preservation of extraterrestrial 3he in 480 ma old marine limestones we have measured the helium abundance and isotopic composition of a suite of lower ordovician marine limestones and associated fossil meteorites from kinnekulle sweden limestone 3he 4he ratios as high as 11 5 times the atmospheric value in fused samples and up to 23 times atmospheric in a single step heat fraction indicate the presence of extraterrestrial helium and demonstrate that at least a fraction of the extraterrestrial 3he carried by interplanetary dust particles must be retained against diffusive and diagenetic losses for up to 480 ma the carrier phase has not been identified but is not magnetic extrapolation of high temperature 3he diffusivities in these sediments is consistent with strong retention of extraterrestrial 3he under ambient earth surface conditions combination of the observed helium concentrations with sedimentation rates estimated from conodont biostratigraphy suggest that the flux of extraterrestrial 3he in the early ordovician was about 0 5 10 12 cm 3 stp cm 2 ka 1 ignoring potential post deposition helium loss this value is indistinguishable from the average 3he flux estimated for the cenozoic era in contrast previous studies of fossil meteorites ir abundances and os isotopic ratios in the limestone suggest that the total accretion rate of extraterrestrial material during the studied interval was at least an order of magnitude higher than the cenozoic average this disparity may reflect significant post depositional loss of 3he from idps within these old limestones if so the match between the ordovician flux and the cenozoic average would be fortuitous alternatively the size distribution of infalling objects during the early ordovician may have been enriched only in extraterrestrial material too large to retain 3he during atmospheric entry heating 30 m the fossil meteorites themselves also preserve extraterrestrial helium meteorite 3he concentrations of 2 to 9 10 12 cm 3 stp g 1 are several orders of magnitude lower than found in most modern meteorites suggesting very substantial helium loss probably 99 9 from these chemically altered objects the meteorites carry 3he concentrations only a factor of a few higher than the host limestones the meteorites themselves cannot be the source of the extraterrestrial 3he observed in the limestones

evolution of large normal faults evidence from seismic reflection data recent advances have been made in understanding how extensional faults and basins develop as faults propagate and link evidence for these linkage patterns in seismic reflection data can be seen in data from east africa early fault linkage patterns for boundary faults can follow three possible paths fault linkage and propagation occur either 1 prior to significant basin formation 2 after minor faulting has created an extensive area of subsidence or 3 during basin development the data from east africa show examples mainly of paths 1 and 2 transverse anticlines anticlines developed parallel to and in the hanging wall of the strike of faults associated with boundary faults are common features they represent either the sites of old synthetic transfer zones or a region of low fault displacement along the strike of a fault where two or more depocenters of different ages overlap as fault activity decreases over time displacement tends to be concentrated on progressively narrower parts of the fault this pattern is developed particularly well in continental rifts and may help discriminate late synrift sedimentation from postrift sedimentation where strike lines across the hanging wall of the fault are observable

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

influence of rift obliquity on fault population systematics results of experimental clay models abstract we use clay models to simulate how fault population systematics vary as a function of rift obliquity rift obliquity is related to the acute angle between the rift trend and the displacement direction so that the value of is inverse to the degree of obliquity the range of azimuths in a fault population increases as rift obliquity increases i e as decreases the length of the longest faults the sum of fault lengths and the width of the deformed zone all increase as rift obliquity decreases i e as increases the majority of faults in our models are segmented and have highly tortuous traces tortuosity is maximum when 30 as segments of widely varying azimuth link during fault growth significant changes occur in the fault patterns between 30 and 45 at 30 two fault populations of equal importance develop in the center of the rift zone one approximately rift parallel and the other displacement normal between 30 and 45 the number of faults more than doubles and between 45 and 60 summed fault length more than doubles fault patterns for all models are fractal with fractal dimensions that increase with increasing and that are comparable to those found in the field fault populations are not multi fractal because the cumulative frequency distributions of fault lengths do not generally follow a power law relationship an exponential distribution best describes the data for whole faults with the characteristic length increasing with increasing segment lengths also follow an exponential distribution with characteristic length varying very little with the fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid ocean ridge segments and are similar in geometry to those in oblique continental rift basins at the rift margins tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions resulting in the formation of two distinct sub populations of faults during oblique rifting

search for the origin of exotic helium in deep sea sediments merrihue1 first observed helium with a very high 3he 4he isotope ratio 10 4 here termed exotic helium in a magnetic separate of a deep ocean sediment and attributed it to input of extraterrestrial material krylov et al 2 subsequently reported the existence of exotic he in several deep sea sediments while tilles3 measured argon isotope ratios in magnetic separates of two ocean sediments observing that the 3he 4he ratio is inversely correlated with the sedimentation rate ozima et al 4 concluded that the exotic he is carried by interplanetary dust particles idps we have now examined other deep sea sediments to characterize the exotic he and to identify its carrier and have confirmed that the exotic he resides mainly in a strongly magnetic fraction stepwise degassing experiments suggest that the exotic he consists of a single component the carrier of the he is likely to be chondritic idps the higher level of exotic he in magnetic fractions of sediments can be explained by secondarily produced magnetite in idps as a result of thermal shock during atmospheric entry

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

on the fluxes and fates of 3he accreted by the earth with extraterrestrial particles abstract the influx of extraterrestrial et matter on the earth results in appreciable fluxes of 3 he originating from several different processes we have analyzed the published data on the observed concentrations of 3 he in interplanetary dust particles idps sampled in the stratosphere in polar ice samples and in meteorites this information considered together with production of secondary fragments during the atmospheric entry of meteoroids allows us to make plausible estimates of fluxes of the extra terrestrial fluxes of the two principal 3 he types accreted along with the extraterrestrial materials i solar wind sw and solar energetic particle sep 3 he implanted in et particles and ii 3 he produced in the et matrix by interactions of galactic cosmic ray gcr particles an appreciable fraction of 3 he brought to the earth by et particles is expected to be lost from the particles during ablation and fragmentation of the particles during their transit through the atmosphere by comparing the measured 3 he fluxes in two terrestrial archives polar ice and marine sediments and based on supplementary data 3 he 4 he and 20 ne 22 ne ratios we have attempted to determine the relative fluxes of solar sw and sep and gcr 3 he that are preserved in them our analyses establish the fact that the reported 3 he concentrations in particles in relatively small size polar ice samples from greenland and antarctica are primarily due to the solar 3 he accreted via idps of 2 5 m radii scavenged in the polar ice we predict that if larger ice samples were analyzed greater contributions would be found from gcr 3 he present in fragments of meteoroids produced during their entry through the atmosphere it follows therefore that long term fluxes of idps can be estimated from studies of 3 he in relatively small samples of polar ice sheets one observes appreciable concentrations of 3 he in marine sediments extensive observational data are available for sediments in the atlantic and pacific oceans we show that besides idps there is an important source which is an important contributor to the flux of 3 he in marine sediments this is the gcr produced 3 he in meteoroids of sizes up to 1 m we compare the measured 3 he fluxes in marine sediments with those predicted from solar 3 he in idps and from gcr produced 3 he in secondary fragments of meteoroids of sizes up to 1 m produced during their transit through the atmosphere our studies allow us to reach plausible conclusions regarding the predominant source of 3 he in marine sediments

extra terrestrial noble gases in deep sea sediments abstract magnetic fractions separated from four deep sea sediments from the pacific ocean were analysed for elemental abundances and isotopic ratios of noble gases by stepwise heating a 3 he 4 he ratio of 1 8 0 4 10 4 was observed in most samples in the high temperature fractions in all the samples 20 ne 22 ne ratios are constant 11 6 0 6 in most temperature fractions these ratios are different from he a ne a and he b ne b and appear to come from a unique component the 40 ar 36 ar ratios in high temperature fractions are lower than the atmospheric ratio indicating the existence of extra terrestrial ar while the 38 ar 36 ar ratios are indistinguishable from atmospheric planetary and solar ar indicating negligible spallogenic ar the isotopic ratios of kr and xe are indistinguishable from atmospheric ratios from the 20 ne 22 ne ratios and from thermal release patterns of he we conclude that the extra terrestrial noble gases are implanted solar flare particles the energy range of the solar flares is estimated from the relation between the grain size of idps interplanetary dust particles and the penetration depth of particles impinging on to a grain which gives less than 1 mev n the uniformity of isotopic ratios of he and ne can be explained by a grain size sieving effect of idps during atmospheric entry and by the subsequent diffusion loss of shallowly implanted solar wind noble gases

unusual neon isotopic composition in neoproterozoic sedimentary rocks fluorine bearing mineral contribution or trace of an impact event abstract extraterrestrial materials have he and ne isotopic compositions that are distinct from those of the earth s surface in order to track the extraterrestrial material accreted onto earth during the ediacaran period we have analyzed the he and ne isotopic composition of thirteen sedimentary rocks in the age range 550 600 ma coming from the huqf supergroup in oman for which carbon and sulfur isotopic data have been characterized previously 3he 4he ratios range between 0 006 0 003 and 0 27 0 01 ra with ra being the atmospheric ratio 3he 4he ratios show a positive relationship with 3he contents ranging between 0 6 and 31 10 13 cm3 stp g 1 the 3he contents are within the literature data for 3 to 480 myr old samples with evidence of idp 3he idp for interplanetary dust particles suggesting that extraterrestrial 3he is still retained in such old samples 20ne 22ne ratios are close to or below the modern atmospheric ratio of 9 8 with the minimum value equal to 9 05 0 03 21ne 22ne ratios show a high range of variation going from 0 0345 0 0009 to 0 0935 0 0023 the ne isotopic compositions follow a nucleogenic trend similar to that of crustal fluids from the literature and predicted continental crust however one sample 3404 shows an unusual ne isotopic composition with a lower 20ne 22ne at similar 21ne 22ne compared to the other samples two hypotheses can explain this singular ne isotopic composition first it could be the result of a particular nucleogenic trend due to the presence of f bearing minerals in this sample sem eds elemental mapping showed that although f and ca rich phases which could correspond to fluorites are present in sample 3404 however their abundance of 0 15 seems too low to explain the unusual ne isotopic composition however due to the high uncertainty of the calculations we cannot totally rule out this hypothesis alternatively the singular ne isotopic composition could be due to the presence of a ne a component a component characterizing pre solar diamonds contained in chondrites this would indicate that a major object impacted the earth at the time the sediment was forming between 600 and 550 ma which is coherent with the estimated age range of the acraman impact in australia

size fractionation reproducibility and provenance of helium isotopes in north equatorial pacific pelagic clays helium isotope compositions were measured in late quaternary pelagic clays from the north equatorial pacific where terrestrial and extra terrestrial end member components are identified and quantified in different grain size populations the results provide important constraints on the nature of interplanetary dust particles idps in sedimentary environments and on their application as constant flux proxies in paleoceanographic studies approximately 92 95 of the extra terrestrial 3 he 3 heet is concentrated in particles smaller than 20 mm within this grain size group 44 of 3 heet is concentrated in the 20 10 and 10 2 mm fractions while more than 50 of the total 3 heet is found in fine particles smaller than 2 mm the peak concentration of 3 heet in the very fine particle size population explains a consistently better reproducibility of 3 heet in sediments compared to model predictions that is related to the relatively homogenous distribution of clay size particles in sediments compared to coarser particles especially in deep sea sediments and ice cores the 3 he 4 he ratios are highest in the coarser than 20 mm fractions peaking between 63 and 38 mm 1 7871 13 10 4 compared to the bulk and the o20 mm fractions 3 2072 62 10 5 and 2 1170 88 10 5 respectively the isotopic composition of helium in each of the size fractions reflects a mixture between a pure extraterrestrial end member and a terrigenous component which is itself comprised of varying mixtures of volcanic material and eolian material associated with the fine grain size fraction of loess deposits in south east asia poor 3 heet reproducibility observed here and elsewhere inhibiting its application as a constant flux proxy stems from the presence of a small number of aliquots displaying significantly higher 3 heet contents compared to the average of their reference group these helium rich particles hrps are probably characterized by a o20 mm diameter and carry significant amounts of extraterrestrial 3 he and 4 he careful rejection of such aliquots improves the signal reproducibility by more than two fold

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

4 d evolution of rift systems insights from scaled physical models the four dimensional 4 d evolution of brittle fault systems in orthogonal oblique and offset rift systems has been simulated by scaled sandbox models using dry cohesionless fine grained quartz sand extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model the results of these analog model studies are compared with natural examples of rift fault systems rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction segmentation of the rift margin increases with increase in obliquity of the rift axis resulting in a consequent increase in displacement on intrarift fault systems offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone along strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones high relief extension parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults the second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low relief oblique slip zone or grabens these typically are found in highly oblique rift systems 45 degrees and above right steps in offset rift models

kinematic analysis of faults in a physical model of growth faulting above a viscous salt analogue abstract syn sedimentary faults overlying an extended ductile layer within a sand model provide an analogue for studying some aspects of the geometry and kinematics of basin scale extensional listric faults associated with salt rollers the displacement analysis techniques used to examine displacement variations over fault surfaces within the sand model show that displacement varies in a regular and predictable manner deviations from simple fault displacement patterns are due to transfers of displacement onto adjacent synthetic faults aggregation of the displacements on adjacent synthetic faults produces more regular patterns of displacement distribution and demonstrates the geometric coherence of the aggregated faults rapid upward decreases in displacement on the model faults are due to fault growth during sedimentation elsewhere in the model post sedimentary faulting is associated with very low upward displacement gradients and pre and syn faulting sedimentation are readily distinguished on cross sectional fault traces in three dimensions a non horizontal line separating the pre faulting and syn faulting sequences tracks the lateral extent of a fault through time backstripping of fault displacements by subtracting the fault displacement of a sedimentary horizon from the displacements of underlying horizons gives the displacement distribution on a fault surface at the time of deposition of the backstripped horizon backstripped fault displacement distributions also allow examination of the general fault structure through time backstripping of fault displacements on one of the larger faults within the model and on an associated hanging wall synthetic splay shows that the two structures originated as a relay zone which was breached by lateral propagation of one of the overlapping faults which form the relay zone the backstripping technique allows determination of the geometrical changes during growth of a fault system

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

maximum link distance between strike slip faults observations and constraints the maximum fault link distance lateral separation between two interacting strike slip faults is studied in relation to fault length combined length of the two interacting faults data collected for laboratory generated strike slip faults indicate that fault linkage takes place when the lateral separation between two strike slip faults is less than one tenth of the combined length of the two faults strike slip faults developed in clay models require a smaller link distance than those developed in gouge materials about 98 of the data collected for natural strike slip faults and 93 of earthquake rupture data derived from knuepfer 1989 follow the rule these observations support a simple scaling relationship for strike slip faults i e w 0 1 l where wis lateral link distance and l the combined fault length an and sammis 1996a two possible explanations are discussed assuming a fault continues to be dominantly strike slip after various fault coalescences the fault link distance for both compressional coalescence leading to restraining bend and extensional coalescence leading to releasing bend can be constrained within 0 134 l assuming fault coalescence is caused by interaction between breakdown zones at fault tips then the scaling relationship b 0 1 l between the size of a breakdown zone b and the individual fault length l scholz et al 1993 leads to the scaling relationship w max 0 1 l where w max is the maximum link distance the above observed relationship between w max and l may not hold if faults coalesce through preexisting faults if a fault has a mixed mode other than strike slip or if a fault bend is caused by cross faulting rather than fault coalescence limited exposure of natural fault traces may also lead to errors in link distance measurements

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

extraterrestrial 3he in seafloor sediments evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust orbital parameters and quaternary climate we have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the ontong java plateau western equatorial pacific ocean odp site 806 these results provide a two million year record of the burial flux of extraterrestrial 3he which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles the 3he burial flux prior to 700 ka was relatively low 0 5 pcc cm 2 kyr 1 but from 700 ka to the present the burial flux gradually increased to a value of 1 0 pcc cm 2 kyr 1 100 kyr periodicity in the 3he burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate with high 3he burial fluxes associated with interglacial periods this periodicity and phase are consistent with previous 3he measurements in north atlantic sediments although 100 kyr periodicity in 3he burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris the measurements and predictions differ by one half cycle in phase nevertheless our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and or inclination relative to the solar system invariable plane such control is a necessary but not sufficient condition for the hypothesis of muller and macdonald 1995 that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period we also identify several brief 25 kyr intervals of strongly enhanced 3he burial possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

helium isotopes in ferromanganese crusts from the central pacific ocean abstract helium isotopes have been measured in samples of two ferromanganese crusts va13 2 and cd29 2 from the central pacific ocean with the exception of the deepest part of crust cd29 2 the data can be explained by a mixture of implanted solar and galactic cosmic ray produced gcr he in extraterrestrial grains and radiogenic he in wind borne continental dust grains 4he concentrations are invariant and require retention of less than 12 of the in situ he produced since crust formation loss has occurred by recoil and diffusion high 4he in cd29 2 samples older than 42 ma are correlated with phosphatization and can be explained by retention of up to 12 of the in situ produced 4he 3he 4he of va13 2 samples varies from 18 5 to 1852 ra due almost entirely to variation in the extraterrestrial he contribution the highest 3he 4he is comparable to the highest values measured in interplanetary dust particles idps and micrometeorites mms helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in falling terrestrial and extraterrestrial grains of fe mn crusts the extraterrestrial 3he concentration of the crusts rules out whole undegassed 4 40 m diameter idps as the host instead it requires that the extraterrestrial he inventory is carried by numerous particles with significantly lower he concentrations and occasional high concentration gcr he bearing particles

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

extraterrestrial 3 he as a tracer of marine sediment transport and accumulation the deposition rate of deep sea sediments and their focused redeposition by deep sea currents can be evaluated from analyses of sedimentary 230th with a temporal resolution limited only by bioturbation6 7 10 11 230th is produced uniformly throughout the ocean by radioactive decay of dissolved 234u and is removed sufficiently fast by sorption onto sinking particles to act as a constant flux tracer of sedimentation rates but the half life of 230th 75 kyr limits its use for this purpose to the past 200 250 kyr here we explore the use of extraterrestrial 3he from interplanetary dust particles1 4 idps as a constant flux proxy that is free from this limitation a comparison of 3he with 230th in two cores from the equatorial pacific ocean indicates that the variability in the mean flux of idps over the past 200 kyr is less than 75 but in contrast to this relatively constant rate of supply of 3he to the deep sea the local burial rates of 3he and 230th have varied by a factor of five over the past 450 and 200 kyr respect ively we interpret this variability as reflecting sediment focusing with a temporal pattern that suggests regular cycles of climate driven reorganization of near bottom currents in the deep pacific ocean

evaluating the accretion of meteoritic debris and interplanetary dust particles in the gpc 3 sediment core using noble gas and mineralogical tracers abstract extraterrestrial et noble gases helium and neon in 35 sediment samples from central pacific core ll 44 gpc 3 demonstrate the variable flux of interplanetary dust particles idps and major meteorite impacts over the past 70 ma 21 72 ma spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of idps including the well established cretaceous tertiary k t and late eocene e o impact boundaries no spinel grains with chemical or mineralogical evidence of a distinctly et origin were found in an extensive survey of background samples i e non e o or k t boundary suggesting that either the carrier grains for et noble gas occur within the fe ti oxide mineral fraction observed in this study found to include ilmenite and ulvospinel or are too small for identification by sem the presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for et noble gas rich particles noble gas analysis on both the emf extractable magnetic fraction and the bulk minus emf bulk emf show that the he and ne compositions are consistent with partially degassed noble gas signatures of zero age magnetic grains z mag and stratospheric interplanetary dust particles idps conclusive evidence for a planetary ne a noble gas signature is found only in the bulk sediments at the k t boundary although all gpc 3 k t fractions bulk emf and hf digestion plot along a mixing line between planetary ne a and solar wind sw spinels from major impact boundaries k t e o exhibit dendritic texture and elevated ni consistent with previous reports in contrast to the otherwise consistent 3he signal from idps the 3he at the known impact boundaries k t and e o actually decreases these anomalously low 3he are accompanied by significantly elevated ne and significantly lower 3he 20ne solar ratios 10 lower produced by both preferentially degassing of he relative to ne at times of increased flux of larger et material degassed degassed he enriched ne profile noble gas characteristics occur in two sample intervals that do not correspond to any known impact events 47 and 71 ma explained by an influx of larger particles sem analysis of the 47 ma sample shows spinels with dendritic textures but without distinctive markers of large meteorite impacts e g elevated ni particle size increases and degassed signatures may be caused by major bolides micrometeorites comet showers or simply a flux of larger idps potentially with a different source

sand box modelling of basement controlled transfer zones in extensional domains the interaction modalities of transfer zones connecting rift segments may be influenced by several factors amongst these the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies sand box models were made to investigate how the orientation geometry and kinematics of transfer zones depend upon pre existing basement anisotropies analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults strike slip faults are present inside the transfer zone provided that angle between the extension direction and the axis of the transfer zone is less than 50 narrower transfer zones striking oblique to the extension direction occur for angle between the direction perpendicular to the extension direction and the direction of the basement anisotropy less than 90 wider transfer zones subparallel to the extensional direction occur for 90 increasing the overstep induces narrower transfer zones striking subparallel to the extension direction similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts the comparison between experimental and natural data shows how the geometry kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies transfer zones along inherited basement structures set at lower angles to the extension direction and display lower aspect ratios than transfer zones where inherited anisotropies are absent

high 3 he 4 he ratio in ocean sediments two decades ago merrihue1 reported 3he 4he ratios of 10 4 in ferromagnetic separates from a pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to 1 of the sediment a decade later krylov et al 2 compiled 3he 4he isotopic data on ocean sediments measured in the soviet union and observed that the 3he 4he ratio is generally higher in pelagic sediments where the sedimentation rate is lower they suggested that the high 3he 4he ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor however these important discoveries were almost completely neglected until we re examined the problem we have measured 39 sediments from 12 different sites 10 sites from the western to central pacific and two sites from the atlantic ocean we find 3he 4he ratios 5 10 5 for six sites well above the values generally observed in common terrestrial materials the very high 3he 4he ratio in the sediments is probably due to input of extraterrestrial materials input of stratospheric dust of 1 p p m which corresponds to a fallout rate of 2 000 tons per year can explain the observation

oil and gas fields transfer zone relationships thrace basin nw turkey in the thrace basin oil and gas are produced from carbonate and sandstone reservoirs of eocene and oligocene age detailed geological and geophysical studies reveal that earlier and eocene structures were influenced by basement paleotopography resulting from late cretaceous movements between the arabian and anatolian plates while latest miocene structures were generated by splays of the north anatolian fault in the thrace basin the eocene structures which contain the hamitabat gas and devecatagi and k osmancik oil fields trend in a ne sw direction however miocene structures which are gas producers in the karacaoglan and umurca fields are oriented in a nw se direction parallel to the strike of the main faults which accommodate the transfer zones in the thrace basin the transfer zones occur between faults that dip in opposite directions conjugate and in the same directions synthetic the formation of reservoir zones improvement of reservoir characteristics and generation of oil and gas in the oil window zone were influenced by the timing of faulting and appearance of the transfer zones in the thrace basin

terrigenous helium in deep sea sediments abstract we have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep sea sediments these materials include ice rafted debris from the north atlantic chinese loess and sediment collected at or near the mouths of three large rivers the amazon the ganges and the yangtze we observe terrigenous 3he 4he ratios that vary from 1 8 10 9 to 4 6 10 7 i e values that are higher than the theoretically derived range of 10 9 to 10 8 atlantic and pacific deep sea sediment 3he 4he ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources terrigenous sources for north atlantic and north pacific sediments are characterized by 3he and 4he contents that are higher and 3he 4he ratios that are lower than those for central and eastern equatorial pacific sediments this is consistent with the supply to the north atlantic and north pacific oceans of older cratonic continental material that contains high amounts of in situ produced nucleogenic and radiogenic helium terrigenous material transported to central and eastern equatorial pacific sediments contains lower amounts of 3he and 4he and higher 3he 4he ratios indicative of supply from a more juvenile andean source in the equatorial atlantic core v31 135 we have used previously published 230th data to determine an extraterrestrial 3he flux of 1 16 0 15 10 12 cm3stp cm 2 ka 1 within error of that previously determined in sediments from the equatorial pacific ocean 0 78 0 29 10 13 cm3stp cm 2 ka 1 marcantonio et al 1996

accretion of extraterrestrial matter during the last 80 million years and its effect on the marine osmium isotope record the flux of particulate extraterrestrial et matter to the deep sea has been calculated using a four component mixing model based on osmium concentrations and isotope ratios in slowly accumulating pelagic sediments from the pacific ocean nineteen published bulk leach osmium isotope data pairs that cover the last 80 million years have been used for the calculation the calculated annual particulate flux averaged over several 100 ka by slow accumulation and bioturbation ranges from 18 000 to 67 000 tons with a mean value of 37 000 13 000 tons the data indicate no significant variability in the flux with time except at the k t boundary and are thus compatible with results based on ir accumulation in deep sea sediments the inferred constancy of the et matter flux is in contrast to recent results based on the et 3he flux that integrates only a small size fraction of the entire et matter flux 3he data indicate variability by a factor of 6 over the past 70 ma and a fivefold increase in the et flux between 3 ma and 1 ma that is not seen in the bulk et matter flux based on osmium isotope data the apparent constancy of the cosmic matter flux over the past 80 ma stands in marked contrast to the dynamic nature of the marine osmium isotope record and indicates that dissolution of cosmic matter does not drive changes in this record except at the k t boundary a comparison between osmium isotope and ir data from two pelagic sediment cores from the pacific dsdp site 596 and ll44 gpc3 indicates that the seawater soluble fraction of et os cannot exceed 36 kg a and most likely is significantly smaller the maximum value of 36 kg a can account for approximately one half of the seawater soluble et matter flux necessary to balance the radiogenic continental runoff of os without any additional unradiogenic source simple mass balance calculations indicate that an additional unradiogenic source of os to the oceans most likely alteration of oceanic crust is required to balance the present day seawater osmium isotopic composition this source is probably more important in balancing radiogenic continental runoff than is dissolution of cosmic matter in seawater

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

propagation and linkage of oceanic ridge segments we have investigated the propagation of spreading ridges and the development of structures that link ridge segments using an analogy between ridges and cracks in elastic plates the ridge propagation force and a path factor that controls propagation direction were calculated for echelon ridge segments propagating toward each other the ridge propagation force increases as ridge ends approach but then declines sharply as the ends pass so ridge segments may overlap somewhat the sign of the path factor changes as ridge ends approach and pass so the overlapping ridge ends may diverge and then converge following a hook shaped path the magnitudes of shear stresses in the plane of the plate and orientations of maximum shear planes between adjacent ridge segments were calculated to study transform faulting for different loading conditions simulating ridge push plate pull and ridge suction we identify a zone of intense mechanical interaction between adjacent ridge ends in which stresses are concentrated for all conditions the shear stress in the interaction zone increases as ends approach and remains large as the ends overlap thus crust in this zone may fracture and weaken in preparation for the formation of a through going transform fault the calculated shear planes rotate toward an orientation about 90 from the strike of ridges as the ends pass thus favoring the orthogonal arrangement of ridges and transforms the magnitudes of mean stresses in the plane of the plate and orientations of principal stress planes were also calculated the mean stress is tensile in the interaction zone so basins may form there except in the case of ridge push loading the planes across which the maximum tension acts are oblique to ridges thus favoring obliquely oriented normal faults bounding the transform valley

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

temporal variation of 3he 4he ratio recorded in deep sea sediment cores he isotopic composition was measured for 72 deep sea sediments from 16 sites in the pacific and atlantic oceans and the japan sea 3he content ranges from less than 10 12 cm3 stp g to about 6 10 11 cm3 stp g and 3he 4he from about 10 8 to 1 67 10 4 the he isotopic data lie fairly well on a mixing curve in a 3he 4he 3he diagram where the end members of the mixing curve are extraterrestrial he in interplanetary dust particles idp s and terrestrial he in ocean sediments this confirms the view that there are significant amounts of extraterrestrial he in ocean sediments from global fallout of idp s into deep sea sediments 3he flux estimated for two long cores gives a value of about 1 5 1 0 10 15 cm3 stp cm2 yr over a time span of 40 m y which corresponds to an annual fallout rate of idp s size 10 m of about 400 t in one of the cores we observe a 3he spike which is associated with anomalously high ir content 1 86 0 56 ppb this may be due to meteorite debris

an alternative age model for the paleocene eocene thermal maximum using extraterrestrial 3he a continuous age model for the brief climate excursion at the paleocene eocene boundary has been constructed by assuming a constant flux of extraterrestrial 3he 3he et to the seafloor 3he et measurements from odp site 690 provide quantitative evidence for the rapid onset few kyr and short duration 120 kyr of global warming and of the associated disturbance to the earth s surficial carbon budget at this time these observations support astronomically calibrated age models indicating extremely rapid release of isotopically light carbon possibly from seafloor methane hydrate as the proximal cause of the event however the 3he et technique indicates a previously unrecognized and extreme increase in sedimentation rate coincident with the return of climate proxies to pre event values the 3he et based age model thus suggests a far more rapid recovery from the climatic perturbation than previously proposed or predicted on the basis of the modern carbon cycle and so may indicate additional or accelerated mechanisms of carbon removal from the ocean atmosphere system during this period 3he et was also measured at odp site 1051 to test the validity of the site 690 chronology comparison of these data sets seems to require removal of several tens of kyr of sediment within the climatic excursion at site 1051 an observation consistent with sediment structures and previous age modeling efforts the site 1051 age model shows a 30 kyr period in which climate proxies return toward pre event values after which they remain invariant for 80 kyr if this rise represents the recovery interval identified at site 690 then the 3he et based age models of the two sites are in good agreement however alternative interpretations are possible and work on less disrupted sites is required to evaluate the reliability of the proposed new chronology of the climate excursion regardless of these details this work shows that the 3he et technique can provide useful independent evidence for the development and testing of astronomically calibrated age models

laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he to earth helium isotopes have been measured in 45 putative micrometeorites in the size range 50 to 400 m that were extracted from antarctic ice forty of the antarctic micrometeorites amms are irregular in shape and display varying degrees of partial melting whereas five are spherical and thought to represent completely melted particles of the 30 50 to 100 m suspected micrometeorites only 8 released neither 3he or 4he sixteen released 3he and 4he 0 004 3 5 10 3 cm3 stp g having 3he 4he in the range 1 5 10 4 to 4 1 10 4 six particles released only 3he yielding minimum 3he 4he of 1 6 to 10 6 10 4 5 of which fall in the range of values for amms that had measurable 3he and 4he only 2 of the 15 100 to 400 m amms released measurable 4he 4 44 10 7 cc g 1 these have 3he 4he of 6 10 4 and 20 10 4 respectively significantly higher than the majority of the ratios of 50 to 100 m amms implanted solar energetic particles are the dominant source of he in the 50 to 100 m amms with small contributions of solar wind and or spallogenic he present in 10 particles the absence of sw he is likely due to the loss of the outermost layers of the micrometeorites by ablation during atmospheric entry and abrasion during residence in ice and sampling spallation he is most important in the 100 to 400 m amms consistent with their smaller surface area to volume ratio maximum spallation 3he exposure ages of six particles are close to that predicted from poynting robertson spiral in times of asteroidal micrometeorites the two terrestrial particles identified by chemical composition and mineralogy released only 4he yielding radiogenic he isotope compositions an inverse relationship between particle sizes and solar he abundance is explained by he loss during entry helium loss is likely controlled by phyllosilicate breakdown where present and diffusion controlled bubble growth and rupture rather than melting the 50 to 100 m micrometeorites deliver 8 10 8 cm3 stp 3he g to earth this is more than 100 times less than transported by 20 m particles 2 10 5 cm3 stp 3he g but because 50 to 100 m micrometeorites deliver approximately 20 times more extraterrestrial matter to earth than 20 m particles the larger particles may contribute 5 of the total flux of extraterrestrial 3he to terrestrial sediments the mean he concentrations of the 50 to 100 m amms antarctic micrometeorites and the five spherules 2 1 10 9 cm3 stp 3he g 1 overlap those of the magnetic fraction of deep sea sediments 0 8 68 10 9 cm3 stp g a combination of this material could provide the extraterrestrial 3he measured in ancient deep sea sediments without calling upon the survival of friable 20 m chondritic interplanetary dust particles for tens of millions of years

analog modeling of pull apart basins scaled sandbox models have successfully simulated the geometries and progressive evolution of pull apart basins developed in a weak sedimentary cover above right stepping releasing dextral strike slip fault systems in rigid basement synkinematic basin infill was added progressively as the models were deformed vertical and horizontal sectioning of the completed models has allowed the full three dimensional architecture of the pull apart system to be analyzed we present three representative end member experiments 30 underlapping releasing sidestep 90 sharp nonoverlapping releasing sidestep and 150 overlapping releasing sidestep the pull apart basin geometries are typically sigmoidal to rhombic grabens the geometries of which are dependent upon the architecture of the underlying basement fault systems underlapping releasing sidestepping faults offset angles of 30 75 typically form elongate rhomboidal grabens 90 releasing offset sidesteps produce shorter squat rhomboid pull apart basins and overlapping releasing sidesteps 115 150 produce box like grabens with highly kinked basin sidewalls the pull apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones pdz of the main dextral strike slip faults the sidewall faults show changes in kinematics from dominantly dip slip extension in copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received june 24 1996 revised manuscript received november 20 1996 final acceptance june 10 1997 2fault dynamics project geology department royal holloway university of london egham surrey tw20 oex united kingdom supported by the fault dynamics project sponsored by arco british limited petrobras u k ltd bp exploration conoco u k limited mobil north sea limited and sun oil britain we also acknowledge additional funding from sun oil britain landsat thematic mapper data courtesy of rtz mining and exploration limited south american division jurrian reijs is thanked for field data relating to the salina del fraile pull apart basin chris elders is thanked for reviewing earlier versions of this manuscript and j crowell c morley and k biddle are thanked for constructive reviews of the manuscript the contents of this paper have been

a 35 myr record of helium in pelagic limestones from italy implications for interplanetary dust accretion from the early maastrichtian to the middle eocene we have determined the helium concentration and isotopic composition of a suite of early maastrichtian through middle eocene pelagic limestones in the italian apennines the results provide a 35 myr record of the implied flux of extraterrestrial 3he which is a proxy for the accretion rate of interplanetary dust particles idps our measurements show that the 3he flux was fairly constant in the maastrichtian except for possible minor increases factor of two or less from 70 5 ma to 68 ma and at 66 ma which probably reflect transient increases in the accretion rate of asteroidal and or cometary idps we find no evidence for an increase in idp accretion at or immediately before the k t boundary implying that the k t impact was not associated with enhanced solar system dustiness this observation precludes the possibility that the k t impactor was a member of a major comet shower and is more consistent with impact of a lone comet or asteroid our data suggest a 2 to 4 fold increase in idp accretion between 57 and 54 ma followed by a factor of three decrease over an 4 myr period in the early to middle eocene the duration and magnitude of this variability is inconsistent with previous observations attributed to a shower of long period comets and is more likely the result of collisions in the asteroid belt and or kuiper belt in the entire 35 myr record we find no evidence for major enhancements of the idp accretion rate of the type expected from comet showers our results in combination with earlier 3he measurements do not support models that predict recurrent comet showers with periods of 38 myrs if there is a periodicity in the cratering record that is caused by periodic modulation of the oort cloud it is not evident in the apennine sediment data along with the 3he measurements we also obtained 4he concentrations which record temporal changes in the flux or composition of terrigenous matter the most significant change in 4he occurs in the last 4 myrs of the cretaceous over which the concentration of 4he in the detrital component rises by 300 this rise tracks a strong increase in the seawater 87 sr 86 sr ratio suggesting a globally significant change in the composition of continental detritus delivered to the oceans possibly arising from increased continental weathering

scales of structural heterogeneity within neotectonic normal fault zones in the aegean region abstract structural heterogeneities within neotectonic normal fault zones in the aegean region are important controls on the geomorphic expression of their uplifted footwall blocks contrasting fault rocks and slip plane phenomena corrugations comb fractures slip parallel fractures and pluck holes are responsible for mesoscopic inhomogeneities within fault scarps along strike disruptions in the continuity of fault scarps step over zones step up zones step down zones footwall cross faults and hangingwall cross faults are products of perturbations in slip plane geometry or intersections with transverse faults large scale variations in range front morphology reflect contrasting patterns of deformation within fault zones some major fault zones and many minor fault zones are characterized by a migration of the active slip plane towards the hangingwall resulting in a distributed network of high angle faults and a range front which is stepped in profile the presence of a localized mass of uneroded bedrock hangingwall salients protruding from the hangingwall of some major fault zones results in intense deformation along a principal slip plane and a range front which is ramp like in profile as a consequence of structural heterogeneities within neotectonic normal fault zones fault scarps and range fronts in the aegean region are subject to a temporally and spatially variable pattern of degradation

the accretion rate of extraterrestrial 3he based on oceanic 230th flux and the relation to os isotope variation over the past 200 000 years in an indian ocean core abstract in the eastern equatorial indian ocean the flux of extraterrestrial 3 he a proxy of interplanetary dust particles idps has been relatively constant over the past 200 ka the flux is equal to 1 1 0 4 10 12 cm 3 stp cm 2 ka 1 a value obtained using the xs 230 th profiling method variations in mass accumulation rates mars derived assuming a constant extraterrestrial 3 he flux have a 40 ka periodicity similar to that observed in the 18 o derived mars this frequency is similar to that of the earth s obliquity measured 187 os 188 os ratios are less radiogenic than present day seawater 0 49 0 98 reflecting the mixing of os derived from extraterrestrial terrigenous and hydrogenous sources when coupled with he data measured on the same samples os isotope data yield important information about the terrigenous component supplied to the eastern equatorial indian ocean the amount of os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics once corrected for the extraterrestrial component of os os isotope signatures in conjunction with the 4 he concentrations suggest a supply of terrigenous material from indonesian ultramafic and himalayan crustal sources that clearly varies through time

abrupt intensification of the sw indian ocean monsoon during the last deglaciation constraints from th pa and he isotopes abstract sediments from western arabian sea core 74kl representing the last 23 ka were analyzed for helium thorium and protactinium isotopes assuming global average fluxes of extraterrestrial 3he and 230th the average 3he derived sediment mass accumulation rate mar is a factor of 1 8 higher than the average 230th derived mar 3he and 230th derived mars converge however during the younger dryas yd and during the peak of the early holocene humid interval these features not seen anywhere else in the world probably reflect a combination of climate driven changes in the flux of 230th and 3he ratios of xs231pa xs230th proxies of paleoproductivity are lowest during the last glacial maximum lgm and increase abruptly during the bolling allerod later following a sudden decrease to near lgm values during the yd they rise abruptly to maximum values for the entire record in the early holocene we hypothesize that low xs231pa xs230th ratios reflect low productivity due to the decreased intensity of the sw monsoon whereas the opposite is true for high ratios the correlation between arabian sea productivity and monsoonal upwelling on the one hand and north atlantic climate variability on the other suggests a linkage between high and low latitude climates caused by changing patterns of atmospheric circulation

estimation of the extraterrestrial 3 he and 20 ne fluxes on earth from he and ne systematics in marine sediments abstract sediments contain interplanetary dust particles idps carrying extraterrestrial noble gases such as 3 he which have previously been used to estimate the idp accretion flux over time and the duration of past environmental events however due to its high diffusivity he can be lost by diffusion either due to frictional heating during entry in the atmosphere or once it has been incorporated in the sediments therefore the absolute values of 3 he idp fluxes cannot be known due to its lower diffusivity ne is less likely to be lost by diffusion than he and can potentially provide an absolute idp flux value here we studied the ne and he isotopic composition of 21 sediments of different ages 3 to 38 myr 56 myr and 183 myr in order to better constrain the retention of 3 he in such deposits the samples are carbonates from 2 sites of the integrated ocean drilling program iodp which previously showed evidence of detectable extraterrestrial 3 he and from the sancerre core in the paris basin the 3 he 4 he 20 ne 22 ne and 21 ne 22 ne ratios of decarbonated residues vary respectively from 0 09 10 6 to 76 5 10 6 9 54 0 08 to 11 30 0 60 and from 0 0295 0 0001 to 0 0344 0 0003 these isotopic compositions can be explained by a mixing between two terrestrial components atmosphere and radiogenic he and nucleogenic ne present in the terrigenous fractions and an extraterrestrial component the linear relationship between 20 ne 22 ne and 3 he 22 ne ratios shows that the extraterrestrial component has a unique composition and is similar to the he and ne composition of implanted solar wind this composition is different from the individual stratospheric idps for which the ne and he isotopic compositions have been measured we suggest that this difference is due to a bias in the sampling of the individual idps previously analyzed toward the largest ones that are more likely to lose he during entry in the atmosphere our data further constrains the size of the majority of the idps to be less than 10 m in diameter in addition the constant 3 he 22 ne ratio of the extraterrestrial component present in the samples which is similar to the implanted solar wind composition suggests that no diffusive loss of 3 he occurred in the atmosphere or on the seafloor thus neglecting any non fractionating he and ne loss by weathering and or alteration of the host phases on the seafloor the extraterrestrial 3 he and 20 ne fluxes between 3 to 38 myr ago are respectively 0 2 0 1 10 12 cm 3 cm 2 kyr 1 and 0 2 0 1 10 11 cm 3 cm 2 kyr 1 during the sharp increases of the late eocene and late miocene the idp 3 he and 20 ne fluxes reach values up to five times higher

4he as a tracer of continental dust a 1 9 million year record of aeolian flux to the west equatorial pacific ocean the mass accumulation rates mars of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past the incorporation of other terrigenous components such as volcanic ashes in seafloor sediments however often obscures the aeolian dust record we describe a new approach which uses the delivery rate of crustal 4he to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components we have determined the flux of crustal 4he delivered to the seafloor of the ontong java plateau ojp in the western equatorial pacific over the last 1 9 myrs crustal 4he fluxes vary between 7 7 and 30 ncc cm 2 kyr 1 and show excellent correlation with global climate as recorded by oxygen isotopes with high crustal 4he fluxes associated with glacial periods over the entire interval studied furthermore the onset of strong 100 kyr glacial interglacial climate cycling is clearly seen in the 4he flux record about 700 kyrs ago these data record variations in the supply of asian dust in response to climate driven changes in the aridity of the asian dust sources consistent with earlier work on asian dust flux to the northern pacific ocean however in contrast to previous studies of sites in the central and eastern equatorial pacific ocean there is no evidence that the inter tropical convergence zone an effective rainfall barrier to the southward transport of northern hemisphere dust across the equator in the central and eastern pacific has influenced the delivery of asian dust to the ojp the most likely carrier phase for crustal helium in these sediments is zircon which can reasonably account for all the 4he observed in the samples as a first order estimate these results suggest that the mass accumulation rate of asian dust on the ojp over the last 1 9 myrs varied from about 4 to 15 mg cm 2 kyr 1 in contrast previous studies show that over the same interval the total mar of terrigenous dust i e asian dust plus local volcanics on ojp varied between about 34 and 90 mg cm 2 kyr 1

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

insights into the effects of oblique extension on continental rift interaction from 3d analogue and numerical models abstract continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity these rift segments arise from rift initiation at non aligned crustal heterogeneities and need to interact to develop a full scale rift system here we test the effects of 1 oblique extension and 2 initial heterogeneity seed offset on continental rift interaction with the use of an improved analogue model set up x ray computer tomography ct techniques are used to analyse the 3d models through time and the results are compared with additional numerical models and natural examples the experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike slip dominated systems towards the strike slip domain we also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface initial soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal rather than linking at depth the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough orthogonal extension promotes the formation of intra rift horsts which may provide hydrocarbon traps in nature

fault growth by linkage observations and implications from analogue models abstract using time sequence analyses of extensional fault models we demonstrate the pivotal role played by fault segmentation in the accumulation of displacement and length during the growth of faults experiments are described in which incremental steps during the development of individual faults have been reconstructed from time lapse photographs taken during model deformation these records confirm the composite segment hierarchy of fault structure a pattern that is frequently recognised in many natural arrays they reveal the progressive enlargement of individual faults to be the product of a repetitive cycle of tip line propagation overlap and linkage between nearest neighbours by contrasting the displacement patterns of successive increments during growth convincing evidence is also presented to suggest that individual segments of faults may remain kinematically independent once they are physically linked this behaviour is shown to be responsible for the characteristic saw tooth patterns often recognised in strike parallel fault displacement profiles such patterns are believed to arise where relict segment boundaries remain preserved as asperities to slip so that displacement is confined to discrete parts of a fault plane surface growth in this way also causes the maximum displacement d and surface length l of faults to continually change by different proportions incremental displacement records presented here corroborate field evidence which shows that linkage between fault segments during growth is responsible for a significant component of the spread of values often recorded in d versus l compilations finally we speculate that linkage between fault segments also accounts for transient irregularities recorded in the frequency distribution of the fault length populations of each model

transtensional faulting patterns ranging from pull apart basins to transform continental margins an experimental investigation abstract seventeen small scale experiments were performed to study the deformation induced in brittle ductile models by a releasing stepover between two transform faults and by intersection between transform and divergent plate boundaries in both cases faulting depends on the rheological layering presence and strength of a ductile layer at depth and the width of the basal stepover successive types of pull apart basins are observed in the first set of experiments which are compared with natural examples firstly a lazy z shaped basin appears which is bounded by y faults above transform boundaries and r faults propagating from each corner of the stepover then r faults replace r faults leading to a rhomb shaped graben at every stage the basin length to width ratio remains between 2 2 and 3 8 suggesting that scale independence of pull apart basins is related to the geometrical shape of bounding faults in a second set of experiments deformation at the end of a transform boundary is characterized by a horsetail splay bounding a divergent basin within the horsetail splay block tilting and block rotation about vertical axes lead to a surface slope perpendicular to the slope of the divergent basin a feature that can be compared with marginal ridges at transform margins

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

electrical hydraulic properties of unsaturated ottawa sands abstract laboratory measurements of moisture equivalent saturated hydraulic conductivity and electrical resistivity were made on a series of uniform very fine to medium grain size ottawa sands for these sediments the unsaturated form of archie s law is shown to be adequate the experimental results are combined and interpreted to demonstrate a relationship between the apparent formation factor and or resistivity at the moisture equivalent and saturated hydraulic conductivity for the same sediment these laboratory results provide a limited experimental basis for relationships between the resistivity of vadose zone sediments and their saturated hydraulic conductivity and hence a basis for estimating the potential for artificial recharge from surface resistivity measurements

sandbox models of relay ramp structure and evolution relay ramps are a common feature formed during the growth of normal fault systems we performed analogue experiments to investigate the structure and evolution of relay ramps an extending rubber sheet induces extension at the base of a sand pack brittle crust analogue silicone bars between the rubber and the sand control the location of fault nucleation we tested the role of fault spacing fault length overlap length and fault strike in the evolution of relay ramps the modelled relay ramps evolved in three stages characterized by the growth of the normal faults their interaction and linkage interaction and linkage occurred only when the combined length of the two interacting faults was larger than eight times their spacing the length to width ratio of the relay ramps during the interaction stage showed preferred geometries clustering around three the propagation of the fault tips was observed both before and after the linkage stage overlap length and spacing relations of the modelled relay ramps are similar to those in nature at different scales and can be explained using existing mechanical models nevertheless the further propagation of the fault tips after linkage has not been described previously

a late miocene dust shower from the break up of an asteroid in the main belt throughout the history of the solar system earth has been bombarded by interplanetary dust particles idps which are asteroid and comet fragments of diameter approximately 1 1 000 microm the idp flux is believed to be in quasi steady state particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution dynamical ejection and planetary or solar impact because idps are rich in 3he seafloor sediment 3he concentrations provide a unique means of probing the major events that have affected the idp flux and its source bodies over geological timescales here we report that collisional disruption of the 150 km diameter asteroid that created the veritas family 8 3 0 5 myr ago also produced a transient increase in the flux of interplanetary dust derived 3he the increase began at 8 2 0 1 myr ago reached a maximum of approximately 4 times pre event levels and dissipated over approximately 1 5 myr the terrestrial idp accretion rate was overwhelmingly dominated by veritas family fragments during the late miocene no other event of this magnitude over the past approximately 10 8 yr has been deduced from main belt asteroid orbits one remarkably similar event is present in the 3he record 35 myr ago but its origin by comet shower or asteroid collision remains uncertain

geodynamic significance of the trm segment in the east african rift w tanzania active tectonics and paleostress in the ufipa plateau and rukwa basin abstract the tanganyika rukwa malawi trm rift segment in western tanzania is a key sector for understanding the opening dynamics of the east african rift system ears in an oblique opening model it is considered as a dextral transfer fault zone that accommodates the general opening of the ears in an nw se direction in an orthogonal opening model it accommodates pure dip slip normal faulting with extension orthogonal to the rift segments and a general e w extension for the entire ears the central part of the trm rift segment is well exposed in the ufipa plateau and rukwa basin within the paleoproterozoic ubende belt it is also one of the most seismically active regions of the ears we investigated the active tectonic architecture and paleostress evolution of the ufipa plateau and adjacent rukwa basin and in order to define their geodynamic role in the development of the ears and highlight their pre rift brittle tectonic history the active fault architecture fault kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the trm segment two pre rift brittle events are evidenced an older brittle thrusting is related to the interaction between the bangweulu block and the tanzanian craton during the late pan african early paleozoic it was followed by a transpressional inversion during the early mesozoic this inversion stage is the best expressed in the field and caused dextral strike slip faulting along the fault systems that now control the major rift structures it has been erroneously interpreted as related to the late cenozoic ears which instead is characterized by pure normal faulting our third and last stress stage

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

an alternative model for the growth of faults conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length we propose an alternative growth model where fault lengths are near constant from an early stage and growth is achieved mainly by increase in cumulative displacement the model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures this growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly the model may also apply to some non reactivated fault systems near constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips the model is validated using kinematic constraints from growth strata which are displaced by a system of reactivated normal faults in the timor sea nw australia

geometrical analysis of the refraction and segmentation of normal faults in periodically layered sequences normal faults contained in multilayers are often characterised by dip refraction which is generally attributed to differences in the mechanical properties of the layers sometimes leading to different modes of fracture because existing theoretical and numerical schemes are not yet capable of predicting the 3d geometries of normal faults through inclined multilayer sequences a simple geometric model is developed which predicts that such faults should show either strike refraction or fault segmentation or both from a purely geometrical point of view a continuous refracting normal fault will exhibit strike i e map view refraction in different lithologies if the intersection lineation of fault and bedding is inclined an alternative outcome of dip refraction in inclined multilayers is the formation of segmented faults exhibiting en echelon geometry the degree of fault segmentation should increase with increasing dip of bedding and a higher degree of segmentation is expected in less abundant lithologies strike changes and associated fault segmentation predicted by our geometrical model are tested using experimental analogue modelling the modelling reveals that normal faults refracting from pure dip slip predefined faults into an overlying sand cover will as predicted exhibit systematically stepping segments if the base of the cover is inclined

noble gases in ocean waters and sediments noble gases are widely used in studies of the basic properties and dynamics of natural systems including the ocean this chapter describes some of the more extensive applications of noble gases mainly helium isotopes to studies of oceanographic problems they include the modern oceanic circulation paleo oceanography hydrothermal and cold brine systems in the deep ocean and ocean atmosphere gas exchange originally oceanic noble gas studies were focused on geochemical problems such as the question of the existence of excess helium in seawater derived from radioactive decay of the elements of the uranium and thorium series in ocean sediments suess and wanke 1965 bieri et al 1966 after the discovery of mantle primordial helium during the 1960s clarke et al 1969 craig and weiss 1971 craig et al 1975 and 3he derived from decay of tritium in the near surface waters of the ocean jenkins and clarke 1976 large programs were developed to exploit helium isotopes frequently in combination with tritium for studies of water mass formation circulation and variability e g jenkins 1987 schlosser et al 1991 more recent studies started to explore the use of helium isotopes to paleoceanographic objectives these studies build upon the discovery of extraterrestrial helium in deep sea sediments in the early 1960s merrihue 1964 this signal is now being systematically examined for its potential as a tool for investigations of sediment accumulation rates on very long time scales millions of years and their correlation with changes in paleoceanography e g farley 1995 farley and patterson 1995 marcantonio et al 1995 1996 2001 soon after the discovery of mantle helium in seawater noble gases were applied to studies of hydrothermal systems originally these studies were focused on the source of the fluids emanating from hydrothermal vents as well as

3he4he ratio noble gas abundance and k ar dating of diamonds an attempt to search for the records of early terrestrial history the 3he4he ratios measured in 27 southern africa diamond stones four from premier mine and the rest of unidentified origin range from 4 2 10 8 to 3 2 10 4 with three stones above 1 10 4 we conclude that the initial helium isotopic ratio 3he4he 0 in the earth was significantly higher than that of the planetary helium a 3he4he 1 42 10 4 but close to the solar helium 3he4he 4 10 4 the apparent k ar ages for the twelve diamonds of unidentified origin show enormously old age indicating excess argon 40 3he4he evolution in diamonds suggests that the diamonds with the high 3he4he ratio 2 10 4 may be as old as the earth noble gas elemental abundance in the diamonds relative to the air noble gas abundance shows monotonie decrease with a decreasing mass number this paper discusses the implications of these observations on the early solar system and the origin of diamonds

extraterrestrial accretion from the gisp2 ice core the rate of extraterrestrial accretion for particles in the size range 0 45 m to 20 m was determined from dust concentrates extracted from greenland ice sheet project 2 gisp2 ice core samples using instrumental neutron activation analysis inaa we determined the iridium ir content of the dust following a core specific correction for terrestrial ir and assuming a chondritic ir abundance of 500 ppb we measure an average accretion rate for 0 45 m to 20 m particles over the entire earth of 0 22 0 11 109 g yr kton yr for 317 years of ice through the interval 6 to 20 ka this is consistent with the interplanetary dust accretion rate of 0 17 0 08 x 109 g yr that we derive from published 3he data for the gisp2 core accounting for particles that are larger and smaller than those detected by or experiment our best estimate of the total accretion rate including particle sizes up to about 4 cm in diameter is 2 5 109 g yr the uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error based on monte carlo simulations we estimate the upper limit for total extraterrestrial accretion to earth of 6 25 109 g yr 95 confidence level this accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice estimates from ground based radar studies and with accretion estimates of 3he bearing interplanetary dust particles assuming that 3he is correlated with particle surface area it is however lower than estimates based on platinum group element studies of marine sediments the conflict may indicate systematic errors with either the marine or the non marine samples departures from the assumed particle spectrum of grun and coauthors or time variable accretion rates with the early holocene period being characterized by low levels of accretion

a record of the extraterrestrial 3he flux through the late cretaceous abstract late cretaceous 100 73 ma pelagic limestones were measured for helium concentration and isotopic composition to characterize the interplanetary dust flux using 3he as a tracer in the bottaccione section near gubbio italy three intervals of elevated 3he concentration were detected k1 in the campanian stage at 79 ma k2 in the santonian stage at 85 ma and k3 in the turonian stage at 91 ma all three of these episodes are associated with high 3he 4he and 3he non carbonate ratios consistent with their derivation from an enhanced extraterrestrial 3he flux rather than decreased carbonate sedimentation or dissolution while k2 is modest in magnitude and duration and thus is of limited significance k1 and k3 are each identified by a few myr interval with an 4 fold enhancement in mean 3he flux compared with pre event levels samples from odp hole 762c in the indian ocean spanning both k2 and k3 93 83 ma confirm the presence of a peak in the turonian stage suggesting that k3 is a global event the k1 and k3 3he events are similar in most respects to the two peaks previously detected in the cenozoic suggesting a similar origin these have been attributed to a major asteroid collision in the late miocene and to a shower of either comets or asteroids in the late eocene based on the age and temporal evolution of k1 we suggest that it most likely records the collision which produced the baptistina asteroid family independently dated at 80 ma the k3 event is less easily explained it is characterized by an unusually spiky and erratic temporal progression suggesting an unusual abundance of very 3he rich particles not previously seen in the sedimentary 3he record we suggest this episode arises either from a comet shower or from an asteroid shower possibly associated with dust producing lunar impacts

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model