 cm which compared favorably with photographs of the bottom taken with a camera mounted on the tripod the ripple patterns were stable during periods of low combined wave current bottom stresses but changed significantly during high stress events such as the february storm two different sonic altimeters recorded changes in bed elevation of 10 to 20 cm during the periods of measurement these changes are thought to have been caused principally by the migration of low amplitude long wavelength sand waves into the measurement area

correlation of sidescan backscatter with grain size distribution of surficial seabed sediments the dependence of acoustic backscatter on sediment grain size distribution is examined using dual frequency 100 and 410 khz sidescan sonar and 22 sediment grab samples from the loch linnhe artificial reef site on the west coast of scotland the sidescan data were processed to remove an empirically estimated average grazing angle dependence on backscatter the processed data were analysed by forming histograms of pixels extracted from a 20 m 2 box around each ground truth site a positive correlation r 0 73 between mean backscatter intensity and mean grain size was obtained i e the coarsest samples had the brightest backscatter a positive correlation r 0 59 was also found between the standard deviations of the backscatter and grain size distributions i e poorly sorted sediments gave the most variable backscatter the performance of the sidescan data was compared to results from a co incident single beam echo sounder roxann survey the roxann roughness index e1 compared well with the sidescan whilst the roxann hardness index e2 did not this may be due to a physical link between the acoustic measures the comparison showed the sidescan to have delivered a significantly higher resolution image of the seabed for a similar amount of ship time imaging of the artificial reef modules themselves was found to be frequency dependent d 2004 elsevier b v all rights reserved

the morphology of lava flows in planetary environments predictions from analog experiments the rates of surface cooling and lateral spreading are evaluated for lava flows on the surface of earth venus mars the moon and io differences between the flow morphologies expected in these environments are then predicted under the assumption that the results of recent laboratory simulations of lavas using wax extruded beneath cold water fink and griffiths 1990 can be carried over these experiments involved the spreading of viscous liquid under gravity in the presence of a solidifying surface crust and revealed a set of four distinct surface morphologies transitions from one morphology to the next occurred in a well defined sequence when the relative rates of surface solidification and lateral spreading were varied in comparison with subaerial flows on earth the surface of lavas solidifies faster on venus where the dense atmosphere provides additional convective heat transfer lateral flow of lava is much slower under the smaller gravity of the moon hence for a given extrusion rate and viscosity solid crust is predicted to form closer to the vent on both the moon and venus than on earth equivalently faster extrusion rates than those on earth are required on the moon io and venus in order to produce a given surface morphology a comparison of observed structure flow rates and estimated viscosities for some well documented terrestrial lava flows shows reasonable agreement with the predictions of the model extrapolation to extraterrestrial lavas is then illustrated by constraining the effusion rate for a martian lava flow whose composition is assumed spectral information on the chemical composition of martian flows to be sought by the thermal emission spectrometer on the mars observer mission in conjunction with detailed morphologic observations from the mars observer camera could allow much more accurate estimates of effusion rates for martian lavas

seafloor characterization and backscatter variability of the almeria margin alboran sea sw mediterranean based on high resolution acoustic data abstract an acoustic study of the almeria margin eastern alboran sea sw mediterranean was carried out by means of an integrated dataset comprising deep towed tobi sidescan sonar swath bathymetry topas high resolution seismics and sediment gravity cores the almeria margin is a complex system dominated by volcanic and tectonic features shaped by recent sedimentary processes in which a regional hemipelagic sedimentation is intercalated by sporadic gravitative events the aim of this work is twofold the first objective is to recognize the sedimentary and tectonic features that shape the almeria margin the main morpho sedimentary and morpho structural elements observed are the almeria canyon channel turbidite system the dalias tributary valley system landslides active faults and folds and volcanic banks the second aim is to explore and quantify the surface and subsurface seafloor parameters that characterize the backscatter of the study area sedimentary geomorphological and biological elements that play a role in the acoustic signature of the almeria margin were detected nevertheless it should be noted that seafloor acoustic backscatter is also affected by geometrical and instrumental factors despite the wide variability of the environments studied a low average acoustic backscatter reflects a diffused draping of hemipelagic sedimentation in the area higher backscatter values were observed at the top of some volcanic banks along the almeria canyon and in the shallower sector of the dalias tributary valley system the influence of subseafloor properties in the acoustic signature of the area was revealed by backscatter grain size correlations which were carried out for different depth intervals in sediment cores collected in the almeria turbidite system a poor relationship was found between backscatter and superficial silty sediments of the area whereas a higher correlation resulted in the upper 50 cm the presence of subsurface turbidites in the cores associated with the higher backscatter values suggests volume interface scattering of these sandy layers as a controlling factor of the acoustic signature obtained in the almeria margin on the other hand in rough settings such as the ones observed in the dalias tributary valley system topographic relief of up to 1 1 5 m strongly interacted with the tobi acoustic pulse this suggests that large scale roughness is an additional parameter that characterizes the acoustic strength of the area extremely high reflective patches distributed along some of the volcanic banks chella and pollux banks for depths ranging from 230 to 470 m coincide with areas where carbonate cold water coral mounds develop in the tobi images coral facies reach the maximum value of acoustic backscatter which is probably related to the rough morphology of coral ecosystems the position of coral mounds in the banks suggests that the occurrence of strong bottom currents and reduced sedimentary inputs are environmental factors that favour their settling and development in the almeria margin

magmato tectonic cyclicity at the ultra slow spreading southwest indian ridge evidence from variations of axial volcanic ridge morphology and abyssal hills pattern on axis deep tow side scan sonar data are used together with off axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra slow spreading southwest indian ridge differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments we interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development including periods of volcanic construction and periods of tectonic dismemberment using off axis bathymetric data we identify numerous abyssal hills with a homogeneous size for each segment these abyssal hills all display an asymmetric shape with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away we suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment i e a magmato tectonic cycle we observe that large abyssal hills are in ridge sections of thicker crust whereas smaller abyssal hills are in ridge sections of thinner crust this suggests that the magma supply controls the size of abyssal hills the abyssal hills in ridge sections of thinner crust are regularly spaced indicating that the magmato tectonic cycle is a pseudoperiodic process that lasts 0 4 m y about 4 to 6 times shorter than in ridge sections of thicker crust we suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long lived segments by contrast when magma supply strongly decreases and becomes highly discontinuous regular abyssal hills patterns are no longer observed