the 7be 210pbxs ratio as an indicator of suspended sediment age or fraction new sediment in suspension reported here is a technique to use the 7be 210pbxs ratio as a measure of suspended sediment age or as an indicator of the fraction of the suspended sediment that is recently eroded from the landscape although both 7be and 210pbxs are delivered seasonally and stochastically to the landscape by precipitation the ratio of the two radionuclides varies substantially less the 7be 210pbxs ratios measured in three different catchments decrease in the following manner precipitation c 16 suspended sediments in rivers 6 7 suspended sediments in estuaries 4 6 sediment collected in sediment traps in the estuary c 1 surface sediment of the estuary c 0 5 decreases in the 7be 210pbxs ratio in suspended sediments can be interpreted to be the result of increased age of the sediment since 7be decays faster than 210pb alternatively a decrease in the 7be 210pbxs ratio in suspended sediments can be interpreted to be the result of dilution of newly tagged 7be rich sediment by 7be dead sediment for example by erosion of soil below the 7be enriched surface layer or by resuspension of 7be dead bottom sediment presented here is a model which uses the 7be 210pbxs ratio in suspended sediments to determine the time since the particles were tagged by precipitation derived radionuclides i e the age of the suspended sediment in addition an alternative model is presented to determine the fraction of the sediment that is newly tagged these two models are applied to three catchments old woman creek ohio weeks bay alabama and south slough oregon and yield similar findings at all three sites sediment ages increase from 0 in newly tagged material to 50 80 days in rivers to about 80 100 days in the estuaries to about 200 days in the sediment traps to about 300 days on surface bottom sediments alternatively the percentage new sediment decreases from 100 per cent in newly tagged material to about 35 50 per cent in rivers to 25 35 per cent in the estuary to less than 10 per cent in the sediment traps to 1 4 per cent on the surface of the bottom sediments

joint isotopic mass balance a novel approach to quantifying channel bed to channel margins sediment transfer during storm events the important role of floodplains and the broader riparian zone in providing temporary storage for a large fraction of the annual sediment load of rivers is well established but this understanding is largely based on observations of the long term average behavior of the catchment here we combine measurements of the fallout radionuclides 7be and 210pb and the stable isotopes of hydrogen in water to quantify fine sediment mobilization and storage in a stream and its channel margins during individual intermediate sized storm events with recurrence intervals of a few months or less we demonstrate this method using five storm events in a small 15 km2 undeveloped gravel bedded tributary of the connecticut river usa we estimate that in each storm the mass of sediment deposited onto the margins accounts for almost 90 of the sediment mobilized from the bed with the remainder of the mobilized bed sediment transported downstream as suspended load the result that the bed is a net source of sediment to the stream and the margins a net sink is robust but estimates of the mass of material eroded from the bed and deposited on the margins are less certain the source of sediment to the bed remains unclear as consistent with earlier studies we observe only limited deposition of sediment to the bed during the storm events the suspended sediment is organic rich and thus its source may be associated with in channel organic decay between storm events understanding the coupled interactions between discharge magnitude and frequency and sediment resupply at the event time scale has important implications for stream restoration efforts seeking to connect the channel and the broader riparian zone and for the development of accurate sediment budgets and predictions of sediment flux from a watershed copyright 2015 john wiley sons ltd

faunal characteristics and sediment accumulation processes in the james river estuary virginia abstract this study was designed to relate fauna characteristics and sediment accumulation processes in the james river va and was conducted during june 1981 physical sedimentary and benthic biological parameters as well as sediment structure and radionuclide profiles were evaluated for 11 stations faunal distribution patterns reflected species response to salinity changes along the estuarine gradient but not to differences in sediment accumulation rates levels of bioturbation could not be predicted easily on the basis of faunal characteristics alone results suggest that the physical processes of erosion and deposition strongly influence the ability of macrobenthos to bioturbate sediments in this estuary areas of rapid deposition 3 cm y 1 exhibit little evidence of bioturbation as do areas where erosion or relatively constant physical reworking of sediments dominate areas with low sediment accumulation rates 0 5 3 cm y 1 exhibit the highest levels of mixing as evidenced in x radiographs estuarine organisms inhabiting soft bottoms are typically opportunistic shallow living and short lived species and the composition of their communities is not strongly influenced by rates of deposition physical reworking of sediments is most likely to occur near to the sediment water interface where reworking by shallow living organisms is most intense sediment mixing processes should be characterized using a range of approaches the phasing of interactions among erosion physical transport deposition and biological mixing must be resolved on the appropriate time scales if the mechanics of processes governing the formation of the sedimentary record are to be elucidated

detachment and steep normal faulting in atlantic oceanic crust west of africa improved images of the internal structure of early cretaceous north atlantic crust reveal both probable detachment faults and more steeply dipping normal faults the detachment faults occur as subhorizontal structures passing 1 5 km beneath fault blocks without offset several steeper block bounding faults appear to detach onto these structures however the detachments are bounded to the west ridgeward by presumably younger more steeply west dipping normal faults in one possible interpretation the detachment and the steep faults belong to the same rolling hinge extension system an intriguing alternative is that a phase of detachment faulting perhaps related to increased magmatic activity was succeeded by localized amagmatic extension along steeper and more deeply penetrating faults

geochemistry and thermodynamics of an earthquake a case study of pseudotachylites within mylonitic granitoid abstract pseudotachylites are melts produced by frictional heating during seismic slip understanding their origin and their influence on slip behavior is critical to understanding the physics of earthquakes to provide insight into this topic we conducted a case study in the proto mylonitic to mylonitic asbestos mountain granitoid in the eastern peninsular ranges batholith california which records both ductile mylonites and brittle deformation features pseudotachylites and ultracataclasites u pb chronology and zr thermometry of titanite porphyroblasts in the mylonites indicate that mylonitization of the plutons occurred at near solidus conditions 750 c over a 10 ma interval from 89 to 78 ma mylonitization resulted in recrystallization of quartz plagioclase and biotite with the biotite concentrated into biotite rich foliation planes subsequent brittle deformation is superimposed on the ductile fabrics micro xrf elemental mapping and in situ la icp ms analyses on these brittle deformation products show that the pseudotachylites are more mafic lower si but higher fe and k rich than the host mylonite while the ultracataclasites are intermediate between the host and the pseudotachylites inverse mass balance calculations show that both brittle deformation products are depleted in quartz but enriched in biotite with the pseudotachylites showing the most significant enrichment in biotite indicating preferential involvement of biotite during brittle deformation we suggest that biotite rich layers generated during ductile deformation may have been the preferred locus of subsequent brittle deformation presumably because such layers represent zones of weakness frictional heating associated with slip along such planes results in melting which causes a decrease in viscosity in turn leading to further strain localization during the short time span of an earthquake frictional melting appears to be a disequilibrium process in which the minerals are melted in order of their melting points from biotite 800 c to plagioclase 1400 c and finally to quartz 1700 c rather than by equilibrium melting which results in silicic eutectoid melts at lower temperatures 650 c thus with progressive slip melt composition should evolve from mafic to felsic eventually approaching the bulk composition of the host rock the mafic composition of the pseudotachylites thus indicates that they formed between the melting point of biotite and plagioclase 800 1400 c our chemical and modeling analyses on the pseudotachylites suggest that the chemical composition of pseudotachylites can potentially be used to constrain the thermodynamic conditions in the shear zone as well as earthquake source mechanics

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

incipient subduction at the contact with stretched continental crust the puysegur trench a seismic benioff zone and plate kinematics show puysegur trench south of new zealand transitioning to subduction because the local structure and its influence on subduction initiation is poorly understood we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles our early results show that the overriding pacific plate beneath the solander basin is composed of block faulted and thinned continental crust and the inner trench wall of northern puysegur ridge is composed of folded and faulted sediment the megathrust interface has been imaged and shows 500 m of downgoing undisturbed sediments combining plate kinematic history with seismic velocity inferred density we show that the density difference across the plate boundary changed as oblique strike slip plate motion juxtaposed dense oceanic crust with thinned continental crust the density difference rapidly increased 18 to 15 ma coincident with subduction initiation suggesting that compositional differences have a large influence on subduction initiation

mineralogy and pressure temperature time path of cretaceous granulite gneisses south eastern san gabriel mountains southern california mid cretaceous granulite gneisses crop out in a narrow belt in the cucamonga region of the south eastern foothills of the san gabriel mountains southern california interlayered mafic granulites and pelitic carbonate calc silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700 800 c regional deformation and formation of banded gneisses ceased by c 108 ma although mafic intermediate magmatism and high grade metamorphism continued locally as late as c 88 ma garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near isobaric cooling but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo chronological data it is more likely that the entire terrane remained at granulite facies p t conditions until 88 ma followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes uplift occurred between c 88 and 78 ma at rates of approximately 1 2 km ma 1 the geotectonic evolution of the cucamonga granulites is similar to mid cretaceous high p granulites in the sierra nevada and salinian block of central california late cretaceous uplift common to these granulites may provide an important tectonic link between dismembered mesozoic batholithic terranes in the california cordillera

internal structure of the oceanic lithosphere at a melt starved ultraslow spreading mid ocean ridge insights from 2 d seismic data extensive outcrops of serpentinized peridotite in melt starved spreading corridors of the ultraslow easternmost southwest indian ridge are hypothesized to be due to slip on successive long offset normal faults that alternate polarity flip flop detachment faults we investigate the nature of the oceanic crust which forms under these conditions using seismic reflection data acquired during the sismosmooth 2014 cruise using 3 d binning the seismic profiles were binned elastically while three of the profiles shot closely were merged into one to take advantage of the larger air gun source volume using a poststack imaging sequence we observe several types of reflectors at crustal and infracrustal depths in the axial valley and off axis correlating our seismic observations with residual mantle bouguer gravity anomalies and seafloor observations we find that our results are explicable in the framework of the flip flop hypothesis of detachment faulting reflectors imaged down to 5 km into the basement and interpreted as due to damaged zones outlining the detachment faults dip 50 at the early stages while at late stages after developing offsets 10 km they dip 25 other reflectors observed in the crust are interpreted as moderate offset 200 m normal faults accommodating deformation and alteration in the hanging wall and channeling the sparse melt to the seafloor we interpret these and other observed seismic reflectors in the frame of a two phase evolutionary sequence over the lifetime of two successive flip flop detachment faults exhumation footwall flexure damage serpentinization and incipient magmatism in the footwall of one detachment fault followed by further tectonic damage alteration and incipient magmatism in the hanging wall of the next detachment fault

observations of event scale sedimentary dynamics with an instrumented bottom boundary layer tripod abstract a new approach to examine event scale sedimentation is described that establishes a direct connection between water column signals and changes in the seabed the po river italy shelf is a good case study due to the dominant impact of winter storm events on sediment transport and the availability of coincident measurements necessary for the analysis repeated deployments of an instrumented bottom boundary layer tripod at this site have produced a nearly continuous record of sediment transport over 2 5 y sediment cores were collected every 3 mo during this time and seabed inventories of 7 be half life 53 3 d were used to determine net sediment deposition or erosion during each deployment comparison of these values with the net elevation change obtained from the tripod altimeter shows good agreement varying from 2 cm deposition to 1 cm erosion the altimeter s use is extended to quantify the net seabed change that results from individual storm induced sediment resuspension events lasting from days to weeks within each event analysis of the altimeter record has proved challenging likely due to changes in sediment porosity that occur during a storm event however net erosion and deposition that occurs over an event can be evaluated and was found to be

three dimensional geometry of axial magma chamber roof and faults at lucky strike volcano on the mid atlantic ridge we present results from three dimensional 3 d processing of seismic reflection data acquired in june 2005 over the lucky strike volcano on the mid atlantic ridge as a part of the seismic study for monitoring of the mid atlantic ridge survey we use a 3 d tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time migrated seismic volume and provide 3 d geometry of the axial magma chamber roof fault reflectors and layer 2a gradient marker we also generate a high resolution bathymetric map using the seismic reflection data the magma chamber roof is imaged at 3 4 0 4 km depth beneath the volcano and major faults are imaged with dips ranging between 33 and 50 the magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10 of the total plate separation shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces

environmental chemistry of beryllium 7 in addition to the stable isotope 9be be is also formed as two cosmogenic isotopes of interest to earth scientists cosmogenic be is formed primarily in the stratosphere from cosmic ray spallation of oxygen and nitrogen but some is produced in the troposphere and in situ on the surface of the earth production of cosmogenic be was hypothesized by peters 1955 soon after naturally occurring 7be was identified in precipitation by arnold and al salih 1955 and essentially concurrently and independently by goel et al 1956 10be t1 2 1 5 106 yr was first detected by arnold 1956 in marine sediment cores the 10be 7be production ratio is approximately 0 5 lal and peters 1967 after its formation in the atmosphere cosmogenic be adsorbs electrostatically to aerosols that may be washed out by precipitation and delivered to ecosystems the amount of cosmogenic be that reaches the surface of the earth is a function of production rate cosmic ray intensity stratosphere troposphere mixing circulation and advection within the troposphere and efficiency of removal from the troposphere wet and dry deposition feely et al 1989 since the discovery of cosmic ray produced be numerous researchers have gathered data on its 1 production 2 fluxes to the oceans and terrestrial ecosystems 3 distribution and inventory in soils snow sediments and vegetation and 4 geochemical behavior in 1955 peters suggested that the relatively long half life of 10be could make it useful for quantifying tertiary sedimentation rates and other surficial processes the focus of this chapter is the short lived radionuclide 7be t1 2 53 12 0 07 days jaeger et al 1996 due to its short half life relative ease of measurement and well defined source term 7be serves as a useful tool for tracing and quantifying environmental processes on the

seasonal variations in 7be activity in the sediments of cape lookout bight north carolina abstract short term sediment accumulation rates were determined using activity distributions of 7be in the surficial sediments of a station a 1 in cape lookout bight north carolina usa this semienclosed coastal marine basin is characterized by high annual accumulation rates protected from high energy conditions by its geomorphology the relatively quiescent waters of the bight s interior cause it to act as an efficient settling trap for fine grained sediment lack of bioturbation in the surface sediments allowed for variations in depth integrated activity profiles of 7be to be interpreted as short term accumulation events beryllium 7 inventories exhibited seasonal cyclicity with inaxima occurring during spring february through june the inventory of 7be ranged from 5 8 to 32 6 dpm cin 2 and was always well in excess of the atmospherically supported value x 3 8 dpm cm 2 n 15 accumulation rates calculated froin 7be activity profiles indicate that the delivery of particulate matter to the sediments of cape lookout bight is not constant throughout an annual cycle the highest monthly accumulation rates appeared to be associated with north northeast storm activity inputs were generally highest during late winter early spring when storm frequency is greatest short term accumulation rates derived from this study agree with 210pb rates calculated for a ten year period of accumulation the annual sediment accumulation rates for each of the two years over which the study was conducted was found to be 4 8 0 8 g cm 2y 1 and 3 7 1 2g cm 2y 1

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

recognizing non steady state weathering conditions in ancient submarine fan systems correcting for k metasomatism reveals a late cretaceous non steady state weathering regime point loma formation southwestern california u s a abstract during steady state weathering the rate of removal of the exposed weathering zone is balanced by the rate of generation of mineralogically identical material through chemical weathering as steady state conditions prevail the mineralogical and chemical composition of sediments derived from the weathering profile should be invariant over time in a cn k space a steady state weathering condition is characterized by tightly grouped compositions with dispersion between sands and muds a consequence of sorting during fluvial transportation the purpose of this study was to assess whether such a condition was present during the development of the upper cretaceous point loma formation published paleotectonic reconstructions portray the upper campanian lower maastrichtian point loma formation as being deposited in a forearc basin developed westward of a continental margin magmatic arc in such accounts during the subduction of an oceanic plateau or aseismic ridge or during constant dip subduction rocks in the magmatic arc were uplifted and the volcanic cover nearly stripped clean exposing its plutonic roots published detrital zircon data derived from samples of the point loma formation indicate that exposed igneous rocks in the source area were mainly from 135 ma to 100 ma plutons located in the western zone of the peninsular ranges batholith new point count xrd and chemical data show that detritus in mudstones and some fine grained feldspathic sandstones of the point loma formation following deposition and shallow burial were subjected to a k metasomatic event after correcting for introduction of k and consideration of the effects of hydraulic sorting resulting mudstone and fine grained sandstone data plot in a cn k space in overlapping fields whereas samples of coarse grained sandstone lie between rays extending through the most and least weathered mudstones hence detritus in the point loma formation was derived mainly from regolith during non steady state weathering conditions

a review of batholiths and other plutonic intrusions of mexico abstract granitic plutons constitute a major portion of the phanerozoic continental crust of mexico with the great majority ca 90 associated to the laramide late cretaceous eocene orogeny and the eastward subduction of the pacific ocean plates as well as to magmatic arcs essentially built since the early mesozoic at the western margin of north america exposed mainly as a wide up to 300 km and over 3000 km long batholithic belt at the mexican pacific margin from baja california to chiapas granitoids conform large intrusive complexes and hundreds of smaller plutons the age of which vary from ca 1400 ma mesoproterozoic to ca 10 ma late miocene in many cases uplift and erosion have revealed the deep roots of the batholiths whereas in other places many intrusions were emplaced in upper crustal environments as suggested by the extremely variable cooling rates of 200 c ma very shallow to 1 10 c ma very deep lithologies and isotopic data indicate unambiguously the central participation of the local lower crust in the genesis of the batholiths and plutons imprinting on them marked petrologic geochemical and structural zoning across the paleozoic paleomargins and through the present nw trending mexican continental edge according to the lithospheric component involved laurentia in the northern and northwestern regions of mexico accreted mesozoic terranes in western mexico and oaxaquia gondwana in eastern and southern mexico major problems related to the evolution of the mexican main granitoids are outlined in time slices for the proterozoic paleozoic permo triassic jurassic early cretaceous late cretaceous paleogene and neogene which represent in mexico major epochs of crustal growth and continental recycling mainly associated not only with pacific north america convergence but also with extensional and transpressional events that altogether marked not only the lithological and structural evolution of most of the country but also its extraordinary mineral wealth finally some preliminary comparisons differences and similarities are made between the mexican batholiths and other plutonic complexes in central asia lhasa terrane and japan

modern sediment dispersal and accumulation on the outer poverty continental margin abstract the poverty margin was sampled in 2005 and 2006 as part of an international initiative to examine the terrestrial and marine sedimentary response to natural and human impacts on dispersal systems at mud dominated coasts the nsf margins source to sink initiative 210pb accumulation rates generally decrease from 1 g cm2 y in the mid shelf depocenter on the outer shelf to 0 1 g cm2 y on the mid slope plateau range 0 04 2 53 g cm2 y higher accumulation rates are observed all along the outer poverty shelf extending over the shelf break onto the upper poverty slope in canyons and gullies rates are fast in gullies that incise into the shelf edge 0 75 1 g cm2 y particularly in the area between waipaoa canyon and poverty canyon and in the axis of poverty canyon 1 29 1 89 g cm2 y below 1200 m water depth rates in the axis of poverty canyon are no more rapid than those found at open slope cores in similar water depths 0 12 0 15 g cm2 y excess 210pb profiles generally exhibit steady state characteristics except in the axis of poverty canyon where non steady state 210pb profiles are observed 137cs was not found above minimum detection limits or above statistical background levels in any cores from the poverty margin 7be was found distributed widely along and across the margin during both summer and winter periods observed to depths of up to 4 cm widespread presence of 7be on the margin in both winter and summer suggests that terrestrially derived sediment is reaching the outer shelf and upper slope throughout the year a sediment budget based on 210pb accumulation rate data shows that approximately 13 18 of the waipaoa annual discharge is accumulating on the outer poverty margin as a whole and approximately 11 15 of the annual discharge is accumulating on the continental slope alone notably approximately 28 of the sediment annually accumulating in the outer margin is being sequestered in slope gullies and canyons although they represent only 6 of the area of the outer margin

columbia river sediment in quinault canyon washington evidence from artificial radionuclides sediments collected from beneath 1500 2000 m of water in quinault canyon off the coast of the state of washington contain 60co and 152eu which indicate that they are derived from the columbia river the presence of these radionuclides confirms that sediment from the columbia river is transported to and deposited in quinault canyon on a time scale of a few years or less fallout derived 137cs is also present in quinault canyon sediments 137cs can probably be used at other locations to distinguish canyons where deposition of river or shelf derived sediments is active from dormant canyons

the use of 7be to identify event and seasonal sedimentation near the po river delta adriatic sea in october 2000 a major flood event of the po river occurred the resulting seabed deposit was initially sampled in december 2000 and most sites were reoccupied on subsequent cruises over a 3 yr period using the maximum penetration depth of the short lived radioisotope 7be half life 53 3 d the flood deposit was found to be up to 15 cm thick individual depocenters of thick strata were located immediately adjacent to the distributaries at the river mouth in relatively shallow water 30 m these flood deposit thicknesses are minimum estimates because the depth of 234th half life 24 1 d exceeds that of 7be in physically stratified flood sediment indicating that the first sediment deposited during the flood event likely originated from the river channel 30 55 of the estimated sediment load delivered to the shelf during the flood event can be accounted for using 7be penetration depths seasonal deposition also can be quantified using 7be after removing the effects of biological mixing seasonal deposition rates are a maximum of 6 cm yr 1 near the pila distributary decreasing to 2 cm yr 1 in the southern portions of the dispersal system

a detailed geophysical study in the canary basin eastern atlantic implications for the internal structure of 130 ma oceanic crust summary we report the results of a detailed geophysical survey sited on two previously imaged dipping reflectors one dipping towards and the other away from the present day mid atlantic ridge axis within mesozoic aged crust of the east atlantic the new data include gravity magnetic and seismic refraction and reflection profiles collected in a grid that constrain the crustal structure as well as the reflector geometry a second order axial discontinuity which is associated with anomalous crustal structure is located within the survey area the westward dipping reflector can be traced from the moho to the top of the igneous crust where it coincides with a step of 800 m in the basement this reflector is approximately planar with a dip of 28 2 w and occurs 8 km north of the axial discontinuity we interpret this reflector as being a whole crustal normal fault that was generated within the median valley the eastward dipping reflector cannot be traced across seismic layer 2 and does not correlate with any feature of the basement it penetrates 2 3 km into the upper mantle its reflection strength is about twice that of the moho reflection this reflector which is approximately planar strikes parallel to the isochrons has a length of 18 2 km and a dip of 28 2 e we speculate that this reflector is also a fault that was generated on the western wall of the median valley but was captured from the north american plate by a lateral shift in the axis of spreading possibly associated with a major change in plate motion

suspended sediment sources and transport distances in the yellowstone river basin the activity of fallout radionuclides 7 be 1 3 7 cs and 2 1 0 pb was measured on upland and floodplain soils and on suspended sediments to quantify sources of fine sediment and to estimate sediment transport distances in stream channels in the yellowstone river basin samples were collected seven times during snowmelt and runoff at nine locations from the headwaters of soda butte creek to billings montana a 423 km long reach of channel the inventory of radionuclides in soil increases with precipitation and is highest in the headwaters the activity of radionuclides in suspended sediment decreases downstream and more activity is observed earlier than later in the flood hydrograph the radionuclide activity of sediment derived from erosion of upland soils differs from that derived from bank erosion fine suspended sediment has an intermediate radionuclide signature that is quantified in terms of the relative contribution of these two sources of fine sediment at sites high in the drainage soils contribute 50 to the suspended load and this value decreases to 11 26 downstream fine sediment transport distances were calculated from the exponential decrease in radionuclide concentration below a point source transport distances increase from a few kilometers in the headwaters to hundreds of kilometers downstream these estimates are consistent with transport distances estimated from the settling velocity of the particles and from the distribution of mine tailings downstream from a dam failure this study of a large watershed confirms earlier results from smaller basins and suggests that transport distances increase with basin size

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

fractional changes in elemental mass ω and relative variation curves unravelling elemental mass redistribution patterns within the regolith abstract within the arid climate of southern california usa we uncovered through trenching a granodioritic tonalitic corestone a partially encircling fractured rind and an overlying ah2 horizon composed of sandy soil using the geometric mean of chemical data derived from an adjacent erosionally exhumed corestone as a proxy for unweathered bedrock the compositional linear trend derived from weathering was calculated average weathering intensities t values and 95 confidence intervals are as follows uncovered corestone 0 56 0 13 inner fractured rind 0 93 0 08 outer fractured rind 1 5 0 05 and ah2 1 7 0 09 ti was used as a reference frame for calculating the modeled fractional change in a given elements mass resulting relative variation curves plotted on 10 bivariate graphs where the x axis represents t values and the y axis is show that with the exception of p 2 o 5 samples from each of the above textural groups cluster and plot closely about the 10 relative variation curves the clustering and ordering of samples analyzed from the uncovered corestone inner rind outer rind and ah2 horizon about the relative variation curves suggests that weathering of the section of regolith uncovered by trenching occurred in an orderly and sequential manner as the result of increasing ph and decreasing surface area for fluid rock reactions mass balance calculations are consistent with the modeled relative variation curves and the weathering of plagioclase to smectite and kaolinite and biotite and chlorite to vermiculite

10be concentrations and the long term fate of particle reactive nuclides in five soil profiles from california concentration depth profiles of cosmic ray produced 10 be t 1 2 1 5m y have been measured by accelerator mass spectrometry in five soil profiles these measurements were made in an effort 1 to understand the retentivity of soil surfaces for particle reactive tracers depositing from the atmosphere on time scales of 10 4 10 6 years and 2 to explore the application of 10 be as a chronometer of geomorphic surface age the profiles sampled are from two wave cut terraces located near mendocino california a table mountain top and an alluvial fan both located near friant california the ages of the mendocino terraces are inferred to be 1 5 10 5 years based on amino stratigraphic correlations and models of terrace evolution those of the table mountain top and alluvial fan are 9 5 10 6 years and 6 0 10 5 years respectively based on k ar analyses all the surfaces sampled are nearly flat and exhibit few erosional features in addition to 10 be we measured 210 pb 239 240 pu and 7 be to ascertain the retentivity of the soils for particle reactive nuclides and to assess the present day delivery rate of nuclides from the atmosphere the 7 be inventory is 4 0 dpm cm 2 similar to those observed at nearby locations the inventories of 210 pb and pu isotopes conform to those predicted from model calculations and suggest that the soil surfaces sampled retain the entire burden of particle reactive nuclides delivered to them over short time scales 100 years the 10 be concentrations in the sample range between 0 2 and 7 10 8 atoms g soil and show strong correlations with leachable fe and or al the inventory of 10 be in the soil domain sampled is 1 2 orders of magnitude lower than that expected from the geological age of the surface and an average delivery rate of 10 be from the atmosphere 5 2 10 5 atoms cm 2 yr the low inventory of 10 be is attributed to its loss from the soil domain sampled by solution transport based on a simple model type calculation with a first order removal process for 10 be the residence time of 10 be in the soil domains sampled is determined to be of the order of 10 4 years the low residence time of 10 be in the soil domains sampled requires that it be found either deeper in the regolith or in ground waters in either case the application of 10 be as a chronometer of geomorphic surface age is severely constrained however the study of 10 be in soils provides the only entry into the long term 10 4 10 6 years behavior of particle reactive nuclides in soils and hence could be important for understanding the behavior of analogous nuclides introduced into soils by natural and anthropogenic processes

direct determination of7be in sediments cosmic ray produced7be 53 days half life is directly measurable in freshly collected samples of river and coastal zone sediments using a lithium drifted germanium detector calibration of the detector is discussed and an assessment of overall accuracy and sensitivity presented the detection limit for a 250 g dry weight sediment sample contained in a marinelli beaker and counted for 200 minutes is approximately 0 4 pci g applications toward locating zones of rapid sedimentation as well as in understanding shallow water sedimentation dynamics are suggested

th pb ion microprobe dating of allanite allanite which is a common accessory mineral in a wide variety of rock types typically contains high concentrations of th and u thus an in situ method of u th pb dating of this phase would have broad application we describe a method to permit th pb ages of allanite to be determined with approximately 10 accuracy using a high resolution ion microprobe knowledge of the composition and substitution mechanisms of this complex mineral is key to understanding the relative ion ization efficiencies of th and pb the chemical compositions of three allanite samples used as age standards cima d asta pluton 275 5 1 5 ma atesina volcanic complex 276 3 2 2 ma la posta pluton 94 2 ma were determined using an electron microprobe permitting an assessment of matrix effects on ionization an ion microprobe calibration curve involving elemental and oxide species of th and pb i e 208 pb th vs tho 2 th yields highly scattered apparent ages when allanite age standards with different fe contents are used however a three dimensional plot of 208 pb th vs tho 2 th vs feo sio improves the accuracy of the calibration to about 10 even though this level of uncertainty is substantially greater than that expected for u th pb ion microprobe analyses of zircon or monazite th pb ages of allanite can still be used to address important geologic questions we used this method to date two metamorphic allanite grains from the footwall of the main central thrust nepal himalaya and an allanite grain from the pacoima canyon pegmatite california allanite inclusions in garnet from nepal yield significantly older ages than the coexisting monazite indicating that allanite formation in these rocks records a previous metamorphic cycle that predates slip along the fault the pacoima canyon allanite grain yields a younger age than that reported for zircon implying pb loss during cooling of the pegmatite

mineralogy of the la posta pluton implications for the origin of zoned plutons in the eastern peninsular ranges batholith southern and baja california the 94 ma la posta pluton is located in the eastern zone of the peninsular ranges batholith and is exposed over an area of approximately 1400 km2 it is zoned from a sphene hornblende biotite tonalite rim inward to a core of muscovite biotite granodiorite with internal contacts gradational over distances of several tens to hundreds of meters this pluton is texturally distinct from the older synkinematic plutons of the western zone of the batholith in that it is undeformed and contains sharply euhedral mafic minerals microprobe analyses indicate that hornblendes from the eastern side of the pluton have higher ait and lower si than those from the western side this variation in composition yields geologically unreasonable differences in hornblende geobarometry and reflects hornblende equilibratipn under different thermochemical conditions on either side of the pluton compositional variations within individual mineral suites are consistent with a general model involving inward crystallization of a static magma chamber however biotites in the interior facies are more oxidized and have abruptly higher fe fe mg than those in the marginal fabes these data suggest that the muscovite bearing core facies represents the tail of a diapir that intruded a ballooned magma chamber prior to its complete solidification a longer residence time for the tail adjacent to a cratonal source resulted in its enrichment in at least fe3 and fe fe mg

laser ablation rb sr dating by online chemical separation of rb and sr in an oxygen filled reaction cell abstract the rb sr beta decay dating system is one of the most attractive tools in geochronology as rb is sufficiently abundant in common k bearing minerals like biotite muscovite and k feldspar this allows dating of a wide variety of rocks e g volcanic magmatic metamorphic sedimentary and hydrothermal environments without the need of preconcentration as is often required for u pb dating of zircon however this advantage was to date negatively counteracted by the lack of a suitable in situ technique as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope a reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer icp ms allows exactly this the online chemical separation of two different elements coupled to a laser ablation la system in situ rb sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates sr from rb we present here a simple procedure in which rb sr ages can be obtained from a suite of individual phases in regular thin sections using the most established reaction gas oxygen o 2 it is possible to react part of the ablated sr to sro while no rbo is formed measurements of feldspars and micas with a spot size of 80 m were calibrated against glass standards nist srm 610 bcr 2g and biotite from la posta california 91 6 ma results are presented for a variety of magmatic rocks with well established thermal records a sample each from the klokken syenodiorite greenland 1161 ma the ulvo alkaligabbro sweden 1256 ma and a pegmatite from the bohus granite sweden 920 ma obtained in situ rb sr isochron ages are accurate 87 sr 86 sr ratios are accurate the methodology outlined in this study has significant implications for rb sr studies due to the high spatial resolution 1 quality of measurements can be better controlled by avoiding alterations and inclusions 2 large sample numbers can be investigated simply by using regular thin sections 3 several mineral generations can be targeted that would not be distinguishable in mineral separates and 4 isotope zonation within single crystals can be revealed this will open the rb sr dating system to new fields of study for example thermochronologic studies can use 87 sr 86 sr and 87 rb 86 sr zonation within crystals to better work out cooling and or reheating paths further applications are in the field of provenance studies and temporal evolution of shear zones

tectonic implications of exposure of lower continental crust beneath the iberia abyssal plain northeast atlantic ocean geophysical evidence we present new seismic data from a basement high beneath the southern iberia abyssal plain that appears from the latest ocean drilling program odp results and indirectly from various seismic observations to consist of lower continental crustal material this is a unique opportunity to use seismic profiles and other geophysical evidence to investigate the tectonic process which led to the exhumation of these rocks we infer that the lower crust was exhumed at the seafloor and then uplifted toward the end of rifting our results lead to the following new observations low angle detachment faults previously reported along the east west lusigal 12 profile across the southern iberia abyssal plain penetrate both the 3 10 km thick continental crust and unusually the uppermost mantle processing of new multichannel seismic reflection profiles across the basement high on which odp sites 900 1067 and 1068 are located reveals that the high is bounded by a previously unsuspected steep landward dipping normal fault on its east flank basement cores from the above odp sites also reveal that the high is not capped by the previously predicted early syn rift sediment but by rocks from the lower continental crust these unexpected observations are incorporated into a new tectonic model of the development of this part of the west iberia margin which is also consistent with other geophysical observations a novel feature of this model is the proposal that the seaward edge of the continental crust is thinned to 3 6 km by a currently poorly understood process the model implies three stages of deformation 1 lithospheric extension principally by symmetric pure shear which leads to a 10 km thick crust in which the lower crust is largely absent over the axial zone and the crust mantle boundary forms a shear zone 2 further thinning and then dissection of the most distal continental crust by seaward dipping low angle normal faults 3 inception of a high angle landward dipping normal fault that offsets the tectonized crust mantle boundary and uplifts the lower crust to the crest of the basement high on which odp sites 900 1067 and 1068 were drilled