the ochil fault and the kincardine basin key structures in the tectonic evolution of the midland valley of scotland the ochil fault one of the most prominent tectonic features in the midland valley of scotland juxtaposes lower devonian volcanic rocks against late westphalian strata implying a possible vertical displacement of up to 4 km the kincardine basin in its hanging wall although actively subsiding during the greater part of the silesian trends generally n s perpendicular to the ochil fault and its silesian sedimentary record shows little sign of tectonic control by that fault it is proposed that the ochil fault was initiated possibly as a sinistral strike slip feature in the devonian but acted as a sidewall fault during the early evolution of the kincardine basin attributed to extension on a hypothetical pre brigantian fault along the bo9ness line on the east side of the basin until late namurian times when active extension ceased both faults were probably buried during late namurian and westphalian times reactivation of the ochil fault during end carboniferous n s extension dated by quartz dolerite emplacement was responsible for probably at least 2 km displacement including the presently visible footwall uplift it is suggested that the hanging wall may have contained a permian basin now removed

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

inferring sites of subglacial erosion using the pb isotopic composition of ice rafted feldspar examples from the weddell sea antarctica the delivery of ice rafted debris ird from glaciated margins is a function of ice sheet dynamics shifts in supply and sourcing of ird can therefore identify episodes of ice sheet instability however records can be difficult to correctly interpret because the subglacial geology of the catchment areas which controls ird composition may be obscured importantly variations can also result from shifts in erosion sites due to changes in the basal ice sheet conditions this study evaluates where subglacial erosion has occurred in catchments that flow into the southern weddell sea antarctica by determining the pb isotopic compositions of individual ice rafted feldspars from late holocene marine sediments feldspar compositions match those of rock units inferred through extrapolation of outcrop magnetic and gravity data to compose areas where ice velocity bed roughness and shear stress are high significantly signals from areas where ice velocities are high but bed roughness and shear stresses are low were not recorded suggesting that there is reduced bedrock erosion in these regions major variations in ird composition in the weddell sea can result from changing the loci of subglacial erosion and do not necessarily correspond with major ice sheet instability

the evolution of the subglacial landscape of antarctica the aim is to investigate the evolution of the subglacial landscape of antarctica using an ice sheet and erosion model we identify different stages of continental glaciation and model the erosion processes associated with each stage the model links erosion to the basal thermal regime and indicates that much of the antarctic interior may have been subject to less than 200 m of erosion the depth of erosion reflects the presence or absence of warm based ice and the consistency of ice flow direction this information linked with knowledge about landscapes of glaciation in the northern hemisphere and some simple but robust assumptions about initial topography is used to generate a map of the subglacial antarctic landscape in which much of the lowland interior resembles the landscapes of areal scouring typical of the laurentian and scandinavian shields near the continental margins selective linear erosion has overdeepened pre existing river valleys by as much as 2 8 km high elevation plateaus adjacent to such drainage systems have survived largely unmodified under cold based ice high erosion rates result from steep thermal gradients in basal ice mountain regions such as the gamburtsev mountains uplands in dronning maud land and massifs in west antarctica are likely to bear features of local alpine glaciation such landscapes may have been protected under cold based ice for the last 34 myrs or possibly longer

spatial coherence of the ground motion produced by the 1979 imperial valley earthquake across el centro differential array abstract spatial coherence of digitally recorded acceleration time histories across five elements of the el centro differential array recorded during the october 15 1979 imperial valley california earthquake is studied recorded within a total spacing of 214 m the three component time histories at five stations of this linear array provide a unique set of free field data at sites of similar geology in the near field of a strong earthquake the observed peak ground accelerations vary by as much as 43 13 and 40 on the e w n s and vertical components it is shown that the incoherence of high frequency waves brought about by wave scattering near the receiver is partially responsible for the observed variation this is evident from the frequency dependent reduction of the peak cross correlation amplitudes of the strong phase of ground shaking the observed reductions increase from 1 below 1 hz to 20 30 in the 1 10 hz band and to nearly 80 above 10 hz investigation of the polarization angle of p wave coda indicates that high frequency waves 10 hz approach the recording station randomly from nearly all directions the peak cross correlation of the polarization angle in general is lower than that of amplitudes the successive sampling of this angle in the low frequency band suggest a northwestward propagation of the source during the faulting process

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

integrating satellite observations with modelling basal shear stress of the filcher ronne ice streams antarctica using inverse methods constrained by recent satellite observations we have produced a comprehensive estimate of the basal shear stress beneath the filchner ronne ice streams the inversions indicate that a weak bed approx 4 20 kpa underlies much of these ice streams compared to the ross ice streams the distribution of weak subglacial till is more heterogeneous with sticky spots providing much of the resistance to flow a weak bed beneath recovery ice stream extends several hundred kilometres inland with flow along this ice stream discrepancies between thickness measurements and flux estimates suggest the existence of a deep 1400 m trough not resolved by existing maps of subglacial topography we hypothesize that the presence of this and other deep troughs is a major influence on this sector of the ice sheet that is not fully incorporated in current models of ice sheet evolution

seismic evidence for a water filled canal in deforming till beneath rutford ice stream west antarctica 1 seismic data from rutford ice stream west antarctica show high amplitude reversed polarity reflections from parts of the bed only interfaces between ice and water or between water and sediment have the acoustic properties to produce such strong reflections ice sediment interfaces produce much weaker echoes the evidence suggests that an area of the ice stream measuring at least 1 km by 0 2 km is underlain by water with a depth of less than 1 m the data are consistent with the presence of a sub ice drainage system comprising shallow canals cut into the sedimentary substrate as predicted by theory

organized flow from the south pole to the filchner ronne ice shelf an assessment of balance velocities in interior east antarctica using radio echo sounding data 1 ice flow through central antarctica has the potential to transmit accumulation changes from deep interior east antarctica rapidly to the shelf but it is poorly constrained owing to a dearth of ice velocity observations we use parameters derived from airborne radio echo sounding res data to examine the onset areal extent and englacial conditions of an organized flow network tributaries feeding an ice stream draining from the south pole to the filchner ronne ice shelf we classified res flight tracks covering the region according to whether englacial stratigraphy was disrupted i e internal layers diverged significantly from the surface and bed echoes or undisrupted i e internal layers closely parallel surface and basal topography and we calculated subglacial roughness along basal reflectors where satellite measured surface ice flow speeds are available covering 39 of the study region regions of fast and tributary flow correspond with res flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow this suggests that disrupted internal layering and smooth subglacial topography identified from res profiles can be treated as indicators of past or present enhanced flow tributaries where neither satellite nor ground based ice flow measurements are available we therefore use these res derived parameters to assess the balance flux modeled steady state flow regime between the south pole and filchner ronne ice shelf the res analysis confirms that an organized flow network drains a wide region around the south pole into the filchner ronne ice shelf however the spatial extent of this network as delineated by the res data diverges from that predicted by currently available balance flux models

quantifying subglacial bed roughness in antarctica implications for ice sheet dynamics and history abstract glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice sheet history superimposed over and in turn modifying preglacial topography lithology and geological structure insights into the formation of glaciated landscapes can in principle be obtained by analysing modern ice sheet beds but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed it is however possible to quantify roughness the vertical variation of the subglacial interface with horizontal distance along two dimensional images of the bed obtained from radio echo sounding res here we collate several case studies from antarctica where roughness calculations have been used as a glaciological tool to infer basal processes and ice sheet history over large 500 km 2 areas we present two examples from west antarctica which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments glacial dynamics and former ice sheet size we also present two examples from east antarctica which illustrate how roughness provides knowledge of ice sheet dynamics in the interior and pre quaternary ice sheet histories in modern ice sheet settings characterising bed roughness along res tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field satellite and modelling investigations the technique also offers significant potential for the comparison of modern and former ice sheet terrains contributing to an improved understanding of the formation and evolution of glaciated landscapes

coats land crustal block east antarctica a tectonic tracer for laurentia undeformed rhyolite and granophyre in the coats land crustal block of east antarctica dated as 1112 4 ma are identical in age to both the umkondo large igneous province lip of the kalahari craton southern africa and the early keweenawan lip of laurentia north america although marine and satellite data demonstrate that coats land was close to kalahari within the gondwana supercontinent the coats land rocks yield pb isotope compositions strikingly distinct from those of the umkondo province yet indistinguishable from rocks of the keweenawan province the anorogenic red bluff granitic suite along the present day southern laurentian margin in the franklin mountains texas usa is of comparable age general rock type and pb isotope composition to rocks of coats land and may provide a piercing point for a coats land laurentia link paleomagnetic poles permit the coats land block to be close to this part of laurentia ca 1100 ma and allow juxtaposition of kalahari and southern laurentia ca 1000 ma the coats land crustal block may therefore be a critical tectonic tracer for placing laurentia within late mesoproterozoic and neoproterozoic paleogeographic reconstructions if this hypothesis is correct laurentia collided with the kalahari craton along antarctica s maud orogen which would represent a continuation of the ca 1000 ma grenville orogen of eastern and southern laurentia

measurements of coseismic deformation in southern california 1972 1982 since 1972 a number of static strain changes have been measured at pinon flat observatory southern california as a result of regional faulting as recorded by a collection of long baselength strainmeters and tiltmeters these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half space in contrast colocated short baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration calculations show that the root mean square rms of the coseismic signals from several different instruments at one site or from several sites at known hypocentral distances provide a good estimate of the seismic moment a result confirmed by the observations static moment estimates for the four largest events in this period are imperial valley 1979 m0 9 0 1018nm mexicali valley 1980 m0 6 1 1018nm westmorland 1981 m0 5 3 1017nm and brawley 1979 m0 2 3 1017 nm these estimated moments generally exceed those determined seismically by about 25 which is as expected since they include some postseismic deformation results from the mexicali valley earthquake are particularly interesting indicating an average fault strike approximately 9 different from the historic fault trace several small to moderate earthquakes ml 3 7 5 2 were also well enough recorded to estimate source parameters for the only other detectable events in the period 1972 1982 a total of three others for which the strain change erms was greater than 0 4 ne high accelerations caused most of the instruments at the site to malfunction no precursory signals were evident for any of the earthquakes recorded at this site

a glaciological map of filchner ronne ice shelf antarctica a set of landsat 5 multispectral scanner mss images of filchner ronne ice shelf was commissioned by the institut fur angewandte geodasie and recorded over a short period in early 1986 a mosaic of digitally enhanced images was constructed using all available ground control in due course it is planned to use the data for the production of a series of digital image maps at a scale of 1 1 000 000 meanwhile a line map has been prepared at a scale of 1 2 000 000 to show the principal glaciological features the new mosaic shows much more detail than earlier landsat images and extends coverage to higher latitudes more of the poorly known southern boundary of the ice shelf can be identified but floating ice still extends beyond the limit of coverage extensive ice rumples are revealed many of them undetected in earlier imagery flow bands can be tracked without a break from their ice stream source to the ice front over distances of up to 800 km even minor tributaries and sheet flow from the inland ice sheet between the ice streams yield narrow but identifiable flow bands ice thickness velocity and mass flux data are reviewed in order to underline their dependence on good mapping

evidence for iceberg armadas from east antarctica in the southern ocean during the late miocene and early pliocene abstract sediments from ocean drilling program site 1165 in the indian ocean sector of the southern ocean off prydz bay contain a series of layers that are rich in ice rafted debris ird here we present evidence that ird rich layers at site 1165 at 7 4 8 and 3 5 ma record short lived massive discharges of icebergs from wilkes land and adelie land more than 1500 kilometers to the east of the depositional site this distant source of icebergs is clearly defined by the presence of ird hornblende grains with 40ar 39ar ages of 1200 1100 ma and 1550 1500 ma ages that are not found on the east antarctic continent in locations closer to site 1165 this observation requires enormous amounts of detritus carrying drifting icebergs most likely in the form of large icebergs these events probably reflect destabilization surge and break up of ice streams on the wilkes land and adelie land margins of the east antarctic ice sheet in the vicinity of the low lying aurora and wilkes basins they occurred under warming conditions but each coast seems to have produced ice rafting events independently at different times the data presented here constitute the first evidence of far traveled icebergs from specific source areas around the east antarctic perimeter launch of these icebergs may have happened during quite dramatic events perhaps analogous to heinrich events in the north atlantic

crustal deformation across the imperial fault results from kinematic gps surveys and trilateration of a densely spaced small aperture network kinematic global positioning system gps surveys in 1991 and 1993 of a 66 station small aperture network across a 12 km long segment of the imperial fault near el centro california reveal a fairly uniform secular displacement field consistent with right lateral slip along a single vertical dislocation surface total relative motion across the array equals 20 3 mm yr at n49 7 w for sites 5 6 km away from the fault trace on opposite sides these rates agree with simple dislocation models that assume 10 mm yr of creep along the fault trace from the surface to a locking depth of 9 km and a plate boundary rate of 45 mm yr oriented n40 w at depth model fit does not significantly improve if creep rates are permitted to vary along strike by several millimeters per year a comparison of 1991 1993 gps with 1987 high precision electronic distance measurements validates the dislocation models but suggests a smoother deformation field than that defined by the 1991 1993 velocities the difference can be attributed to the poorer 1991 kinematic positioning accuracy rather than to temporal variations in strain there is evidence for 20 mm of sympathetic coseismic slip from the 1987 superstition hills earthquake although aseismic slip along both faults appears to be completely decoupled

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

east antarctic ice stream tributary underlain by major sedimentary basin marine and rift sediments exert a fundamental control on ice stream flow in the west antarctic ice sheet and hence on its mass balance and stability in contrast most ice streams in the much larger east antarctic ice sheet are thought to be relatively stable features resting on till perhaps underlain by crystalline rock any geological controls on east antarctic ice sheet enhanced flow remain largely unknown we present aerogeophysical evidence indicating that a region of enhanced ice flow in the interior of the east antarctic ice sheet is underlain by subglacial sediments 3 km thick and that these are influencing the flow regime of the overlying ice we show that subglacial sediments are important in modulating ice dynamics not just for the west antarctic ice sheet but also for its much larger neighbor and suggest that the sedimentary basin identified here may contain information on the neogene glacial history of this part of the east antarctic ice sheet

analysis of strong motion s wave polarization of the 15 october 1979 imperial valley earthquake abstract this article illustrates the use of near source s wave polarization analyses for constraining some rupture parameters of large earthquakes in the particular case of the 1979 imperial valley mainshock the ten sma1 strong motion records closest to the source have been studied in the 1 to 3 hz frequency range the records were first corrected following the analysis of zollo and bernard 1989 from some anisotropy that produced strong s splitting at a few sites the rotation of the s polarization with time at most of the sites was then interpreted in terms of the northwestward propagation of the rupture we assumed a constant focal mechanism on the fault plane vertical strike slip the polarization data and the absolute times at some stations allowed the identification of a set of subsources their location in space and time assuming a depth of 8 km provided a mean rupture velocity of 3 1 km sec 0 25 km sec i e 90 of the shear velocity the high frequency radiation 3 hz during most of the rupture thus is very likely to have been generated about 1 sec before the beginning of the low frequency radiation 0 5 to 1 hz studied by archuleta 1984 the final rupture model fits the s polarization history well at most sites it also constrains the anisotropy characteristics of the sediments under some sites in particular em0 where it is consistent with the alignment of vertical cracks in the ns stress field applying such a method to other earthquakes requires a careful selection of the distance and frequency range in particular the effect of perhaps pervasive ubiquitous anisotropy in the upper crust must be taken into account which may require the analysis of aftershock records at the same sites

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

analysis of near source static and dynamic measurements from the 1979 imperial valley earthquake gross features of the rupture mechanism of the 1979 imperial valley earthquake ml 6 6 are inferred from qualitative analysis of near source ground motion data and observed surface rupture a lower bound on the event s seismic moment of 2 5 1025 dyne cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture 40 5 cm far field estimates of moment suggest an average slip over the fault plane of 105 cm from which a static stress drop of 11 bars is obtained an alternative slip model consistent with the far field moment has 40 5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km this model suggests a static stress drop of 39 bars from the lower estimate of 11 bars an average strain drop of 32 strain is derived this strain drop is four times greater than the strain that could have accumulated since the 1940 el centro earthquake based on measured strain rates for the region hence a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940 unusually large amplitude 500 to 600 cm sec2 vertical accelerations were recorded at stations e05 e06 e07 e08 eda of the ei centro array and the five stations of the differential array near eda although the peak acceleration of 1705 cm sec2 at e06 is probably amplified by a factor of 3 due to local site conditions these large amplitude vertical accelerations are unusual in that they are evident on only a few stations all of which are near the fault trace and at about the same epicentral range two possible explanations are considered first that they are due to a direct p wave generated from a region about 17 km north of the hypocenter or second that they are due to a pp phase that is unusually strong in the imperial valley due to the large p wave velocity gradient in the upper 5 km of the imperial valley based on the distribution of both the horizontal and vertical offsets it is likely that the rupture went beyond stations e06 and e07 during the main shock by exploiting the antisymmetry of the parallel components of particle velocity between e06 and e07 and by examining polarization diagrams of the particle velocity at e06 and e07 an average rupture velocity in the basement of 2 5 to 2 6 km sec between the hypocenter and station e06 is obtained in addition several lines of evidence suggest that the imperial fault dips about 75 to the ne

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

antarctic sediment chronology by programmed temperature pyrolysis methodology and data treatment 1 we report a detailed programmed temperature pyrolysis combustion methodology for radiocarbon 14c dating of antarctic sub ice shelf sediments the method targets the autochthonous organic component in sediments that contain a distribution of acid insoluble organic components from several sources of different ages the approach has improved sediment chronology in organic rich sediments proximal to antarctic ice shelves by yielding maximum age constraints significantly younger than bulk radiocarbon dates from the same sediment horizons the method proves adequate in determining isotope ratios of the pre aged carbon end member however the isotopic compositions of the low temperature measurements indicate that no samples completely avoided mixing with some proportion of pre aged organic material dating the unresolved but desired young end member must rely on indirect methods but a simple mixing model cannot be developed without knowledge of the sedimentation rate or comparable constraints a mathematical approach allowing for multiple mixing components yields a maximum likelihood age a first order approximation of the relative proportion of the autochthonous component and the temperature at which allochthonous carbon begins to volatilize and mix with the autochthonous component it is likely that our estimation of the cutoff temperature will be improved with knowledge of the pyrolysis kinetics of the major components chronology is improved relative to bulk acid insoluble organic material ages from nine temperature interval dates down to two but incorporation of inherently more pre aged carbon in the first division becomes more apparent with fewer and larger temperature intervals

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

provenance and dispersal of glacial marine surface sediments in the weddell sea and adjoining areas antarctica ice rafting versus current transport abstract mineralogical and granulometric properties of glacial marine surface sediments of the weddell sea and adjoining areas were studied in order to decipher spatial variations of provenance and transport paths of terrigenous detritus from antarctic sources the silt fraction shows marked spatial differences in quartz contents in the sand fractions heavy mineral assemblages display low mineralogical maturity and are dominated by garnet green hornblende and various types of clinopyroxene cluster analysis yields distinct heavy mineral assemblages which can be attributed to specific source rocks of the antarctic hinterland the configuration of modern mineralogical provinces in the near shore regions reflects the geological variety of the adjacent hinterland in the distal parts of the study area sand sized heavy minerals are good tracers of ice rafting granulometric characteristics and the distribution of heavy mineral provinces reflect maxima of relative and absolute accumulation of ice rafted detritus in accordance with major iceberg drift tracks in the course of the weddell gyre fine grained and coarse grained sediment fractions may have different origins in the central weddell sea coarse ice rafted detritus basically derives from east antarctic sources while the fine fraction is discharged from weak permanent bottom currents and or episodic turbidity currents and shows affinities to southern weddell sea sources winnowing of quartz rich sediments through intense bottom water formation in the southern weddell sea provides muddy suspensions enriched in quartz the influence of quartz rich suspensions moving within the weddell gyre contour current can be traced as far as the continental slope in the northwestern weddell sea in general the focusing of mud by currents significantly exceeds the relative and absolute contribution of ice rafted detritus beyond the shelves of the study area

deeper penetration of large earthquakes on seismically quiescent faults why many major strike slip faults known to have had large earthquakes are silent in the interseismic period is a long standing enigma one would expect small earthquakes to occur at least at the bottom of the seismogenic zone where deeper aseismic deformation concentrates loading we suggest that the absence of such concentrated microseismicity indicates deep rupture past the seismogenic zone in previous large earthquakes we support this conclusion with numerical simulations of fault behavior and observations of recent major events our modeling implies that the 1857 fort tejon earthquake on the san andreas fault in southern california penetrated below the seismogenic zone by at least 3 to 5 kilometers our findings suggest that such deeper ruptures may occur on other major fault segments potentially increasing the associated seismic hazard

basal conditions beneath enhanced flow tributaries of slessor glacier east antarctica radio echo sounding data are used to investigate bed roughness beneath the three enhanced flow tributaries of slessor glacier east antarctica slow moving inter tributary areas are found to have rough beds while the bed of the northernmost tributary is relatively smooth a reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times so marine sediments may have accumulated here together these factors are further support for the dominance of basal motion in this tributary reported elsewhere conversely although the other two slessor tributaries may have water routed beneath them they would not have been below sea level before the growth of the ice sheet so cannot be underlain by marine sediments they are also found to be rough and within the range of uncertainties it is likely that basal motion does not play a major role in the flow of these tributaries perhaps the most interesting area however is a deep trough where flow rates are currently low but the bed is as smooth as the northern slessor trough it is proposed that although ice deformation currently dominates in this trough basal motion may have occurred in the past when the ice was thicker

basal ice sequences in antarctic ice stream exposure of past hydrologic conditions and a principal mode of sediment transfer 1 the brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in kamb ice stream the upper sequence 7 3 m comprises clear ice and layers with dispersed and stratified debris the lower sequence 8 2 m consists of accretion ice with alternating layers of stratified and solid debris the two sequences have volumetric debris contents of about 5 and 20 respectively we infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the siple coast plain where basal freezing currently dominates the basal thermal regime the basal ice layer contains the equivalent of 2 1 0 4 m of frozen sediment the high volume of sediment is a result of debris rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on off cycles fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the ross sea our results show that sediment wedges near grounding lines may form by melt out of basal ice debris this is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override coulomb plastic substrates

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

hypocenter for the 1979 imperial valley earthquake using p and s wave arrival times with the laterally varying p wave velocity structure derived from analysis of a refraction survey of the imperial valley a hypocenter is ascertained for the october 15 1979 imperial valley earthquake latitude 32 sup 0 39 50 n longitude 115 sup 0 19 80 w depth 8 0 km time 23 16 54 40 gmt

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

estimates of seismic potential in the marmara sea region from block models of secular deformation constrained by global positioning system measurements we model the geodetically observed secular velocity field in north western turkey with a block model that accounts for recoverable elastic strain ac cumulation the block model allows us to estimate internally consistent fault slip rates and locking depths the northern strand of the north anatolian fault zone nafz carries approximately four times as much right lateral motion 24 mm yr as does the southern strand in the marmara sea region the data show strain accu mulation to be highly localized we find that a straight fault geometry with a shallow locking depth of 6 7 km fits the observed global positioning system velocities better than does a stepped fault geometry that follows the northern and eastern edges of the sea this shallow locking depth suggests that the moment release associated with an earthquake on these faults should be smaller by a factor of 2 3 than previously inferred assuming a locking depth of 15 km online material an updated version of velocity field data

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

crustal structure of the filchner ronne shelf and coats land antarctica from gravity and magnetic data implications for the breakup of gondwana understanding the tectonic evolution of the weddell sea embayment provides valuable insights on the breakup of gondwana existing geodynamic models for this region differ mainly in the nature of the crust beneath the filchner ronne shelf and the movement of the ellsworth whitmore mountains ewm crustal block new gravity data presented here constrain the area and timing of this ewm movement from its prebreakup position to its present day position the crustal thickness estimates of 29 5 km and 27 4 km derived from power spectral analysis as well as forward gravity modeling make the existence of oceanic crust beneath the filchner ronne shelf unlikely a new crustal unit consisting of precambrian crust has been identified with gravity and magnetic data and extends 175 km off the coast of coats land this crustal unit conflicts with the late mesozoic movement of the ewm in this area the effective elastic thickness of t e 35 5 km has been estimated from coherence spectra for the ronne shelf together with the crustal thickness of 27 km from refraction seismic studies and comparison with theoretical models the thermal age is estimated to be between 165 and 230 ma for the ronne shelf this age marks the last significant tectonic event in this area and is in good agreement with the breakup of gondwana it further makes later movements of microplates like the ewm in this region unlikely thus a filchner block consisting of continental crust should be considered in any geodynamic model of the early weddell sea evolution

rock debris in an antarctic ice shelf we have discovered a band of stones and coarse sand in the ronne ice shelf antarctica some 60m above the ice shelf s base 40km from its seaward edge and 420km from the point where the ice originally went afloat a study of ice sounding radar data from across the ronne ice shelf has revealed other areas likely to contain debris in significant quantities it appears that basal debris at the margins of ice streams feeding the ice shelf can be buried in the ice shelf by sea water freezing on at the ice shelf base these findings are evidence for a mechanism active in a present day ice sheet shelf system which enables icebergs to transport large volumes of ice rafted debris and which also provides a potential mechanism for the formation of ice rises near ice fronts we anticipate that a seismics study of debris melted from the ice shelf and deposited beneath will provide a valuable control on the history of ice shelf ocean interactions

bedform topography and basal conditions beneath a fast flowing west antarctic ice stream a grid of seismic reflection lines has been used to image basal topography and infer basal conditions and flow processes beneath similar to 140 km 2 of rutford ice stream west antarctica the subglacial topography in this region consists of two troughs flanking a central high and the bed is composed of water saturated sediments the two troughs are filled with deforming sediment whereas the bed in the central region appears to undergo a transition from largely deforming conditions upstream to basal sliding downstream the deforming bed is very flat along flow but undulates across flow sliding areas show rougher bed topography cross stream bed topography is characterised by streamlined mounds of deforming sediment aligned in the ice flow direction these bedforms occur superimposed on the bed in regions of both basal sliding and sediment deformation in places they form finger like mounds of material which extend into the sliding region further downstream mean bedform height is 22 m mean width is 267 m and many of them extend for at least 1 2 km along flow we interpret most of these bedforms as drumlins and one as a mega scale glacial lineation the juxtaposition of different basal conditions is consistent with models proposed from terrestrial studies in which the glacier bed is a mosaic of stable and deforming bed areas variable both spatially and temporally any theory of subglacial sediment rheology must also be able to account for our conclusion that at any given time pervasive deformation extends at least a few metres into the bed and can persist over a considerable area many km 2 bedform geometry and basal conditions concur with interpretations of former ice streams with evidence for increasing elongation ratio with distance downstream however those studies also identified bedrock cropping out at the ice bed interface for which there is no evidence on rutford ice stream c 2008 elsevier ltd all rights reserved