the laidlaw volcanics a late silurian point on the geological time scale mapping mineralogical and chemical studies of an area west and northwest of canberra southeastern australia have shownthat the laidlaw volcanics are unique in the region and that their age is tightly controlled stratigraphically as they lie both above and below early ludlow late silurian fossiliferous sediments k ar mineral analyses previously published rb sr mineral analyses and new rb sr wholerock and mineral analyses together give the laidlaw volcanics an age of 420 7 2 2 m y this result for the early ludlow is identical numerically with a recently published age of 421 3 m y for the ashgillian late ordovician stockdale rhyolite and suggests either that the time interval between the ashgill and ludlow does not exceed a few million yearsor that the age estimate for the stockdale rhyolite is too young

rb sr systematics of permian volcanites in the schwarzwald sw germany from the quartz porphyry occurrences of haigerach and geisberg central schwarzwald which were dated by minerals and whole rock samples the whole rock samples generally yielded lower ages than the minerals this result agrees with other studies on apparently rejuvinated whole rock ages of acid volcanic rocks compare mckerrow et al 1980 petrographic and geochemical observations show that two processes disturbed the rb sr relations 1 fast alterations immediately after extrusion such as sericitization during autohydrothermal bleaching in the pipe area of the volcanoes gain of rb addition of common sr 2 loss of radiogenic 87sr due to long lasting exchange processes with groundwaters

geochronology of late caledonian magmatism in northern britain rb sr k ar ar ar and sm nd isotopic data are presented for mineral separates and whole rock samples from old red sandstone ors lavas associated minor intrusions and 9newer granite9 intrusive complexes from the cheviot hills north to shetland these data yield 16 internally consistent rb sr mineral ages and one each of ar ar sm nd and k ar ages when considered with precise and reliable published data it is clear that there are systematic differences in the age of late caledonian magmatism through the geological regions of scotland in the grampian highlands and the scottish midland valley magmatism was concentrated during the period 423 to 410 ma ors volcanism in the highlands occurred around 420 ma with plutonism continuing to about 408 ma volcanism and plutonism in the midland valley took place between 415 and 410 ma in the southern uplands magmatism occurred earlier in the north with all magmatism close to the southern uplands fault dated around 408 ma and that close to the iapetus suture dated at around 394 ma as no evidence exists for ocean closure prior to c 400 ma the age of igneous activity north of the southern uplands fault is consistent with chemical evidence for a relationship to active nw directed subduction the younger magmatism of the s part of the southern uplands cannot be subduction related and the c 408 ma plutons nearer the southern uplands fault can only be subduction related if tectonic construction of the southern uplands was a two stage process

pockmarks in the witch ground basin central north sea marine sediments host large amounts of methane ch4 which is a potent greenhouse gas quantitative estimates for methane release from marine sediments are scarce and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios here we use 2d and 3d seismic reflection multibeam bathymetric geochemical and sedimentological data to i map and describe pockmarks in the witch ground basin central north sea ii characterize associated sedimentological and fluid migration structures and iii analyze the related methane release more than 1500 pockmarks of two distinct morphological classes spread over an area of 225 km2 the two classes form independently from another and are corresponding to at least two different sources of fluids class 1 pockmarks are large in size 6 m deep 250 m long and 75 m wide show active venting and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources class 2 pockmarks which comprise 99 5 of all pockmarks are smaller 0 9 3 1 m deep 26 140 m long and 14 57 m wide and are limited to the soft fine grained sediments of the witch ground formation and possibly sourced by compaction related dewatering buried pockmarks within the witch ground formation document distinct phases of pockmark formation likely triggered by external forces related to environmental changes after deglaciation thus greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present day fluxes but require an analysis of the pockmark forming processes through geological time

three s type volcanic suites from the lachlan fold belt southeast australia the concept that granitoids of the lachlan fold belt in southeast australia are derived from either igneous or sedimentary source rocks i or s type can be extended to volcanic rocks of the same age three suites of late silurian s type volcanics are described two of which can be matched quite closely with plutonic equivalents a large part of the paleozoic continental margin volcanic activity in southeast australia consisted of the magmatic recycling of old metasedimentary crust probably of late proterozoic age the three volcanic suites are moderately to strongly peraluminous with the corresponding presence of al rich minerals variation within the volcanic suites is ascribed chiefly to progressive removal of restite or source material residual from partial melting the most mafic suite the hawkins suite contains plagioclase cordierite orthopyroxene biotite and quartz as restite components with less abundant almandine these rocks are chemically equivalent to mafic biotite rich and cordierite bearing granitoids from parts of the berridale and murrumbidgee batholiths garnet is absent from the goobarragandra suite volcanics which are a little more felsic and close in compositon to granitoids of the young and maragle batholiths the s type character of the laidlaw suite is less pronounced although a sedimentary source seems to be established such a source would be less mature than those for the other two volcanic suites the laidlaw suite resembles but cannot be closely identified with felsic s type granitoids of the murrumbidgee and berridale batholiths low grade regional metamorphism in most of the volcanic rocks has resulted in much mineralogical alteration and mobility of alkali and alkaline earth elements despite this alteration unaltered rocks are present in all three suites compositional data for the phenocryst phases within the unaltered rocks has allowed estimation of some intensive parameters hawkins suite volcanics were extruded directly from their source region and preserve phenocryst equilibria established at 5 6 kbar at 800 c the laidlaw and goobarragandra suites reequilibrated at lower pressures than their source before extrusion the laidlaw suite reequilibrated at estimated temperatures of 725 730 c o2 of 10 15 5 to 10 14 bars and h2o of 2 to 2 5 kbar differing mineral compositions in the three suites are related to differing source rock compositons and oxygen fugacities relative biotite and orthopyroxene mg is possibly pressure dependent

late quaternary palaeontology sedimentology and geochemistry of a vibrocore from the witch ground basin central north sea abstract results are detailed from analytical procedures carried out on a single closely sampled 5 7 m long vibrocore from the witch ground basin central north sea information about the environmental conditions during the deposition of the witch ground formation of late weichselian to holocene age was obtained by combining the various palaeontological sedimentological and geochemical results this has suggested that a complete glaciomarine to marine sequence from before 13 000 to after 10 000 b p is present in the central north sea an ash layer equated with the vedde ash of western norway has been identified for the first time in the uk sector

resistivity and induced polarization surveys for slope instability studies in the swiss alps a geoelectrical investigation has been carried out in the swiss alps near the dent de nendaz within the southern part of the rhone valley in the swiss alps to delineate groundwater circulation within an unstable slope this unstable zone is characterized by a landslide area with clear scarps and hummocky ground at the bottom of the slope large scale apparent resistivity mapping provided information on the main pathways for water transport within the slope subsequently 2d resistivity and induced polarization imaging supplied details on the region of instability located at the bottom of the slope polarizable units identified in the region of instability could be linked to a clay rich unit or to local enrichment of graphite within the bedrock our results illustrate how low permeability clay graphite units and permeable water bearing units could be distinguished using a combined interpretation of resistivity and ip data in slope instability studies

analysis of lightning electromagnetic field propagation in mountainous terrain and its effects on toa based lightning location systems in this paper we analyze the propagation effects on lightning radiated electromagnetic fields over mountainous terrain by using a three dimensional 3 d finite difference time domain fdtd method we also discuss the time delay error in the time of arrival toa technique currently used to locate lightning in detection networks specifically furthermore the accuracy of different approximate methods presented in the literature is discussed and tested by using our 3 d fdtd method it is found that 1 the time delays and amplitudes of the lightning radiated electromagnetic fields can be significantly affected by the presence of a mountainous terrain and associated diffraction phenomena 2 for a finitely conducting ground the time delay shows a slight increase with the increase of the observation distance but the time delay resulting from the finite ground conductivity appears to be smaller than that caused by the mountainous terrain and 3 the timing error associated with the toa technique depends on the threshold times threshold times of 10 and 20 of the peak provide very similar results compared to those corresponding to the peak of the first derivative of the magnetic field and the threshold time exceeds 50 of the initial rising amplitude of the signal furthermore we have assessed the accuracy of two simplified methods terrain envelope method and tight terrain fit method to account for the time delays resulting from the propagation in a mountainous terrain it is found that both methods result in time delays that are in reasonable agreement but always overestimating the results obtained using the full wave 3 d fdtd approach for the perfectly conducting ground these two methods represent interesting alternatives to account for the time delay over a nonflat terrain using the terrain model

possible two stage 87sr evolution in the stockdale rhyolite the rb sr total rock data for the stockdale rhyolite of significance for the palaeozoic time scale are more scattered about a single stage isochron than expected from experimental error two stage 87 sr evolution for several of the samples is explored to explain this as an alternative to variation in the initial 87 sr 86 sr which is customarily used in single stage dating models the deletion of certain samples having very high rb sr removes most of the excess scatter and leads to an estimate of 430 7 m y for the age of extrusion there is a younger alignment of rb sr data within each sampling site at 412 7 m y we suggest that the stockdale rhyolite is at least 430 m y old that its original range in rb sr was smaller than now observed and that it experienced a net loss in sr during later hydrothermal alteration at ca 412 m y

palaeomagnetism of caledonian intrusive suites in the northern highlands of scotland constraints to tectonic movements within the caledonian orogenic belt abstract to evaluate the scale of tectonic movements within the northern sector of the 500 400 ma caledonian orogenic belt and its precambrian foreland zone between the great glen fault ggf zone to the southeast and the laurentian block to the northwest we have studied the palaeomagnetism of minor intrusive rocks within the northern highlands terrain these rocks include 1 1 amphibolites and other metamorphic rocks predating deformation 2 2 microdiorities dolentes and related suites emplaced and probably magnetised between 450 and 420 ma and 3 3 lower middle devonian lamprophyres a range of predominantly nne negative and ssw positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority wnw ese axis isolated southerly negative directed hematite held components suggests limited but no widespread remagnetisation in devonian carboniferous times comparison is made with data from other tectonic divisions in the caledonian orogenic belt and the bordering forelands palaeopoles from the northern highlands closely conform in part with north american ordovician poles and in part with the post ordovician palaeopoles from britain south of the ggf the definitive motions of the british caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the caledonian terrain in early ordovician times small relative motions during the remainder of ordovician times followed by large clockwise and then anticlockwise rotations during late ordovician to early silurian times contemporary with the last major movements on the moine thrust ca 430 ma late silurian devonian movements along the ggf were probably below the limits of palaeomagnetic detectability the collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by north american studies and the path for lower palaeozoic times is more complex than recognised hitherto

propagation effect on field waveforms in relation to time of arrival technique in lightning location the propagation effect on electromagnetic pulses associated with negative return strokes is evaluated from data observed by direction finders for lightning location equipped with gps clocks nine direction finders constitute a network in the northern part of the honshu island of japan which locates lightning by a method combining the direction finding and the time of arrival toa technique the delay of the peak of an electromagnetic pulse was evaluated to be negligible in propagation over seawater for distances less than 300 km and its average was about 1 8 s in the east west propagation over land for the distance of about 130 km the timing error of 1 8 s may cause maximum location error of about 0 5 km within a toa lightning location network and larger error is anticipated outside of the network the evaluated propagation delay in this area corresponds to the average ground conductivity of about 0 003 s m except for the southwest part with slightly lower conductivity of about 0 002 s m these findings were confirmed by an analysis based on ground truth data obtained by still cameras equipped with precise clocks the measured delay in the north south propagation corresponded to the average ground conductivity of about 0 003 s m the estimated arrival time of an electromagnetic pulse based on the arrival time of the peak and the trigger time gives smaller delay than the simple arrival time of the peak but the difference is not significant it would be better to compensate the peak time by evaluating the propagation effect in obtaining an estimate of the arrival time to improve the accuracy of lightning location by using the toa technique

the propagation effects of lightning electromagnetic fields over mountainous terrain in the earth ionosphere waveguide in this paper a full wave two dimensional finite difference time domain model is developed to evaluate the propagation effects of lightning electromagnetic fields over mountainous terrain in the earth ionosphere waveguide in the model we investigate the effect of the earth ionosphere waveguide structure and medium parameters including the effect of the ionospheric cold plasma characteristics the effect of the earth curvature and the propagation effects over mountainous terrain for the first time the obtained results are validated against simultaneous experimental data consisting of lightning currents measured at the santis tower and electric fields measured in neudorf austria located at 380 km distance from the tower it is shown that both the time delays and amplitudes of the lightning electromagnetic fields at 380 km distance can be strongly affected by the ionospheric electron density profile the mountainous terrain and the earth curvature after taking into account the effect of the irregular terrain between the santis tower and the field measurement station the vertical electric fields calculated by using our model are found to be in good agreement with the corresponding measured cases occurred in both daytime and nighttime the ideal approximation used in either the classical solutions or the simplified models might lead to inaccuracies in the estimated reflection height furthermore we discuss the sensitivity of our results by considering different return stroke models as well as different typical values of the return stroke speed and of the ground conductivity

a relict ice scoured erosion surface in the central north sea abstract the interpretation of shallow seismic records from the central north sea has revealed the existence of an irregular erosion surface within late pleistocene sediments a morphological study of this surface has identified two main types of relief 1 an irregular rough topography with depressions varying in depth from 1 to 6 m and in width from 25 to 300 m and 2 a much smoother topography with relatively few depressions on a palaeobathymetric map the rough topography extends from ca 110 to 160 m below sea level od while the smoother topography extends beyond the 160 m below od contour this surface is interpreted as an ice scoured erosional feature formed by the grounding of sea ice in a shallow shelf environment the stratigraphic position of the ice scoured surface shows it to be a relict late weichselian feature formed at ca 17 000 18 000 b p

relict sea floor ploughmarks record deep keeled antarctic icebergs to 45 s on the argentine margin abstract this paper reports ploughmarks on the argentine continental margin up to 44 50 s the most northerly morphological evidence of giant icebergs in the southern atlantic more than 2500 ploughmarks up to 32 km long 685 m wide and more than 20 m deep have been observed at modern water depths up to 646 m taking into account these iceberg sizes and ocean current directions which appear to have remained similar from the last full glacial to the present interglacial the icebergs that produced the ploughmarks are interpreted to be calved from fast flowing ice streams draining huge basins of the west antarctic ice sheet and the western antarctic peninsula during the last glacial maximum lgm about 20 000 years ago ploughmarks at the deepest water depths correspond to icebergs more than 500 m thick and at least 2 km wide given a sea level rise of 120 m since the lgm the icebergs after having drifted northeast in the antarctic circumpolar current were transported northward by the falklands malvinas current a total distance of 2000 to 4000 km the waters north of the falkland islands in particular were probably several degrees colder than today to prevent rapid iceberg melting and deterioration

the statistical analysis and interpretation of imperfectly fitted rb sr isochrons from polymetamorphic terrains abstract rb sr isotopic data for large relatively homogeneous whole rock samples of uivak 1 gneiss from the saglek hebron area of northern labrador exhibit a scatter which exceeds that predicted by experimental error isotopic analyses of adjacent compositionally different layers of uivak gneiss 1 2 cm in width define secondary isochrons with slopes corresponding to an age of ca 1800 ma as field evidence combined with previous isotopic dating demonstrates that the compositional layering did not form at this time the secondary isochrons are interpreted as resulting from localized sr isotopic homogenization along 87sr abundance gradients generated by ageing in the previously layered gneisses the geological scatter in the larger gneiss specimens is therefore attributed to the same phenomenon on a reduced scale viz sr isotopic equilibration at 1800 ma between adjacent volumes of gneiss however regional differences in mean 87 sr 86 sr and mean 87 rb 86 sr are assumed to be unchanged this interpretation has led us to the development of a weighted least squares regression technique that utilizes the geologically induced error structure in the whole rock rb sr data the method encompasses three models in the first the local isotopic equilibrium or free line model it is assumed that the error structure in the whole rock samples is the same as that in the layered gneisses except for an unknown scaling factor common to both 87 rb 86 sr and 87 sr 86 sr in the other two models the initial strontium isotopic composition is constrained to values greater than primordial sr and corresponding to that expected for the contemporary bulk earth by forcing the least squares fit for the data to pass through a fixed point that corresponds to the present day 87 rb 86 sr and 87 sr 86 sr composition of the bulk earth viz approximately 0 085 and 0 7047 respectively in the first of these models bulk earth model 1 no assumptions are made about the error structure of the primary data an estimate of the age can be obtained but no estimate of its uncertainty in the second bulk earth model 2 the first two methods are combined and estimates for both the age and its 95 confidence limits may be found in developing the method nineteen whole rock samples plus the means of the slabbed gneisses yielded the following results for the age and initial 87 sr 86 sr respectively of the uivak 1 gneisses 1 1 free line model 3621 410 686 ma 0 70006 565 354 2 2 bulk earth model 1 3606 ma 0 70020 3 3 bulk earth model 2 3606 175 213ma 0 70020 27 22

location accuracy evaluation of toa based lightning location systems over mountainous terrain in this paper we analyze the location error of time of arrival toa based lightning location systems llss caused by propagation effects over mountainous terrain around the santis tower located in the swiss alps the study is based on a full wave three dimensional 3 d finite difference time domain approach using the topographic map including the santis tower and the nearby sensors belonging to llss it is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity and that the location error associated with the toa technique depends strongly on the used onset time estimation technique the evaluated location errors associated with amplitude thresholds of 10 and 20 and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest about 300 m or less finally we assess the accuracy of two simplified methods terrain envelope method and tight terrain fit method to account for the location error due to propagation over mountainous terrain these two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a nonflat terrain by using available topographic data in addition a possible real time location error compensation algorithm using the elongated propagation path method to improve the location error of the llss in mountainous regions is presented and discussed

the significance of isotopic dates from the english lake district for the ordovician saurian time scale the lower palaeozoic time scale is discussed in the light of a new date for the threlkeld microgranite and a reassessment of other published dates from the lake district which suggest ages of c 410 ma for the base of the devonian c 421 ma for the ashgill c 429 ma for the late caradoc c 439 ma for the early llandeilo and c 468 ma for the early llanvirn

propagation effects on the lightning generated electromagnetic fields for homogeneous and mixed sea land paths the influences of the path of propagation on the shape and amplitude of the electric fields and electric field derivatives generated by lightning return strokes were investigated results are presented for the cases where 1 both the lightning return stroke and the point of observation are located over finitely conducting homogeneous ground and 2 the lightning return stroke is located over sea and the point of observation is located over land at some distance from the sea land boundary for propagation paths over finitely conducting ground the electric field derivative can decrease significantly in propagation distances as small as 1000 m when the path of propagation is partly over sea and partly over land the propagation effects on the electric radiation field derivative are significant unless the width of the land portion of the propagation path is less than a few tens of meters