inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

gravitational potential energy and regional stress and strain rate fields for continental plateaus examples from the central andes and colorado plateau abstract the commonly observed extension in areas of elevated and thickened crust is an expected consequence of having excess gravitational potential energy gpe compared to the low gpe of the surrounding crust while this conceptual model is well founded it is less clear how well gpe related stress orientations compare quantitatively to observed stress and strain rate orientations and what any inconsistency tells us about the presence of other competing forces we estimate the gpe distribution for the central andes and the greater colorado plateau area using topography and crustal thickness variations respectively and compare the related stress fields with the world stress map as well as with a geodetic strain rate field for the colorado plateau only in both areas deviatoric stresses associated with gpe variations alone cannot fully account for the observed deformation rate field for the central andes only a combination of deviatoric stresses associated with gpe and relative plate motions can account for the near n s tensional stress observed in the peruvian andes and the margin normal compressional stress along the eastern cordillera and sub andean fold and thrust belt the observed deformation field around the colorado plateau shows e w extension largely inconsistent with the deviatoric stresses associated with gpe variations except for the area east of the rio grande rift the ne sw oriented stress observed on the southwestern colorado plateau is consistent with the orientations of tensional deviatoric stresses associated with gpe variations we argue that this consistency could be haphazard stress observations may not reflect the current state of stress due to inherited structure or could result from the relative high strength of colorado plateau that allows for regional gpe variations and possibly basal shear to be more significant forces than far field plate interactions for the central andes and colorado plateau stresses associated with gpe variations have a strong influence on the total stress field and can thus be used to calibrate the overall level of deviatoric stress acting within the lithosphere

normal faulting and the topographic roughness of mid ocean ridge flanks the ocean floor on the flanks of mid ocean ridges is covered by abyssal hills topographic features elongated perpendicularly to the direction of relative plate motion these topographic features are interpreted as normal fault blocks and or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading the purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults given a fault population and a mechanical model for the response of the lithosphere to faulting we obtain expressions for the variation in root mean square roughness with profile length and for the power spectral density of a profile given three parameters a fault density number of faults per unit length crossed by the profile an average squared fault scarp height and a characteristic length of flexure to keep matters simple we make a number of assumptions and approximations namely that the lithosphere behaves as an elastic plate that faults have an infinite length and a vertical dip that the response of topography to a number of faults is simply the sum of the responses to each fault and that faults have random locations and scarp heights independently chosen from some statistical distribution the theory predicts that the roughness length relationship power spectral density should follow power laws for scales wavelengths less than a characteristic scale proportional to the length scale of flexure we compare the predictions of the theory with actual measurements of mid ocean ridge flank roughness and find good first order agreement in particular we use independent estimates of fault densities average squared fault scarp heights and flexural length scales to predict the topographic roughness of the east pacific rise and we find that normal faulting can explain all the observed topographic roughness nevertheless there are some differences between predictions and observations these differences are likely to be due to processes other than faulting that create topographic relief e g volcanism and to spatial correlations of fault scarp heights despite these shortcomings the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief

along axis variability in crustal accretion at the mid atlantic ridge results from the ocean study the ocean experiment is an integrated geophysical study of a region of the cape verde abyssal plain that formed at 140 ma deep seismic reflection and ocean bottom hydrophone obh refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey the igneous basement is overlain by about 1 3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the mid atlantic ridge axis we describe the results of a 150 km long profile oriented parallel to magnetic anomalies m15 and m16 along which deep seismic reflection data collected by the british institutions reflection profiling syndicate are complemented by refraction data constrained by four obhs the line spans an entire spreading segment between two fracture zones the northern of which has an offset of 40 km and the other central has an offset of only 10 km away from the fracture zones the mean igneous crustal thickness is 7 2 km near both fracture zones thinning of up to 4 km is observed giving a mean igneous crustal thickness over the whole segment of approximately 6 5 km differences are seen between the two fracture zones in their seismic velocity structure in the associated basement topography and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone the boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity a number of planar reflections up to 10 km in length are present within the middle and lower crust dipping outward from beneath low amplitude basement highs at 15 these appear to be present only within layer 3 the moho has several expressions in the reflection data including isolated reflection events a local increase in reflected amplitudes and a downward decrease in coherent reflections at the center of the segment there is a zone at the base of the crust within which both high and low velocity materials are present this zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge

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

a tectonic boundary for the southern colorado plateau abstract a tectonic boundary should be defined by changes in tectonic elements tectonic elements would include such parameters as structural style stress orientations volcanism heat flow seismicity and changes in crustal thickness examination of these tectonic elements for the southern colorado plateau suggests that the southwestern part of the physiographic plateau appears to be tectonically part of the basin and range province i therefore propose a boundary at 112 w longitude the locus of seismicity and recent volcanism and also the site of crustal thickening and a change of structural style the tectonic boundary of the southern colorado plateau is suggested to form part of the eastern boundary of the southern basin and range seismotectonic region this seismotectonic region is bounded on the north by the las vegas shear zone nevada seismic zone and has a different stress orientation than the northern basin and range from which it began to evolve separately 13 m y ago the present principal stress orientation of the southern basin and range of arizona appears to be the result of compound movements of the san andreas transform and garlock las vegas shear zone resulting in a shear couple affecting the crust in arizona and an associated ne sw least principal stress orientation

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

effects of short flexible seagrass zostera noltei on flow erosion and deposition processes determined using flume experiments innovative flume experiments were conducted in a recirculating straight flume zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth the aims of this study were i to quantify the combined effects of leaf flexibility and development characteristics of zostera noltei canopies on their interaction with hydrodynamics and ii to quantify the role of zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses velocity within the canopy was significantly damped the attenuation in velocity ranged from 34 to 87 compared with bare sediments and was associated with a density threshold resulting from the flow induced canopy reconfiguration the reduction in flow was higher in dense canopies at higher velocities than in less dense canopies in which the reduction in flow was greater at low velocities these contrasted results can be explained by competition between a rough wall boundary layer caused by the bed and a shear layer caused by the canopy the velocity attenuation was associated with a two to three fold increase in bottom shear stress compared with unvegetated sediment despite the increase in near bed turbulence protection of the sediment against erosion increased under a fully developed meadow while sediment properties were found to be the main factor controlling erosion in a less developed meadow deposition fluxes were higher on the vegetated bed than on bare sediments and these fluxes increased with leaf density fewer freshly deposited sediments were resuspended in vegetated beds resulting in an increase in net sediment deposition with meadow growth however in the case of a very high leaf area index sediment was mostly deposited on leaves which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow

from continental extension to seafloor spreading crustal structure of the goban spur rifted margin southwest of the uk we present a 169 km wide angle velocity model across the goban spur rifted margin southwest of the uk a 120 km wide intermediate region is identified between the first clear seafloor spreading magnetic anomaly anomaly 34r and thinned continental crust where velocities increase from 4 5 km s1 to 6 8 km s1 in the top 4 km beneath acoustic basement at depth it can be divided into a region where a 1 5 km thick high velocity layer 7 2 7 6 km s1 exists and a region where this layer is absent but velocities of 7 km s1 are present wide angle pmp arrivals are observed across the whole of the intermediate region but a normal incidence reflection moho is not present based on these velocities and combined with seismic reflection gravity and magnetic modelling along this transect we interpret the intermediate region to consist of a 70 km wide zone of exhumed mantle with high poisson s ratio at top basement 0 34 an extremely low topographic expression and a high velocity deep layer empirical relationships between velocity and degree of serpentinization suggest that serpentinite content decreases with depth from 100 per cent at top basement to 25 per cent from 5 7 km into basement the observed magnetic anomaly is best fit by a thin magnetized layer 1 km of magnetization 2 3 a m1 the intense magnetization may be due to magnetite formation during a prolonged interaction of serpentinite with sea water between this exhumed mantle and anomaly 34r is a 50 km wide region raised 400 m above the adjacent exhumed mantle consisting of a series of basement ridges poisson s ratio is not well defined in this region which may be composed of anomalous oceanic crust or exhumed mantle

imaging mid ocean ridge transitions with satellite gravity gravity maps derived from satellite altimeter measurements provide unprecedented me dium resolution coverage of sparsely surveyed mid ocean ridges in the southern oceans a spectral analysis of 76 000 km of coincident shipboard and satellite gravity measurements shows that satellite altimeters can accurately resolve features with half wavelengths as short as 13 km the coverage and resolution of these gravity data allow us to determine accurately both the location of poorly charted ridge axes and the variation in axial anomaly character along the ridge axis although their detailed morphology is not resolved the results of this study support earlier studies that showed a transition from spreading rate dependent axial gravity lows to rate independent axial highs with increasing spreading rate four such transitions are imaged on the southeast indian ridge and pacific antarctic ridge we expect that these transitions are the result of a temperature sensitive threshold phenomenon and may be influenced by nearby hot spots

effects of bed material properties on cohesive sediment erosion a straight benthic in situ flume was used to measure the erosion rate of cohesive sediments in freshwater and saltwater environments in new zealand the data show an exponential decay of erosion rate with time which is indicative of depth limited erosion two approaches for the calibration of an erosion formula are used both leading to similar results the data show the dependency of erosion rate on bed material properties such as dry bulk density water content organic content and sand content both methodologies also highlight differences between data from freshwater and saltwater environments also shown is a requirement for direct measurements of bed density profiles and bed load transport during in situ investigations

comparing different methods of bed shear stress estimates in simple and complex flow fields bed shear stress is a fundamental variable in river studies to link ow conditions to sediment transport it is however dif cult to estimate this variable accurately particularly in complex ow elds this study compares shear stress estimated from the log pro le drag reynolds and turbulent kinetic energy tke approaches in a laboratory ume in a simple boundary layer over plexiglas and over sand and in a complex ow eld around de ectors results show that in a simple boundary layer the log pro le estimate is always the highest over plexiglas the tke estimate was the second largest with a value 30 per cent less than the log estimate however over sand the tke estimate did not show the expected increase in shear stress in a simple boundary layer the reynolds shear stress seems the most appropriate method particularly the extrapolated value at the bed obtained from a turbulent pro le in a complex ow eld around de ectors the tke method provided the best estimate of shear stress as it is not affected by local streamline variations and it takes into account the increased streamwise turbulent uctuations close to the de ectors it is suggested that when single point measurements are used to estimate shear stress the instrument should be positioned close to 0 1 of the ow depth which corresponds to the peak value height in pro les of reynolds and tke shear stress copyright 2004 john wiley sons ltd

transition between a valley and a high at the axis of mid ocean ridges an examination of 339 topographic profiles crossing the world mid ocean ridge system indicates that the axial topographic signature is correlated to both the local spreading rate and the absolute depth of the ridge axis the spreading rate which controls the temperature structure at the ridge axis and the depth of the ridge axis which is an indicator of crustal thickness are both expected to influence the rheology of the axial lithosphere which in turn should control the shape of the axial topographic signature the topographic signature of the axis is determined by fitting an axial high or an axial valley on a trend that follows the thermally driven subsidence of the ridge flanks at a given axial depth the transition between an axial valley and a high takes place abruptly over a relatively small change in spreading rate 15 mm yr

crustal structure of the sw moroccan margin from wide angle and reflection seismic data the dakhla experiment part a wide angle seismic models abstract a total 1500 km of seismic reflection and wide angle profiles were acquired off the southern moroccan margin during the dakhla cruise a joint project of ifremer the universities of brest el jadida and lisbon and total the shots along two profiles parallel to the margin and two profiles perpendicular to the margin were also recorded by ocean bottom seismometers obs the profiles perpendicular to the margin were additionally extended on land using 14 stations on the northern profile and 11 stations on the southern profile modelling of the reflection and wide angle seismic data reveals a 10 km deep sedimentary basin including two high velocity carbonate layers lateral crustal thinning is observed from a 27 km thick crystalline continental crust to a 7 km thick oceanic crust occurring over less than 100 km the crystalline continental crust can be divided into two distinct layers of 12 and 15 km thickness the oceanic crust east of the magnetic anomaly m25 displays higher velocities in layer 3 than west of the magnetic anomaly the change in seismic velocity suggests a possible link to changes in accretionary processes of the oceanic crust some regions show seismic velocities between 6 8 and 7 4 km s which could be explained by slightly elevated mantle temperatures during accretion of the crust

sedimentation on intertidal mudflats in the lower part of macrotidal estuaries sedimentation rhythms and their preservation the objective of this study is to propose an original approach to the analysis of the formation of intertidal rhythmites their preservation and the evaluation of sedimentation rates on estuarine mudflats three mudflats one from each of three estuaries were analysed using a combination of long term a year high frequency 1 burst 10 or 20 min and high resolution 0 2 cm altimeter datasets and x ray images of sediment cores collected during topographic surveys the results highlight the roles played by sediment supply hydrodynamics and morphology of the lower estuaries on the sedimentation rhythms while the sediment starved medway estuary kent uk remains stable at different time scales the annual sedimentation rates on the sheltered authie mudflat pas decalais france and the open seine mudflat normandy france are relatively high at 18 and 15 cm yr 1 respectively on the authie mudflat sedimentation rhythms correspond to the semi lunar cycle with a good correlation between tidal range and deposit thickness sedimentation occurs at the beginning of the recovery mean value of 0 25 cm per semi diurnal cycle and is not disturbed by wind induced waves in the lower seine estuary semi diurnal mechanisms of deposition occur mainly when the mudflat is covered by a minimal water height tidal range threshold value 7 1 m sedimentation rhythms are discontinuous and deposition occurs only during the highest spring tides mean deposit thickness is 0 6 cm per semi diurnal cycle controlled by the turbidity maximum and the long high tide slack 2 3 h the fluid mud is sensitive to wind waves in this open funnel shaped estuary which undergoes wind induced erosion 0 2 to 2 cm about 10 times per year comparison of altimeter datasets and lithology of the sediments cored at the same points provided improved understanding of the sediment record rhythms and the sedimentation events on the sediment starved medway mudflat the result of sediment processes is a single superficial lamina the elementary deposit in the authie bay corresponds to a semi lunar linked layer typical of sheltered environments on the lower seine mudflat sedimentation rhythms are linked to the highest spring tides i e the lunar cycle resulting from increased sediment availability related to the high suspended matter concentration in the turbidity maximum these results underline the complex response of intertidal mudflats to hydrodynamics and sediment supply conditions from the semi diurnal to the annual scales

the influence of current speed and vegetation density on flow structure in two macrotidal eelgrass canopies the influence of eelgrass zostera marina on near bed currents turbulence and drag was investigated at three sites in two eelgrass canopies of differing density and at one unvegetated site in the san juan archipelago of puget sound washington usa eelgrass blade length exceeded 1 m velocity profiles up to 1 5 m above the sea floor were collected over a spring neap tidal cycle with a downward looking pulse coherent acoustic doppler profiler above the canopies and two acoustic doppler velocimeters within the canopies the eelgrass attenuated currents by a minimum of 40 and by more than 70 at the most densely vegetated site attenuation decreased with increasing current speed the data were compared to the shear layer model of vegetated flows and the displaced logarithmic model velocity profiles outside the meadows were logarithmic within the canopies most profiles were consistent with the shear layer model with a logarithmic layer above the canopy however at the less dense sites when currents were strong shear at the sea floor and above the canopy was significant relative to shear at the top of the canopy and the velocity profiles more closely resembled those in a rough wall boundary layer turbulence was strong at the canopy top and decreased with height friction velocity at the canopy top was 1 5 2 times greater than at the unvegetated sandy site the coefficient of drag cd on the overlying flow derived from the logarithmic velocity profile above the canopy was 3 8 times greater than at the unvegetated site 0 01 0 023 vs 2 9 10 3

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

heat flow observations on the bermuda rise and thermal models of midplate swells the bermuda rise is a broad topographic swell which is apparent in both residual depth and geoid anomaly maps of the western north atlantic the magnitudes of the depth and geoid anomalies associated with the bermuda rise are similar to the anomalies associated with other swells surrounding recent volcanic islands e g hawaii suggesting that despite the lack of recent volcanism on bermuda the rise has a similar origin to other midplate swells results are reported from 171 new heat flow measurements at seven carefully selected sites on the bermuda rise and the surrounding seafloor off the bermuda rise the basement depths are generally shallower and the heat flow higher than either the plate or boundary layer models predict with the measured heat flow apparently reaching a uniform value of about 50 mw m 2 on 120 m y old crust on the bermuda rise the heat flow is significantly higher 57 4 2 6 mw m 2 than off the swell 49 5 1 7 mw m 2 the magnitude of the anomalous heat flux 8 10 mw m 2 is comparable to that previously found along the older portion of the hawaiian swell near midway the existence of higher heat flow on both the hawaiian swell and bermuda rise indicates that these features fundamentally have a thermal origin the differences in the shape uplift and subsidence histories of the hawaiian swell and bermuda rise can be quantitatively explained by the different absolute velocities of the pacific and north american plates moving across a distributed heat source in the underlying mantle two dimensional numerical convection models indicate that the observed depth geoid and heat flow anomalies are consistent with simple convection models in which the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection

analysis of the 2009 earthquake swarm near sunset crater volcano arizona abstract a swarm of microearthquakes occurred on october 31 2009 within 5 km of the sunset crater arizona volcano a detailed study of the swarm was warranted because of its location near a young volcanic construct and its proximity to the population center of flagstaff arizona the question posed in this study was whether the swarm was the result of tectonic stress release during fault slip or due to stresses driven by magmatic processes this question was addressed by analyzing and comparing the physical and seismic characteristics of the swarm to the regional tectonic environment and to the characteristics of tectonic swarms in arizona and magmatic volcanic swarms elsewhere this analysis included swarm duration frequency of events b value focal depths and epicentral pattern of the swarm the comparison of the salient features of the 2009 sunset crater swarm to both magmatic and tectonic swarms indicates that the sunset crater swarm has features similar to magmatic swarms and is a potential magmatic swarm candidate

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

a global analysis of mid ocean ridge axial topography summary current views of mid ocean ridges are strongly influenced by extensive mapping of the mid atlantic ridge and east pacific rise the global picture of the mid ocean ridge system particularly in the sparsely surveyed southern oceans is still based primarily on underway bathymetry profiles collected over the past 40 years this study presents a quantitative analysis of global mid ocean ridge morphology based on 156 of these underway bathymetric profiles thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis an empirical orthogonal function eof analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth it is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti symmetric modes the remainder is considered stochastic maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr 1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr 1 the stochastic component of the axial morphology referred to as bathymetric roughness also decreases with spreading rate for rates less than 80 mm yr 1 and remains relatively constant at higher rates although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence they are weakly correlated at slow spreading centres the distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges these observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid ocean ridges

subsidence and crustal roughness of ultra slow spreading ridges in the northern north atlantic and the arctic ocean summary five basin wide seismic reflection profiles of up to 550 km each were acquired in the arctic ocean and the northern north atlantic in 2001 and 2002 the main objective was to investigate the depth to the basement and to analyse the crustal structure morphology and roughness of ultra slow spreading ridges of the gakkel molloy and knipovich ridges to date little is known to date of the ultra slow spectrum of such spreading ridges the seismic profiles of all investigated ridges show similar morphological characteristics with deep axial valleys and rough basement topography magnetic data compilation and interpretation suggests that the ultra slow spreading systems are fairly stable and existed during the entire evolution of the basins to the north of the greenland fracture zone the thermal subsidence curve was calculated and corrected for sediment loads and crustal roughness values are estimated for all five profiles the resulting roughness values append the global roughness data set for ultra slow spreading systems the results are higher than those predicted by interpolating existing global roughness this study confirms the presence of a global relationship between crustal roughness ridge morphology and spreading rates new curve fits supporting the global relationship are discussed data on present spreading rates ridge morphology subsidence and roughness provide a better insight into the development of the axial ridge morphology in the study area the results show that the basins to the north of the greenland fracture zone were formed at ultra slow spreading axial rift valleys and continued spreading at ultra slow rates to the present day configuration

seismicity and tectonics of the blue ridge area of the mogollon plateau arizona abstract the seismicity of the blue ridge area of the mogollon plateau exists within a framework of tertiary volcanics and normal faulting the seismicity includes earthquake sequences in 1989 and 2005 with principal events as large as richter local magnitude m l 4 6 national earthquake information center neic analysis of this seismicity indicates that it is characterized by normal faulting on northwest trending planes the normal faults form part of the cataract creek fault system that has historically generated earthquakes as large as surface wave magnitude m s 6 2 brumbaugh 1991 bausch and brumbaugh 1997 regionally these northwest trending fracture systems faults of the southern colorado plateau appear to pose the greatest seismic hazard to the grand canyon national park with almost five million visitors per year and the rapidly growing flagstaff area

three dimensional mantle convection beneath a segmented spreading center implications for along axis variations in crustal thickness and gravity segmentation and along axis variations within individual segments indicate the inherently three dimensional nature of mantle up welling and melting beneath oceanic spreading centers numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center the experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher viscosity mantle half space a periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt the relative magnitudes of the buoyant and plate driven components of mantle flow are controlled by the spreading rate and mantle viscosity with buoyant flow more important at lower spreading rates and viscosities buoyant flow beneath the spreading axis amplifies along axis variations in upwelling near a ridge transform intersection and distributes the variations along the entire spreading axis buoyant flow may thus be responsible for the more three dimensional character of slow spreading centers away from the spreading axis thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along axis wavelength controlled by the prescribed thickness of the asthenosphere however the position and stability of rolls are influenced by the segmentation geometry in cases where the spreading center geometry does not allow a stable configuration of rolls the flow is time dependent along axis variations in upwelling cause variations in melt production which imply large variations in crustal thickness that dominate the surface gravity signal the crustal thickness distributions implied by these numerical experiments produce bulls eye shaped negative mantle bouguer anomalies centered over spreading segments as observed at several spreading centers the amplitude of the anomaly increases with decreasing spreading rate

new seismic images of oceanic crustal structure remarkable new images of the internal structure of oceanic crust have been recorded from a two ship multichannel seismic survey in the western north atlantic results include strong planar dipping reflectors in the lower crust evidence of whole crustal failure generating reflectors from the top of the basement to the moho and banded layering in the lower crust and in some places in the upper mantle anomalous crust in the blake spur fracture zone is documented from both normal incidence and wide angle seismic profiles the internal structure of the crust records the magmatic and tectonic history of the crust as it was generated at and then moved away from the spreading center

morphology and crustal structure of the kane fracture zone transverse ridge the kane fracture zone kfz transverse ridge is an anomalously shallow ridge which parallels the kfz for over 200 km east of its intersection with the mid atlantic ridge rift valley sea beam bathymetry and gravity data have been used to determine the morphology and density structure of the ridge and travel time data from two seismic refraction experiments have been modeled to constrain its seismic velocity structure the transverse ridge first appears on older lithosphere opposite the eastern ridge transform intersection it rises to a maximum height of about 2000 m above the bordering kfz trough 40 km east of the intersection area although bathymetry data along the western limb of the kfz are relatively sparse a ridge of similar dimensions is not present the kfz transverse ridge has a decidedly flexural shape in cross section with a high steep south facing wall and a gently sloping northern side the grain of the ridge parallel topography north of the kfz continues undisrupted across the transverse ridge suggesting that the ridge has formed by vertical uplift of this crust seismic ray tracing and gravity analyses indicate that the kfz transverse ridge is not locally compensated by an overthickened crust but is underlain by crustal thicknesses and velocities similar to that of normal oceanic crust on either side of the kfz anomalously thin crust is only present immediately beneath the kfz trough and along the south facing wall of the transverse ridge these constraints and the formation of the kfz transverse ridge on the older plate preclude several previously proposed models for its origin including flexural bending due to differential subsidence across the fracture zone constructional volcanism and serpentine diapirism although no single mechanism provides a completely satisfactory explanation for the origin of the kfz transverse ridge the most likely mechanism appears to be a combination of thermal and viscodynamic forces operating near the ridge transform intersection

active faulting and seismicity in a prefractured terrane grand canyon arizona structural and seismic data for the eastern grand canyon were analyzed to determine the relationship between basement fractures surface faulting and seismicity published structural data were compared with the results of focal mechanism analysis of tremors occurring in the eastern canyon region during the period 1989 2004 previous studies of the geology of this area indicate that the fault and joint trends exposed in the surface rocks and through the entire sedimentary section were first established with predominant northeast and northwest trends in precambrian time the northwest trends of this prefractured terrane correlate with a dominant northwest trend of nodal planes in the seismic data suggesting that the present stress system in the region is favoring the reactivation of the preferentially oriented pre existing northwest trending fractures

a global and regional stochastic analysis of near ridge abyssal hill morphology this paper presents the results of a global and regional stochastic analysis of near ridge abyssal hill morphology the analysis includes the use of sea beam data for the estimation of stochastic parameters up to order 4 these parameters provide important quantitative physical information regarding abyssal hills including their rms height azimuthal orientation characteristic width aspect ratio hausdorff dimension skewness tilt and peakiness the global data set consists of 64 sea beam swaths near the rivera cocos and nazca spreading sections of the east pacific rise the mid atlantic ridge and the central indian ridge in one form of analysis the parameters are averaged among spreading rate bins each of the spreading rate subsets can be identified as unique from the others in at least one aspect the slowest spreading rate subset mid atlantic data exhibit the largest scales rms height and characteristic width and length of abyssal hills these parameters generally decrease as spreading rate increases up to the fast spreading rate data pacific cocos but increase going from fast to very fast pacific nazca spreading rate data this indicates some complexity in the relationship between spreading rate and abyssal hill morphology the plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data the fractal dimension is nearly identical for all spreading rate subsets the vertical skewness is positive for the slow and medium spreading rate data indicating larger peaks than troughs and negative for the fast spreading rate data indicating larger troughs than peaks the kurtosis or peakiness is everywhere larger than the gaussian value of 3 and tends to be larger in the atlantic than the pacific the tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data but only marginal evidence for it in the slow spreading rate data from an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set in particular while over the global data set the characteristic width exhibits a well resolved positive trend when plotted versus rms height these parameters exhibit a more gradual positive trend in the mid atlantic data and a negative trend in the pacific cocos data in addition the plan view aspect ratio while generally uncorrelated with rms height for the global data set is positively correlated with rms height for the pacific cocos data set these results emphasize a strong uniqueness of the pacific cocos data relative to the rest of the data global set the pacific cocos data consist of 27 swaths concentrated between the siquieros and orozco fracture zones these data provide very good abyssal hill coverage of this well mapped and well studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters in this analysis it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis indicating that the relative abundance in magma supply which likely controls the ridge axis depth may also be an important influence on the formation of abyssal hills

seismic structure of oceanic crust in the western north atlantic we examine the seismic structure of cretaceous aged ocean crust north of the blake spur fracture zone in the western north atlantic by using a combination of multichannel seismic and wide angle reflection refraction data although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure it displays large variations in crustal reflectivity on both ridge normal and ridge parallel profiles the upper crust is highly reflective and contains both subhorizontal or shallow dipping events and more steeply dipping reflectors subhorizontal reflectors are typically present between 1 5 and 2 km depth and are in some cases laterally continuous for distances of 15 to 20 km generally these events do not correlate with the depth of the seismic layer 2 layer 3 boundary determined from refraction data and their origin is still poorly understood the steeply dipping events are generally confined to the upper 2 5 km of the crust we interpret these reflectors as the subsurface expression of ridge parallel extensional faults commonly mapped at mid ocean ridges the lower crust in this area exhibits alternating regions of high and low reflectivity the highly reflective zones are made up of packages of linear or concave upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust some of these dipping reflectors appear to cut through the whole crustal section although most fade out in the acoustically transparent midcrust on ridge normal profiles the majority of these events dip to the east toward the paleo spreading center whereas on ridge parallel profiles the events typically dip south toward the blake spur fracture zone the base of the crust is generally not associated with a strong discrete moho reflection but is a comparatively indistinct boundary usually associated with a 1 to 2 km thick band of diffuse high reflectivity we interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8 10 km with the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge normal and ridge parallel profiles we may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults alternatively these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere

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

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

seismicity and active faulting of the kanab fredonia area of the southern colorado plateau 1 detailed study of the historic seismicity of the kanab fredonia area adds information to the location of the colorado plateau tectonic boundary in this area and to the nature of contemporary seismicity and faulting on this part of the colorado plateau historic seismicity over 45 years 1959 2004 defines a band of activity which extends through fredonia arizona and into southern utah around kanab the area is dominated by north trending high angle normal faults the spatial distribution of epicenters consists of clusters associated with the west kaibab big springs and toroweap sevier faults indicating that these faults may be active four focal mechanism solutions show basin range style high angle normal faulting on north to north northwest trending surfaces that are parallel to subparallel in most cases to mapped quaternary faults this suggests a present east west extension occurring on the southern colorado plateau for the kanab fredonia area and an expansion of the adjacent basin range province eastward into the plateau as far as the west kaibab big springs faults the west kaibab big springs faults are proposed to be the tectonic boundary of the southern colorado plateau in this area since they mark the limit of significant seismic activity that is combined with basin range type faulting

sediment retention by a mediterranean posidonia oceanica meadow the balance between deposition and resuspension the role of posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at fanals point nw mediterranean spain by comparing particle deposition within the meadow and adjacent bare sediment small sediment traps were used to measure deposition within and above the meadow and over bare sand a model based on measurements of particle deposition at increasing distances from the bottom was used to partition the total depositional flux between primary sediment particles deposited for the first time at the measuring site and resuspended deposition sediment particles that have been previously deposited at the measuring site measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition and to form a balance of particle transport at the annual time scale significant differences in total deposition were found over time ranging from 1 5 to 500 g dw m 2 d 1 including those between bare and vegetated sediments the effect of p oceanica in increasing primary deposition at an annual scale was modest however p oceanica significantly buffered sediment resuspension which was reduced more than three fold compared to the unvegetated bottom the annual flux of deposition was dominated by settling of resuspended materials which represented 85 of the total flux within the meadow but 95 of the total deposition on bare sand thus seagrass meadows reduce resuspension in the nw mediterranean littoral thereby contributing to increased sediment retention and therefore reducing erosion in the coastal zone

rough smooth basement boundary in the western north atlantic basin evidence for a seafloor spreading origin a distinctive change in the character of acoustic basement has been mapped within the mesozoic keathley magnetic anomaly sequence in the western north atlantic basin this boundary known as the rough smooth basement boundary separates relatively smooth acoustic basement on the older western side of the boundary from much rougher basement to the east south of bermuda the boundary is well defined and nearly isochronous corresponding to anomaly m 12 or m 13 128 m y b p north of bermuda the boundary is diachronous and more transitional with the change in basement relief occurring in a zone between anomaly m 21 and m 16 age crust 132 to 142 m y b p both the well defined and transitional changes in basement relief found in the western basin are also identifiable in seismic reflection records from the eastern canary basin and an analysis of magnetic anomaly patterns in the central north atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge these results strongly suggest a primary seafloor spreading origin for the rough smooth basement boundary with the greater basement relief associated with slower spreading rates changing directions of relative plate motion and ridge jumps of isolated accretionary segments beginning in late jurassic early cretaceous time

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

long term survival of the axial valley morphology at abandoned slow spreading centers results of a lithospheric stretching model of mid ocean ridges suggest that the axial valley topography created at active slow spreading centers may be preserved for tens of millions of years after cessation of spreading we simulate the evolution of a slow spreading center using a finite element model that incorporates a temperature dependent rheology and discrete faulting model results show that significant valley morphology persists after regional extension ceases because the high lithospheric viscosities associated with a slow spreading center prevent ductile relaxation and because no effective mechanism exists to reverse rift bounding normal faults removal of the axial valley can only be accomplished by reversal of the regional extensional strains immediately upon cessation of active spreading however such a process does not seem likely these results suggest that the persistence of axial valleys at extinct spreading centers is consistent with a lithospheric stretching model for active slow spreading ridges

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

ecosystem engineering by annual intertidal seagrass beds sediment accretion and modification seagrasses are generally known as ecosystem engineers as they reduce flow velocities in their canopies in perennial subtidal meadows this usually leads to increased net sedimentation rates and reduction of the grain size the present study aims to describe the contribution of annual seagrass populations to these processes and elucidate the temporal dynamics sediment accretion and grain size modification were experimentally tested by transplanting seedlings of an annual intertidal eelgrass population to an unvegetated tidal flat within the planting units 79 shoots m 2 4 7 mm of sediment accreted whereas in the most dense parts of these units 199 shoots m 2 accretion amounted to 7 1 mm the silt fraction 63 m increased and the sand fraction 63 500 m decreased in the eelgrass beds which provides evidence that higher silt content in seagrass beds is the result and not the cause of seagrass presence annual intertidal eelgrass beds significantly contribute to the immobilisation of sediment during the growing season with its magnitude depending on canopy density during winter the accumulated sediments were released again and could even induce additional erosion possible consequences of these sediment dynamics for the larger scale functioning of estuarine ecosystems are discussed

sediment deposition and production in se asia seagrass meadows abstract seagrass meadows play an important role in the trapping and binding of particles in coastal sediments yet seagrass may also contribute to sediment production directly through the deposition of detritus and also the deposition of the associated mineral particles this study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition to inorganic carbonate and non carbonate and organic sediment production total daily deposition measured with sediment traps varied from 18 8 2 0 g dw m 2 d 1 in silaqui philippines to 681 1 102 g dw m 2 d 1 in bay tien vietnam these measurements correspond to a single sampling event and represent sedimentation conditions during the dry season in se asia coastal areas enhalus acoroides was the most common species in the seagrass meadows visited and together with thalassia hemprichii was present at sites from low to very high deposition halodule uninervis and cymodocea species were present in sites from low to medium deposition the mineral load in seagrass leaves increased with age and was high in e acoroides because it had the largest and long lived leaves up to 417 mg calcium carbonate per leaf and 507 mg non carbonate minerals per leaf and low in h uninervis with short lived leaves 4 mg calcium carbonate per leaf and 2 mg non carbonate minerals per leaf in se asia seagrass meadows non carbonate minerals accumulate at slower rates than the production of calcium carbonate by the epiphytic community consequently the final loads supported by fully grown leaves were as average lower than calcium carbonate loads our results show that organic and inorganic production of the seagrasses in se asia represents a small contribution maximum of 15 of the materials sedimented on a daily base by the water column during the sampling period the contribution of the carbonate fraction can be locally significant i e 34 in silaqui in areas where the depositional flux is low but is minor

gravity anomalies and crustal structure at the mesozoic blake spur fracture zone a number of recent studies have used gravity data from mid ocean ridges to infer along axis crustal thickness variations and hence variations in melt supply a problem with these studies is that it is difficult to distinguish the effects of crustal thickness variations from those of crustal and mantle density variations we analyze gravity data from the 12 13 km offset blake spur fracture zone in 140 ma crust in the western north atlantic the advantage of studying this area is that mantle density variations are likely to be much smaller than those at the ridge axis and crustal thickness variations are well constrained by expanding spread profiles esps thus we are able to separately quantify the contributions made to the gravity anomaly by crustal thickness and crustal density variations after removal of the gravity effects of the seabed and the oceanic basement topography as defined by a grid of multichannel seismic reflection profiles the gravity data show a linear 5 mgal high along the fracture zone after removal of the effects of crustal thickness variations defined by the esps and of a basal high velocity layer beneath the fracture zone interpreted to be partially serpentinized upper mantle the data show a 5 mgal gravity low at the fracture zone an alternative interpretation of the basal layer as a thickened moho transition zone also leads to a residual gravity low these results together with other recent results from fracture zones on the mid atlantic ridge suggest that crustal thickness variations at fracture zones may be consistently understimated by studies based on gravity data the discrepancy may be explained by variations in crustal density due to enhanced fracturing in the vicinity of fracture zones and to disproportionate thinning of oceanic layer 3 near segment boundaries

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles the iam 9 transect at 40 20 n we present a crust and mantle velocity structure for the west iberia passive continental margin derived from a 320 km long wide angle seismic profile acquired in the southern iberia abyssal plain we observe a 170 km wide ocean continent transition zone which includes a pair of overlapping peridotite ridges and is bounded by oceanic crust and landward by fault bounded blocks of continental crust the profile lies 40 km south of the transect sampled by ocean drilling program odp legs 149 and 173 the transition zone structure can be divided into an upper layer 2 4 km thick with velocities of between 4 5 and 7 0 km s 1 and generally a high velocity gradient 1 s 1 and a lower layer up to 4 km thick with a velocity of 7 6 km s 1 and a low velocity gradient a weak moho reflection in this zone was seen only on wide angle profiles at an offset of 30 km the upper layer has a distinctly lower velocity than thinned continental crust adjacent to the continental slope conversely the lower layer has too high a velocity to be magmatically intruded or underplated lower continental crust on the coincident seismic reflection profile fault bounded crustal blocks identified in unequivocal extended continental crust are not observed in the transition zone the upper layer has velocity bounds and gradient similar to oceanic layer 2 observed west of the peridotite ridges but no oceanic layer 3 velocity structure is present while magnetic anomalies have been identified within the transition zone they have not been modeled successfully as seafloor spreading magnetic anomalies nor do they generally form long linear margin parallel features finally odp boreholes 40 km north of our profile and within the interpreted transition zone have recovered up to 140 m thick sections of serpentinite and serpentinized peridotites with little evidence of mafic igneous material we conclude that the transition zone cannot be dominantly composed of either extended continental crust or oceanic crust although current melting models predict a considerably thicker crust of decompression melt products we interpret this region as exposed upper mantle peridotite with little or no synrift extrusive material and limited amounts of synrift material intruded within the serpentinized peridotite

estimating bottom stress in tidal boundary layer from acoustic doppler velocimeter data bed stresses in the bottom boundary layer of the york river estuary virginia were estimated from three dimensional near bottom velocities measured by acoustic doppler velocimeters advs and also by a profiling array of electromagnetic current meters by assuming the measurements were made in a constant stress layer four methods of stress estimation were evaluated using advs direct covariance cov measurement turbulent kinetic energy inertial dissipation utilizing the kolmogorov spectrum and log profile the four methods yielded similar estimates of frictional velocity u based on adv output from both 14 cm and 44 cm above bed all eight estimates of average u were consistent with the overall mean of 1 10 cm s to within the 95 confidence interval for individual burst estimates the cov method worked slightly better nearer the bed possibly because of the sensitivity of cov to the upper limit of the constant stress layer the inertial dissipation method performed marginally well at 14 cm above bed likely because of sediment induced stratification and insufficient separation of turbulent production and dissipation scales the log profile method was the most variable and appeared most sensitive to stratification and to the thickness of the constant stress layer the turbulent kinetic energy method was the most consistent at both heights and appears most promising for further development results encourage future use of the adv in estuarine environments but also favor the simultaneous use of several methods to estimate bottom stress

the crustal structure of the canary basin accretion processes at slow spreading centers multichannel seismic reflection and gravity data define the structure of mesozoic ocean crust of the canary basin formed at slow spreading rates single and multichannel seismics show a transition from smooth to rough basement topography from jurassic to cretaceous crust and a coeval change in crustal structure internal reflectivity of the rough basement area comprises upper upper middle or whole crust cutting discrete dipping reflections lower crustal reflectivity is almost absent and reflections from the crust mantle transition are short and discontinuous or absent for several kilometers in contrast crust in the smooth basement area is characterized by sparse lower crustal events and common reflections from the crust mantle boundary the crustal structure of fracture zones in the rough basement area is associated with depressions in the basement top and in most cases with thin crust in the smooth basement area fracture zones exhibit neither a clear topographic expression nor crustal thinning we interpret these characteristics as indicative of an increase in extensional tectonic activity and decrease in magmatic activity at the spreading ridge associated with a general decrease of spreading rate from jurassic to cretaceous times in addition the crust imaged across the path of the cape verde hot spot in the canary basin exhibits a widespread lower crustal reflectivity very smooth topography and apparently thick crust our data document significant changes in the structure of crust formed at slow spreading rates which we attribute to thermal changes in the lithosphere due either to variations in spreading rate or to the presence of a hot spot beneath the mesozoic mid atlantic ridge

flow characteristics within different configurations of submerged flexible vegetation summary the effects of three configurations aligned staggered and columnar of submerged flexible vegetation on flow structure are investigated experimentally in the laboratory time averaged flow velocity and turbulence behavior are evaluated at different positions in each configuration by using a 3d acoustic doppler velocimeter adv according to the hydrodynamic regimes in experimental results the vertical distribution of streamwise velocity can be separated into three layers the upper non vegetated layer middle vegetation layer and lower sheath layer this three layer model which is associated with different logarithmic equations can be applied to describe the vertical distribution of streamwise velocity the local maximum velocity within vegetation occurs at the sheath section of a plant clump 0 10 0 15 vegetation height h v where the frontal width is minimal turbulent intensities in the streamwise u rms and spanwise v rms directions peak at the sheath section and at the approximate top of the canopy 0 9 1 2 h v the maximum reynolds stresses exist at roughly 0 9 1 2 h v which may be migrated vertically as the frontal width of a plant clump is increased this high frontal width also increases streamwise velocity above vegetation leading to increase variations in reynolds stresses around the canopy top on the vertical turbulent velocity scale w rms the vortices above a still canopy rotate faster than those above a waving canopy therefore the experimental results demonstrate that the flow field can vary significantly at the sheath section and at the top of a plant clump due to altered flow pass these analytical findings will likely prove useful when designing ecological habitats and preventing riverbed erosion

geometry of extensional faults developed at slow spreading centres from pre stack depth migration of seismic reflection data in the central atlantic canary basin we present depth images from portions of profiles that are close to flow lines of cretaceous oceanic crust in the eastern central atlantic compared with post stack time migrations the images illustrate the improvement resulting from the application of pre stack depth migration the images document the scale and geometry of normal faulting in oceanic crust formed over 25 myr at a half spreading rate of less than 10 mm yr 1 and the variation in extensional style with position within the spreading segment away from major fault zones fzs most faults are subplanar dip more than 35 are associated with moderate basement relief 0 2 1 km relief and may penetrate to deep crustal levels these faults could be related to the lifting of the lithosphere out of the median valley to the flanking mountains also observed away from fzs are gently dipping to subhorizontal reflections in the upper crust which resemble detachment faults in contrast the inside corner crust is more rugged with basement highs rising up to 2 km above the intervening basins this larger scale topography is associated with a different style of faulting the depth images reveal gently dipping 35 faults that are rooted in the deep crust and that project to the ridgeward flank of the dome shaped large basement highs 1 2 km vertical relief the faults seem to continue as the ridge facing flank of these highs and some may extend over the crest of the high to breakaways beyond in this case the domal highs that form the exhumed footwall to the faults can be described as oceanic core complexes these controlling faults are up to 20 km long and have a heave of 10 km sufficient to have accommodated up to 50 per cent extension and to have exhumed deep crustal and perhaps even mantle rocks we suggest that similar faults can explain the structure and lithologies found at megamullion structures oceanic core complexes at inside corners near the present day spreading ridge

seabed stress determinations using the inertial dissipation method and the turbulent kinetic energy method direct measurements of seabed stress are difficult especially in field conditions several methods for estimating these stresses using current meter data are available two of these methods the inertial dissipation method and the turbulent kinetic energy method are described below and a matlab program is used to analyse data from a wave current environment

contemporary seismicity faulting and the state of stress in the colorado plateau the contemporary seismicity of the colorado plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude and contrary to earlier views of a low to moderate rate of occurrence with earthquakes widely distributed concentrations of earthquakes have been observed in a few areas of the plateau the most seismically active area of the colorado plateau is the eastern wasatch plateau book cliffs where abundant small magnitude seismicity is induced by coal mining the largest earthquakes observed to date of estimated richter magnitude m l 5 6 have generally occurred in northern arizona although very few earthquakes can be associated with known geologic structures or tectonic features in the colorado plateau seismicity appears to be the result of the reactivation of pre existing faults lacking surficial expression but favorably oriented to the tectonic stress field the small to moderate size of the earthquakes and their widespread distribution are consistent with a highly faulted precambrian basement and upper crust and a moderate level of differential tectonic stress earthquakes in the plateau generally occur in the upper crust from the near surface to a depth of 15 20 km although events have been observed in both the lower crust and uppermost mantle in areas of low to normal heat flow the latter suggests that temperatures are sufficiently low at these depths that brittle failure and hence earthquakes are still possible the predominant mode of tectonic deformation within the plateau appears to be normal faulting on northwest to north northwest striking faults with some localized occurrences of strike slip faulting on north west or northeast striking planes at shallow depths the contemporary state of stress within the plateau is characterized by approximate northeast trending extension in contrast to the previous belief that the plateau was being subjected to east west tectonic compression one area of the plateau the eastern wasatch plateau and book cliffs may still be characterized by compressive stresses however the nature of these stresses is not well understood

on the roughness of mesozoic oceanic crust in the western north atlantic summary seismic reflection profiles from mesozoic oceanic crust around the blake spur fracture zone bsfz in the western north atlantic have been widely used in constraining tectonic models of slow spreading mid ocean ridges these profiles have anomalously low basement relief compared to crust formed more recently at the mid atlantic ridge at the same spreading rate profiles from other regions of mesozoic oceanic crust also have greater relief the anomalous basement relief and slightly increased crustal thickness in the bsfz survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation if so the intracrustal structures observed may be representative of an atypical tectonic regime

decrease in oceanic crustal thickness since the breakup of pangaea earth s mantle has cooled by 6 11 c every 100 million years since the archaean 2 5 billion years ago in more recent times the surface heat loss that led to this temperature drop may have been enhanced by plate tectonic processes such as continental breakup the continuous creation of oceanic lithosphere at mid ocean ridges and subduction at deep sea trenches here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time we find that oceanic crust formed in the mid jurassic about 170 million years ago is 1 7 km thicker on average than crust produced along the present day mid ocean ridge system if a higher mantle temperature is the cause of thicker jurassic ocean crust the upper mantle may have cooled by 15 20 c per 100 million years over this time period the difference between this and the long term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss we also find that the increase of ocean crustal thickness with plate age is stronger in the indian and atlantic oceans compared with the pacific ocean this observation supports the idea that upper mantle temperature in the jurassic was higher in the wake of the fragmented supercontinent pangaea due to the effect of continental insulation thicker oceanic crust forms from a hot mantle observations of unusually thick oceanic crust that formed 170 million years ago in the atlantic and indian oceans suggest that the ancient supercontinent pangaea helped insulate and warm the mantle

seismic and tectonic analysis of the 2014 2015 flagstaff arizona earthquake sequence abstract the 1 december 2014 moment magnitude m w 4 7 arizona earthquake occurred 13 km southwest of the city of flagstaff arizona flagstaff is located within the northern arizona seismic belt and the san francisco volcanic field the flagstaff area is subject to infrequent moderate magnitude earthquakes such as the 1993 cataract creek m w 5 3 and the 1912 surface wave magnitude m s 6 2 events the epicenter of the m w 4 7 earthquake lies 2 km to the east of the oak creek normal fault this is the most proximal event to the fault in the last 40 years since the seismographic networks were first deployed the earthquake was recorded throughout northern arizona by stations of the arizona integrated seismic network data were also acquired by the network from two foreshocks of duration magnitude m d 3 2 and 2 8 and 27 aftershocks m d 1 4 3 0 locations were obtained using hypoinverse 2000 and epicenters were accurate in most cases to within 3 0 km or less fault plane solutions were obtained from the mainshock and from seven of the other events of the sequence analysis of the data indicates that release of the stress was controlled by the presence of pre existing structures including the oak creek fault and prominent fractures in the hanging wall block of the fault this is the first well documented fault slip sequence in northern arizona that has shown 1 active northeast ne trending fracture and 2 a pattern of alternation of stress release on ne and northwest nw trending fractures the alternation of stress release accommodates southeast se directed slip on both ne and nw trends the reactivation of the oak creek fault in northern arizona appears to be the result of the westward motion of the colorado plateau producing a strain rate field that implies nw se extension

quantitative characterization of abyssal hill morphology along flow lines in the atlantic ocean the morphology of the seafloor when sampled along flow lines of relative plate motion represents an indirect geologic time series record of the processes which form it eventually we hope to be able to interpret this information so that we may improve our understanding of the workings of ridge crest and offridge processes through time this study takes an important step toward that goal by quantitatively characterizing through stochastic modeling the abyssal hill morphology along the available sea beam flow line data sets in the atlantic the six sea beam flow line data sets used in this study include two near the kane fracture zone and four near the rio grande and moore fracture zones stochastic parameters estimated from these flow lines such as the mis height lineament azimuth characteristic widths and planview aspect ratio are plotted as a function of crustal age in addition to testing available hypotheses the purpose of this work is to investigate how the abyssal hill morphology changes with age and to correlate this behavior to known changes in ridge characteristics with age these results place important constraints on our understanding of the process of seafloor creation and begin to establish a baseline against which morphology may be interpreted in the absence of other information along the kane fracture zone variations in the estimated abyssal hill lineament azimuth rms height and plan view aspect ratio with crustal age appear to be correlated in various ways with the segmentation history and also possibly with large changes in plate motion direction that have occurred there within the past 80 m y along the rio grande and moore fracture zones variations in the lineament azimuth rms height and other parameters as a function of crustal age are often significant on a 5 10 m y time scale these variations are difficult to interpret as yet to emphasize longer time scale variations the estimated rms heights from the south atlantic flow lines are averaged over common age bins this demonstrates a strong negative correlation of rms height with the spreading rate however when characteristic widths are averaged in this manner no strong correlation with spreading rate is found large variations in this parameter occur on a 25 m y time scale

the influence of waves and seagrass communities on suspended particulates in an estuarine embayment resuspension of bottom sediments by waves corresponding changes in suspended particulate material spm concentrations in the overlying water column and transport pathways of spm were investigated in a shallow estuarine embayment colonized by seagrass communities in chesapeake bay in shallow 2 m unvegetated regions significant resuspension of bottom sediments was evident when northerly winds exceeded 25 km h 1 increasing spm concentrations up to 10 fold high concentrations of spm generated by resuspension dissipated rapidly within 24 h after winds became calm patterns of spm along the embayment s depth gradient suggest that part of this resuspended material was transported offshore into deeper reaches of the estuary in areas of the embayment colonized by seagrasses wave energy was attenuated by the vegetation suppressing resuspension and enhancing deposition as a result spm concentrations were significantly lower inside seagrass beds than in adjacent unvegetated areas during periods of high winds when wave induced resuspension occurred in unvegetated areas spm concentrations remained unchanged inside the bed at normal water levels however when water levels were elevated by spring tides or storm surges plants were less effective at attenuating wave energy and spm concentrations increased inside the seagrass bed due to resuspension and advective processes calculations based on the results of this study indicate that sedimentation rates are substantially higher in seagrass communities than in unvegetated areas

rapid source mechanism analysis of the april 29 1993 cataract creek mw 5 3 northern arizona earthquake abstract the source mechanism of two moderate size normal faulting earthquakes located slightly south of the grand canyon in arizona are studied using several near real time seismic wave analysis procedures the events are found to have source depths near 10 to 15 km and the smaller of the two events m b 4 8 is used to provide empirical green9s functions to analyze the rupture directivity of the larger m w 5 3 mainshock these events indicate complexity in the crustal stress environment on the western margin of the colorado plateau