coda of long range arrivals from nuclear explosions short period three component recordings from peaceful nuclear explosions pne of the profile quartz located in russia are used to constrain the nature of the coda of pne arrivals in particular we examine the unusually strong and extensive coda of the long range p n interpreted as a whispering gallery wg phase propagating to beyond 3000 km energy balance considerations in three dimensions show that such an extensive coda is inherent not only to wg but to all other p wave phases and can be explained by crustal scattering the long coda is a result of excitation of short period scattered waves pg sg lg rg within the crust by the waves incident from the mantle or conversely by generation of mantle phases from crustal guided waves within the source region the resulting estimates of coda q range between q 380 near 2 hz and q 430 near 5 hz and can be associated with crustal attenuation including the sediments our coda model also explains quantitatively the observed build up of the arrival amplitude with time and the apparent lack of a pronounced coda of the body wave arrivals from the mantle transition zone these effects result from adding up the energy of the later arrivals arriving during the codas of earlier arrivals we deconvolve the overlapping coda patterns and show that the true relative energies of the arrivals are significantly lower than the apparent energies measured from the raw records a whispering gallery interpretation of the long range p n and crustal scattering accounts for the entire range of observations of kinematic spectral and amplitude pattern of the pne wavefield and allows the derivation of constraints on attenuation

migration of chlorinated hydrocarbon solvents through coventry sandstone rock columns the migration in the coventry sandstone aquifer system of aqueous solutions of various chlorinated hydrocarbon solvents chss 1 1 1 trichloroethane tca trichloroethylene tce and tetrachloroethylene tece was studied by means of percolation experiments with undisturbed rock cores the retardation factor rf was very low always less than 2 for tca less than 3 for tce and less than 5 for tece the rf varied slightly according to the nature of the solid sample but could not be related to the solid organic carbon content the solvent adsorption was always almost completely reversible and little affected by the percolation flow rate 0 08 0 56 m day 1 the chs breakthrough curves were typical of non equilibrium processes caused by sorption by the solids and possibly by diffusion between mobile and immobile water the rf values obtained experimentally were much lower than those calculated from batch adsorption studies showing that percolation experiments are necessary to evaluate the filtering properties of consolidated rocks these results indicate that in the coventry sandstone aquifer relatively high vertical and especially lateral hydraulic conductivities two carbon chs aqueous solutes should migrate readily with water with very little retardation

crustal growth during island arc accretion and transcurrent deformation natal metamorphic province south africa new isotopic constraints the natal metamorphic province consists of from north to south the tugela mzumbe and margate terranes these were accreted to the southeastern margin of the kaapvaal craton in the late mesoproterozoic and followed by intrusion of a large suite of a type granitoid bodies new u pb data from zircon titanite and monazite further constrains the temporal framework of these geological events the tugela and mzumbe terranes record protracted magmatism in an island arc complex from 1200 ma to 1160 ma followed by the accretion of these terranes onto the southern margin of the kaapvaal craton at 1150 ma arc magmatism in the margate terrane continued until 1120 ma and was followed by extension and bimodal volcanism immediately prior to accretion to the kaapvaal mzumbe continental margin at 1090 ma this accretion was accompanied by high pressure and high temperature metamorphism juxtaposition of the mzumbe and margate terranes along the melville thrust and the formation of a number of syntectonic intrusive units derived from melting of the pre existing arc crust after accretion extensional collapse is evidenced by the intrusion of mafic ultramafic and alkaline intermediate magmatic suites at 1085 ma resulting from mafic underplating and or lower crustal delamination nd and hf isotopic data imply the magmatic rocks of the natal metamorphic province were derived from relatively juvenile continental crust initially generated by island arc magmatism and subsequently reworked during the accretion event s the combined kaapvaal nmp region the southern margin of the enlarged kalahari craton then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the kaapvaal cratonic margin to the southernmost portion of the natal metamorphic province this deformation is accompanied by low pressure ultra high temperature metamorphism isobaric cooling and intrusion of the voluminous a type oribi gorge suite porphyritic granites and charnockites throughout the mzumbe and margate terranes

frequency dependent attenuation of shear waves in the crust of the southern kanto area japan the attenuation characteristics of shear waves in the crust of the southern kanto area central japan were estimated using strong motion data including acceleration data recorded in the pre tertiary basement rocks by means of downhole observation the quality factor qs f was determined for a range of discrete frequencies from 0 5 to 16 hz from the analysis of data from 13 local earthquakes with focal depths of less than about 50 km that occurred in the philippine sea plate and in the boundary zone between the lower part of the eurasian plate and the upper part of the philippine sea plate the estimated 1 qs f shows a peaked structure in this frequency range on the assumption that the geometrical spreading exponent is 1 body waves the estimated peak 1 qs f is of the order of 10 2 at 0 8 hz

postscript summary of the coventry groundwater investigation and implications for the future abstract the broader implications of the coventry groundwater investigation are discussed in this postscript the complex interbedded sandstone and mudstone sequence under coventry is seriously polluted by chlorinated hydrocarbon solvents investigations in such terrain will require much greater expenditure than was available for this project if they are to describe the distribution and movement of pollutants in enough detail to evaluate quantitatively options for aquifer clean up it is not realistic to consider clean up of coventry s groundwater at the present time because of the lack of detailed knowledge it is probable at least for the present that clean up is not feasible because of hydrogeological constraints arising from the extreme variability of permeability in the system and an absence of operational techniques for clean up stronger more pro active protection of groundwater is needed a major recommendation for future investigations is a procedure to avoid open boreholes which can allow pollution to penetrate deeper into the aquifer and may confuse the results of sampling

temporal variations of qc 1 and b values associated to the december 2000 january 2001 volcanic activity at the popocatepetl volcano mexico abstract we correlated temporal variations of coda attenuation q c 1 at 6 hz and b values with the 11 december 2000 23 january 2001 major eruptive activity at the popocatepetl volcano mexico we interpreted a total of 225 volcanic earthquakes with magnitudes between 1 9 and 3 3 located between 2 and 8 km from the summit of the volcano and registered from january 1999 to june 2001 q c 1 was estimated using a single scattering model and values of b were obtained applying the method of maximum likelihood our results indicate that from january 1999 to march 2000 period 1 q c 1 varied within the same range without significant changes then values of q c 1 steadily decreased during april mid december 2000 period 2 from about mid december 2000 onset of the major volcanic activity up to february 2001 period 3 q c 1 sharply increased reaching its peak value from march to june 2001 period 4 q c 1 decreased and increased in response to the post eruptive phase of dome formation and destruction process that took place during periods 2 and 3 average b values behaved opposite to the q c 1 trend within period 2 when q c 1 decreased b slowly increased at the start of period 3 b reached its peak value and returned to low values by the end of this time interval we associate the temporal changes of the two analyzed parameters to variations of pressure and or the variation in the degree of saturation of pore fluids that took place before during and after the major eruption during periods 2 and 3 q c 1 correlated negatively with b with a correlation coefficient of 0 61 and 0 83 respectively the significance of correlation cannot be rejected at the confidence level of 0 975 our observations may have important implications for future prediction studies of large eruptions at the popocatepetl volcano

temporal change in coda q before the tangshan earthquake of 1976 and the haicheng earthquake of 1975 a new earthquake precursor called coda q is demonstrated for the tangshan earthquake of 1976 and for the haicheng earthquake of 1975 the precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake during the 3 year period preceding the tangshan earthquake the coda q was about 3 times lower than it was before or after this period a comparable change also occured at the time of the haicheng earthquake the observed change cannot be due to the change in the source parameters such as the focal depth or the source spectrum the coda q precursor appears to detect the same physical process detected by other precursors such as b value quiescence radon water quality vp vs and animal behavior but has advantages over them in its clearer physical mechanism easier sampling and better defined anomalous region

investigation of a solvent polluted industrial site on a deep sandstone mudstone sequence in the uk part 2 contaminant sources distributions transport and retardation abstract a single industrial site has been investigated in detail to gain information on sources and movement of chlorinated hydrocarbon solvent chs pollutants in groundwater under the industrialized city of conventry in the uk a soil gas survey identified the most polluted area of the site where two types of investigation borehole were drilled open boreholes lined with plain and slotted casing and a single dedicated borehole incorporating gas samplers and gas driven water samplers specially designed sampling techniques using pumps and packers were employed to determine vertical pollution profiles in the open boreholes core material was collected for sorption and diffusion experiments an assessment of field investigation procedures suggests that soil gas sampling is a useful method for reconnaissance surveying of pollution by volatile organic compounds but the results reflect only shallow pollution in the vertical profile open boreholes were found not to be satisfactory for accurate groundwater profile sampling and dedicated boreholes are recommended for such studies many uncertainties are involved in such a study of solvent pollution owing to the limited number of boreholes drilled and the complex spatial and temporal variations of pollutants despite the limitations of the investigation several useful conclusions are drawn concerning sources and movement of solvent pollution the main pollution source at the site is identified as the solvent storage tanks and pollution has resulted from poor use practices vertical pollution profiles indicate that the downward migration of 1 1 1 trichloroethane is following that of trichloroethene reflecting their history of use on the site the overall quality of groundwater abstracted in the system is set to deteriorate as high pollution loads presently at shallow levels in the system penetrate deeper little attenuated by sorption or degradation diffusion of pollutants between mobile fissure and immobile pore waters will result in poor quality groundwater under this site for the foreseeable future this trend is expected to be reflected throughout the urban area as a whole with most groundwaters falling below drinking water standards for chss

the devonian of the parana basin brazil sequence stratigraphy paleogeography and sw gondwana interregional correlations abstract the devonian period is characterized by an overall mean sea level higher than is observed today this pattern is recognized in several basins worldwide and permitted for example the flooding of the rheic ocean through sw gondwana s cratonic domain nonetheless this flooding was not purely eustatic the ordovician devonian geodynamic famatinian episode also played its role this tectonic eustatic interplay favored shallow marine deposition recorded in several sw gondwana basins such as the parana basin in brazil even so the evolution of the parana basin through the devonian and the factors responsible for the accommodation space remain speculative which leads to bias regarding the correlation of transgressive regressive events in sw gondwana this work aims by integrating outcrop and subsurface data to provide a basin scale stratigraphic framework and a paleogeographical reconstruction of the ponta grossa early devonian and sao domingos formations middle to late devonian of the parana basin it also correlates the studied interval with adjacent basins to identify the impact of tectonic and eustatic processes on the evolution of sw gondwana tectono sedimentary cycles we identified two 2nd order transgressive regressive sequences that correspond to the lithostratigraphic occurrence of ponta grossa and sao domingos formations respectively and whose maximum transgressive and regressive surfaces and high hierarchy unconformities are traceable through adjacent basins the ponta grossa sao domingos boundary also delineates an important tectonic event associated with far field stresses probably caused by precordilleran orogeny related phenomena the ponta grossa and sao domingos formations share several similarities with coeval strata in sw gondwana basins indicating that they can be extrapolated and correlated to contemporaneous processes of interregional scale the first namely precordilleran i early devonian was dominated by tectonics and encompasses the ponta grossa and las pavas composite sequence set the second precordilleran ii middle to late devonian was mostly eustatic and encompasses sao domingos and aguarave composite sequence sets both intervals are part of the famatinian geodynamic cycle

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

temporal variations of the coda decay rate on kamchatka are they real and precursory temporal variations of the decay rate of the coda of local earthquakes are studied for four kamchatka stations to suppress any bias caused by the coda q versus lapse time dependence i use the deviation of an individual coda decay function from the empirical reference decay function and the analysis is done over a fixed lapse time window for each individual coda record a decay rate parameter is determined by the following procedure first the reference log coda decay function is subtracted from an individual one giving a residual function second the slope of this residual versus lapse time is determined over the fixed lapse time window giving an estimate the data used are records of 150 250 kamchatka earthquakes of 1967 1990 ml 3 5 5 obtained by four stations employing three component 1 s seismographs the processed data show apparent temporal variation of with high statistical significance several possibilities are then investigated of mimicking the genuine temporal variation of by systematic variation of other parameters first of all epicenter wandering and nodal plane rotation this analysis does not reveal any strong bias and thus suggests that the observed variation is genuine then i show the significance of five apparently precursory anomalies identified retroactively also i describe briefly the real time prediction experiment conducted on kamchatka for 1982 1990 using coda decay aimed at the intermediate term forecasting of large benioff zone earthquakes this experiment resulted in the successful forecast of the august 17 1983 mw 7 0 event with an accuracy of 2 months in time and 100 200 km in location magnitude was overestimated by 0 5 later a false alarm was also issued thus the experiment confirms the reality of the coda decay rate precursor but also shows that it needs further improvement to become reliable physically the variations of the coda decay rate are associated with time dependent local variations of scatterer density in the lithosphere

frequency dependence of coda q part i numerical modeling and examples from peaceful nuclear explosions frequency dependent coda attenuation values are often reported however such measurements usually depend on the types of the q f models employed we use numerical modeling of peaceful nuclear explosion pne coda at far regional to teleseismic distances to compare two such models namely the conventional frequency dependent quality factor q coda f q f and frequency independent coda attenuation q c with geometrical attenuation the results favor strongly the q c model and illustrate the mechanisms leading to apparent q coda f dependencies tests for variations of the crustal velocity structures show that the values of are stable and related to lithospheric structural types and the inverted q c values can be systematically mapped into the true s wave attenuation factors within the crust modeling also shows that could increase in areas where relatively thin attenuating layers are present within the crust such areas could likely be related to younger and active tectonics by contrast when interpreted by using the traditional q approach the synthetic coda shows a strong and spurious frequency dependence with 0 5 which is also similar to the observations observed lg coda from two pnes located in different areas in russia show similar values of 0 75 10 2 sec 1 which are also remarkably close to the independent numerical predictions at the same time coda q c values vary strongly from 850 in the east european platform to 2500 within the siberian craton this suggests that parameters and q c could provide stable and transportable discriminants for differentiating between the lithospheric tectonic types and ages and also for seismic coda regionalization in nuclear test monitoring research

estimates of q in central asia as a function of frequency and depth using the coda of locally recorded earthquakes digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the hindu kush region of afghanistan were used to determine the attenuation factors of the s wave coda qc and primary s waves q an anomalously rapid decay of the coda shortly after the s wave arrival observed also in a study of coda in central asia by rautian and khalturin 1978 seems to be due primarily to depth dependent variations in qc in particular we deduce the average qc in the crust and uppermost mantle 100 km depth is approximately four times lower than the deeper mantle 400 km depth over a wide frequency range 0 4 to 24 hz further while qc generally increases with frequency at any depth the degree of frequency dependence of qc depends on depth except at the highest frequency studied here 48 hz the magnitude of qc at a particular frequency increases with depth while its frequency dependence decreases for similar depths determinations of q and qc agree suggesting a common wave composition and attenuation mechanism for s waves and codas comparison of these determinations of qc in afghanistan with those in other parts of the world shows that the degree of frequency dependence of qc correlates with the expected regional heterogeneity such a correlation supports the prejudice that qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments

hydrogeology of the coventry region uk an urbanized multilayer dual porosity aquifer system abstract the coventry groundwater investigation has researched the sources and movement of chlorinated hydrocarbon solvents in the groundwater system underlying the coventry region the hydrogeological scene is set in this paper as a basis for the other aspects of the investigation which are reported in the other papers in this volume the deep intercalcated sandstones and mudstones of the permo carboniferous which underly the study area behave as a multilayer aquifer although individual aquifer units are not regionally extensive the sandstone units are fractured and behave as dual porosity aquifers with rapid tracer movement in fractures and diffusion into the low permeability but porous matrix the water balance of the groundwater system has changed from natural conditions because of man s influence although direct recharge has been reduced by the impermeable surfaces of roads and towns substantial extra recharge arises in the urban areas from leaking water mains

comment on attenuation in west bohemia evidence of high attenuation in the nový kostel focal zone and temporal change consistent with co2 degassing by m wcislo l eisner j malek t fischer j vlcek and g kletetschka

investigation of a solvent polluted industrial site on a deep sandstone mudstone sequence in the uk part 1 site description and groundwater flow abstract the sources and subsurface movement of chlorinated hydrocarbon solvents have been investigated at an industrial site as part of a wider study of groundwater pollution by these solvents in the area of coventry uk this paper presents a description of the site and groundwater flow as a prelude to a second paper describing the distribution transport and fate of pollutants both open boreholes and a single borehole incorporating a multilevel sampler were constructed and pumping and tracer tests performed the latter borehole was the most successful in defining hydraulic heads for individual aquifer units the cyclic pumping of the production borehole was analysed using a computer model to gain hydraulic data for aquifer and aquitard units the tracer test defined qualitatively the role played by matrix diffusion in solute transport within the dual porosity aquifer it is concluded that horizontal groundwater flow is dominant along fissured moderately permeable sandstone layers the low permeabilities of mudstone layers maintain high vertical hydraulic gradients sorption of contaminants on to the aquifer matrix is likely to be low owing to a low fraction of organic carbon however matrix diffusion will serve to attenuate peak concentrations of pollutants and stretch pollution pulses in time

a temporal change in coda wave attenuation observed during an eruption of mount st helens during the past few years there have been numerous reports of changes in coda wave attenuation occurring before major earthquakes these observations are important because they may provide insight into stress related structural changes taking place in the focal region prior to the occurrence of large earthquakes the results of these studies led us to suspect that temporal changes in coda wave attenuation might also accompany volcanic eruptions by measuring power decay envelopes for earthquakes at mount st helens recorded before during and after an eruption that took place during september 3 6 1981 we found that coda q sup 1 for frequencies between 6 and 30 hz was 20 30 higher before the eruption than after the change is attributed to an increase in the density of open microcracks in the rock associated with inflation of the volcano prior to the eruption q sup 1 was found to be only weakly dependent on frequency and displayed a slight peak near 10 hz the weak frequency dependence is attributed to the dominance of intrinsic attenuation over scattering attenuation since it is generally accepted that intrinsic attenuation is constant with frequency whereas scattering attenuation decreases strongly at higher frequencies the weak frequency dependence of q sup 1 more at mount st helens contrasts with results reported for studies in nonvolcanic regions the peak in q sup 1 near 10 hz at mount st helens is attributed to the scale length of heterogeneity responsible for generating backscattered waves results for nonvolcanic regions have shown this peak to occur near 0 5 hz thus a smaller scale length of heterogeneity is required to explain the 10 hz peak at mount st helens copyright american geophysical union 1988 less

a geological map of the scotia sea area constrained by bathymetry geological data geophysical data and seismic tomography models from the deep mantle abstract the scotia sea is one of the tectonically most complex and least understood back arc basins on earth which partly results from its remote location making the acquisition of data challenging here we provide a review of current publicly available geophysical and geological data in the scotia sea realm including magnetic bouguer gravity anomaly high resolution bathymetric heat flow and reflection seismic data and rock type data from cored and dredged samples with this inter disciplinary data set we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged scotia sea domain cross sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area the data review revealed that basalt is the dominant lithology in the scotia sea area occupying most of the west and east scotia sea wss and ess andesitic and more felsic lithologies are identified in the central scotia sea css and the northern east scotia sea ess mesozoic palaeozoic metamorphic sedimentary lithologies are reported from the highs along the north and south scotia ridges nsr and ssr these highs originate from a land bridge that until the late mesozoic connected the south american and antarctic continents scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the scotia plate but our mapping exercise allowed us to confirm the presence of the early oligocene late miocene ancestral south sandwich arc assa in the east of the css setting the minimum age of the older segment of the css crust to eocene earliest oligocene three cross sections cross cutting the scotia sea reveal two high velocity zones indicating cold mantle material one is situated below the structural highs along the ssr which we interpreted as remnant slab material of the assa and another below south georgia and the css which we interpreted as the front of the slab that currently subducts at the south sandwich trench ssat the neutral velocity discontinuity below the wss implies mantle conditions of a recently extinct spreading centre the upper mantle low velocity anomaly below the css is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the ssat we have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote inaccessible offshore domains this is a less time consuming economically advantageous exercise which re uses existing geological and geophysical data for a new purpose it is the most data inclusive study there is today of the scotia sea region and serves as a guideline for future expeditions targeting the css and the structural features along the nsr to identify their age and origin a georeferenced version of the map is provided in the supplementary material

temporal variations of coda q an attenuation coefficient view abstract spatial and temporal variations of coda wave attenuation were identified in many studies and particularly in relation to major earthquakes and volcanic eruptions both the coda quality factor qc and its frequency dependence often change following such events which is often attributed to variations in the properties of large volumes of the subsurface however qc is also strongly sensitive to the assumed theoretical models which are usually insufficiently accurate for constraining the actual relationships between the geometrical spreading anelastic dissipation and scattering this inaccuracy often leads to significant exaggeration of attenuation effects and complicates the interpretation of temporal variations to resolve this problem this study uses a phenomenological approach based on the temporal attenuation coefficient instead of qc the attenuation coefficient often linearly depends on frequency f with intercept 0 related to the geometrical spreading and slope giving the effective quality factor qe as d df q e 1 two published examples of temporal variations of local earthquake coda are revisited a non volcanic stone canyon in central california and volcanic area mount st helens washington in both cases linear f patterns are found with the effects of on coda decay rates being significantly stronger than those of q e 1 at stone canyon ranged from 0 035 to 0 06 s 1 and qe varied from 3000 to 10 000 with increasing and qe decreasing during the winter season at mount st helens remained constant at 0 18 s 1 and qe changed from 400 before the eruption to 750 after it the observed temporal variations are explained by the near surface changes caused by seasonal variations in the non volcanic case and gas magma and geothermal system related in the volcanic case scattering attenuation does not appear to be a significant factor in these areas or otherwise it may be indistinguishable due to its fundamental trade off with the background structure and anelastic attenuation in the data

basement geology and palaeozoic mesozoic mafic dykes from the cape meredith complex falkland islands a record of repeated intracontinental extension mafic dykes groups a d intruded into mesoproterozoic basement amphibolites gneisses and granitoids of the cape meredith complex on the southern tip of west falkland provide an important record of at least three periods of lithospheric extension during palaeozoic and jurassic times group a dykes are calc alkaline lamprophyres that were generated by partial melting of an enriched subcontinental lithospheric mantle in cambro ordovician times group b dykes are ordovician dolerites derived from an asthenospheric mantle source perhaps during the same extensional episode as group a dykes group c dykes were also derived from an asthenospheric source and are possibly of silurian age the youngest group d dykes are part of the widespread jurassic gondwana province this group contains an oceanic island basalt like sample and an enriched sample similar to both group a lamprophyres and to the jurassic ferrar province in antarctica these correlations have interesting implications for the composition and evolution of mantle sources through time the co existence of cambrian lamprophyres and jurassic ferrar type magmas in the cape meredith complex demonstrate for the first time that the enriched lithospheric mantle source postulated for the ferrar magmas existed as far back as cambrian times

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

factors controlling the distribution and transport of trichloroethene in a sand aquifer hydrogeology and results of an in situ transport experiment groundwater underlying the perth metropolitan area was found to be contaminated by extensive plumes of trichloroethene tce ammonium and sulphate and regions of high dissolved organic carbon concentration at the study site approximately 600 m downgradient of the presumed source area depth specific sampling in the vertical plane across the plumes showed tce concentrations up to 1300 g l 1 and ammonium and sulphate concentrations up to 90 and 470 mg l 1 respectively the plumes were approximately 100 150 m wide and between 3 9 m thick no significant degradation of tce was observed also depth specific sampling suggests that separate tce plumes are present within the shallow aquifer which may be indicative of multiple tce sources depth specific sampling enabled a better definition of the plumes which is important for possible risk analysis or remediation to determine the transport characteristics of the tce plume a natural gradient field transport experiment using deuterium labelled tce tcedl and bromide was carried out over a displacement distance of 13 m tcedl was stable and non retarded no in situ transport experiment making use of deuterated tce has been reported previously also no previous field experiment has shown tce to move conservatively modelling of the field transport experiment indicated a longitudinal dispersivity of 0 005 m which is the lowest longitudinal dispersivity ever reported for the scale of the experiment i e for a vertical interval of 0 8 m and a horizontal distance of 13 m although not measured the vertical transverse dispersivity is expected to be less than the longitudinal dispersivity the aquifer at the site therefore is characterised by exceptionally little heterogeneity no aquifer however is homogeneous and the tracer experiment revealed the aquifer to be hydraulic conductivity stratified the data suggest that the longitudinal dispersivity estimated from depth integrated concentrations will increase with both the vertical and the horizontal scale and is not expected to approach a constant value hydrogeological investigations indicated that horizontal transverse spreading of the plume was negligible since annual variations in the flow direction 3 were minor therefore any significant transverse spreading of the plumes must result from disturbances of the mean advective flow e g due to localised recharge or extensive pumping these effects are site specific preventing the use of generalised dispersion coefficients together with the results of the transport experiment these data indicate that tce within the acquifer moves as a conservative compound is highly mobile and that high concentrations will be preserved to substantial distances downgradient of the source areas

temporal change in coda q at nevado del ruiz volcano colombia abstract an analysis of coda q q c intrinsic q q i scattering q q s and total q q t was made at nevado del ruiz volcano nrv for local earthquakes recorded during 1992 1994 coda q has been calculated for the first time at nevado del ruiz volcano nrv using the single scattering model proposed by aki and chouet 1975 estimations of q c were made for five frequency bands centred at 1 5 3 0 6 0 12 0 and 24 0 hz the estimated q c varies from around 60 at 1 5 hz to around 980 at 24 hz depending on the frequency with q c 30 1 2 f 1 03 0 001 in general q c values at nrv are rather similar to those obtained in other volcanic areas in the world a steady trend in q c 1 was found from january 1992 to january 1994 this variation is coincident with the stability in the volcanic activity since the last phreato magmatic eruption on 1 september 1989 in the middle of april 1994 a new activity started at nrv and a small volcanic crisis occurred continuous increasing was observed in q c 1 around two months before the crisis this suggests a possibility that the temporal variation of q c can be useful for monitoring the volcanic activity at nrv q i and q s were separated from q t by using a multiple scattering model called multiple lapse time window analysis mltwa fehler et al 1992 estimations for q i q s and q t were made for four frequency bands 3 0 6 0 12 0 and 24 0 hz q i values were found to be near to those of q c for frequencies higher than 6 0 hz for low frequencies q s values were near to q c values this can suggest that the intrinsic absorption could be the predominant attenuation phenomenon in the nrv area at frequencies higher than 6 hz and the scattering phenomenon is predominant for low frequencies therefore the change in q c at high frequencies is considered to reflect variations in the inner conditions of the volcano such as changes in temperature movement of fluids and so on

strain dependent attenuation observations and a proposed mechanism the measured attenuation q 1 of a rock is a function of a number of parameters one of those being the applied strain amplitude it is important to understand the effect that strain amplitude has on q 1 for several reasons different measurement techniques use differing strain amplitudes and may measure a dissimilar q 1 near source large strain wave propagation may behave highly non linearly and the strain amplitude dependence can provide insight into the attenuation mechanism a physical model based on the contact friction between crack surfaces in the rock has been developed to describe rock deformation and dissipation under large applied strain the three dimensional crack surfaces are characterized by a statistical distribution of asperity heights the sliding contact of these spherically tipped asperities dissipates frictional energy hertzian theory is applied to the average asperity contact and predicts that the large strain attenuation is given by q 1 k e p4 3 where k is a constant consisting of the matrix elastic parameters is the crack density e is the strain amplitude and p is the confining pressure the total attenuation measured appears to be the sum of this strain dependent term and a strain independent term the results of ultrasonic pulse transmission experiments are compared with the model s prediction both p and s waves with strain amplitudes from 10 8 to 10 5 were employed frequencies from 0 4 to 1 5 mhz were used in conjunction with rock confining pressures of 2 to 580 bars on dry berea sandstone and lucite samples the spectral ratio method and rise time technique were applied to deduce the q 1 values the observed data and other observations from the literature compare well with the model s prediction for the dependence of q 1 on large strain amplitude crack density and pressure

q determined from local earthquakes in the south carolina coastal plain abstract coda q or qc is determined from the rate of coda decay in the coastal plain near charleston south carolina the mean of qc calculated from individual estmates is near 190 at 1 hz and increases to over 1650 at 10 hz a possible temporal change in qc is identified high values being associated with increased seismic activity during late 1977 temporal changes have been observed elsewhere and laboratory measurements of q under different stress states correlate high q with high stress a spatial variation in qc is identified increasing from west qc at 10 hz equal to 850 to east qc at 10 hz greater than 2100 across the coastal plain the demarcation of high to highest zones of qc is coincident with the western boundary of the innermost isoseismal of the charleston 1886 earthquake it is hypothesized that the 1886 earthquake was mislocated originally because of this spatial variation in energy attenuation that was unrecognized at that time

temporal change in attenuation intensity before and after the eastern yamanashi earthquake of 1983 in central japan a crutal earthquake of magnitude 6 0 took place on august 8 1983 in the middle of the kanto tokai microearthquake observation network of the national research center for disaster prevention in central japan by analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock attenuation change was estimated by using two different kinds of analyses in the first analysis station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before in the second analysis s coda duration from the origin time was measured and was compared with the average magnitude the increase of s coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter the simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 hz qs 1 1 4 10 2f 1 since other precursors appeared in the preparation stage it is possible to interpret the change in qs 1 as a precursor the attenuation intensity would have been anomalously large before the main shock

interpretation and chronostratigraphic mapping of multichannel seismic reflection profile i95167 eastern falkland plateau south atlantic abstract the falkland plateau fp is a submarine high representing the eastern prolongation of the south america continent it separates the argentine basin of the south atlantic ocean from the scotia sea of the antarctic region a south north seismic section across the falkland plateau has been interpreted using information from two wells sited on the margin of the maurice ewing bank meb on the plateau the sedimentary cover has a regionally horizontal configuration with high lateral continuity of the reflecting horizons the older sequence is dominated by a normal fault system of the mesozoic gondwana break up tectonics partially reactivated by the following tectonic events between them the one mainly affecting the studied area is the transpression of the north scotia ridge nsr producing some thrusting in the sedimentary cover of the falkland trough ft visible only on the south extremity of the interpreted section and a prominent tilting of its whole crust the interpreted seismic section outlines a gradually thinner crust from the maurice ewing bank margin to the falkland plateau and to the falkland trough during the interpretation particular attention was given to picking the main unconformities to analysing the relationships of reflections within the sequences to the unit boundaries and to identifying by the seismic facies analysis the different depositional units these last labelled with symbols a g from mesozoic to present age represent the whole sedimentary cover lying on the pre cambrian basement of the falkland plateau in the reconstructed chronostratigraphic section the whole data set has been displayed as a diagram wherein age decreases upward and the lateral extent of units is on the horizontal scale illustrating the main sedimentary and erosional events in the area erosion nondeposition and deposition of sediment drifts by the sea bottom currents particularly active from the cainozoic to the present are mostly linked to the gondwana breakup with the mesozoic opening of the south atlantic ocean and the cainozoic opening of the drake passage during the mesozoic time the proto south atlantic and indian oceans connection caused extensive erosion and nondeposition resulting in absence of important portions of the sedimentary cover

the falkland plateau in the context of gondwana breakup abstract during the jurassic the falkland plateau was part of gondwana and occupied a position between the african and antarctic plates several contrasting models exist for the breakup of gondwana that depend on assumptions about the currently unknown crustal structure of the falkland plateau here we present the results of recently acquired wide angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions in contrast to published crustal models the new data show that the falkland plateau basin consists of up to 20 km thick oceanic crust which is bounded to the east by a continental fragment the maurice ewing bank in a refined geodynamic model rifting started between the falkland islands and the maurice ewing bank at 178 ma and ceased at around 154 ma the plateau s exceptionally thick oceanic crust likely results from its position in an extensional back arc regime situated over a mantle thermal anomaly that was also responsible for the extensive onshore karoo ferrar and chon aike volcanic provinces

time space variation of seismic s wave coda attenuation qc 1 and magnitude distribution b values for the petatlan earthquake average s wave coda q qc at 6 hz and to about 10 sec after the s wave arrival shows 1 temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 petatlan mexico earthquake 2 spatial variation through the aftershock area positively correlated with b high qc b values at the two activity centers intermediate values in between and landward and low values toward the middle america trench which remain low for two weeks after the main shock and 3 five foreshock average qc 135 1 6 and eleven aftershock average qc 175 1 3 for the eastern asperity the b values and possibly qc values seem to correlate with the level of aftershock activity

the gondwana connections of northern patagonia a multidisciplinary study u pb sensitive high resolution ion microprobe geochronology hf and o isotopes in zircon sr and nd isotopes in whole rocks as well as major and trace element geochemistry has been carried out on granitoid samples from the area west of valcheta north patagonian massif argentina these confirm the cambrian age of the tardugno granodiorite 528 4 ma and the late permian age of granites in the central part of the yaminue complex 250 ma together with petrological and structural information for the area we consider a previously suggested idea that the cambrian and ordovician granites of northeastern patagonia represent continuation of the pampean and famatinian orogenic belts of the sierras pampeanas respectively our interpretation does not support the hypothesis that patagonia was accreted in late palaeozoic times as a far travelled terrane originating in the central transantarctic mountains and the arguments for and against this idea are reviewed a parautochthonous origin is preferred with no major ocean closure between the north patagonian massif and the sierra de la ventana fold belt supplementary material u pb shrimp analytical data geochemical analyses and sample global positioning system locations are available at www geolsoc org uk sup18722

geochemistry and isotopic evolution of the mesoproterozoic cape meredith complex west falkland whole rock major and trace element geochemical and rb sr sm nd isotopic data are presented for the mesoproterozoic 1 0 ga metamorphic and igneous rocks of the cape meredith complex west falkland the data indicate that the oldest rocks the 1 1 ga supracrustal gneisses of the big cape formation which form three petrographic and geochemical groups mafic amphibolite quartz plagioclase biotite hornblende intermediate gneiss and acid gneiss probably represent a juvenile calc alkaline basalt andesite rhyolite volcanic sequence with epsilon endt values and nd t dm ages of 3 to 6 and 1100 to 1400 ma respectively it is argued on geochemical grounds that these metavolcanics were extruded in an island arc at around 1120 ma the big cape formation was intruded by granitoids during and after a collisional orogenic event at around 1090 ma the oldest foliated g1 granodiorite was emplaced as thin sheets at approximately 1090 to 1070 ma and is characterized by end values of 1 5 to 4 t dm 1200 to 1400 ma showing its juvenile nature the 1070 ma g2 syntectonic granitoid gneisses and 1000 ma g3 post tectonic granites also exhibit juvenile characteristics e nd 0 to 5 and t dm 2200 to 1200 ma respectively the granitoids show a time composition evolution from na rich g1 granodiorite to potassic high high field strength element granites g3 the geochemical and isotopic characteristics and geological evolution of the cape meredith complex is comparable with that of the adjacent gondwana crustal blocks in natal se africa and dronning maud land east antarctica supporting models that demonstrate these areas evolved in a contiguous juvenile arc environment prior to and during a major orogenic event at 1 1 ga these events were associated with the birth of the rodinian supercontinent the three areas remained juxtaposed during rodinia break up and were subsequently incorporated into gondwana in the same relative positions

a routine high precision method for lu hf isotope geochemistry and chronology a method for chemical separation of lu and hf from rock meteorite and mineral samples is described together with a much improved mass spectrometric running technique for hf this allows i geo and cosmochronology using the176lu 176hf decay scheme and ii geochemical studies of planetary processes in the earth and moon chemical yields for the three stage ion exchange column procedure average 90 for hf chemical blanks are 0 2 ng for lu and hf from 1 g of hf a total ion current of 0 5 10 11 ampere can be maintained for 3 5 h yielding 0 01 0 03 precision on the ratio176hf 177hf normalisation to179hf 177hf 0 7325 is used extensive results for the johnson matthey hf standard jmc 475 are presented and this sample is urged as an international mass spectrometric standard suitable aliquots prepared from a single batch of jmc 475 are available from denver lu hf analyses of the standard rocks bcr 1 and jb 1 are given the potential of the lu hf method in isotope geochemistry is assessed

age of basement rocks from the maurice ewing bank and the falkland malvinas plateau abstract the maurice ewing bank is located east of the falkland malvinas islands and is one of the most intriguing regions of southern gondwana the interaction of several microplates with major cratonic areas resulted in complex displacements between paleozoic and cretaceous times here we present combined u pb and lu hf zircon isotopic analyses of the maurice ewing bank basement to constrain the age and tectonic setting to aid in paleogeographic reconstruction the high grade paragneisses in the bank present dominant late mesoproterozoic detrital zircon age distributions with populations at 1032 12 ma 1068 16 ma and 1233 8 ma and juvenile signature positive ehf t values the gneisses are cutting by anatectic pink granite crystallized at 1006 13 ma which formed after syn collisional high grade metamorphism and shows juvenile and some crustal component ehf t 1 6 to 3 the maurice ewing bank basement rocks have same stenian age and dominant juvenile signature as the arc related basement rocks from the falkland malvinas islands and are probably part of the same continental plateau of the south america continent this new finding enables future reconstructions in paleogeographic models where the basement of the falkland malvinas islands and the maurice ewing bank formed a single block during the mesoproterozoic and were partly separated by crustal stretching during the opening of the atlantic ocean in the late jurassic cretaceous the block played a major role in the deformation of the cape fold belt from the late paleozoic through the mesozoic

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

temporal change in coda associated with the round valley california earthquake of november 23 1984 seven thousand seismograms of small earthquakes in the mammoth lakes bishop area were used to measure values of q from the decay of the earthquake coda these measurements were compared between events that occurred before and after the round valley earthquake m 5 7 we found that in regions near the main shock epicenter measurements of coda q 1 for earthquakes that occurred after the main shock were higher than for those earthquakes that occurred prior to the main shock the opposite behavior was found for regions farther away from the main shock namely lower coda q 1 after the main shock than before measurements of coda q 1 in the long valley caldera outside of the immediate source region of the earthquake were higher than in surrounding areas before the main shock but the difference disappeared after the occurrence of the main shock this indicates that the temporal variation in coda q 1 is comparable to its spatial variation the doughnut model seismicity quiescence surrounded by a zone of activity which was invoked for explaining the precursory seismicity pattern appears to be similar with the observed coda q 1 variation associated with the round valley earthquake the observed spatial variations in coda q 1 also help to reconcile conflicting results published in previous studies of the coda q 1 precursor

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

investigation of point source pollution by chlorinated solvents in two different geologies a multi layered carboniferous sandstone mudstone sequence and the chalk

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

falkland plateau evolution and a mobile southernmost south america assessment of southwest gondwana break up and its implications for regional hydrocarbon prospectivity must now take into account the origin of the southeast margin of the falkland islands as a volcanic rifted continental margin and of the floor of the major part of the falkland plateau basin as elevated oceanic crust a reconstruction of the falkland plateau against southern africa shows a southward extensional widening of the outeniqua basin across the line of the falkland agulhas fracture zone changing from stretched continental to oceanic crustal structure such a model for outeniqua basin opening and the independent westward and clockwise rotation of the falkland islands block suggests that southernmost south america was also a collection of microplates moving independently within a generally extensional environment in the late triassic early jurassic this is incompatible with assumptions of a rigid southernmost south america over this time and a dominant role for a continuous dextral strike slip gastre fault

pre lamprophyre mafic dykes of the cape meredith complex west falkland new field petrographic geochemical and rb sr sm nd isotope data are presented from early mafic dykes which intrude the mesoproterozoic cape meredith complex west falkland the dykes which have been previously regarded as being ordovician in age are seen in the field to cut a suite of lamprophyre sheets new k ar data from biotite separates from two lamprophyres suggest a miminum age of emplacement of 520 ma confirming previous work the rb sr and sm nd analyses of the pre lamprophyre mafic dykes suggests that they were probably intruded during the late neoproterozoic at 600 ma this new data has considerable implications for the previously published estimates of the longevity of intracontinental extension events in this part of gondwana

quantification of groundwater recharge in the city of nottingham uk groundwater is an important and valuable resource for water supply to cities in order to make full and wise use of the asset value a clear understanding of the quantities and sources of urban groundwater recharge is needed the water supply and disposal network is often an important source of recharge to urban groundwater through leakage from water mains and sewers an approach to establishing the spatial and temporal amounts of the three urban recharge sources precipitation mains and sewers is developed and illustrated using the nottingham uk urban aquifer a calibrated groundwater flow model is supplemented by calibrated solute balances for three conservative species cl so4 and total n thus providing four lines of evidence to use in the recharge estimation nottingham is located on a triassic sandstone aquifer with average precipitation of 700 mm year using the models current urban recharge is estimated to be 211 mm year of which 138 mm year 40 is from mains leakage and 10 mm year 100 is from sewer leakage the wide confidence intervals result from the scarcity of historical field data and the long turnover time in this high volume aquifer and should be significantly lower for many other aquifer systems

the crustal structure of the continental margin east of the falkland islands abstract the 1500 km long falkland plateau is the most prominent morphological structure in the southern south atlantic ocean which crustal composition and development is mainly unknown at the westernmost boundary of the plateau the falkland islands precambrian geology provides the only insight into basement structure and age the question of whether continental basement of a similar age and origin underlies the falkland plateau further east is strongly disputed we present new high quality constraints on the crustal fabric of the plateau east of the falkland islands based on wide angle seismic and potential field data acquired in 2013 the p wave velocity model supported by amplitude and density modelling shows that the falkland plateau basin is filled with 8 km of sediments continental crust of 34 km thickness underlies the falkland islands the eastern continental margin of the falkland islands can be classified as a volcanic rifted margin the falkland plateau basin is floored by up to 20 km thick oceanic crust the exceptionally thick igneous crust and its high lower crustal velocities up to 7 4 km s indicate the influence of a regional thermal mantle anomaly during its formation which provided extra melt material the wide angle model revises published crustal models which predicted thin oceanic or thick extended continental crust below the falkland plateau basin our results provide a sound basis for future tectonic interpretations of the area

mesozoic rifting evolution of sw gondwana a poly phased subduction related extensional history responsible for basin formation along the argentinean atlantic margin abstract the opening of the south atlantic in the early cretaceous was the final stage of the complex rifting history of sw gondwana in this contribution we reassess the chronology of mesozoic basin formation in southern south america and africa and integrate it in the long term breakup history of sw gondwana triassic rifting is characterized by intracontinental rifting in africa karoo i phase and retro arc extension on the sw margin of gondwana in the early jurassic the impingement of the karoo plume triggered rifting in eastern africa producing the karoo ii basins and the colorado and salado basins on the argentinean shelf east africa rifting ultimately lead the breakup of eastern from western gondwana in the middle jurassic in patagonia the austral malvinas and other related basins formed in association with the synextensional emplacement of the chon aike magmatic province in the patagonian retro arc in the late jurassic the rocas verdes back arc basin opened in southern patagonia while oblique rifting in the core of the late paleozoic gondwanides orogen produced the outeniqua and rawson valdes basins the south atlantic rift initiated in the early cretaceous associated with present day e w extension rifting occurred diachronically from south to north initiating in the previously thinned rawson valdes outeniqua segment a precursor oblique rift system and a larger degree of extension in this segment could explain the lack of seaward dipping reflectors sdr south of the colorado cape fracture zones rifting and sdr emplacement occurred progressively to the north along different rift segments producing strongly asymmetric conjugate margins