a sea floor spreading event captured by seismometers two thirds of earth s surface is formed at mid ocean ridges yet sea floor spreading events are poorly understood because they occur far beneath the ocean surface at 9 50 n on the east pacific rise ocean bottom seismometers recently recorded the microearthquake character of a mid ocean ridge eruption including precursory activity a gradual ramp up in activity rates since seismic monitoring began at this site in october 2003 suggests that eruptions may be forecast in the fast spreading environment the pattern culminates in an intense but brief 6 hour inferred diking event on 22 january 2006 followed by rapid tapering to markedly decreased levels of seismicity

segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

an early permian coastal flora dominated by germaropteris martinsii from basinal sediments in the midland basin west texas abstract fossils found in cores from wells in the midland basin of west texas include several kinds of terrestrial plants and a variety of marine animal remains depositional settings ranged from basin slope to deep water basin floor hence the presence of land plants was unexpected the fossil plant assemblage is depauperate dominated by germaropteris martinsii a permian age peltasperm other specimens include the peltasperm supaia sphenopteris germanica axes of uncertain affinity and incertae sedis remains presumed to be terrestrial plants fossil plants are found predominantly in fine grained siliceous mudrocks between coarser grained calcareous floatstones and wackestones packstones interpreted as debrites and turbidites suggesting that the plants were carried from land by surface currents before sinking to the basin floor and being buried by slowly accumulating hemipelagic sediment specimens were examined from drillcores in 14 wells spanning an interval from the lower wolfcamp through the lower leonard this record of g martinsii in lower permian wolfcamp rocks is among the earliest occurrences of these plants which have been found most abundantly in upper permian strata of western europe

how many vent fields new estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations abstract decades of exploration for venting sites along spreading ridge crests have produced global datasets that yield estimated mean site spacings of 12 220 km this conclusion demands that sites where hydrothermal fluid leaks from the seafloor are improbably rare along the 66 000 km global ridge system despite the high bulk permeability of ridge crest axes however to date exploration methods have neither reliably detected plumes from isolated low temperature particle poor diffuse sources nor differentiated individual closely spaced clustered within a few kilometers sites of any kind here we describe a much lower mean discharge spacing of 3 20 km revealed by towing real time oxidation reduction potential and optical sensors continuously along four fast and intermediate rate 55 mm yr spreading ridge sections totaling 1470 km length this closer spacing reflects both discovery of isolated sites discharging particle poor plumes 25 of all sites and improved discrimination at a spatial resolution of 1 km among clustered discrete and diffuse sources consequently the number of active vent sites on fast and intermediate rate spreading ridges may be at least a factor of 3 6 higher than now presumed this increase provides new quantitative constraints for models of seafloor processes such as dispersal of fauna among seafloor and crustal chemosynthetic habitats biogeochemical impacts of diffuse venting and spatial patterns of hydrothermal discharge

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

seismic architecture of a lower cretaceous platform to slope system santa agueda and poza rica fields mexico two three dimensional seismic data sets over the albian western golden lane margin and time equivalent basinal deposits of poza rica field allowed us to investigate the linked architecture of a steep sided carbonate platform el abra formation and a thick accumulation of redeposited carbonate sediment at the toe of the slope and in the basinal area tamabra formation regional seismic cross sections show that the most aggrading albian platform has an eroded platform top a scalloped margin and a channelized slope that are equivalent to a 20 km 12 4 mi wide westward thinning thick toe of slope apron made of chaotic contorted mounded moderate to high amplitude reflections detailed reflection geometries in the albian toe of slope and basinal deposits consist of chaotic to short discontinuous low amplitude reflection at the toe of the slope of the golden lane platform laterally changing to a discontinuous mounded shingling reflection which ultimately turns into high amplitude parallel reflections we interpret this lateral change to reflect the seismic signature of the change from the block and debris flow dominated toe of slope area to debris flow and concentrated density flow deposits in the basin that ultimately grade laterally into pelagic deposits on a flattened seismic slice mounded reflections correspond to lobate to fan shaped seismic events several kilometers wide that are interpreted as a carbonate basin floor fan comparison between core and seismic data shows a dominance of debris flows in the lower two albian sequences albian 1 and albian 2 that grade vertically into more lobate concentrated density flows and turbidites in the upper two albian sequences albian 3 and albian 4 seismic data used in this study combined with core observations do not support the interpretation of the albian tamabra formation being of shallow water origin seismic features identified as basin floor fan channel and debris flow deposits have a shape and size that are similar to those of other redeposited basinal carbonate deposits elsewhere the seismic architecture shows that the poza rica field is a typical example of thick accumulation of grainy porous carbonate deposits in a basinal setting this example shows the potential of a large hydrocarbon accumulation in a tectonically modified stratigraphic trap around shallow water carbonate platforms

waxing and waning volcanism along the east pacific rise on a millennium time scale microbathymetric maps of the southern east pacific rise reveal subtle field relations between volcanic features and provide new insight on seafloor spreading processes along one of the shallowest and broadest sections of ridge at 17 28 s lavas have erupted from a fissure system and flooded the axis through a network of lava tubes and lava channels along the neighboring ridge segment at 18 15 s the axial area has subsided and formed a broad tectonized trough a swath of newly accreted crust has since widened that trough late stage volcanism consists of small circular pillow mounds we propose that these contrasting eruptive styles reflect the waxing and waning phases of a common magmatic evolution spanning a few millennia

high resolution surveys along the hot spot affected galapagos spreading center 2 influence of magma supply on volcanic morphology the galapagos spreading center gsc at 89 95 w exhibits large gradients in magma supply at a relatively constant intermediate spreading rate making this area an ideal natural laboratory to study the effects of magma supply on volcanism at seafloor spreading ridges prior work shows that the gsc develops from axial valley to shallow axial rise and a shallow magma sill much like a typical fast spreading ridge as the contribution of the hot spot increases the volcanic morphology varies with magma supply in a predictable manner that we divide into three terrains based on the characteristic style of volcanic emplacement and edifice construction within each terrain the volcanic cone terrain comprises most of the gsc and is characterized by prominent volcanic cones within a 1 km wide and 100 m deep axial graben approaching the area of maximum mantle plume influence at 91 w the gsc axis lies along an elevated axial rise split by a 1 km wide and 100 m deep axial graben and the style of volcanism shifts to axial volcanic ridge terrain characterized by axis elongate low relief ridges of pillow lava the lava channel terrain comprises only one segment on either side of the maximum magma supply at 91 w where sheet lava flows and lava channels are relatively widespread a general lengthening of seafloor fissures with increasing magma supply suggests a greater tendency toward linear source eruptions in agreement with the volcanic observations these results suggest that magma supply rather than magma chamber depth or rate of tectonic extension is the primary influence on lava morphology hence eruptive processes at seafloor spreading ridges in general in both the axial volcanic ridge and lava channel terrains a single prominent volcanic cone exists within each volcanic segment suggesting a segment centered magma focusing

last interglacial and holocene altithermal environments in south africa and namibia micromammalian evidence abstract climatic interpretations are derived indirectly from evidence for vegetation changes based on the remains of micromammals small rodents and shrews from archaeological sites thus far only border cave has provided evidence apparently referable to the earlier altithermal in oxygen isotope substage 5e vegetation zones and by implication climatic zones apparently extended 3 and more south of their present southern limits rainfall may have been up to 100 more than it is today and much more seasonal with drier winters mean annual temperatures were perhaps about 1 c higher on other evidence eight other sites provide evidence concerning conditions during the holocene altithermal 7000 6000 yr b p the evidence seems to indicate relatively low annual precipitation in the main summer rainfall region and an extended rainfall season in the southwestern winter rainfall region it is less clear whether or not annual rainfall was relatively high at this time in the latter region although in the namib desert conditions were apparently wetter this tends to agree with the dry period analogue of cockcroft et al 1987 and would imply reduced seasonality of precipitation throughout the region in contrast to the situation during the earlier altithermal apparent conflict with palynological evidence may be explainable but requires further examination

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

continuous mantle melt supply beneath an overlapping spreading center on the east pacific rise tomographic images of upper mantle velocity structure beneath an overlapping spreading center osc on the east pacific rise indicate that this ridge axis discontinuity is underlain by a continuous region of low p wave velocities the anomalous structure can be explained by an approximately 16 kilometer wide region of high temperatures and melt fractions of a few percent by volume our results show that oscs are not necessarily associated with a discontinuity in melt supply and that both osc limbs are supplied with melt from a mantle source located beneath the osc we conclude that tectonic segmentation of the ridge by oscs is not the direct result of magmatic segmentation at mantle depths

crustal fissuring and its relationship to magmatic and hydrothermal processes on the east pacific rise crest 9 12 to 54 n the deep towed argo i optical acoustical vehicle and a geographic information system gis have been used to establish the abundance widths and spatial distribution of fissures as well as the relative age distribution of lavas along the narrow 500 m wide axial zone of the east pacific rise epr from 9o12 to 9o54 n on a second order scale 78 km long wider but less numerous fissures are found in the northern portion of the survey area this changes to narrower more abundant fissures in the south a profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike near 9o49 9o35 and 9o15 n where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle filling and covering older fissures on a fourth order scale 5 15 km long the mean density of fissuring on a given segment is greater where relative axial lava age is greater fissure density also correlates with hydrothermal vent abundance and type increased cracking toward segment tips is observed at the second order scale whereas fourth order segments tend to be more cracked in the middle cracking on a fourth order scale may be driven by the propagation of dikes rather than by the far field plate stresses the above relations constrain the model of haymon et al 1991 in which individual fourth order segments are in different phases of a volcanic hydrothermal tectonic cycle

magmatic subsidence of the east pacific rise epr at 18 14 s revealed through fault restoration of ridge crest bathymetry 1 the fault restoration technique of de chabalier and avouac 1994 is applied to an ultra high resolution bathymetry data set from the east pacific rise epr at 18 14 s fault offsets are calculated and subtracted from the original seafloor bathymetry to examine the morphology of the unfaulted seafloor surface within an area encompassing the ridge axis 400 1600 m in dimension the restored topography reveals a gently sloping seafloor 200 m wide which slopes 5 inward toward the spreading axis we attribute this inward sloping seafloor to subsidence within the axial trough due to subsurface magmatic deflation the vertical deformation field extracted from the bathymetry is used to characterize the ridge axis fault population present in the area median fault throws 9 m for inward facing and 8 m for outward facing faults are comparable to values measured for nearby mature abyssal hill fault populations suggesting a genetic link however median fault spacings 70 and 46 m are an order of magnitude smaller and estimated total extensional strain is 3 4 greater than values measured for ridge flank faults these differences indicate the axial fault population at 18 14 s cannot be rafted onto the ridge flanks to form abyssal hill faults without significant modification presumably via volcanic burial we attribute the dense faulting observed in this area to slip on axial fissures during subsidence of the crestal region the surface subsidence of a volcanic edifice can be modeled in terms of volume change in the magma source reservoir and volume of magma withdrawn from the reservoir using the relationship derived for a sill like magma body we estimate that the axial depression at 18 14 s could represent magma withdrawal associated with a small number 4 22 of the frequent dike injection and eruption events required to build the upper crust during the evolution of the trough the subsidence volumes represented by the axial topography at 18 14 s are a small percentage of the underlying upper crustal sections 3 4 suggesting that only a minor mismatch between magma recharge and withdrawal from the axial melt lens during ongoing plate separation could account for this pronounced axial depression

crustal thickness and moho character of the fast spreading east pacific rise from 9 42 n to 9 57 n from poststack migrated 3 d mcs data we computed crustal thickness 5740 270 m and mapped moho reflection character using 3 d seismic data covering 658 km2 of the fast spreading east pacific rise epr from 9 42 n to 9 57 n moho reflections are imaged within 87 of the study area average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last 180 ka however individual crustal thickness measurements differ by as much as 1 75 km indicating that the mantle melt delivery has not been uniform third order but not fourth order ridge discontinuities are associated with changes in the moho reflection character and or near axis crustal thickness this suggests that the third order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth order ridge segmentation arises from midcrustal to upper crustal processes in this light we assign fourth order ridge discontinuity status to the debated ridge segment boundary at 9 45 n and third order status at 9 51 5 n to the ridge segment boundary previously interpreted as a fourth order discontinuity our seismic results also suggest that the mechanism of lower crustal accretion varies along the investigated section of the epr but that the volume of melt delivered to the crust is mostly uniform more efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half this south to north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the lamont seamounts

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

volcanic eruptions on mid ocean ridges new evidence from the superfast spreading east pacific rise 17 19 s uniform sediment cover were recovered from lava that buries older faulted terrain the boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector consistent with magmas from two separate reservoirs being erupted in the same event chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments a comparison of our results to global data on submarine mid ocean ridge eruptions suggests consistent dependencies of erupted volume activated fissure lengths and chemical heterogeneity with spreading rate consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths index terms 3035 marine geology and geophysics midocean ridge processes 8414 volcanology eruption mechanisms 8439 volcanology physics and chemistry of magma bodies 3655 mineralogy and petrology major element composition keywords lava flow chemical heterogeneity erupted volume lava morphology side scan sonar

breaking new ground estimates of crack depth along the axial zone of the east pacific rise 9 12 54 n using simple fracture mechanics models the depths of fissures that were observed along the axial zone of the east pacific rise epr from 9 12 to 54 n with the deep towed argo z vehicle are estimated the widest cracks 4 8 m along this fast spreading portion of the epr are the deepest and are spatially correlated with the broadest youngest and most hydrothermally active portions of the ridge crest if the widest deepest cracks are not primarily eruptive they should be most abundant in areas of older lava flows having increased in width with time this is not observed some of the widest cracks are located where the 1991 eruption of the ridge crest occurred at 9 45 52 n and may be deep enough to reach the sheeted dikes of layer 2b these cracks may have tapped melt during the eruption and facilitated the flux of vapor rich hydrothermal fluids through overlying lava flows the narrowest presumably shallowest cracks correspond to the narrowest oldest and least hydrothermally active portions of the ridge crest we interpret the wide deep cracks as primarily eruptive in origin and suggest that they may be associated with the inflation of an axial magma chamber whereas the narrow shallow cracks are interpreted as primarily tectonic and are thought to be associated more with far field plate stresses

three dimensional geological and synthetic seismic model of early permian redeposited basinal carbonate deposits victorio canyon west texas the lower permian outcrops of victorio canyon in the sierra diablo mountains in west texas show undisturbed stratigraphy of carbonate toe of slope and basinal deposits these rocks consist of a vertical stack of carbonate debris flow deposits and hyperconcentrated density flow deposits thick skeletal ooid grainstone deposited as basin floor fans various reworked muddy carbonate deposits and fine siliciclastic siltstones this succession was deposited within five sequences that include the upper hueco formation late wolfcampian through the bone spring formation middle leonardian using a ground based light detecting and ranging generated high resolution digital outcrop model dom aged as a template we mapped and digitized the stratigraphy of the toe of slope and basinal deposits on the 5 cm 2 in precision dom on the basis of the digitized stratigraphic contacts several regional surfaces were constructed and a 3 d geocellular model was built facies information within this model is extrapolated from measured section data using both a kriging algorithm and stochastic simulations using impedance values extracted from a subsurface analog a 3 d impedance model was created for both the kriged and the stochastic models both models incorporate fine scale stratigraphic architecture in addition the stochastic impedance model incorporates spatially correlated noise resulting in more realistic synthetic seismograms three dimensional synthetic seismograms were calculated at 20 40 and 80 hz the reservoir prone facies is skeletal ooid grainstone deposited as a 1 5 km 750 m 0 93 mi 2460 ft basin floor fan up to 15 m 49 ft thick this basin floor fan is subtly imaged in vertical seismic section at a frequency below 80 hz it is however better recognized on time slices with peak frequency as low as 20 hz and even better delineated on horizon slices that parallel the stratigraphy

impact of strong easterly trade winds on carbonate petroleum exploration relationships developed from caicos platform southeastern bahamas abstract brisk persistent easterly trade wind influences define holocene patterns of carbonate sedimentation across the caicos platform southeastern bahamas resultant predictive sedimentary facies models based on trade wind influences are more widely applicable to the exploration for subsurface carbonate plays than are existing models based on the northern bahamas facies patterns which are characterized by gentle trade winds and strong platform margin related oceanic processes swells and tidal currents the caicos platform relationships may be more applicable because many ancient shallow carbonate depositional environments were within the trade wind belts and commonly within broad intracratonic seas that were little influenced by oceanic processes the grainstone dominated caicos platform exhibits reservoir potential over much of its surface in contrast to northern bahamian platforms where oceanic tidal currents or swells and gentle easterly trade winds confine higher energy environments with reservoir potential to platform margins strong easterly trade winds across caicos platform promote widespread holocene platform interior oolitic skeletal and grapestone grainstone bodies on this platform orientations of ooid sand bodies vary depending on preexisting topography water depth and bottom energy shallow subtidal ooid sand shoals orient parallel to these winds ooid sands developed along older shorelines orient parallel to the shorelines but prograde perpendicular into these winds deeper platform interior oolitic sands exist as widespread sheet like deposits these trade winds allow reefs and ooids to coexist in many settings permit isolated linear reefs to flourish on certain leeward platform margins and promote effective off bank transport of carbonate sands that create onlapping grainstone wedges these relationships are very applicable to the rock record strong trade wind influences such as seen on caicos platform better explain the occurrence of cretaceous reef and or oolitic grainstone reservoirs developed well in from platform margins fairway field in east texas black lake field in louisiana than do existing northern bahamian models depositional models based on conditions in the northern bahamian models would predict low energy facies in these platform interior settings a trade wind driven depositional model characterized by strong persistent easterlies also better explains the origin of the onlapping wedge of skeletal grainstones at poza rica oilfield in mexico the caicos platform easterly trade wind driven depositional model should generally apply to any ancient shallow carbonate environment that developed 5 22 north or south of the paleoequator whether or not a platform margin was near by future carbonate exploration or exploitation should always factor in not only geological age but also physiographic latitudinal and climatic setting at a global and local scale

geomorphological variations at hydrothermal sites in the southern mariana trough relationship between hydrothermal activity and topographic characteristics abstract this study presents the first detailed geomorphological characterization of field scale geological features associated with hydrothermal systems in the southern mariana trough using near bottom swath mapping data collected by the autonomous underwater vehicle auv urashima during cruise yk09 08 and dive observation data acquired by the submersible shinkai6500 during cruise yk10 11 the motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area two of the hydrothermal sites in the study area near 12 57 n 143 37 e are located on the active backarc spreading axis the snail and yamanaka sites one is located at the eastern foot of the axial high the archean site and two are located on an off axis knoll about 5 km from the spreading axis the pika and urashima sites the on axis area is divided into tectonically dominant and volcanically dominant zones volcanically dominant zones are characterized by mounds height 5 30 m diameter 250 320 m cut by fissures the snail and yamanaka sites are located adjacent to these fissures and are possibly represented local activity associated with a 4th order segment scale diking event on the basis of comparisons with previously studied cases on the east pacific rise with similar on axis geological characteristics in contrast to the on axis sites the off axis sites show no evidence of faulting the archean site at the foot of the axial high is characterized by a single mound height 50 100 diameter 250 300 m pronounced off axis lava flows and the presence of high amplitude rugged seafloor features the site is located at the top of the mound numerous ridge lines height mainly 2 6 m extend radially from the top of the mound and several chimney like structures up to approximately 6 m high occur on the top and slopes of the mound the pika site is located on the western peak of an off axis knoll and the newly discovered urashima site is located at the northern foot of the western peak of the same knoll the western peak is characterized by bumpy seabed textures formed by numerous smaller scale mounds and ridge lines however the eastern peak has a very smooth top and slope and shows no signs of hydrothermal activity numerous mounds heights 5 75 m diameters 50 350 m are developed on the comparatively gentle slope of the knoll in contrast to the numerous ridge lines height mainly 1 6 m developed on the relatively steep slopes of the knoll on the basis of the associated geomorphological features the three off axis sites archean pika and urashima were identified as localities created by relatively long term large scale hydrothermal activity as compared with sites in the on axis area the sustained activity at off axis sites appears closely related to an off axis upwelling magma system the three off axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting these areas are characterized by numerous ridge lines conical mounds and bumpy seabed texture whereas the on axis sites are characterized by the absence of ridge lines and the presence of white smoker and shimmering observed on dome shaped pillow mounds hence the distribution of ridge lines mound morphology and bumpy seabed texture is likely to correlate with hydrothermal activity

high resolution surveys along the hot spot affected galapagos spreading center 3 black smoker discoveries and the implications for geological controls on hydrothermal activity 1 to explore effects of hot spots on mid ocean ridge hydrothermal systems we conducted nested sonar hydrothermal plume and near bottom photographic surveys along the portion of the galapagos spreading center gsc influenced by the galapagos hot spot from longitude 95 89 5 w we report the first active high temperature black smokers to be found on the gsc at longitudes 94 4 5 w and 91 56 2 54 3 w describe two areas of recently inactive smokers at longitudes 91 23 4 23 7 w and 91 13 8 w and document an older inactive site at longitude 90 33 4 w all imaged vents issue either from dike induced fissures along linear axial volcanic ridges and collapses or from a caldera magmatic control of hydrothermal systems also is revealed by spatial clustering of plumes within the topographically elevated middles of volcanic ridge segments with inferred centralized melt supply in searched areas smokers are more typical than diffuse flow vents but total gsc plume incidence is half of that expected from the spreading rate why dike fed fissures provide permeable pathways for efficient hydrothermal extraction of magmatic heat but cones without calderas do not among many point source cones surveyed only the two with calderas had detectable plumes possibly dominance of point source over linear source melt delivery on the gsc decreases plume incidence also similar maturities of observed vents and their host lava flows indicate that hydrothermally active volcanic segments along the western gsc are contemporaneously in a waning phase of volcanic hydrothermal activity perhaps ridge hot spot interaction produces melt pulses that drive near synchronous volcanic hydrothermal activity on the volcanic segments spanning the hot spot during active periods hydrothermally active dike fed fissures and calderas may be more abundant than we currently observe

play analysis and leading edge oil reservoir development methods in the permian basin increased recovery through advanced technologies the permian basin of west texas and southeast new mexico remains an important oil producing province accounting for 17 of united states production 327 million bbl in 2002 with a resource base of such size increased understanding of reservoir geology and improved use of enhanced recovery practices in the basin can have a substantial impact on united states oil production thirty two oil plays covering both the texas and new mexico portions of the permian basin were defined on the basis of reservoir stratigraphy lithology depositional environment and structural and tectonic setting one thousand three hundred and thirty nine significant sized reservoirs cumulative production of 1 million bbl 1 59 105 m3 of oil through 2000 were assigned to a geologic play cumulative production from these reservoirs was 28 9 billion bbl 4 59 109 m3 or 95 of the basin s total examples of successful reservoir development practices are listed by play because methods demonstrated to work well in one reservoir should be applicable to other reservoirs in the play the permian basin is dominantly a carbonate province carbonate reservoirs account for 75 of total oil production clastics 14 mixed clastics and carbonates 8 and chert 3 the plays having the largest cumulative production are the northwest shelf san andres platform carbonate play 4 0 billion bbl 6 31 108 m3 the leonard restricted platform carbonate play 3 3 billion bbl 5 25 108 m3 the pennsylvanian and lower permian horseshoe atoll carbonate play 2 7 billion bbl 4 29 108 m3 and the san andres platform carbonate play 2 2 billion bbl 3 42 108 m3 the permian system dominates production accounting for 73 of cumulative production followed by the pennsylvanian 13 and the ordovician 6 the estimated remaining reserves from that component of the resource base that is already discovered and producing is 3 25 billion bbl 5 17 108 m3

carnivore size and quaternary climatic change in southern africa abstract the relationship between carnassial length and latitude south is analyzed for 17 african carnivore species to determine if individuals tend to be larger in cooler climates as predicted by bergmann s rule with modern data in support middle and late quaternary temperatures might then be inferred from mean carnassial length in fossil samples such as those from equus cave elandsfontein sea harvest duinefontein and swartklip in the cape province of south africa one problematic aspect of the study is the use of carnassial length and latitude as necessary but imperfect substitutes for body size and temperature respectively for some species another difficulty is the relatively small number of available modern specimens combined with their uneven latitudinal spread still in 14 of the species carnassial length does tend to increase with latitude south while mean carnassial length in the same species tends to be greater in those fossil samples which accumulated under relatively cool conditions as inferred from sedimentologic palynological or geochemical data given larger modern samples from a wide variety of latitudes refinement of the mathematical relationship between carnassial length and latitude in various species may even permit quantitative estimates of past temperatures in southern africa

permian wolfcamp limestone reservoirs powell ranch field eastern midland basin deep water carbonate channel reservoirs form important oil reservoirs along the toe of the eastern shelf of the permian basin in west texas in northwestern glasscock county these wolfcamp reservoirs are leonardian early permian in age and define high energy channels incised into surrounding carbonate detritus and basinal shale porous grain flow material filling these channels along with encasing detritus was derived from the shallow shelf located six miles to the east reservoirs are in packstone and grainstone facies and have significant interparticle and moldic porosity relevant exploration began in the 1960s but expanded slowly thereafter due to lack of success caused by complex patterns of channel occurrence results of a three dimensional 3 d seismic survey conducted in 1990 have greatly enhanced the identification and mapping of productive channels in the powell ranch field complex wells in this complex are capable of flowing 400 1200 bbl of oil per day and have reserves ranging from 0 2 to 1 3 mbo the new 3 d data have improved the relevant geologic model and dramatically increased rates of drilling success application of such data to this setting offers a potential model for other parts of the permian basin

do sea level variations influence mid ocean ridge magma supply a test using crustal thickness and bathymetry data from the east pacific rise abstract recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the pleistocene influence mantle melting and volcanic eruptions at mid ocean ridges with models predicting variations in oceanic crustal thickness and seafloor bathymetry linked to sea level change analyses of seafloor bathymetry have found evidence of significant spectral energy at frequencies consistent with milankovic cycles of 1 23 1 41 and 1 80 1 120 ka 1 however other studies emphasize the need for crustal thickness observations to test the sea level hypothesis here we investigate the hypothesis of climate driven periodicity in mid ocean ridge magmatism through analysis of a unique bathymetry and crustal thickness dataset derived from a 3d multi channel seismic investigation of the east pacific rise from 9 42 to 57 n crustal thickness data spans the last 235 ka in age and reveals three axis parallel zones of 200 800 m thicker crust the amplitude and spacing of these thick crust ridges which are most prominent on the east flank of the ridge are consistent with predictions of sea level modulated mantle melting similarly spaced ridges are apparent in the longer duration 470 ka seafloor bathymetry data spectral analysis of these datasets shows peaks centered near 1 80 ka 1 and locally near 1 41 ka 1 on the east flank in both bathymetry and crustal thickness west flank spectral results show intermittent peaks near 1 100 ka 1 and 1 41 ka 1 in crustal thickness and no coherent peak frequencies in bathymetry data we attribute differences between the east and west flank to the impacts of spatially variable asymmetric spreading and small changes in the locus of accretion observed half spreading rates are dominantly faster to the east with small ridge jumps transferring crust from the west flank lagged cross correlations between sea level and crustal thickness indicate a maximum when the latter is lagged by 45 ka which align thick crust zones with the 100 ka periods of lower sea level crustal thickness is also directly compared with seafloor bathymetry indicating a component of compensated topography with rms relief at the seafloor of 10 to 29 of crustal thickness variations however complexity inherited from variable asymmetric spreading and seafloor faulting is also apparent and results provide new insights into how the crustal accretion filter modulates the recording of magma supply variations in the crust and in seafloor relief while the significance of the statistical analysis of these ridge records is limited by the short duration of the available crustal thickness dataset and effects of asymmetric spreading the novel observations of crustal thickness varying at timescales of 80 100 ka require a mechanism and the sea level hypothesis provides a plausible explanation

permian wolfcamp limestone reservoirs powell ranch field eastern midland basin discussion the open quotes ep however its economic importance and geological complexity and the question of whether his model can be extended to other areas in the permian basin warrant this discussion

fine scale segmentation of volcanic hydrothermal systems along fast spreading ridge crests abstract fine scale 3rd and 4th order segments of the mid ocean ridge mor are morphologically defined units of crustal accretion with enigmatic origins at fast spreading centers 3rd order segments are bounded by discontinuities in the structure of the axial high most 3rd order segments are 20 10 km long and terminate at ridge axis discontinuities rads exhibiting 0 2 3 km lateral offsets and or 20 m axial depth increases nested within 3rd order segments are shorter 4th order segments bounded by smaller rads along the fast spreading east pacific rise epr at 10 02 9 08 n and 17 11 42 s spatial distributions of active high temperature hydrothermal vents biological communities and lava flow morphology are known accurately from extensive near bottom surveys and submersible dives along a total of seven 3rd order segments and twenty four 4th order segments the cumulative northern and southern epr data show that 3rd order segments but not 4th order segments consistently exhibit apparently higher rates of lava effusion near segment middles in comparison to segment ends supporting the hypothesis that each 3rd order segment is a discrete centrally fed volcanic system or volcanic segment spatial analyses of the cumulative hydrothermal data reveal a striking increase in abundance of hydrothermal features within the middle portions of 3rd order segments approximately 60 of ongoing high temperature focused flow is concentrated within the middle 40 of 3rd order segments and twice as many biological communities are found at mid segment in comparison to segment ends the two independent data sets volcanic and hydrothermal both indicate that magmatic heat sources are focused beneath the central portions of 3rd order segments and that magmatic hydrothermal systems are spatially and temporally segmented at 3rd order length and time scales we propose that 3rd order segmentation arises from processes controlling melt supply to the middle to lower crust and that 4th order segmentation largely arises from more rapid frequent upper crust processes of crack formation and dike intrusion future work is needed to test the applicability of our results along portions of the fast spreading epr notched by fault bounded axial troughs and in areas farther south where spreading rate reaches a maximum

crustal fissuring on the crest of the southern east pacific rise at 17 15 40 s s with the near bottom dsl 120 andargo ii imaging systems we observe that the youngest lava flows on a relative age scale aresparsely fissured and that there is a cumulative increase in fissure abundance with time thatproduces a strong positive correlation between fissure density and relative age of lava flows average fissure widths were used to estimate fissure depths in the 17 15

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

central anomaly magnetization high documentation of crustal accretion along the east pacific rise 9 55 9 25 n near bottom magnetic data collected along the crest of the east pacific rise between 9 55 and 9 25 n identify the central anomaly magnetization high camh a geomagnetic anomaly modulated by crustal accretionary processes over timescales of 104 years a significant decrease in camh amplitude is observed along axis from north to south with the steepest gradient between 9 42 and 9 36 n the source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption instead magnetic moment models show that it can be accounted for by an observed 50 decrease in seismic layer 2a thickness along axis layer 2a is assumed to be the extrusive volcanic layer and we propose that this composes most of the magnetic source layer along the ridge axis the 9 37 n overlapping spreading center osc is located at the southern end of the steep camh gradient and the 9 42 9 36 n ridge segment is interpreted to be a transition zone in crustal accretion processes with robust magmatism north of 9 42 n and relatively low magmatism at present south of 9 36 n the 9 37 n osc is also the only bathymetric discontinuity associated with a shift in the camh peak which deviates 0 7 km to the west of the axial summit trough indicating southward migration of the osc camh boundaries defined from the maximum gradients lie within or overlie the neovolcanic zone nvz boundaries throughout our survey area implying a systematic relationship between recent volcanic activity and camh source maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the nvz and camh boundaries lava dip angles average 14 toward the ridge axis which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed

morphology and segmentation of the western galapagos spreading center 90 5 98 w plume ridge interaction at an intermediate spreading ridge author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 8515 doi 10 1029 2003gc000609

evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the east pacific rise 15 30 17 n hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the east pacific rise 15 30 17 n are used to assess the relationship between crustal structure morphological indicators of magma supply and ridge segmentation from stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust layer 2a in contrast to other studies correlation between axial morphology and magma lens characteristics is found with a lens that on average is twice as wide and slightly shallower 100 200 m beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment however large local scale variations in lens depth and width are also observed that are not related to morphology we conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment with large variations within a segment due to local processes of magma eruption and delivery a negative correlation between zero age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment in this innermost axial region above the magma lens factors such as magma pressure not magma volume in the lens may govern the thickness of extrusives that accumulate the width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment 5 8 km and a narrower zone 2 5 km of minor thickening less than double along the northern segment and toward ridge segment ends this relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply we find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry indicating a genetic link between the finest scale tectonic segmentation of the ridge and segmentation of the magma lens in several locations discrete magma lenses at different levels within the crust offset by several hundred meters are imaged beneath adjacent fourth order ridge segments

evaluating the depositional environment lithofacies variation and diagenetic processes of the wolfcamp b and lower spraberry intervals in the midland basin implications for reservoir quality and distribution detailed facies characterization in the wolfcamp b and lower spraberry intervals of two drill cores in the midland basin has yielded five main lithofacies with distinctive physical chemical and biologic attributes the main attributes for facies identification include lithology and or mineralogy texture and or fabric porosity hydrogen index and total organic carbon content the methodologies are focused on detailed core description thin section petrography quantitative x ray diffraction and field emission scanning electron microscopy sem porosity data are primarily based on gas research institute core laboratories measurements and field emission sem assessment moreover the depositional and diagenetic controls on facies development are addressed to assess the dominant geological processes that govern reservoir quality and distribution a model of five end member facies is proposed to characterize source and reservoir elements through delineating facies tracts the five end member facies are as follows facies 1 silty mudstone optimal source and optimal to fair reservoir facies 2 muddy siltstone optimal source and optimal reservoir facies 3 silty calcareous mudstone good source and good reservoir facies 4 bioclastic wackestone floatstone fair source and fair reservoir and facies 5 packstone grainstone poor source and poor to excellent reservoir the proposed facies scheme aims to provide a more comprehensive approach to capture the high vertical and lateral variability in this mixed carbonate and fine grained clastic succession through this detailed textural compositional sedimentologic and diagenetic approach this facies model can be used to better understand reservoir quality and distribution throughout the midland basin

correlation between volcanic and tectonic segmentation of fast spreading ridges evidence from volcanic structures and lava flow morphology on the east pacific rise at 9 10 n 1 combined analyses of volcanic features in dsl 120 sonar data and argo i images along the ridge crest of the east pacific rise 9 09 54 n reveal a consistent decrease in inferred lava effusion rate toward the ends of third order segments the correlation of tectonic segmentation and volcanic style suggests that third order segmentation corresponds to the volcanic segmentation of the ridge along axis changes in volcanic structures from collapse troughs to basaltic lava domes and lava morphology from sheet to pillow flows coincide with the boundaries of morphologically defined third order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry pillow lava flows cover 25 of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third order segment ends an additional 25 of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough the remaining terrain consists of lobate lava flows we interpret the spatial correlations of morphologic structural seismic and petrologic data as evidence that individual volcanic plumbing systems are organized at 20 km spacing along the ridge axis third order segment scale in agreement with the hypothesis that volcanic and tectonic segmentations are correlated for fast spreading ridges we estimate that the longevity of volcanic segments is 104 105 years 1 3 orders of magnitude longer than fourth order segments 102 103 years this implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable

hydrothermal venting along earth s fastest spreading center east pacific rise 27 5 32 3 s 1 during march april 1998 we conducted detailed mapping and sampling of hydrothermal plumes along six segments of earth s fasting spreading mid ocean ridge 27 5 32 3 s on the east pacific rise we compared the distribution and chemistry of hydrothermal plumes to geological indicators of long term spreading rate and moderate term ridge inflation variations in magmatic budget in this large offset propagating rift setting these geological indices span virtually the entire range found along fast spreading ridges worldwide hydrothermal plumes overlaid 60 of the length of superfast 130 km myr spreading axis surveyed and defined at least 14 separate vent fields we observed no plumes over the slower spreading propagating segments finer scale variations in the magmatic budget also correlated with hydrothermal activity as the location of the five most intense plumes corresponded to subsegment peaks in ridge inflation along the entire ridge crest the more inflated a ridge location the more likely it was to be overlain by a hydrothermal plume plume chemistry mostly reflected discharge from mature vent fields apparently unperturbed by magmatic activity within the last few years plume samples with high volatile metal ratios generally indicating recent seafloor volcanism were scarce along axis trends in both volatile 3he ch4 ph a proxy for co2 and particulate s and nonvolatile fe mn species showed a first order agreement with the trend of ridge inflation nevertheless a broad correspondence between the concentration of volatile species in plumes and geological proxies of magma supply identifies a pervasive magmatic imprint on this superfast spreading group of ridge segments

early permian deep water allochthonous limestone facies and reservoir west texas conventional cores from six wells in southwest reagan and northern crockett counties texas recovered interbedded limestone conglomerate intraclast and bioclast limestone calcarenite and shale twenty one lithologies are grouped into six lithofacies based on study of slabbed core surfaces lithoclast rudstone floatstone porous bioclast lithoclast rudstone floatstone bioclast wackestone intraclast floatstone rudstone thin bedded grainstone packstone calcarenite and dark gray shale the limestone facies are interpreted on the basis of petrologic characteristics biota regional stratigraphic setting and facies stratigraphy as deep water and allochthonous biotic constituents within the lithoclasts and matrix indicate an early permian wolfcamp age the porous bioclast lithoclast rudstone floatstone facies contains a modest but geologically significant oil reservoir at gunnx field southwest reagan county production is from 10 ft 3 m of perforations in about 42 ft 13 m of pale yellow brown porous limestone with intercrystalline reduced and solution enlarged biomoldic intraskeletal fracture and possible intergranular porosity moldic porosity appears to be mainly within lithoclasts baroque dolomite is rare to common and partly to completely fills former void space in addition to calcite spar and micrite porosity generally ranges 7 18 the producing depth is about 9 450 ft 2 880 m and the trap is stratigraphic the cored sequence at gunnx is interpreted as a basin floor complex of debris flow sheets which contain the reservoir overlain by interbedded thin carbonate turbidites thin debris flow sheets and shale the lowermost debris flow at gunnx was deposited on a thin generally undated shale the lower part of which is late pennsylvanian at west world field the shale is interpreted as representing starved basin floor deposition and abruptly overlies shallow water strawn limestone in core at west world field carbonate detrital components in the limestone at gunnx were probably derived from a wolfcamp platform at least 10 mi 16 km west and were emplaced by various submarine gravity flow processes age and facies determinations from cores in the gunnx area significantly alter earlier stratigraphic interpretations made with wireline logs alone late paleozoic allochthonous carbonate facies may provide significant new reserves in the permian basin

hydrothermal plumes along segments of contrasting magmatic influence 15 20 18 30 n east pacific rise influence of axial faulting 1 vertical profiles of light backscattering and temperature recorded on 133 rock cores and dredge hauls between the orozco and rivera transform faults on the east pacific rise epr 15 20 18 30 n provide an opportunity to compare the hydrothermal environment of three adjacent but distinctly different segments that span the maximum range of axial cross section at a relatively constant spreading rate contrary to predictions based on data from other pacific ridges hydrothermal plumes over the inflated 16 n segment were less extensive and weaker than along the narrower rifted 17 n segment remarkably the 17 n segment has a plume incidence equal to the mean of superfast spreading segments from the southern epr the data suggest that the local permeability environment in this region controls the expression of hydrothermal activity in the water column the 16 n segment which has little or no indication of faulting may have its hydrothermal activity presently suppressed by widespread volcanic flows that act as an impermeable cap over much of the segment activity on the 17 n segment may be tectonically enhanced with hydrothermal fluids circulating through deep faults to a cracking front within each segment intense hydrothermal plumes characteristic of focused discharge seem associated with clearly rifted areas while weaker water column signals characteristic of diffuse discharge are associated with unrifted portions of the ridge axis that appear dominated by magmatism previous studies at intermediate to superfast spreading ridges have emphasized a positive correlation between local magmatic budget and hydrothermal activity our data suggest however that even at fast rates local tectonics can control the extent and nature of hydrothermal activity as documented for several sites on the slow spreading mid atlantic ridge despite the segment scale incongruity between hydrothermal activity and magmatic budget the fraction of total ridge length between 15 20 and 18 30 n overlain by plumes 0 39 follows the existing global correlation between plume incidence and spreading rate

temporal and spatial patterns of biological community development at nascent deep sea hydrothermal vents 9 50 n east pacific rise the april 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 45 n and 9 52 n on the east pacific rise provided a unique opportunity to follow temporal changes in biological community structure from the birth of numerous deep sea hydrothermal vents in march l992 dsv alvin was used to deploy an on bottom observatory the biologic geologic transect to monitor faunal succession along a 1 37 km segment of the axial summit caldera between 9 49 61 n and 9 50 36 n depth 2520 m photo and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the transect were performed in march 1992 december 1993 october 1994 and november 1995 photographic and chemical time series analyses revealed the following sequence of events in low temperature venting areas 1 immediately following the 1991 eruption hydrogen sulfide and iron concentrations in diffuse fluids were extremely high 1 mmol kg 1 and microbially derived material blanketed active areas of venting in the form of thick microbial mats 2 mobile vent fauna e g amphipods copepods octopods and galatheid and brachyuran crabs and non vent fauna e g nematocarcinid shrimp proliferated in response to this increased biological production 3 within 1 yr of the eruption areal coverage of microbial mat was reduced by 60 and individuals of the vestimentiferan tube worm tevnia jerichonana settled gregariously in areas where diffuse flow was most intense 4 two years after the eruption maximum levels of h2s decreased by almost half from 1 90 to 0 97 mmol kg 1 and dense thickets of the vestimentiferan riftia pachyptila dominated vent openings previously inhabited by tevnia jerichonana 5 three years after the eruption maximum hydrogen sulfide levels declined further to 0 88 mmol kg 1 and mussels bathymodiolus thermophilus were observed on basaltic substrates 6 four years after the eruption galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0 72 mmol kg 1 also by this time mussels had colonized on to tubes of riftia pachyptila 7 between 3 and 5 yr after the eruption there was a 2 to 3 fold increase in the number of species in the faunal assemblages in the absence of additional volcanic tectonic disturbance we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5 10 yr following the eruption we also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase we hypothesize that the above series of events 1 7 represents a general sequence of biological successional changes that will occur at newly formed low temperature deep sea hydrothermal vents along the northern east pacific rise and contiguous ridge axes megafaunal colonization at deep sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life history strategies of individual species physical oceanographic processes and the dynamic hydrothermal environment our observations indicate that the successful sequential colonization of dominant megafaunal vent species from tevnia jerichonana to riftia pachyptila to bathymodiolus thermophilus also may be strongly influenced by temporal changes in geochemical conditions additional evidence demonstrating the close link between diffuse vent flux fluid geochemistry and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow these correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast spreading mid ocean ridges should consider not only the interplay of species specific life history strategies community productivity and physical oceanographic processes but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

geological chemical and biological evidence for recent volcanism at 17 5 s east pacific rise the superfast spreading portion of the east pacific rise at 17 5 s is a magmatically robust section of mid ocean ridge plumes characterized by high volatile metal ratios which has been hypothesized to be indicative of recent magmatism were found between 17 22 s and 17 35 s in november december 1993 dives with the french submersible nautile in december 1993 observed young sheet flows and widespread venting centered on the segment s shoalest point at 17 26 s eight submersible dives made one year later with the japanese submersible shinkai 6500 in september november 1994 found glassy unsedimented lavas a range of diffuse and high temperature vents and several types of biologic communities on the ridge crest beneath the volatile rich 1993 plumes the diffuse vents range from small areas in the youngest lavas characterized by very low temperature flow 10 c and a relatively low macrofaunal diversity to larger areas where the apparent older surface was covered with a dense and diverse biotope including anemones tubeworms mussels and serpulids several inactive vents marked by dead mussel clam beds are present in the southernmost portion of the site in the older lavas submersible mapping indicates that the youngest lava l0 unit erupted for at least 4 km along the strike of the ridge at the southern site and for 5 10 km at the northern site the flow is up to 400 m in width at the southern site and may be as much as 1 2 km in width at the northern site comparison of the observed 1993 fe s h2s heat and 3he heat plume values with time series measurements at sites on the northern east pacific rise and the juan de fuca ridge where the onset of the magmatic event is known implies that the last magmatic event at 17 5 s occurred within several years prior to 1993 continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives the type and diversity of the vent biota found on the l0 vents also suggests a young system the spatial and temporal scales of the accretion event at the 17 5 s epr site are similar in scale to eruptions on the intermediate rate spreading north cleft segment juan de fuca ridge but the 17 5 s event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

geological indexes of hydrothermal venting hydrothermal venting occurs on mid ocean ridge axes with a diverse array of morphological structural and petrological characteristics consequently it is not clear if certain geological environments are more conducive than others to hydrothermal activity this paper uses complementary data sets recently collected on multiple tectonic segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue the spatial density of venting is gauged by plume incidence the fraction of linear ridge crest overlain by hydrothermal plumes five geological indexes are compared with plume incidence ridge axis elevation ridge axis cross sectional area the percentage of ridge axis underlain by an axial magma chamber amc reflector mgo wt in basaltic glass and spreading rate a mean value for plume incidence and each index where available is calculated for each of 14 second to fourth order tectonic segments from the juan de fuca ridge 44 30 to 48 45 n and the northern 9 to 11 50 n and southern 13 50 to 18 40 s east pacific rise plumes are most common on segments with a cross sectional area 3 5 km2 a net elevation 0 35 km amc coverage 60 and mgo 7 wt spreading rate is not deterministic on the segment scale the data suggest two classes of ridge segments segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget those with high index values have a plume incidence ranging from low to complete coverage apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity the best individual predictors of hydrothermal activity are cross sectional area and net elevation applying these indexes to the east pacific rise between 18 n and 33 s suggests that the unexplored segments with the highest probability of activity are those at 15 30 16 n 6 9 n 5 6 s 27 28 s and 30 30 33 s

small non overlapping offsets of the east pacific rise high resolution bathymetric surveys1 2 of the east pacific rise epr reveal that in addition to overlapping spreading centres oscs the crest of the epr is offset tiny amounts by smaller features recently langmuir et al have shown that despite their subtle bathymetric expression these small offsets bends and kinks of the epr collectively termed deviations from axial linearity devals represent important petrological and tectonic boundaries some devals are small non overlapping offsets snoos that offset the epr axis by up to a few hundred metres bathymetric contours near snoos are usually disturbed from their axis parallel trend in ways that we interpret as evidence for strike slip faulting and volcanism at high angles to the trend of epr axis we offer a model for the formation of snoos and their associated faults that invokes independent magma supply stochastic active spreading and limited non rigid behaviour of the neovolcanic zone

basaltic lava domes lava lakes and volcanic segmentation on the southern east pacific rise meter scale dsl 120 sonar mapping and coregistered argo ii photographic observations reveal changes in eruptive style that closely follow the third order structural segmentation of the ridge axis on the southern east pacific rise 17 11 18 37 s near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology the ubiquity of pillow lava suggests low effusion rate eruptions the abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices near segment centers we observe fewer but larger lava domes voluminous drained and collapsed lava lakes and smooth lobate and sheet lava flows with very little pillow lava the abundance of sheet flows suggests that high effusion rate eruptions are common fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources this pattern was observed on eight third order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale the third order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across axis width of the region of seismic layer 2a thickening along the one segment where sufficient cross axis seismic lines exist the geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries although rock sampling density is not adequate to constrain the variation across all the third order volcanic segments that we identify these observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading 140 km myr full rate mid ocean ridges

calciclastic submarine fans an integrated overview abstract calciclastic submarine fans are rare in the stratigraphic record and no bona fide present day analogue has been described to date possibly because of that and although calciclastic submarine fans have long intrigued deep water carbonate sedimentologists they have largely been overlooked by the academic and industrial communities to fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans thus offering an updated view of this type of carbonate slope sedimentary system calciclastic submarine fans range in length from just a few to more than 100 km three different types can be distinguished 1 coarse grained small sized most calciclastic submarine fans were formed on low angle slopes and were sourced from distally steepened carbonate ramps subjected to high energy currents under these conditions shallow water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows these conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters nutrient enrichment or oligophoty another circumstance that contributes to the transfer of shallow water sediments to the distal ramp slope is a low sea level forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore water pressure however the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point sourced sedimentary accumulation in most cases this occurred through a major slope depression associated with tectonic structures an inherited topography or large scale mass failures

synthesis and thermal decomposition of tetragonal rbclo4 and volume of fluid o2 from 2 to 9 gpa 1 orthorhombic o rbclo4 transforms to a new tetragonal t form at pressures above 20 kbar thermal decomposition proceeds through the reversible equilibrium reaction t rbclo4 b2 rbcl 2o2 at modestly increasing temperatures centered 550 c over the pressure range investigated from 20 to 90 kbar cell volumes for t rbclo4 and b2 rbcl were determined by high pressure high temperature in situ x ray diffraction on station 16 4 of the daresbury synchrotron radiation source from which the partial molar volume of o2 along the decomposition curve was determined this volume 10 cm3 mol is in close agreement with volumes determined from studies of the thermal decomposition of kclo3 and kclo4 but is difficult to reconcile with fluid o2 volumes determined using shock wave techniques if these new o2 partial molar volumes are relevant to molten iron solutions then storage of substantial amounts of o2 in the core becomes feasible

recent tectonic magmatic and hydrothermal activity on the east pacific rise between 17 s and 19 s submersible observations the objective of the naudur cruise december 1993 of the submersible nautile was to study the interaction among magmatic tectonic and hydrothermal processes at a very fast spreading mid ocean ridge axis twenty three dives were completed both along and across the axis in four areas located between 17 10 and 18 45 s on the east pacific rise rock sulfides water and biological samples have been collected along each of the segments two main types of segments have been distinguished characterized either by the predominance of present day volcanic activity or by predominant tectonic activity linked to both types of activity 69 hydrothermal sites have been discovered and sampled they comprise four types interpreted as successive evolutionary stages the first are shimmering water sites which occur immediately after the formation of lava lakes and are characterized by large surface area and poorly developed associated fauna the second in areas dominated by recent volcanic activity have waters venting directly from lava fissures and more focused discharge areas through black smoker chimneys the third stage is represented by more mature hydrothermal vents and deposits along the faults bounding the eastern side of the axial graben in tectonic dominated areas the associated fauna is well developed the fourth stage corresponds to the reactivation of volcanic activity with lava flows young black smokers and diffuse venting associated with the faults bounding the axial graben fluids collected range from 200 to 340 c and show a wide variability in chemical and gas composition within each of the explored areas evidence of recent volcanic activity has been observed

exploring the ocean for hydrothermal venting new techniques new discoveries new insights abstract enumerating active hydrothermal fields on the seafloor has been a challenge since their discovery almost 40 years ago high temperature hydrothermal fields are readily discoverable primarily by detecting mineral laden plumes but low temperature particle poor vent fields resist discovery decades of exploration for vent fields have covered though often cursorily about one third of the global lengths of both oceanic spreading ridges osrs and volcanic arcs identifying some 630 active vent fields about 80 of these fields are on osrs and the spatial frequency of those fields is currently estimated as 0 5 5 100 km generally increasing with spreading rate over the last decade however a few detailed surveys have added sensors capable of detecting ephemeral chemical tracers oxidation reduction potential in addition to standard sensors that detect quasi conservative optical tracers such as light backscattering this approach has revealed a new view of the distribution of venting fields along fast spreading 55 mm yr osrs studies of four such ridge sections totaling 1470 km length suggest that the present inventory of vent fields may underestimate the true global population of vent fields on fast spreading osrs by a factor of 3 6 this increase implies that ridge axes are unexpectedly leaky reservoirs from which hydrothermal fluids escape at far more sites than presently assumed that the supply of dissolved hydrothermal iron which may be fertilizing the primary production of the southern ocean is higher than now calculated and that present estimates of recoverable sulfide tonnage from ridge axes may be too low along slow spreading ridges which account for 60 of the global osr length and 86 of known sulfide tonnage expansive axial valleys present special exploration challenges that will not be easily overcome

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions

size variation in the rock hyrax procavia capensis and late quaternary climatic change in south africa abstract the average adult size of the rock hyrax varies greatly across south africa regression analysis suggests that mean hyrax size is more closely linked to precipitation than to temperature probably because precipitation has a much greater impact on preferred hyrax food plants the relationship between mean size and precipitation is curvilinear such that size increases up to about 700 mm annum and declines thereafter this parallels a tendency for less palatable grasses to replace more palatable ones where rainfall exceeds 700 mm annum in conjunction with other indicators of past climate hyrax size variation can be used to reconstruct precipitation history near deeply stratified south african late quaternary sites including elands bay cave die kelders cave 1 and nelson bay cave