cenozoic tectonic extension and inversion of the jizhong basin hebei northern china abstract seismic reflection profiles have revealed three major phases of tectonic extension in the jizhong basin during tertiary time the basement faults characterized by listric planar rotational normal faults controlled the development of the sedimentary basin which migrated progressively eastward inversion structures including partially reactivated normal faults a hangingwall back thrust and newly developed reverse faults and folds have involved all sedimentary sequences identifiable in the seismic section probably indicating quaternary tectonic inversion this inversion was probably related to the e w directed shortening in this part of china induced by the overall india eurasia collision

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

the permian to jurassic stratigraphy and structural evolution of the central cheshire basin new seismic reflection data reveal details of the permo triassic basin fill in the central cheshire basin the seismic stratigraphy indicates that the permian and triassic succession is far thicker than previously estimated seismic data and backstripping of the preserved permian to early jurassic sedimentary succession suggest that at least two phases of fault controlled subsidence occurred in early permian and early triassic times an intra triassic unconformity is seen on the seismic data near the top of the sherwood sandstone group this may equate with the hardegsen unconformity it is possible that this unconformity was generated by syn extensional regional uplift accompanying lithospheric thinning the gross morphology and location of the faults on the eastern margin of the cheshire basin are interpreted to be at least locally controlled by extensional re activation of a westwards dipping reverse fault possibly an extension of the pontesford linley lineament

a shallow seismic reflection survey in basalts of the snake river plain idaho the objective of this feasibility study was to determine if the seismic reflection method could help to optimize placement of water quality monitoring wells near a radioactive storage facility seismic reflections from depths less than 30 m were recorded along a 500 m long line over a basalt rhyolite and sedimentary sequence in the snake river plain some shallow reflections at 40 to 50 ms on the field files are of exceptional quality with frequency exceeding 150 hz reflections and refractions from selected seismograms along the line possess vastly different normal moveout nmo and apparent velocities as well as wavelet characteristics extreme variations in quality seismic character and reflector geometries observed on seismograms give the appearance of varying geologic settings and are uncommon for such short distances severe surgical muting was necessary for accurate velocity and statics analyses the seismic reflection data show apparent structural lows in a sedimentary layer sandwiched between

internal structure of the dead sea leaky transform rift in relation to plate kinematics abstract the structures along the dead sea transform rift are related to the motions of the sinai and arabia plates which border it and to the irregularities of their boundaries the total slip was 105 km left lateral but the present structures were formed mainly during the last 40 km of slip which probably occurred in the plio pleistocene along the southern half of the transform the strike slip motion takes place on en echelon faults this produces rhomb shaped grabens or pull aparts which are sometimes composite and in which there is local crustal separation thus much of the transform is leaky these structures occur in a morpho tectonic rift valley delimited by normal faults which express a small component of transverse extension along a few segments the shape of the transform is such that lateral motion produces local transverse compression the geometric relations of the structures along the transform define an eulerian pole of relative plate motions at 32 8 n 22 6 e 0 5 the older motion was somewhat different and is described by a pole located about 5 west of the above then the component of transverse extension and crustal separation was much smaller than now while local transverse compression was more important the northern half of the dead sea transform has an irregular shape and the bordering plates did not remain rigid as lateral motion continued here transverse compression is often important

shallow seismic investigation of a site with poor reflection quality a shallow seismic reflection experiment was performed on a construction site to determine the feasibility of using reflection seismology to investigate the shallow structure in a weathered sand gravel interlayered zone that was known to be a poor transmission of high frequency seismic energy field recording parameters were designed to fit the limited space of the urban construction survey area a 7 kg sledgehammer was used to generate p waves and sh waves single 100 hz geophones were deployed at 1 0 m 0 5 m group intervals and 200 100 hz low cut filters were applied prior to a to d conversion to attenuate ground roll for sh wave reflections single 14 hz geophones and a 70 hz low cut filter on the seismograph were used the dominant frequency bands ranged from 33 to 275 hz and were centered around 110 hz for p waves lower dominant frequency bands 20 to 160 hz with a dominant frequency of around 85 hz were observed on sh wave records four seismic lines three p wave recordings and one sh wave recordi

shallow seismic reflection mapping of the overburden bedrock interface with the engineering seismograph some simple techniques where unconsolidated overburden exceeds 20 m in thickness the reflection method may be efficiently used with a 12 channel engineering seismograph to map topography on the overburden bedrock interface as well as possible structure within the overburden the two techniques we suggest are the simplest forms of reflection profiling which can be applied with a minimum amount of equipment which we suggest is a 12 channel enhancement seismograph a 12 geophone array and a hammer source these techniques require good transmission characteristics of the overburden as well as a sharp velocity discontinuity at the overburden bedrock interface for data processing and display a microcomputer is essential

mapping a bedrock surface under dry alluvium with shallow seismic reflections shallow seismic reflection techniques were used to image the bedrock alluvial interface near a chemical evaporation pond in the texas panhandle allowing optimum placement of water quality monitor wells the seismic data showed bedrock valleys as shallow as 4 m and accurate to within 1 m horizontally and vertically the normal moveout velocity within the near surface alluvium varies from 225 m s to 400 m s all monitor well borings near the evaporation pond penetrated unsaturated alluvial material on most of the data the wavelet reflected from the bedrock alluvium interface has a dominant frequency of around 170 hz low cut filtering at 24 db octave below 220 hz prior to analog to digital conversion enhanced the amplitude of the desired bedrock reflection relative to the amplitude of the unwanted ground roll the final bedrock contour map derived from drilling and seismic reflection data possesses improved resolution and shows a bedrock valley not interpretable from drill data alone

mapping shallow faults at the evrona playa site using high resolution reflection method a shallow high resolution seismic reflection survey was carried out at the evrona playa site in the southern arava valley israel the aim of the survey was to detect and map faults in the shallow subsurface upper 100 150 m and establish the relationship of the morphological features revealed by aerial photographs and surface geological mapping with the faults detected in the subsurface the survey included three seismic lines shot using the p wave technique and one sh wave line which overlapped one of the p wave lines the seismic energy source on all the lines was a sledge hammer the acquired reflection data were of good quality and did not require special processing efforts the seismic sections along the lines show a sequence of reflected events within the 8 150 m range at several locations continuity of the events is interrupted by a system of faults these faults form flower structures apparently related to strike slip motions typical of the region comparison of the faults mapped on the seismic sections with those expressed by surface morphological features generally show good correspondence the results of the seismic survey provide important information for the study of paleoseismicity and seismic hazards in the investigated area

onshore high resolution seismic profiling applied to sedimentology high resolution seismic profiling is a valuable method for the exploration of the shallow subsurface currently a resolution of 0 5 m can be attained this resolution is sufficient to recognize small sedimentary features channels dunes linear ridges etc and to reconstruct the environment of deposition the surveys described in this paper were carried out under favorable conditions on intertidal flats and beaches

seismic reflection investigations into the stratigraphy and structural evolution of the worcester basin two seismic reflection lines recently shot in the western part of the worcester basin suggest that its western boundary is defined by major syn depositional normal faults downthrowing to the east and active throughout permo triassic times farther east other faults can be seen within the basin these moved at various times during the permian and triassic and profoundly affected patterns of sedimentation the subsidence history of the basin can be explained by two phases of lithospheric stretching one in the permian and the second in early triassic times giving a cumulative stretching factor of about 1 2 this is consistent with the regional pattern of normal faulting provided that stretching was either one dimensional oblique to the n s and e w directions or two dimensional

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

inversion tectonics of the variscan foreland of the british isles late westphalian inversion structures on the variscan foreland display wide variations in orientation that are the product of the underlying basement grain the orientation of the major basement lineaments in relation to the direction of maximum shortening also determined the severity of deformation of the inverted basins ne sw trending faults were oriented roughly perpendicular to the nw se to nnw sse direction of maximum shortening determined from thrust transport directions in the variscan orogen to the south as a consequence ne sw trending basins in the variscan foreland have been strongly inverted the silesian post rift fill has commonly been expelled from the basins and the syn rift sediments have been deformed by chevron folds dublin and bowland basins in contrast the n s and nw se trending faults were oriented more obliquely to the direction of maximum shortening and as a consequence display a significant component of oblique slip this oblique slip component is manifested as en echelon periclines and flower structures in general the n s and nw se trending basins were less strongly inverted and the silesian post rift fill has been retained

integrated geophysical geological modelling of the caledonian and precambrian basement of southern britian quantitative modelling of potential field data has been used to test and extend geological sections constructed for the new british geological survey tectonic map of britain and ireland three of the profiles cross part of the anglo brabant massif and provide new information on the nature of the pre mesozoic basement a profile across southern england passing just to the west of london suggests that a significant contribution to observed gravity variations in the region results from changes in the thickness of relatively low density lower palaeozoic rocks it also identifies a major deep seated body with relatively high magnetic susceptibility and low density which is interpreted as a precambrian cratonic core underlying the southeastern part of the midlands microcraton profiles across central and eastern england reveal major boundaries within the precambrian basement some of which coincide with structures mapped at surface a number of intrusions of probable caledonian age have been recognized including bodies beneath the widmerpool gulf and the wash those beneath the wash appear to lie in a discrete basement region which separates belts of more magnetic basement lying to the northwest and southeast

active faulting in the dead sea rift abstract manifestations of late quaternary and holocene faulting were studied in a 500 km long segment of the dead sea transform rift most prominent are left slip faults whose characteristic physiographic features are recognizable along most of the studied segment where these faults bend or are stepped to the left rhomb shaped grabens or pull aparts are produced forming depressions in the reverse situation compressional features such as pressure ridges domes and folds form positive topographic features such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers normal faults sub parallel to the left slip faults produce a trough like valley along much of the dead sea transform but are most prominent along the margins of the large rhomb grabens e g the dead sea trough they apparently record a small component of transverse extension generally their motion is slow young slip did not occur along some segments during the last few 10 4 y elsewhere throws of 10 20 m at least occurred in this period the dead sea transform is seismically active the instrumental and historic records indicate a seismic slip rate of 0 15 0 35 cm y during the last 1000 1500 y while estimates of the average pliocene pleistocene rate are 0 7 1 0 cm y either much creep takes place or the slip rate varies over periods of a few 10 3 y

shallow seismic reflection survey across the meers fault oklahoma a high resolution seismic reflection survey over part of the meers fault scarp in comanche county oklahoma provides an improved understanding of the shallow structure associated with the fault zone the objective of this survey was to detect and identify shallow faulting near the meers fault and to appraise the resolving power of the shallow seismic reflection technique in a structurally complex area three reflection profiles were acquired using a downhole 50 caliber rifle energy source and two 100 hz geophones per channel connected in series field parameters were designed to optimize the recording of seismic reflections from geologic units in the 40 to 200 m depth range severe analog low cut filters helped to increase the dominant recorded reflection frequencies into the 100 to 250 hz range providing minimum bed resolution of about 6 m lines 1 and 2 straddle and cross the scarp orthogonally and line 3 was acquired approximately parallel to the scarp seismic data along line 2 show reverse faulting that extends from 250 m southwest to 200 m northeast of the scarp the cdp stacked seismic data are sufficiently good to interpret an up to the north vertical to high angle reverse displacement on the meers fault the amount of vertical displacement on the fault associated with the scarp cannot be determined with these seismic data maximum vertical displacement is less than 10 m on other secondary faults interpreted on seismic lines 1 and 2 normal faulting is common on line 3 the major structural feature on line 3 is a graben with net displacement of between 5 and 30 m the deformation on line 3 may be evidence of strike slip motion along the main fault the meers fault was in a transpressive tectonic setting during quaternary time as shown by strike slip motion in conjunction with high angle up to the north reverse faulting evidence for angular unconformities within the permian section exists on individual unprocessed field seismograms suggesting that some complex bedforms channel sands are present

end carboniferous fold thrust structures oxfordshire uk implications for the structural evolution of the late variscan foreland of south central england interpretation of commercial seismic reflection data from the west of banbury south central england has provided information on the late carboniferous structural evolution of the oxfordshire syncline end carboniferous structures are preserved in the footwall of the main basin bounding faults of the permo triassic worcester graben restoration of major pre triassic normal faults has allowed reconstruction of earlier compressional structures in contrast to the wnw ese trend of structures to the south in the variscan foreland fold thrust belt major faults and folds in oxfordshire are approximately n s trending the structures seen to deform westphalian d stephanian aged sequences on the seismic profiles are a combination of steep 30 40 dip fault bounded uplifts and less dominant thin skinned tectonics apparently showing transport to the east in the south some thin skinned structures are present and in the north cover sequences are uplifted in a narrow zone of deformation on reactivated basement faults the amount of shortening also changes along strike from c 1600 m in the south to c 950 m in the north reactivation of major basement faults with differential basement shortening accounts for the main structures localized detachment in the young and weak sediments close to the palaeo land surface explains the thin skinned structures seen on some of the seismic lines these end carboniferous structures in oxfordshire and structures in the malvern abberley hills represent the structural limits of a large n s striking intra stephanian uplift in the late variscan foreland it is suggested that the structures formed during a discrete phase of intra stephanian e w directed regional shortening prior to the final phase of movement in the variscan belt

short paper evidence of negative structural inversion beneath central england from new seismic reflection data a major seismic reflection transect acquired by the british geological survey between 1984 and 1986 has elucidated the deep geology and structure of some well known surface features of central england the worcester basin a mesozoic extensional graben system is bounded to west and east by major sub planar normal faults which formed by the extensional reactivation of much older structures respectively the malvern axis and a similar though concealed structure forming its mirror image to the east within the worcester basin a thick permo triassic sequence rests directly upon precambrian basement rocks to the west and east permo triassic rocks are thin or absent but relatively thick palaeozoic sequences are preserved above the precambrian these strikingly different successions together with the fault geometry suggest that the worcester basin lies within a major inverted oblique slip fault system or flower structure in palaeozoic times transpressivt stresses led to reverse faulting along the malvern axis and its complementary structure to the east with consequent regional uplift of the intervening tract and the formation of a positive flower structure in permian and early mesozoic times transtensional stresses caused collapse and negative inversion of the flower structure and consequent subsidence of the worcester basin

crustal growth and late precambrian early palaeozoic plate tectonic evolution of england and wales comparison of the basement rocks of britain reveals major contrasts to the n and s of the iapetus suture solway shannon line north of the suture polymetamorphic high grade basement schists and gneisses and igneous rocks date from about 2900 ma to c 400 ma the latter age corresponding to closure of the iapetus ocean by contrast s of the suture in england and wales the basement is dominantly low grade metamorphic rock together with intrusive and extrusive igneous and sedimentary rocks there is little geological or geochemical evidence for the presence of basement older than c 900 ma below this area and the predominant expression of crustal growth in england and wales is indicated by radiometric ages between c 700 ma and 450 ma reflecting late precambrian palaeozoic magmatism and metamorphism palaeomagnetic data from scotland demonstrate that the crust in northern britain developed as an integral part of the laurentian shield during mid proterozoic times 1600 1000 ma the palaeomagnetic record from the pre ordovician rocks of england and wales shows that this area was unrelated to the laurentian shield prior to closure of the iapetus ocean the apparent polar wander a p w path is that of a distinct microplate involved in rapid growth of iapetus during early cambrian times and emplaced in its present configuration adjacent to northern britain as a result of subduction of the ocean basin between cambrian and silurian times we conclude that whereas the basement to the north of the iapetus suture formed an essentially continental unit from 2900 to 400 ma the basement of england and wales is a result of crustal growth from about 900 ma to 400 ma and formed by accretion of island arcs associated accretionary prisms and fore arc basin sediments within the iapetus ocean

seismic facies analysis of westphalian sequences of the southern north sea abstract differing seismic facies are observed from within the westphalian sequences of the southern north sea a review of published synthetic seismograms and seismic data from known coal bearing basins establishes seismic facies types and their relationships similar reflection character is recognized from westphalian sequences in the southern north sea and lithologies confirmed by borehole information probable sand silt or clastic prone upper coal measures westphalian c d stages and coal prone lower and middle coal measures westphalian a b and part c stages can be defined and mapped providing information on the regional distribution of source and reservoir rocks and stratigraphic plays it is suggested that sand prone channel complexes or clastic dominated areas within the mainly a b stages can be recognized and mapped providing information on migration pathways and stratigraphic plays

use of topographic lineaments to extend known structure and facies patterns in the mesozoic of southern england abstract a detailed structural map of southern england was compiled from published geological and geophysical data carefully integrated with and extended by an interpretation of topographic lineament patterns pre permian basement structures controlled the location of permian to lower cretaceous basin margin faults that were subsequently blanketed by post tectonic upper cretaceous chalk late cretaceous to tertiary compression reactivated and inverted earlier mesozoic structures imprinting them into the upper cretaceous cover structural controls on sedimentation are illustrated by maps and sections for key triassic and jurassic units

magnetic character of a large continental transform an aeromagnetic survey of the dead sea fault new high resolution airborne magnetic hram data along a 120 km long section of the dead sea transform in southern jordan and israel shed light on the shallow structure of the fault zone and on the kinematics of the plate boundary despite infrequent seismic activity and only intermittent surface exposure the fault is delineated clearly on a map of the first vertical derivative of the magnetic intensity indicating that the source of the magnetic anomaly is shallow the fault is manifested by a 10 20 nt negative anomaly in areas where the fault cuts through magnetic basement and by a 5 nt positive anomaly in other areas modeling suggests that the shallow fault is several hundred meters wide in agreement with other geophysical and geological observations a magnetic expression is observed only along the active trace of the fault and may reflect alteration of magnetic minerals due to fault zone processes or groundwater flow the general lack of surface expression of the fault may reflect the absence of surface rupture during earthquakes the magnetic data also indicate that unlike the san andreas fault the location of this part of the plate boundary was stable throughout its history magnetic anomalies also support a total left lateral offset of 105 110 km along the plate boundary as suggested by others finally despite previous suggestions of transtensional motion along the dead sea transform we did not identify any igneous intrusions related to the activity of this fault segment

cambrian and ordovician stratigraphy related to structure and seismic profiles in the western part of the english midlands in england the cambrian successions include relatively continuous dominantly argillaceous sequences nuneaton area 900 m thick and arenaceous successions punctuated by hiatuses malvern shropshire the tremadoc lower ordovician succeeds paraconformably it is widespread dominantly argillaceous and locally very thick 2000 m deposition of the lower tremadoc is thought to have been rapid uplift of regional extent occurred in early ordovician post tremadoc times and inversion of the basins such as the tremadoc worcester graben occurred during the variscan orogeny the disposition of the cambrian tremadoc beneath the upper llandovery unconformity is thought to reflect the arrangement of basins and highs at that time evidence from seismic profiles indicates deposition in half grabens in the welsh borderland evidence from outcrops and boreholes suggests that the st david s series middle cambrian is locally thickened against syn depositional faults the thinning and local absence of the st david s series across such faults is attributed to the influence of tilt block highs rifting and tilting allowed thick tremadoc to accumulate e g in a precursor worcester graben but evidence for the direction of thickening is masked by lack of seismic markers and by the effect of pre landovery erosion

shallow seismic reflection profile of the meers fault comanche county oklahoma this is the publisher s version also available electronically from http onlinelibrary wiley com

orthogonal fracture systems at the limits of thrusting an example from southwestern wales abstract a well developed orthogonal vein system is located in the autochthonous foreland just beyond the limits of variscan thrusting in southwestern wales the two vein sets trend 200 and 290 perpendicular and parallel respectively to variscan structures the veins contain stretched quartz fibers indicating horizontal extensions of between 0 3 and 2 5 the 200 set has greater extension with a distinctive morphology including shorter more curvilinear fractures that have components of normal offset vein intersections are simple indicating episodic growth by alternating regional propagation of each set during propagation maximum compression was subvertical and the horizontal principal stresses switched direction with the alternation of set growth the system formed during waning variscan compression when the directions of horizontal principal stresses were still controlled by the tectonic compression the differential stress may have been greater during propagation of the 200 set this example indicates that orthogonal fracture systems in forelands beyond thrusting 1 are initiated by fracturing parallel to tectonic compression 2 form in a few stages that are controlled by changes in regional stress and 3 form during waning tectonic compression that controls fracture orientations

seismic reflection profiling for groundwater studies in victoria australia experimental seismic reflection profiling was employed for groundwater studies in southeastern australia equipment consisted of a simple engineering seismograph and tape recorder and data reduction was carried out on a minicomputer using a graphics based processing system specifically written for the project the investigation area is the site of a proposed induced groundwater recharge scheme in which surface water would be diverted to infiltrate aquifers outcropping several kilometers from a bore field which supplies up to half of the drinking water for the city of geelong the unconsolidated tertiary aquifers of the region are known to be interrupted in places by steep normal and reverse faults since similar faulting had been inferred along the proposed recharge avenue the objective of the seismic study was to verify if possible the assumption of aquifer continuity along the survey line the reflection results reveal monoclinal folding in the upper unconsolidated sediments produced by recent movem

the woolhope and usk basins silurian rift basins revealed by subsurface mapping of the southern welsh borderland subsurface mapping of the southern welsh borderland has utilized over 600 km of seismic data and three exploration boreholes to investigate the subsurface of this classic geological area the survey reveals two silurian basins each about 400 km 2 in area in the western part of the midland platform the woolhope basin underlies the silurian inlier of the woolhope anticline the basin fill is bounded to the northwest by faults of the neath disturbance and was also influenced by wnw striking extensional growth faults the fill thins eastward towards the malvern line lower silurian sequences thin southward before thickening again particularly across wnw striking faults the resulting usk basin underlying the usk inlier has ill defined margins but some additional n s fault control the fownhope 1 and usk 1 wells indicate that rapid subsidence occurred in both basins during rhuddanian and aeronian early to mid llandovery time modelling of this subsidence assuming the finite duration lithospheric extension model yields stretching factors of between 1 3 and 1 6 the later part of the fill to both basins is marine but red conglomerates sandstones and mudstones in their early llandovery intervals suggest continental fluvial environments in the initial stages of basin development the woolhope and usk basins show that the western sector of the midland platform rifted during early to mid llandovery time perhaps during regional dextral transtension the rifting is synchronous with and perhaps genetically related to an episode of mafic volcanism in the southwest part of the platform however this rifting apparently predates the late llandovery telychian fault controlled subsidence seen in the welsh basin to the northwest

structural trends in central britain from images of gravity and aeromagnetic fields digital image processing techniques have been used to analyse the regional gravity and aeromagnetic datasets in central britain colour and shaded relief images have been generated which convey information on both anomaly amplitude as colour and anomaly gradient as relief and highlight structural trends lineaments and textural contrasts not easily discernible on standard contour maps we have identified some of the more important features and have tried to relate these to the evolution of the crust in each region on a broad scale the images emphasize caledonoid ene trends to the north of the solway line and both tornquist se and caledonoid ne to ene trends to the south the pattern of magnetic anomalies over central england seems to define the extent of the shallow late precambrian early palaeozoic basement of the midlands microcraton and delineates crustal elements and characteristic trends within it magnetic lineaments and the grain of gravity anomalies trending in northeasterly and northwesterly directions on either side of the microcraton seem to relate to structures which originated during the evolution of the welsh and eastern english caledonides respectively where lower palaeozoic and precambrian rocks form the concealed basement as in most of england the gravity lineaments tend to reflect the influence of basement fractures on the subsequent pattern of sedimentation whereas many of the features on the magnetic images are more directly related to structures within the basement itself

delta chromium 53 52 isotopic composition of native and contaminated groundwater mojave desert usa abstract chromium vi concentrations in groundwater sampled from three contaminant plumes in aquifers in the mojave desert near hinkley topock and el mirage california usa were as high as 2600 5800 and 330 g l respectively 53 52cr compositions from more than 50 samples collected within these plumes ranged from near 0 to almost 4 near the plume margins assuming only reductive fractionation of cr vi to cr iii within the plume apparent fractionation factors for 53 52cr isotopes ranged from eapp 0 3 to 0 4 within the hinkley and topock plumes respectively and only the el mirage plume had a fractionation factor similar to the laboratory derived value of e 3 5 one possible explanation for the difference between field and laboratory fractionation factors at the hinkley and topock sites is localized reductive fractionation of cr vi to cr iii with subsequent advective mixing of native and contaminated water near the plume margin chromium vi concentrations and 53 52cr isotopic compositions did not uniquely define the source of cr near the plume margin or the extent of reductive fractionation within the plume however cr vi and 53 52cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of cr contamination plumes reductive fractionation of cr vi predominates in plumes having higher eapp these plumes may be suitable for monitored natural attenuation in contrast advective mixing predominates in plumes having lower eapp the highly dispersed margins of these plumes may be difficult to define and manage