vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

seismic velocities in the uppermost oceanic crust age dependence and the fate of layer 2a it has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to typical crustal velocities 5 2 km s 1 over a period of 30 40 million years with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation some recent seismic studies however suggest that the seismic velocities actually rise rapidly nearly doubling in 5 km s 1 and velocities 3 km s 1 are not observed in crust older than 5 ma there is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years median and average velocities increase from 2 3 to 4 4 km s 1 by 7 5 ma and remain unchanged thereafter if layer 2a is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks layer 2b 5 2 km s 1 it follows that layer 2a does not disappear but is a persistent feature of the oceanic crust these results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation

fractal density modeling of crustal heterogeneity from the ktb deep hole fractal or multifractal concepts have significantly enlightened our understanding of crustal heterogeneity much attention has focused on 1 f scaling natures of physico chemical heterogeneity of earth crust from fractal increment perspective in this study fractal density model from fractal clustering point of view is used to characterize the scaling behaviors of heterogeneous sources recorded at german ktb main hole and of special contribution is the local and global multifractal analysis revisited by using haar wavelet transform hwt fractal density modeling of mass accumulation generalizes the unit of rock density from integer e g g cm3 to real numbers e g g cm so that crustal heterogeneities with respect to source accumulation are quantified by singularity strength of fractal density in dimensional space from that perspective we found that the bulk densities of metamorphic rocks exhibit fractal properties but have a weak multifractality decreasing with the depth the multiscaling natures of chemical logs also have been evidenced and the observed distinct fractal laws for mineral contents are related to their different geochemical behaviors within complex lithological context accordingly scaling distributions of mineral contents have been recognized as a main contributor to the multifractal natures of heterogeneous density for low porosity crystalline rocks this finally allows us to use de wijs cascade process to explain the mechanism of fractal density in practice the proposed local singularity analysis based on hwt is suggested as an attractive high pass filtering to amplify weak signatures of well logs as well as to delineate micro lithological changes

impact of micro organisms activity on the natural remanent magnetization of the young oceanic crust microbes have been widely reported in the deep subseafloor environment still it is difficult to detect a global chemical signature of bacterial activity in the oceanic crust we carried out experiments up to 355 days exposing very young oceanic basalts to anaerobe sulfate reducing organisms in an in vitro marine environment the natural remanent magnetization of samples was monitored during the whole duration of experiments and within this time frame the most magnetized sub samples lost up to 30 of their original signal scanning electron microscope observations show cycling of iron from the titanomagnetites to iron sulfide phases our results suggest that microbes can have a major and fast impact on the magnetization of young oceanic basalts and could contribute to a global signal as the central anomaly magnetic high seen along ridges axis

the influence of porosity and crack morphology on seismic velocity and permeability in the upper oceanic crust 1 the purpose of this study is to explore the relationships between porosity crack morphology seismic velocity and permeability in the upper oceanic crust in a theoretical model velocity is a quadratic function of porosity and the coefficients are functions of the fractional area of asperity contact af across the cracks a measure of crack morphology thus v v af fits to data sets from the lava and dike sections in holes 504b and 1256d yield af 0 096 8 and 0 045 3 with rms errors of 1 7 and 0 3 respectively lower values of af account for the high porosities and low velocities observed in layer 2a after 0 2 ma the porosities of both layer 2a and the deeper part of the lava pile remain in the range 8 4 the observed increase of layer 2a velocities after 0 2 ma is caused largely by changing crack morphology as opposed to decreasing porosity at 8 porosity an increase of af from 0 003 to 0 1 accounts for an increase of velocity from 3 to 5 km s an empirical log linear relationship between permeability and v suggests a relationship between and af as well this relationship between and v can be explained if the permeability coefficient o is a closely constrained quadratic function of the area of asperity contact af af is thus the primary variable affecting both seismic velocities and the permeability of the uppermost oceanic crust

an ocean bottom gravity study of the southern juan de fuca ridge we use seafloor and sea surface gravity data to model density structure along the southern juan de fuca ridge we determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models of 63 seafloor gravity measurements obtained 42 observations were distributed over the 3 km left lateral overlapping rift zone orz located at latitude 45 03 n and separating the juan de fuca ridge into the cleft and vance segments a 21 measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44 52 n a region of linear ridge geometry on the cleft segment these seafloor gravity measurements which are sensitive to shallow crustal density variations were augmented by roughly 800 km of sea surface gravity measurements using a bathymetry bouguer anomaly analysis of the seafloor gravity data we determined the average density of the shallow roughly upper 2 km oceanic crust at these two locations to be 2630 kg m 3 50 kg m 3 within the uncertainties of the density determinations there is no difference between the average shallow oceanic crustal density at the linear cleft segment and at the cleft vance orz using the seafloor measurements we modeled the study area s underlying density to provide constraints on its fine scale structure localized porosity of up to 17 within the upper 500 m of oceanic crust layer 2a provide a possible explanation for the observed gravity anomalies at the cleft segment two dimensional density models of the crust underlying the linear cleft segment 44 52 n show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis for the cleft vance orz three dimensional modeling suggests low density material between the rifts in agreement with thickened layer 2a from seismic observations in contrast layer 2a variations do not explain the low density inferred at the northern end of the cleft segment since seismic observations suggest a thin layer 2a the magmatic activity recently observed in this region may have a distinct deeper source

spreading rate dependence of gravity anomalies along oceanic transform faults gravity and bathymetry data from transform fault systems associated with mid ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate and fast spreading ridges if confirmed this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid ocean ridges mid ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge slow spreading mid ocean ridge segments exhibit significant crustal thinning towards transform and non transform offsets1 2 3 4 5 6 7 8 9 10 11 12 which is thought to arise from a three dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1 2 4 5 6 7 9 10 12 in contrast fast spreading mid ocean ridges are characterized by smaller segment scale variations in crustal thickness which reflect more uniform mantle upwelling beneath the ridge axis13 14 15 here we present a systematic study of the residual mantle bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate previous studies have shown that slow slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1 2 4 6 but our analysis reveals that intermediate and fast slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments this finding indicates that there is a mass deficit at intermediate and fast slipping transform faults which could reflect increased rock porosity serpentinization of mantle peridotite and or crustal thickening the most negative anomalies correspond to topographic highs flanking the transform faults rather than to transform troughs where deformation is probably focused and porosity and alteration are expected to be greatest indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed this finding in turn suggests that three dimensional magma accretion may occur near intermediate and fast slipping transform faults

a towed marine gravity meter for near bottom surveys gravity is measured presently on the sea surface and on the sea floor surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer bottom measurements recover this resolution but suffer from poor coverage because of the time needed for each measurement we have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom this gravity sensor is tethered to a ship and towed just above the sea floor the instrument consists of a lacoste and romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability we have tested it in a survey in the san diego trough a 1000 m deep sedimented valley in the pacific ocean in the california continental borderlands multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mgal the new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth

a new model for submarine volcanic collapse formation 1 collapse pits and an associated suite of collapse related features that form in submarine lava flows are ubiquitous on the global mid ocean ridge crest collapse pits the lava tube systems they expose and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100 300 m thick low velocity zone observed at intermediate to fast spreading mid ocean ridges this horizon of low density high porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust in a recent survey of the east pacific rise at 9 37 n we used photographs video and observations from the submersible alvin and dsl 120a side scan data to determine that 13 of the 720 000 m2 of seafloor imaged had foundered to form collapse pits in 98 of the images collapse pits occurred in lobate flows and the rest in sheet flows on the basis of our observations and analyses of collapse features and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean we develop a detailed multistage physical model for collapse formation in the deep ocean in our model lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam the seawater is transformed to vapor at temperatures above 480 c with a 20 times expansion in volume bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow creating gas filled cavities at magmatic temperatures fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point as the vapor pocket cools the pressure within it drops causing a pressure gradient to develop across the upper crust the pressure gradient often causes the roof crust to collapse during cooling though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava alternatively drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars post eruption seismicity lava movement or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption

the importance of planolites in the cambrian substrate revolution abstract early cambrian subtidal shelf substrates were characterized by low water content and steep chemical gradients conditions likely facilitated by the presence of microbial mats as reflected by an abundance of microbially mediated sedimentary structures in lower cambrian strata such substrate conditions would have been unfavourable for burrowing by benthic metazoans a combination of environmental restrictions and a lack of adaptations to vertical burrowing likely prevented most benthic metazoans from burrowing infaunally in early cambrian subtidal shelf substrates the eventual acquisition of burrowing adaptations by benthic metazoans later in the cambrian promoted an increase in the depth and intensity of bioturbation and initiated a transition toward well hydrated substrates in which extensive infaunal activity was possible siliciclastic units of the lower cambrian succession in the white inyo mountains eastern california contain abundant horizontal bioturbation on bedding planes as documented by bedding plane bioturbation indices but little vertical bioturbation as shown by ichnofabric indices and x radiography planolites a simple horizontal trace fossil represents the dominant type of bioturbation in these units planolites is found in a range of diameters indicating that more than one species of tracemaker likely produced this type of trace although these planolites do not have a vertical component their abundance on bedding planes indicates that the activities of planolites tracemakers had a significant impact on subtidal shelf substrates represented by lower cambrian units in the white inyo mountains early in the cambrian substrate revolution

porosity of the upper edifice of axial seamount seamounts are not solid basalt structures but have relatively high porosities in their upper crustal sections at axial seamount off the coast of oregon united states we used on bottom gravity measurements with a bell gravity meter within deep sea submersible alvin to determine a porosity of 31 for the uppermost 100 m of the edifice the southwestern caldera wall has a porosity of 22 and the caldera floor has a slightly higher porosity of 33 seafloor observations and models indicate that these high porosities result from large scale structural features such as lava tubes and cracks large lava drain backs or regions of open pillow basalts in the near subsurface these high porosity zones can affect the subsurface permeability and models of hydrothermal upflow zones explain observed localized gravity anomalies the variety of hydrothermal alteration hydrothermally active areas and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano

hydrothermal activity and the evolution of the seismic properties of upper oceanic crust in order to investigate the impact of off axis hydrothermal circulation on changes of the seismic properties of upper oceanic crust layer 2a we performed an extensive geophysical survey on the eastern flank of the east pacific rise at 14 s seismic refraction and heat flow data were obtained along a 720 km long and 25 to 40 km wide corridor covering thinly sedimented seafloor created since 8 5 ma the seismic data yield a seismic velocity of 2 9 km s at the top of 0 5 m y old basement rocks within about 8 m y the velocity increases gradually to a value of mature oceanic crust 4 3 km s heat flow data derived from 43 in situ thermal conductivity and 86 geothermal gradient measurements suggest that an open hydrothermal circulation system persists for at least 6 7 m y in crust older than 7 ma regional heat flow is close to values predicted by plate cooling models suggesting that hydrothermal circulation is going to cease considering published dating of alteration minerals it appears that the permeability of uppermost oceanic crust has decreased to values insufficient to promote a vigorous hydrothermal circulation within 10 15 m y this idea may explain why seismic velocities in the pacific ocean have not changed significantly in igneous crust older than 8 10 ma in regions where juvenile and consistently hot crust is buried rapidly by sediments the evolution of the seismic properties is quite different velocities increase rapidly and reach values of mature oceanic crust within 1 2 m y we therefore favor a model where basement temperature is governing the evolution of the seismic properties of upper oceanic crust stephen and harding 1983 rohr 1994

on bottom gravity profile across the east pacific rise crest at 21 north twenty on bottom gravity stations were taken with a lacoste romberg model g gravity meter inside the submersible alvin over a 6 km long cross strike profile which straddles the east pacific rise spreading center at 21 n the residual bouguer anomaly shows a 1 5 mgal low with a total half width of 1 7 km centered about the spreading axis and hydrothermal vents uncertainties in the ocean depth are believed to contribute a standard error of about 0 3 to 0 4 mgal for the gravity data the observed elevation factor of 0 1552 mgal m indicates that the upper ocean crust bulk density is 2 62 0 09g cm3 on the other hand 90 rock samples give a density of 2 90 0 01g cm3 this suggests that the largescale porosity of the upper crust is 14 percent the gravity anomaly was inverted to find the ideal body with the smallest maximum density contrast which satisfies the data and their associated errors this body extends from the sea floor to 2 500 m depth and has a density contrast of 0 043g cm3 if the density contr

quantitative estimate of heat flow from a mid ocean ridge axial valley raven field juan de fuca ridge observations and inferences a systematic heat flow survey using thermal blankets within the endeavour segment of the juan de fuca ridge axial valley provides quantitative estimates of the magnitude and distribution of conductive heat flow at a mid ocean ridge with the goal of testing current models of hydrothermal circulation present within newly formed oceanic crust thermal blankets were deployed covering an area of 700 by 450 m in the raven hydrothermal vent field area located 400 m north of the main endeavour hydrothermal field a total of 176 successful blanket deployment sites measured heat flow values that ranged from 0 to 31 w m 2 approximately 53 of the sites recorded values lower than 100 mw m 2 suggesting large areas of seawater recharge and advective extraction of lithospheric heat high heat flow values were concentrated around relatively small hot spots integration of heat flow values over the raven survey area gives an estimate of conductive heat output of 0 3 mw an average of 0 95 w m 2 over the survey area fluid circulation cell dimensions and scaling equations allow calculation of a rayleigh number of approximately 700 in layer 2a the close proximity of high and low heat flow areas coupled with previous estimates of surficial seafloor permeability argues for the presence of small scale hydrothermal fluid circulation cells within the high porosity uppermost crustal layer of the axial seafloor

age dependent porosity of young upper oceanic crust insights from seafloor gravity studies of recent volcanic eruptions 1 an october 1997 alvin program collected a series of seafloor gravity stations over the february 1996 volcanic eruption site on the northern gorda ridge 46 41 n 126 47 w these measurements yielded estimates of the density and porosity of mid ocean ridge crust in the first few decades following formation the estimated crustal density for this pillow eruption of 2270 260 kg m3 is very low but in good agreement with the estimated densities of other recent seafloor volcanic flows rock matrix densities measured from hand samples retrieved from the study area give a bulk porosity for this flow of 34 16 this data when included with similar estimates of upper crustal porosity argue for a rapid evolution of upper crustal porosities from very high values 30 for recent eruptions to much lower values 10 15 where the seafloor is older than 0 5 million years these variations of density and porosity are of a magnitude consistent with previously observed changes in seismic compressional wave velocity and permeability of ocean crust

influence of pressure and mineralogy on seismic velocities in oceanic gabbros implications for the composition and state of the lower oceanic crust 1 we have analyzed the relationship between p wave velocities measured at pressures of 40 100 and 200 mpa and modal mineralogy in oceanic gabbro samples from ocean drilling program holes 735b 894g and 923a at all pressures increasing velocities correspond to increasing pyroxene contents and decreasing alteration phyllosilicate and amphibole content but indicate little or no variation of olivine content in part because the olivine compositions are in the range fo65 to fo73 a voigt reuss hill inverse model reveals that the effective bulk density and elastic moduli of olivine in the gabbros are low even relative to fo73 possibly because the olivine grains contain ubiquitous networks of cracks even if the cracks are not present in situ on average gabbros with velocities typical of seismic layer 3 6 7 7 0 km s 1 contain 5 15 alteration products including 5 15 amphibole and 0 5 phyllosilicates these results suggest that the lower crust is on average slightly to moderately altered

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

high resolution bathymetric surveys using scanning sonars lava flow morphology hydrothermal vents and geologic structure at recent eruption sites on the juan de fuca ridge the coaxial and cleft segments of the juan de fuca ridge have isolated chronic high temperature and focused hydrothermal vent sites both segments also have experienced recent volcanic eruptions which produced extensive ephemeral low temperature and diffuse hydrothermal venting to study the geologic setting of these sites high resolution bathymetric surveys at eight locations on the coaxial and cleft segments were collected between 1993 and 1999 two 675 khz scanning sonar systems were used mesotech on the submersible alvin and imagenex on the remotely operated vehicle jason the bathymetry from these surveys can be gridded at a scale of 2 4 m and contoured at 1 m and thus can resolve many fine scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface bathymetric data at this resolution are particularly useful for identifying geologic features related to diking faulting and lava flow emplacement for example the high resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length and their extrusion rate is low enough that no significant drainback occurs in contrast lobate sheet flows are formed by short lived high effusion rate eruptions in which no localization of output occurs along the eruptive fissure and inflation is quickly followed by drainback resulting in extensive collapse features however if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time it can create lava rises up to 25 m high with distinctive structure and morphology the scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites the fine scale bathymetry establishes that all the hydrothermal vent sites studied at the coaxial and cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor furthermore the fine scale morphology of recent lava flows can be used as a qualitative indication of eruption duration

continuous near bottom gravity measurements made with a bgm 3 gravimeter in dsv alvin on the east pacific rise crest near 9 31 n and 9 50 n a bell bgm 3 gravimeter has been used to collect continuous underway near bottom 3 to 10 m altitude gravity measurements from the deep diving submersible dsv alvin during surveys on the east pacific rise epr crest near 9 31 n and 9 50 n closely spaced 20 to 30 m gravity measurements were made along transects up to 8 km long in both regions repeatability of measurements made at the same location on different dives is 0 3 mgal along track spatial resolution of anomalies is 130 160 m with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible the average upper crustal density of the ridge crest determined from the relationship between depth and free water gravity anomalies varies greatly between 9 31 n and 9 50 n average upper crustal densities of 2410 kg m 3 for the 9 50 n area and 2690 kg m 3 for the 9 31 n area were calculated the different densities are not due to differing geometry of the layer 2a 2b boundary or a regional cross axis gravity gradient differences in porosity of the shallow crustal rocks or a difference in the proportion of low density extrusives to higher density dikes and sills within layer 2a in these two areas are the likely causes of the different upper crustal densities bouguer gravity anomalies near the epr axis are primarily small amplitude 0 5 2 mgal are a few hundred meters across and appear to be lineated parallel to the axis larger amplitude bouguer anomalies of up to 4 mgal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor these ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough asct at the same latitude probable sources of the 0 5 to 2 mgal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through layer 2a to the surface a grid survey of the ridge axis near 9 50 n shows bouguer anomalies lineated along the axis suggesting that dike swarms do contribute to the observed bouguer anomalies the along axis continuity of the gravity anomalies is disrupted at a 75 m offset of the asct suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system this initial study confirms the ability to conduct high resolution near bottom continuous gravity measurements from alvin it also provides important information on how the shallow crustal structure of a fast spreading mid ocean ridge develops and how it varies with the surface morphology

density and porosity of the upper oceanic crust from seafloor gravity measurements the exposure of 1300 meters of upper oceanic crust at the blanco fracture zone allows near bottom gravity measurements to determine the in situ density of the seafloor as a function of depth gravity measurements along the north wall of the blanco depression indicate an outcrop density of 2530 kg m for the upper 800 meters of crust and a calculated porosity of 23 the lower 500 meters of crust 800 to 1300 meters below the sea floor has a measured density of 2710 130 kg m and a porosity of 14 these data indicate that most of the extrusive volcanic oceanic crust is highly porous and can act as an aquifer and large scale reservoir for hydrothermal fluids these direct crustal density measurements also support previous interpretations that low seismic velocities observed in layer 2 are due to the high porosity of the upper extrusive section

direct measurements of oceanic crustal density at the northern juan de fuca ridge density and porosity of oceanic crust are critically important to understanding the circulation of hydrothermal fluid at mid ocean ridges to determine these parameters directly we completed a fixed on bottom gravity survey on the endeavour segment of the juan de fuca ridge which is a well studied spreading center that is both hydrothermally and tectonically active a bell aerospace gravity meter mounted within the submersible alvin was used to make gravity measurements from the floor of the axial valley to the summit of the west axial ridge analysis of these data gave a density of 2240 50 kg m 3 for the upper 130 m of crust comparison of the outcrop density with that of recovered rock samples resulted in a calculated porosity of 38 4 forward modeling of the seafloor gravity data indicated that both the valley floor and bounding walls have unusually low density crust that is flanked by higher density crust

storm surge ebb origin of hummocky cross stratified units of the andrews mountain member campito formation lower cambrian white inyo mountains eastern california the andrews mountain member of the campito formation consists of complexly interbedded and regionally metamorphosed feldspathic quartzites siltstones and mudstones of latest precambrian and early cambrian age extensive outcrops of the member occur in the white inyo mountains of eastern california the andrews mountain member accumulated in a shallow water offshore depositional system within the broadly subsiding cordilleran miogeocline the sedimentology of the member suggests that both storm and fairweather processes were involved in its formation the following idealized sequence of sedimentary structures is recognized within individual beds 1 an erosional base with localized flute and load casts 2 fine grained sands containing parallel and low angle cross lamination hummocky cross stratification rare penecontemporaneous deformation and internal channels and discontinuities 3 these free sands grade into silts and very fine sands containing climbing ripples wave current ripples flaser bedding and locally abundant bioturbation and 4 the uppermost portions of the bed grade into bioturbated muds with cruziana ichnofossil assemblages and local occurrences of linguoid symmetrical and interference ripples the sedimentary structures in these sequences reflect deposition by high energy storm events the flows generated by these events are interpreted to be the product of the ebb of large storm surges an inferred scenario of storm deposition can be reconstructed for the andrews mountain member shelf 1 initial erosion and suspension of fairweather sediments in response to long period storm waves 2 as a low pressure system moves onshore and dissipates a sediment charged return bottom flow is generated that flows across the shelf and further erodes and entrains material 3 fine sands are deposited as parallel laminated and hummocky cross stratified units as the competence and capacity of the flow decreases 4 attenuation of the surge leads to low flow regime deposition of climbing ripples and where deposition is within storm wave base wave current ripples and flaser bedding 5 with the resumption of fairweather conditions softbodied benthic communities recolonize muddy substrates which are locally reworked by tidal and oceanic currents rather than depositing sand sheets over extensive areas multiple storm surges are inferred to have moved sand incrementally across the andrews mountain member shelf thinning fining upward and thickening coarsening upward sequences may reflect changes in both the intensity of the flows and the proximity of a local sand source

propagating rift explanation for the tectonic evolution of the northeast pacific the pseudomovie abstract we have developed a new computer graphic technique to use in reconstructing plate tectonic evolution and to make movies illustrating this evolution using this technique we have deciphered the evolutionary history of the juan de fuca area in the northeast pacific and show that a combination of seafloor spreading and rift propagation can explain this seemingly complex area as a simple consequence of the interaction of two rigid plates

gravity modeling of a volcanically active site on the east pacific rise axis abstract seafloor and shipboard gravity observations are used to study the east pacific rise epr axis at 9 48 n a site of recent volcanic activity a constant crustal density model fits the seafloor and shipboard gravity data within expected errors a layered density model consistent with rise axis seismic constraints also fits the gravity data these constant and layered density models show no evidence for a volumetrically large magma chamber and indicate the importance of the shallow crustal density structure to the marine gravity field density variations in the shallow crust may be explained by porosity and thermal variations the average shallow 0 450 m epr 9 48 n crust has a density of 2420 80 kg m 3 significantly lower than other rise axis sites this density is less than molten basalt suggesting that magma buoyancy will not contribute to ascent through the uppermost crust enhancing the formation of volcanic dikes and sills below the layer 2 a 2 b boundary

surficial permeability of the axial valley seafloor endeavour segment juan de fuca ridge 1 hydrothermal systems at mid ocean spreading centers play a fundamental role in earth s geothermal budget one underexamined facet of marine hydrothermal systems is the role that permeability of the uppermost seafloor veneer plays in the distribution of hydrothermal fluid as both the initial and final vertical gateway for subsurface fluid circulation uppermost seafloor permeability may influence the local spatial distribution of hydrothermal flow a method of deriving a photomosaic from seafloor video was developed and utilized to estimate relative surface permeability in an active hydrothermal area on the endeavour segment of the juan de fuca ridge the mosaic resolves seafloor geology of the axial valley seafloor at submeter resolution over an area greater than 1 km2 results indicate that the valley walls and basal talus slope are topographically rugged and unsedimented providing minimal resistance to fluid transmission elsewhere the axial valley floor is capped by an unbroken blanket of low permeability sediment resisting fluid exchange with the subsurface reservoir active fluid emission sites were restricted to the high permeability zone at the base of the western wall a series of inactive fossil hydrothermal structures form a linear trend along the western bounding wall oriented orthogonal to the spreading axis high temperature vent locations appear to have migrated over 100 m along ridge strike over the decade between surveys while initially an expression of subsurface faulting this spatial pattern suggests that increases in seafloor permeability from sedimentation may be at least a secondary contributing factor in regulating fluid flow across the seafloor interface

porosity of very young oceanic crust from sea floor gravity measurements porosity and density are two of the most important but under determined parameters in studies of igneous oceanic crust lacking quantitative measurements of crustal densities but with plausible expectations of substantial variation with age previous investigators have used estimates that have varied widely to address this problem more rigorously a recent alvin program obtained 133 on bottom gravity stations to calculate crustal densities for three recent volcanic eruptions on the juan de fuca ridge in the summer of 1995 a bell aerospace bgm 3 gravity meter was mounted in alvin and made fixed station gravity measurements on 11 transect lines from three distinct volcanic environments including 1 the recent coaxial segment volcanic eruption that occurred in 1993 2 two nearby flows with less well determined ages but dated post 1981 and 3 several older adjacent areas on the juan de fuca axial ridge densities for the recent eruptive units ranged between 2280 and 2450 kg m while the upper 50 meters of the older unfractured surrounding flows were approximately 2800 kg m laboratory grain densities measured from rocks collected in the study area allow crustal porosities to be calculated which are anomalously high for the recent flows in comparison to the surrounding axial ridge porosities for the three youngest flows range between 29 36 while the adjacent unfractured regions of the surrounding axial region had porosities near 10 two localized regions of intense surficial crustal fracturing in older crust located just south of the 1993 eruption site also had high porosities of 26 28 this high variability indicates that the evolution of crustal porosities from young to older oceanic crust is not related only to the collapse and infilling of macroscopic voids but that tectonic activity can significantly modify the porosity of older crustal structures

upper crustal densities derived from sea floor gravity measurements northern juan de fuca ridge a transect of sea floor gravity stations has been analyzed to determine upper crustal densities on the endeavour segment of the northern juan de fuca ridge data were obtained using alvin along a corridor perpendicular to the axis of spreading over crustal ages from 0 to 800 000 years calculated elevation factors from the gravity data show an abrupt increase in density with age distance for the upper 200 m of crust this density change is interpreted as a systematic reduction in bulk porosity of the upper crustal section from 23 for the axial ridge to 10 for the off axis flanking ridges the porosity decrease is attributed to the collapse and filling of large scale voids as the abyssal hills move out of the crustal formation zone forward modeling of a plausible density structure for the near axis region agrees with the observed anomaly data only if the model includes narrow along strike low density regions adjacent to both inner and outer flanks of the abyssal hills the required low density zones could be regions of systematic upper crustal fracturing and faulting that were mapped by submersible observers and side scan sonar images and whose presence was suggested by the distribution of heat flow data in the same area

dynamics and navigation of autonomous underwater vehicles for submarine gravity surveying we investigated the effect of autonomous underwater vehicle auv dynamics and navigation on underway submarine gravimetry our research was motivated by the need to obtain spatially dense marine gravity measurements close to the source of subkilometer scale geologic features in the shallow oceanic crust such measurements have been previously obtained for instance with piloted submarines and towed sleds however the high cost and in the case of on bottom measurements poor spatial sampling preclude routine acquisition of these measurements continuous underway gravity surveys with auvs is a compelling cost effective option but this method requires separating the auv accelerations from the measured gravity we show that auvs with a large distance between the center of buoyancy and the center of gravity have lower vertical accelerations than torpedo shaped auvs and consequentially are better suited for underway gravity surveys furthermore state estimators which combine sensor measurements and models of the vehicle s motion provide superior estimates of the vehicle s vertical accelerations than methods used in previous underway submarine gravity surveys we simulated the use of these navigation methods in detecting dike swarms at the east pacific rise analysis showed that we can shorten filters used in reducing gravity data and consequentially provide improved measurements of the free water anomaly with a minimal detectable spatial wavelength approximately 65 lower than previously reported results

direct measurement of magnetic reversal polarity boundaries in a cross section of oceanic crust magnetic field measurements made by submersible define the cross sectional geometry of a magnetic polarity rever sal boundary and the vertical variation of crustal magnetization in upper oceanic crust measured polarity boundaries show a systematic pattern of shallow dip towards the spreading axis within the upper extrusive lavas and steeper dip in the lower extrusive lavas this geometry is a consequence of the em placement of extrusive lava at a midocean ridge reversal boundary geometry and magnetization estimates are used to calculate the magnetic contribution of the extrusive lava se quence to the overlying marine magnetic anomaly signal from the forward modeling the highly magnetized extrusive lavas contribute the majority 50 75 of the observed sea surface magnetic anomaly consistent with the extrusive crust forming the primary source layer for young marine magnetic anomalies