sandwave migration in monterey submarine canyon central california abstract repeated high resolution multibeam bathymetric surveys from 2002 through 2006 at the head of the monterey submarine canyon reveal a sandwave field along the canyon axis between 20 and 250 m water depth these sandwaves range in wavelength from 20 to 70 m and 1 to 3 m in height a quantitative measure was devised to determine the direction of sandwave migration based on the asymmetry of their profiles despite appreciable spatial variation the sandwaves were found to migrate in a predominantly down canyon direction regardless of season and tidal phases a yearlong adcp measurement at 250 m water depth showed that intermittent internal tidal oscillations dominated the high speed canyon currents 50 80 cm s which are not correlated with the spring neap tidal cycle observed currents of 50 cm s or higher were predominantly down canyon applying a simple empirical model flows of such magnitudes were shown to be able to generate sandwaves of a size similar to the observed ones

thickness of sets of cross strata and planar strata as a function of formative bed wave geometry and migration and aggradation rate cross strata and planar strata formed by the migration of ripples and dunes or low relief bed waves on upper stage plane beds respectively are the most common internal structures in fluvial sands and sandstones the thickness of cross sets or planar laminae at a given location is controlled by 1 aggradation rate relative to downstream migration rate of bed waves and 2 the succession of bed waves of varying height passing the location a quantitative understanding of this relationship is now appearing based on new theory and experimental studies with ripples dunes and upper stage plane beds in many cases the effects of control 1 can be ignored in such cases only the largest bed waves in a population form cross sets or planar laminae the ratio of mean cross set thickness mean height of formative bed waves is less than 0 4 and the ratio of mean cross set thickness mean height of all bed waves is less than 0 5 a simple quantitative model a modified form of the paola borgman model is presented for predicting the height distributions of bed waves ripples dunes low relief bed waves from the thickness distributions of cross sets or planar laminae if the aggradation rate is appreciable more of the smaller bed waves in a population can form cross sets or planar laminae and the preservation ratios increase to 1 in the case of supercritical climbing ripple cross strata in this case it is necessary to determine the angle of climb and length of the bed waves in order to use the model to predict bed wave height distributions

the morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae experimental studies of subcritical unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves flow depth was 0 11 m mean flow velocities were 0 86 1 0 m s 1 shear velocities were 0 058 0 71 m s 1 and dimensionless shear stresses were 0 56 0 87 bedwaves are asymmetrical in profile and range from 0 75 to 11 mm in height mainly 2 6 mm from 0 7 to 1 3 m in wavelength and have mean celerities of 10 mm s 1 flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs high resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

antidunes and traction carpet deposits in deep water channel sandstones cretaceous british columbia canada abstract sandstone beds intercalated with thick shale of the upper member of the haida formation albian turonian in the queen charlotte islands british columbia canada show thinly laminated graded bedding the sandstones generally are present as successions of more than a few tens of inversely to normally graded thin beds the beds at the base of each succession contain much coarser detritus than the beds at the top each succession of graded beds shows a lens shaped channel geometry wavy bed forms similar in shape to the hummock and swale topography that is thought to form hummocky cross stratification hcs characterize the uppermost part of the succession however grain fabric and spacing of the bed forms indicate that they are of antidune origin it is concluded that the successions of multiple laminated sandstone beds were produced by high density turbidity currents that were probably associated with a gravity transformation in which a successive freezing of traction carpets took place lack of shale in the succession together with the common presence of antidunes suggest that each succession was the product of one sedimentation event without decrease in stream power

seismic frequency laboratory measurements of shear mode viscoelasticity in crustal rocks i competition between cracking and plastic flow in thermally cycled carrara marble in the first phase of a seismic frequency study of the viscoelasticity of cracked and fluid saturated crustal rocks forced torsional oscillation experiments have been conducted on specimens of carrara marble subjected to multiple thermal cycles up to 560 c under confining pressure of 50 mpa the experimental results showed distinctly different behaviour of both the shear modulus and internal friction between the first and subsequent thermal cycles a pronounced reduction in shear modulus g observed during the first series of thermal cycles at temperature t 400 c presumably due to dislocation relaxation contrasts with frequency independent q 1 at lower temperatures the relatively sudden onset of markedly frequency dependence seismic wave attenuation might prove to be a useful diagnostic of temperature under crustal conditions

the depth distribution of seismicity parameters estimated for the south american area abstract two techniques are applied in order to estimate the seismicity parameters and their depth distribution in south america an area of high seismic activity these techniques are 1 gumbel s third asymptotic giii distribution of extremes and 2 the maximum likelihood approach the obtained seismicity parameters through giii distribution of extremes are the value which is considered as the upper bound magnitude and is related to the finite maximum stresses and strains which are currently accumulated and released by the rocks as earthquake in a region the shape parameter and the characteristic value u with p u 1 e and p 1 the most probable maximum magnitude rm of earthquakes for 95 years is also computed the seismicity parameters obtained by the application of the maximum likelihood approach are the activity rate of the seismic events the mean return period rp of earthquakes above a cutoff magnitude m m and the parameter b of the magnitude frequency relation the parameter which is related to b with b log 10 e is also computed the regional maximum magnitude m max this is considered as the maximum possible magnitude in a specific region is also provided under some constraints six depth range subdivisions are defined for the examined area all the seismicity parameters are computed and especially for giii distribution a covariance analysis of its three parameters u and is performed for each individual depth range in all cases values are higher than the regional m max estimates the seismic activity rate decreases with depth while the parameter which ranges between 1 18 to 2 06 does not show a clear regularity in its depth distribution an important notification is that the parameter obtained by the two methods expresses totally different quantities

controls on the erosion of cenozoic asia and the flux of clastic sediment to the ocean rates of continental erosion may be reconstructed from variations in the rate of accumulation of clastic sediment most of which lies offshore global rates of marine sedimentation are usually considered to have reached a maximum after 3 4 ma driven by enhanced erosion in a variable glacial interglacial climate however a new compilation of seismic data from the marginal seas of asia now shows that only the red river reached its historic peak after 4 ma sediment flux from asia first peaked in the early middle miocene 24 11 ma well before the initiation of a glacial climate indicating that rock uplift and especially precipitation are the key controls on erosion at least over long periods of geologic time reconstructions of weathering in east asia show that faster erosion correlates with more humid warm climates in the early middle miocene changing to less erosive drier climates after 14 ma when antarctic glaciation begins average rates of sedimentation on most east asian continental margins since 1 8 ma are 5 6 times less than the modern fluvial flux implying that the flux to the oceans varies sharply on short timescales and is not always buffered over timescales of 10 4 yr by storage in flood plains

small scale hummocky cross stratification in turbidites a form of antidune stratification small scale hummocky cross stratification occurs in upper cretaceous calciclastic turbidites exposed in the western basque pyrenees facies associations and microfossil assemblages indicate slope to base of slope bathyal depositional environments it is developed in the fine grained portion of beds and displays spacings mostly between 0 2 and 0 7 m the beds fine upward with no sharp grain size breaks or mud partings suggesting that deposition occurred during a single flow event hummocky intervals are 0 1 0 8 m thick and consistently grade laterally and vertically into flat planar laminations of bouma b divisions suggesting that deposition occurred under upper flow regime conditions they have wave like geometries with laminae continuous across crests and troughs and display a ratio of wavelength to estimated underflow thickness of 11 3 12 8 combining the above observations and inferences these examples of small scale hummocky cross stratification are interpreted as a form of antidune stratification generated by standing waves along the interface of a thinner denser underflow main body tail of the turbidity current and an overlying thicker low density layer this occurrence is further evidence that small scale hummocky cross stratification is multigenetic and therefore not indicative of a particular flow condition or depositional environment

semiannual patterns of erosion and deposition in upper monterey canyon from serial multibeam bathymetry recently acquired 3 m resolution 244 khz multibeam seafloor bathymetry 0 5 m depth precision reveals geomorphology at sufficient detail to interpret small scale features and short term processes in the upper 4 km of monterey canyon california the study area includes the continental shelf and canyon features from 10 m to 250 m depth the canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor the sand waves are strongly asymmetrical indicating net down canyon sediment transport in this reach terraces including a broad 25 m tall terrace complex near the head of the canyon bear evidence of recent degradation of the canyon floor slump scars and gullies having a variety of sizes and relative ages shape the canyon walls serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period september 2002 to march 2003 and a 24 h period 24 march to 25 march significant changes over the six month period include 1 complete reorganization of the sand waves on the channel floor 2 local channel degradation creating new 2 m tall erosional terraces on the channel margins 3 local channel widening that laterally eroded older channel margin terraces and 4 60 m extension of one minor gully head on a steep canyon wall there were no discernable changes in morphology during the 24 h study period raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study erosion of 320 000 m 3 80 000 m 3 of sediment occurred mainly in the tributaries along the margins of the axial channel and in the lowest 700 m of the analyzed reach this eroded volume was approximately balanced by 260 000 m 3 70 000 m 3 of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head there was no measurable sediment gain or loss during the 24 h study period

hyperconcentrated sand water mixture flows over erodible bed field surveys and experiments in a tilting flume on hyperconcentrated sand water mixture swm flows were conducted at slopes slightly smaller than the equilibrium slope a rapid development of sand bars propagating upstream was observed these bars consist of an almost horizontal terrace with subcritical flow a steep lee side slope with supercritical flow and a hydraulic jump between subsequent bars the erosion processes on the bar are limited by the high sand concentrations and flow velocities on the steep slopes as a result the erosion rate is more moderate than predicted by the pickup functions used in classical sediment transport theory a simple one dimensional model is presented to study the behavior of triangular schematized bars from an analysis of the equations it is concluded that an increase in erosion rate requires an increase in bed slope to maintain equilibrium flow conditions finally simulations with the model are compared with observations in the field and in the tilting flume t

high resolution morphobathymetric analysis and evolution of capbreton submarine canyon head southeast bay of biscay french atlantic coast over the last decade using descriptive and numerical modeling abstract in this study the capbreton canyon head just off the coast is investigated using high resolution multibeam bathymetry datasets sediment samples and numerical modeling the hr bathymetry analysis reveals a morphological connection between the longshore trough and the head of the canyon the analysis of recent sediment samples shows a clear correlation between the sediment of the canyon head and that of the nearshore hydrodynamic modeling a coupled wave flow model shows that for high energy waves the rotational nature of surf zone circulation reverses and wave induced currents have the potential to transport large quantities of nearshore sands toward the canyon head all these arguments support the assumption that the canyon head captures a part of the sand transported by longshore drift over the 15 years of observation 1998 2013 time lapse bathymetry shows that the floor of the canyon head and one lateral gully network experienced significant morphological reworking in terms of hazards despite this strong activity the position of the canyon head and the profile of its longitudinal slope remained stable and appear as a comforting factor however the activity of unusual lateral erosions needs to be monitored

relationship between subduction and seismicity in the mexican part of the middle america trench two catalogs of earthquakes in the mexican part of the middle america trench are analyzed to elucidate principal relations between structure of the subducting cocos plate and seismicity a catalog of historical events that have occurred during the last two centuries with large magnitudes ms 6 0 is used to obtain cumulative seismic moment mo and seismic moment release rate m 0 distributions along the mexican subduction zone the catalog of instrumentally observed earthquakes from 1963 to 1990 international seismological center and u s geological survey with 4 5 mb 6 0 is applied to investigate background seismicity for the region the strength of coupling between the cocos and north american plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate v and age a of the cocos plate this correlates in general with a steady reduction in background seismicity and a slight average increase of m 0 and seismic energy release rate w at the sites where the main fracture zones of the cocos plate tehuantepec o gorman orozco and rivera undergo the subduction the general correlation breaks down the background seismicity increases at fracture zones while m0 and m 0 decrease significantly this feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted seismic slip rates along the trench obtained from m 0 are lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the v model interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments variability of m 0 and seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes an examination of space time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of 10 km yr

worlwide deepwater exploration and production past present and future approximately 58 billion barrels of oil equivalent have been discovered in deepwater defined as waters of at least 500 m from 18 basins on 6 continents figure 1 this consists of 39 million barrels of oil and condensate and approximately 112 trillion ft3 of gas more than half has been discovered since 1995 however only about 25 of the discoveries are developed or under development and less than 5 have been produced indicating that large expenditures remain to be made most resources have been found in the gulf of mexico brazil and west africa see map in pettingill and weimer 2001

seismic hazard evaluation for specific seismic regions of the world tsapanos t m and burton p w 1991 seismic hazard evaluation for specific seismic regions of the world tectonophysics 194 153 169 seismic hazard is estimated for fifty of the most seismically active countries of the world using the technique of gumbel s third asymptotic distribution of extreme values and hazard maps are produced for this purpose a catalogue of 9700 earthquakes with magnitudes m 2 5 5 has been compiled and earthquakes in the time period 1898 1985 are examined here an equal area grid point mesh of about 5 o latitude x 5 o longitude is chosen to represent the local seismicity this cell size corresponds approximately to the area of romania which is the smallest country examined the results estimate the most probable maximum magnitude for a time period of 85 years which exceeds magnitude 7 for all fifty of the cellular seismic regions and the contour maps and tables show its spatial variation and rank the countries in terms of overall relative seismic hazard the maps effectively produce a brief seismic hazard atlas

contemporary squamish river sediment flux to howe sound british columbia squamish river drains 3600 km2 in the southern coast mountains of british columbia to howe sound at squamish some 50 km north of vancouver this study uses differencing of digitized bathymetric surfaces based on the canadian hydrographic services surveys of 1930 1973 and 1984 at the head of howe sound to yield a long term sedimentation rate for squamish river delta the sediment flux from squamish river to howe sound is determined to be 1 29 106 m3 a 1 or 1 81 109 kg a 1 rates consistent with loads calculated from flow and sediment concentration regimes measured in the estuary in 1973 1975 and 1987 1988 the latter data indicate that the modal discharge effectiveness class is 600 700 m3 s 1 moving 13 of the annual suspended sediment load discharges up to 1400 m3 s 1 constitute 99 8 of all flows and are responsible for transporting 81 5 of the annual load a very significant 18 5 of the load is moved by large magnitude floods 1400 m3 s 1 which occur less than 0 2 of the time squam

subduction zone seismicity and the thermo mechanical evolution of downgoing lithosphere in this paper we discuss characteristic features of subduction zone seismicity at depths between about 100 km and 700 km with emphasis on the role of temperature and rheology in controlling the deformation of and the seismic energy release in downgoing lithosphere this is done in two steps after a brief review of earlier developments we first show that the depth distribution of hypocentres at depths between 100 km and 700 km in subducted lithosphere can be explained by a model in which seismic activity is confined to those parts of the slab which have temperatures below a depth dependent critical valuetcr

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

hatteras transverse canyon hatteras outer ridge and environs of the u s atlantic margin a view from multibeam bathymetry and backscatter abstract previously unknown features in hatteras transverse canyon and environs were recently mapped during multibeam surveys of almost the entire eastern u s atlantic continental margin the newly identified features include 1 extensive landslide scarps on the walls of hatteras transverse and hatteras canyons 2 an area of multiple landslide deposits that block lower hatteras transverse canyon 3 a large depositional feature down canyon from the landslide deposits that rises 100 m above the uppermost hatteras fan and has buried the transition from the mouth of hatteras transverse canyon to uppermost hatteras fan 4 a zone of cyclic steps on upper hatteras fan that suggests supercritical turbidity currents performed a series of hydraulic jumps and formed large upstream migrating bedforms 5 several knickpoints in the channel thalwegs of both hatteras transverse canyon and hatteras canyon one 40 m high that suggest both canyon channels are out of equilibrium and are in the process of readjusting either to the channel blockage by the extensive landslide deposits or by a readjustments to increased sedimentation during the last eustatic lowstand 6 a large area of outcrop on the lower margin between pamlico and hatteras canyons that previously was interpreted as an area of slumps blocky slide debris and mud waves 7 headward erosion in the head region of hatteras transverse canyon where it has intercepted the lowest reaches of albemarle canyon channel as well as headward erosion in a small side channel that has eroded into hatteras outer ridge and 8 sections of bedforms on hatteras outer ridge that are partially buried by sediment from washington norfolk canyon channel as well as by sediment transported from hatteras abyssal plain the newly discovered features add a new level of detail to understand the recent processes that have profoundly affected hatteras transverse canyon hatteras canyon and to a lesser degree hatteras outer ridge

sediment slides and turbidity currents on the laurentian fan sidescan sonar investigations near the epicenter of the 1929 grand banks earthquake the epicenter area of the 1929 grand banks earthquake on the continental slope south of newfoundland has been investigated using sea marc i a deeply towed midrange sidescan sonar with a 4 5 khz subbottom profiler shallow slides pass downslope into debris flows on the muddy continental slope east of the epicenter at the head of the eastern valley of the laurentian fan west of the epicenter arcuate slide scars cut undisturbed upper slope sediment and lead downslope to a lineated erosional seabed at a water depth of about 1600 m this erosional seabed passes into extensive fields of 100 m wavelength gravel waves situated on the broad irregular valley floor the gravel waves become better developed downslope and extend at least to water depths of 3000 m all these morphological features appear fresh on the sidescan sonograms suggesting that they date from the 1929 earthquake event and the distribution of slides corresponds to the area of instantaneous cable breaks in 1929 the upper limit of erosion on valley walls suggests that the 1929 turbidity current was less than 300 m thick timing of cable breaks downfan suggests that flow velocities were sufficient to rework gravel deposits into large bedforms during waning flow stages over elevated areas of the valley floor similar cross bedded coarse sands and gravels are common in ancient channel deposits

swept away by a turbidity current in mendocino submarine canyon california we present unique observations and measurements of a dilute turbidity current made with a remotely operated vehicle in 400 m water depth near the head of mendocino canyon california the flow had a two layer structure with a thin 0 5 to 30 m relatively dense 0 04 vol and fast up to 1 7 m s wedge shaped lower layer overlain by a thicker up to 89 m more dilute and slower current the fast moving lower layer lagged the slow moving dilute flow front by 14 min which we infer resulted from the interaction of two initial pulses the two layers were strongly coupled and the sharp interface between the layers was characterized by a wave like instability this is the first field scale data from a turbidity current to show i the complex dynamics of the head of a turbidity current and ii the presence of multiple layers within the same event

the timing of sediment transport down monterey submarine canyon offshore california while submarine canyons are the major conduits through which sediments are transported from the continents out into the deep sea the time it takes for sediment to pass down through a submarine canyon system is poorly constrained here we report on the first study to couple optically stimulated luminescence osl ages of quartz sand deposits and accelerator mass spectrometry 14 c ages measured on benthic foraminifera to examine the timing of sediment transport through the axial channel of monterey submarine canyon and fan offshore california the osl ages date the timing of sediment entry into the canyon head while the 14 c ages of benthic foraminifera record the deposition of hemipelagic sediments that bound the sand horizons we use both single grain and small 2 mm area single aliquot regeneration approaches on vibracore samples from fining upward sequences at various water depths to demonstrate relatively rapid decadal scale sand transport to at least 1 1 km depth and more variable decadal to millennial scale transport to a least 3 5 km depth on the fan significant differences between the time sand was last exposed at the canyon head osl age and the timing of deposition of the sand from 14 c ages of benthic foraminifera in bracketing hemipelagic sediments are interpreted as indicating that the sand does not pass through the entire canyon instantly in large individual events but rather moves multiple times before emerging onto the fan the increased spread in single grain osl dates with water depth provides evidence of mixing and temporary storage of sediment as it moves through the canyon system the ages also indicate that the frequency of sediment transport events decreases with distance down the canyon channel system the amalgamated sands near the canyon head yield osl ages that are consistent with a sub decadal recurrence frequency while the fining upward sand sequences on the fan indicate that the channel is still experiencing events with a 150 250 year recurrence frequency out to 3 5 km water depths

regional variation of the ω values in the circum pacific belt a data set of earthquakes which occurred in the circum pacific regions has been used to compute the values this parameter is obtained through gumbel s third asymptotic distribution of extreme values and is well known as upper bound magnitude the results show relatively higher values in south america central america and mexico than in the western part of the circum pacific belt the results are interpreted in terms of the different mechanical structure heterogeneity of the materials in the eastern and western parts of the examined area

maximum depth of seismicity and thermal parameter of the subducting slab general empirical relation and its application the maximum depth of seismicity dm in subduction zones is limited by some physical mechanisms related to the rheological strength of slab material metastability conditions failure strength etc the main factors controlling these properties are temperature and pressure within the lithosphere sinking into the mantle these p t conditions may be coarsely assessed from thermal models of slab subduction the observed relationship between dm the age of the subducted lithosphere a and the convergence rate v can be used to test the theoretical models and reveal inconsistencies which may indicate changes in failure mechanisms with depth we analyze the relation between dm and the thermal parameter of the descending slab which is a product of the age of the subducted lithosphere and the vertical component of convergence rate v seismicity profiles across the subduction zones of mexico chile kamchatka kuriles japan sumatra new hebrides aleutians tonga and marianas are used to estimate dm the quasi linear part of the dependence dm 240 km and 2000 km corresponds to relatively young and slowly descending slab and is in general agreement with the critical temperature models of deep earthquakes for 2000 km which corresponds to the relatively older lithosphere subducting at a higher rate the dm f dependence is nonlinear the flattening of the empirical dm f curve in the range of 2000 km 3500 km is found to be a direct indication of the influence of the equilibrium ol sp phase transition on dm models invoking the metastable ol sp phase transition as a mechanism which controls the deepest seismicity cannot be definitely constrained by the results of this study the general dependence dm f may be applied as a standard relation between dm a and v for the analysis of deep earthquakes and seismotectonic studies of the subduction zones

in situ measurements of velocity structure within turbidity currents 1 turbidity currents are thought to be the main mechanism to move 500 000 m3 of sediments annually from the head of the monterey submarine canyon to the deep sea fan indirect evidence has shown frequent occurrences of such turbidity currents in the canyon but the dynamic properties of the turbidity currents such as maximum speed duration and dimensions are still unknown here we present the first ever in situ measurements of velocity profiles of four turbidity currents whose maximum along canyon velocity reached 190 cm s two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year long deployment

generation and migration of coarse grained sediment waves in turbidity current channels and channel lobe transition zones large scale sediment waves composed of gravels and sands have been studied using deep water sidescan systems new data are presented from submarine channels off the canary islands and from canyon mouths off portugal data from other areas are briefly reviewed including a re interpretation of data from laurentian fan in order to summarise the varied morphology and setting of these bedforms coarse grained sediment waves are found in the proximal dominantly bypassing areas of deep water turbidite systems within canyons channels and channel lobe transition zones wave heights are in the region of 1 10 m and wavelengths are up to several hundred metres the distribution of waves and sparse sedimentological evidence from modern and ancient sediment wave fields suggests that initial transport and deposition of coarse sediment occurs within a high density turbidity current and not as a non newtonian debris flow in some cases the development of pronounced wave asymmetry and evidence of wave disruption and reworking suggests that the wave morphology is at least partially controlled by a later phase of low density turbidity flow grain size also appears to exert some control on wave morphology for example gravel rich waves have a greater height for the same wavelength than sand rich waves coarse grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents and are equally difficult to recognise in outcrop because of their large size

subduction zone earthquakes and stress in slabs the pattern of seismicity as a function of depth in the world and the orientation of stress axes of deep and intermediate earthquakes are explained using viscous fluid models of subducting slabs with a barrier in the mantle at 670 km 670 km is the depth of a seismic discontinuity and also the depth below which earthquakes do not occur the barrier in the models can be a viscosity increase of an order of magnitude or more or a chemical discontinuity where vertical velocity is zero longn versus depth wheren is the number of earthquakes shows 1 a linear decrease to about 250 300 km depth 2 a minimum near that depth and 3 an increase thereafter stress magnitude in a subducting slab versus depth for a wide variety of models shows the same pattern since there is some experimental evidence thatn is proportional toe where is a constant and is the stress magnitude the agreement is encouraging in addition the models predict down dip compression in the slab at depths below 400 km this has been observed in earlier studies of earthquake stress axes and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion at intermediate depths the models predict an approximate but not precise state of down dip tension when the slab is dipping the observations do not show an unambiguous state of down dip tension at intermediate depths but in the majority of regions the state of stress is decidedly closer to down dip tension than it is to down dip compression chemical discontinuities above 670 km or phase transitions with an elevation of the boundary in the slab predict when incorporated into the models stress peaks which are not mirrored in the profile of seismicity versus depth models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double benioff zones but also yield stress magnitude profiles consistent with observed seismicity models where a nonlinear rheology is used are qualitatively consistent with the linear models

structureless coarse tail graded bouma ta formed by internal hydraulic jump of the turbidity current abstract turbidity currents are commonly super critical if they travel on slopes steeper than 0 001 hence upon deceleration such flows must pass a hydraulic jump it is therefore surprising that sedimentary features related to the hydraulic jump are so poorly known and not clearly related to the bouma sequence to study this missing link a stability diagram of bed forms generated by decelerating initially super critical turbidity currents is presented here to sketch the position of hydraulic jump deposits relative to super and sub critical bed forms observed in turbidites the axes of the diagram are formed by the densiometric froude number the grain size and the particle fall out rate experimental evidence shows that the hydraulic jump is hardly manifested in the deposit if turbidity currents pass the jump as a low concentration single phase suspension flow only those turbidity currents that develop two phase suspension flow with a super critical flowing traction carpet cause vigorous erosion and liquefaction of the substrate at the jump locality to produce rip up of the substrate rip up clasts and flame structures which are trapped in structureless coarse tail graded deposits characteristic of the classic bouma ta unit the location of the jump is sensitive to density velocity variations of the traction carpet and can shift rapidly up and down the slope this would explain why structureless coarse tail graded deposits such as bouma ta develop sheet like

towards a sediment budget for the santa cruz shelf a conceptual model is presented for the northern monterey bay continental shelf in which coarse sediment moves southward along the coast in the littoral zone while fine sediment moves to the north by advection and diffusion along the midshelf data from measurements and estimates of various sediment sources and sinks show that the midshelf mudbelt is the dominant sink for fine grained sediment introduced into monterey bay the principal sources of the fine sediment are the three rivers that enter monterey bay the san lorenzo pajaro and salinas rivers accumulation rates in the midshelf mudbelt are high relative to documented yields of rivers and cliff erosion and also are high relative to other documented mud accumulations of the west coast continental shelves

seismicity and rheology of subducted slabs since the incorporation of the inclined zones of intermediate and deep seismic activity into the theory of plate tectonics1 2 it has been generally accepted that earthquakes with focal depths 100 km occur in subducted slabs of predominantly oceanic lithosphere yet our understanding of the processes which generate intermediate and in particular deep earthquakes is still incomplete this is largely due to our limited knowledge of the rheological behaviour of subducted oceanic lithosphere at depths between 100 and 700 km however recently considerable insight has been gained into the rheology of oceanic lithosphere in near surface conditions and i investigate here whether the seismic activity at depths 100 km can be understood on the basis of this information i first present results of temperature calculations performed for all subduction zones for which adequate data on seismicity relative plate motion and age of the descending oceanic lithosphere exist sub sequently i adapt the temperature conditions governing the rheology near the surface by including depth dependence this leads to a depth dependent critical temperature tcr z above which the subducted lithosphere cannot sustain the stresses necessary to generate seismic events seismicity data and calculated temperature versus depth relationships are in good agreement with the rheological model predictions this implies that the absence of seismic activity at depths 700 km should not be interpreted as direct evidence for the hypothesis that the 650 km discontinuity in the mantle acts as a barrier to vertical motion of subducted slabs

supercritical and subcritical turbidity currents and their deposits a synthesis common facies models of turbidite deposits are based on idealized sequences of turbidite units which are assumed to reflect the depositional processes of a decelerating turbidity current we show how suites of turbidite units i e distinct turbidite facies associations that are easily described from core and outcrop may characterize the entire range of large scale dynamics of turbidity currents enabling estimates of their densimetric froude number fr subcritical versus supercritical and suspension fall out rate stratified versus nonstratified flows the linking of facies associations with large scale flow dynamics resolves process facies links that were hitherto unresolved by the common turbidite facies models

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

an examination of froude supercritical flows and cyclic steps on a subaqueous lacustrine delta lake chelan washington u s a density driven submarine flows including turbidity currents play an important role in the transfer of sediment into deep water these bottom hugging flows often produce flow transverse bedforms along their path a sedimentological and geophysical survey of the stehekin river delta in lake chelan washington reveals a downslope elongate field of bedforms on the delta foreset associated with hyperpycnal discharges of the stehekin river an analysis of the bedform morphologies delta geometry and density contrast between lake and river water suggests that these hyperpycnal flows are froude supercritical the bedforms are likely cyclic steps flow transverse bedforms that are bounded by stable hydraulic jumps between alternating subcritical and supercritical flow regimes the ability to examine the three dimensional bed configuration produced by natural density driven flows adds valuable information to the body of experimental work focused on the behavior of cyclic steps in flumes

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle