electrical resistivity imaging of the central trans hudson orogen magnetotelluric mt measurements were made on a profile across the trans hudson orogen in 1992 as part of the lithoprobe transect the present study includes analysis of results from a 300 km long section of the profile in which allocthonous paleoproterozoic juvenile terranes and arc rocks of the western trans hudson orogen have been juxtaposed against the archean sask craton impedance tensor decomposition of data from the 40 mt sites in the area indicates a geoelectric strike of n28 e two dimensional inversion of the data using a non linear conjugate gradient algorithm provided images of the resistivity structure resistivity images reveal that the crust of the sask craton is relatively resistive 2000 m in contrast the rocks of the flin flon belt glennie domain and la ronge domain are mostly relatively conductive 100 1000 m in the east of the study area the images suggest that the tabbernor fault juxtaposes more conductive rocks of the glennie domain in the west against more resistive

palaeoproterozoic crustal accretion and collision in the southern capricorn orogen the glenburgh orogeny abstract the capricorn orogen in central western australia records the palaeoproterozoic collision of the archaean pilbara and yilgarn cratons until recently only one orogenic event was thought to be the cause of this collision the 1830 1780 ma capricorn orogeny however recent work has uncovered an older event the glenburgh orogeny that occurred between 2000 and 1960 ma the glenburgh orogeny reflects the collision of a late archaean to palaeoproterozoic microcontinent the glenburgh terrane with the archaean yilgarn craton and is therefore tectonically distinct as well as significantly older than the widespread 1900 1800 ma tectonothermal events recorded in northern australia the glenburgh terrane preserves a different history from either the yilgarn or pilbara cratons granitic gneiss protoliths dated at ca 2550 ma were intruded by widespread granite magmatism dated at 2005 1970 ma accompanied by high grade metamorphism and deformation throughout the terrane at ca 1960 ma silicic granite of the bertibubba supersuite intruded the northern margin of the yilgarn craton along the errabiddy shear zone a crustal scale shear zone that today marks the contact of the glenburgh terrane and yilgarn craton at ca 1950 ma silicic dykes intruded the southernmost part of the glenburgh terrane marking the end of the glenburgh orogeny east of the glenburgh terrane the glenburgh orogeny resulted in the cessation of mafic volcanism in the bryah basin and the basin s eventual closure siliciclastic carbonate and chemical sedimentary rocks were deposited in the padbury basin that formed a retro arc foreland basin on top of the bryah basin and probably records the later stages of the glenburgh orogeny collision

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

assembling and reactivating the proterozoic capricorn orogen lithotectonic elements orogenies and significance abstract the capricorn orogen was initiated during palaeoproterozoic suturing events that brought together the archaean yilgarn and pilbara cratons to form the west australian craton the orogen comprises palaeoproterozoic plutonic and medium to high grade metamorphic rocks of the gascoyne complex a series of palaeoproterozoic volcano sedimentary and sedimentary basins including the ashburton blair yerrida bryah padbury and earaheedy basins and the deformed margins of the pilbara and yilgarn cratons major pulses of deformation and metamorphism took place during the ca 2200 ma ophthalmian orogeny the 2000 1960 ma glenburgh orogeny the 1830 1780 ma capricorn orogeny and a more localized unnamed event at the end of the palaeoproterozoic 1670 1620 ma the orogen has been the site of repeated intracratonic reactivation with renewed basin formation magmatism and orogeny during the mesoproterozoic and the neoproterozoic the exhumed palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic mesoproterozoic edmund and collier basins bangemall supergroup which have been deformed during the neoproterozoic edmundian orogeny the ophthalmian and glenburgh orogenies affected the northern and southern margins of the orogen respectively the former resulted in the development of a foreland basin along the pilbara craton margin whereas the latter is associated with accretion of an allochthonous element the glenburgh terrane of the gascoyne complex to the northwestern margin of the yilgarn craton effects of the capricorn orogeny extended across the entire orogen and provide a younger limit for amalgamation of the yilgarn and pilbara cratons a number of basins developed along both the northern and southern margin of the orogen during this event and were filled with sediment eroded from the yilgarn craton the gascoyne complex and a presently unexposed early palaeoproterozoic terrane the capricorn orogeny corresponds with a major phase of continental collision both in australia and world wide which appears to mark a major phase of supercontinent assembly mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings including the world class iron orebodies of the hamersley basin

gravity anomalies crustal structure and isostasy associated with the proterozoic capricorn orogen western australia abstract present knowledge of the proterozoic capricorn orogen in western australia is largely based on surface geological studies geophysical studies of the crust of the orogen are limited to a few regional seismic refraction profiles and regional potential field interpretations the aim of this paper is to present a simple gravity constrained crustal model for the capricorn orogen anomalous lows immediately north of the yilgarn craton are prominent features of the capricorn orogen gravity field and geoid these anomalous lows do not correlate with surface geology or elevated topography their character suggests that they reflect thickened crust in the core of the capricorn orogen preserved in a state of mechanical equilibrium for about two billion years the location of the anomalous lows and associated crustal thickening south of the inferred location of the pilbara yilgarn suture suggest that the pilbara craton was subducted southwards beneath the previously combined yilgarn craton and southern gascoyne complex in a simple two dimensional crustal model features such as the bangemall bryah and yerrida basins are assumed to be dominantly supported by the rigid lithosphere of this region effective elastic thickness 50 100 km the yerrida basin and adjacent marymia inlier are modelled as north dipping features the bangemall basin as a southward deepening basin and the lower boundary of the gascoyne complex is best modelled as a north dipping contact at the northern margin of the orogen the sylvania inlier appears to lie at shallow depths 1 km beneath the eastern ashburton basin the rocks of the south eastern hamersley province reach a maximum thickness of about 7 km and thin toward the north a notable feature of the model is that it does not maintain isostatic equilibrium the prolonged erosion of this region and the prominent upper crustal density anomalies mean that it is difficult to develop gravity models that maintain isostatic balance a simple isostatic analysis suggests that the present day topography of the capricorn orogen is over compensated this over compensation can be explained by resistance of the rigid lithosphere of western australia to rebound in response to mesozoic or cainozoic erosion mantle flow acting to pull the crust downward from its equilibrium position could also lead to the apparent lack of isostatic equilibrium

origin of the oslo graben in relation to the hercynian alleghenian orogeny and lithospheric rifting in the north atlantic abstract the contemporaneous extrusion of basalts in the oslo graben and intrusion of dolentes in northern britain and southern sweden at 295 ma calls for a common explanation two hypotheses are investigated 1 1 the graben and dykes resulted from extensional stresses associated with progressive lithosphere separation to the northwest of europe in the late carboniferous 2 2 the dykes and therefore the graben were related in some way to oblique collision of plates in the hercynian orogeny which was developing to the south the first hypothesis is mechanically more satisfying and makes a number of testable predictions it states that in late carboniferous times the lithosphere separated in two places along the orogenic grain to the west of britain and norway the two embryonic oceanic rifts were divided by thick cold lithosphere with an archaean crust to the north of scotland extensional stresses were focussed in this region fanning out in an arcuate zone to the south and east causing failure where the lithosphere was relatively thin in norway the strain was restricted to a zone previously thinned and weakened in the early palaeozoic i e the oslo graben in britain the caledonoid grain is oblique to the expected direction of extensional strain and a dyke swarm trending e w and about 300 km wide was formed the first hypothesis predicts that the dykes should die out to the west but continue along an arc and widen to the east northeast under the western north sea interpretation of aeromagnetic maps shows that the dykes behave as predicted by hypothesis one but that their trends and extent are at variance with the expectations of hypothesis two the apparent contradiction of rifting to the northwest of europe occurring at the same time as compression and oblique collision in the heart of europe is resolved in principle by two plate tectonic reconstructions of pangaea drawn for late carboniferous and early permian times the oslo graben was conceived in mid carboniferous times and born towards the end of the carboniferous as the thin lithosphere failed and allowed the extrusion of basalt magma it was then abandoned as the two collinear oceanic rifts to the west and northwest joined up but the graben carried on a life of its own as the mafic magma cushion caught in the density trap beneath the crust continued to differentiate and alkali magma pods rose buoyantly to freeze in the upper crust or extrude on the graben floor

precambrian tectonic evolution of part of gondwana southwestern australia southwestern australia has a long dynamic precambrian history of crustal fragmentation and aggregation by continental collision and accretion this interpretation contrasts with the widely held belief that most of australia was part of a proterozoic supercontinent in which all tectonic magmatic and metamorphic activity was intracontinental the pilbara and yilgarn cratons are two quite different fragments of archean continental crust that collided to form the capricorn orogen between 2000 and 1600 ma the albany fraser orogen formed along the southern margin of the yilgarn craton during another major continental collision at 1300 to 1100 ma rifting removed the northeastern part of these combined cratons and orogens between 1100 and 700 ma subsequent continental collision along this rifted margin formed the paterson orogen at 700 to 600 ma therefore the assembly of this considerable part of gondwana may not have been completed until about 600 ma

early permian rotliegendes palaeowinds of the north sea abstract the classical desert sediments of the early permian upper rotliegendes were deposited in a post variscan basin that extends from eastern england to the russo polish border which has been referred to as the southern permian basin these sediments comprise four main depositional facies fluvial wadi aeolian sabkha and lacustrine the lacustrine facies includes important bedded halites the southern permian basin is separated from the smaller northern permian basin by the fragmented mid north sea ringkobing fyn system of highs and beyond the grampian spur lies the even smaller moray firth basin these latter basins contain rock sequences similar to those of the southern basin with the exception that bedded halite has not been recognised in the lacustrine facies the rotliegendes of the southern basin apparently conforms to deposition in a northern hemisphere tradewind desert similar to the modern southern sahara apart from regional palaeomagnetic considerations and the occurrence of sediments of a non arid nature south of the variscan highlands the strongest evidence supporting this interpretation is the pattern of palaeowind directions deduced from the orientations of dune bedding this is seen in both outcrop and core and derived from the continuous dip meter logs of many north sea wells by plotting the poles to the bedding attitudes on polar nets palaeowind directions have also been deduced from the dip meter logs of wells drilled in the northern and moray firth basins in general they indicate winds that blew in a direction opposed to those of the southern basin thus an area of high barometric pressure seems to have been located over the mid north sea high when the bedding attitudes in a well are roughly unidirectional an origin on a transverse dune is normally indicated with some wells the indicated palaeowind direction is regionally anomalous in many cases this can be rectified if the well is presumed to have drilled down through one flank of a seif dune the indicated palaeowind is thus changed by up to 90 and then commonly fits the regional pattern these early permian desert basins were smaller and closer to the equator than the modern sahara their location in the centre of a continent between the caledonian and variscan mountain ranges suggests the analogy of a more tropical equivalent of the central asian takla makan desert

isotopic dating of the migration of a low grade metamorphic front during orogenesis the southern margin of the pilbara craton in northwestern australia underwent regional heating folding and thrusting as well as extensive fluid flow during collision in the paleoproterozoic however the precise timing of this event is uncertain previous geochronologic studies of archean basement rocks have yielded a wide range of younger radiometric dates from ca 2 4 ga to ca 2 0 ga interpreted to record multiple hydrothermal alteration events in situ u pb geochronology of authigenic monazite and xenotime in very low grade archean metasedimentary rocks from across the craton suggests that the pilbara was affected by at least two cryptic thermotectonic events the first between ca 2430 ma and ca 2400 ma and the second between ca 2215 ma and ca 2145 ma the older event is restricted to three localities in the west and its cause is unknown the younger event affected most of the craton 100 000 km 2 spanning a 70 m y period from ca 2215 ma nearest the collisional margin in the south to ca 2145 ma toward the craton interior in the north the widespread geographic and stratigraphic distribution of ca 2 2 ga phosphates suggests that fluid flow was pervasive this event was probably driven by the northward advancing ophthalmian fold and thrust belt that developed during protracted collision the associated low grade metamorphic front migrated over 350 km at an average rate of 5 0 mm yr placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings

extreme fractionation in a granite pegmatite system documented by quartz chemistry the case study of tres arroyos central iberian zone spain abstract the tres arroyos granite pegmatite system is located in the sw margin of the nisa alburquerque variscan batholith two granitic facies monzogranite and marginal leucogranite and three types of aplite pegmatite dykes barren intermediate and highly evolved li rich have been distinguished in the area with a zoned distribution from the granite southwards trace elements in quartz from the five facies have been analysed by la icp ms in order to obtain information about the petrogenetic links among the different lithologies of this system as well as to better understand the regional and individual fractionation processes that led to the distinct rocks aluminium ti li and ge show continuous trends from the monzogranite through the marginal granitic facies the barren and intermediate aplite pegmatites up to the most evolved li rich dykes titanium and ge contents respectively decrease and increase gradually with fractionation in contrast al and li show a more complex trend with an initial descending trend to the marginal granitic facies and then showing the highest al and li contents in the quartz from the most fractionated li rich aplite pegmatites this suggests the influence of different competing factors controlling the incorporation of these trace elements in quartz such as the chemical composition of the melt the p and t conditions and the rate of crystallization based on the good correlation between al and li the substitution si4 al3 li seems to be the dominant mechanism of li incorporation into quartz the negligible amount of other trace elements suggests that the remaining al was mainly compensated with h ions via the si4 al3 h substitution a continuous fractionation trend from the monzogranite up to the most fractionated aplite pegmatites is inferred from geochemical modelling by applying the rayleigh equation for fractional crystallization fractionation rates over 50 are needed to obtain the marginal granite and the barren aplite pegmatites compositions and over 99 for the most evolved dykes no pattern in the chemical variation of the trace elements in quartz from different layers in the layered aplite pegmatites has been found suggesting the lack of internal fractionation processes most probably due to the rapid crystallization of the pegmatitic melt intruded into a colder country rock

fluid inclusion evidence for low temperature thermochemical sulfate reduction tsr of dry coal gas in upper permian carbonate reservoirs zechstein ca2 in the north german basin abstract upper permian zechstein carbonate ca2 gas reservoirs in the southern part of the pompeckj block in the north german basin locally contain up to 36 vol hydrogen sulfide h2s produced by thermochemical sulfate reduction tsr tsr was triggered by migration of dry to extremely dry coal gas from upper carboniferous into the zechstein carbonate reservoirs methane reacted with dissolved sulfate at temperatures of the results of this study decipher three major stages of gas migration in the pompeckj block stage i commenced in the late triassic during burial when zechstein ca2 reservoirs were charged with dry ch4 co2 n2 gas sourced from mature upper carboniferous coals burial continued through the jurassic and caused alteration of ca2 reservoir gas by sulfate reduction reactions due to increasing temperatures entrapment of ch4 h2s co2 n2 gases in fluid inclusions hosted in cements and fracture filling minerals occurred at temperatures between 100 and 152 c and was related to stage ii uplift in the early lower cretaceous in the late cretaceous stage iii deep burial of the pompeckj block led to charge of the zechstein ca2 carbonate reservoirs with upper carboniferous derived ch4 co2 n2 c2 coal gas and or dilution of existing reservoir gas at temperatures of 144 167 c highly variable 13cch4 values from 18 7 to 8 7 and very negative 13cco2 values 22 4 to 18 9 of h2s rich fluid inclusion gases as well as negative 13c values 10 4 to 4 6 of host calcites reveal compelling evidence for participation of methane in tsr fluid inclusions imply that ch4 dominated tsr proceeded at tmin of 135 c in the presence of catalyzers such as h2s and dissolved mg2 this study demonstrates that fluid inclusions serve as an excellent and accurate tool for tracing h2s concentrations in hydrocarbon gases through time and space which is not possible using the present day compositions of natural reservoir gases it also contributes to the understanding of carbonate reservoir hosted hydrocarbon bearing fluid systems and processes that significantly control the quality of reservoir gases

the electrical resistivity structure of archean to tertiary lithosphere along 3200 km of snorcle profiles northwestern canada magnetotelluric mt measurements to image the three dimensional resistivity structure of the north american continent from an archean core to a region of tertiary assembly were recorded at almost 300 sites along 3200 km of profiles on the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in northwestern canada at the largest scale the mt results indicate significant lithospheric thickness variation from 260 km at the southwest margin of the slave craton to significantly 100 km at the southwestern end of the snorcle transect in the cordillera at intermediate scale the resistivity results allow broad terrane subdivisions to be made several anomalously conductive zones along the snorcle transect in rocks ranging in age from archean to tertiary are attributed to the introduction of either water or carbon into the crust and mantle during subduction processes at the local scale the mt data image two major faults crossing the study area the great slave lake shear zone and the tintina fault the resistivity images show that both the tintina fault and great slave lake shear zone form crustal scale features and that the tintina fault has a remarkably uniform resistivity signature over a 400 km strike length in the study area arguably the most controversial conclusion reached is that the mt data do not support the western extension of north american autochthonous basement suggested from interpretation of the seismic reflection data

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

revisiting complete miscibility between silicate melts and hydrous fluids and the extreme enrichment of some elements in the supercritical state consequences for the formation of pegmatites and ore deposits abstract using water and major and trace element data obtained from melt inclusions primarily in pegmatite quartz we have shown in this and previous papers that melt melt fluid immiscibility is widespread and an important process during the generation of granitic pegmatites furthermore we have shown that the formation of pegmatites actually begins in the supercritical melt fluid stage in this study we extend previous work and in particular explore the behavior of 10 different elements be f p s cl as rb sn cs and ta in the supercritical range from preliminary studies we suggest that other elements such as b na k zn nb and sb will behave similarly near the solvus crest of a generalized pseudo binary melt water system this provides important evidence that pegmatite forming processes have already begun at high temperatures in the range of 850 750 c and that may require a re think on the partitioning behavior of metals in late stage residual melts exsolved from granitic magmas the existence of this parental supercritical silicate melt fluid although derived from granitic magmas imposes significant constraints of its own so drawing conclusions about pegmatite forming processes from data gained from slightly modified granite compositions becomes highly problematic we propose that the extremely high concentration of some elements over a small but well defined water concentration range symmetrically distributed around the critical point of the pseudo binary solvus must be derived from the unusual properties of the near critical or supercritical phases these unusual properties include extremely low density viscosity and surface tension and high diffusivity reactivity and mobility these are exactly the qualities that make such supercritical fluids an excellent medium for the formations of pegmatites by reactions with the existing matrix melt crystal mush and for the extreme enrichment of some ore forming elements highly soluble in alkali and water rich supercritical fluids a very important point in mineral crystallization is the non uniform local enrichment of rare and economically significant elements at specific locations in pegmatites granites and other rocks given the properties of the supercritical phase identified from melt inclusions the process of ostwald ripening can operate very effectively the driving force for ostwald ripening arises because the concentration of solute in the vicinity of small crystals or clusters is greater than and in the vicinity of larger crystals or clusters less than the average supersaturation the solute in the supercritical fluid therefore flows from the small crystals to the larger crystals and permits the growth of very large crystals at the expense of smaller ones this is favored by the extraordinary properties of supercritical fluids given the low surface tension the supercritical fluid can move also through grain boundaries and small channels or dislocation pipes aiding incompatible element transport additionally their consequences for the formation of some important granite related ore deposits are briefly outlined

crustal rheology of the himalaya and southern tibet inferred from magnetotelluric data the collision between the indian and asian continents which began about 70 million years ago involved a convergence of at least 1 400 km and raised the tibetan plateau since crustal material is too buoyant to be transported downwards into the earth s interior the mechanisms at work in plateau formation are important in understanding the evolution of the continents yet still a matter of debate geophysical observations in the himalaya and tibet using ultra low frequency radio waves now reveal that a partially molten layer is present along the entire length of the himalaya this weak layer implies that widespread crustal flow is occurring and explains many features of plateau evolution the cenozoic collision between the indian and asian continents formed the tibetan plateau beginning about 70 million years ago since this time at least 1 400 km of convergence has been accommodated1 by a combination of underthrusting of indian2 and asian lithosphere crustal shortening3 horizontal extrusion4 and lithospheric delamination5 rocks exposed in the himalaya show evidence of crustal melting1 6 and are thought to have been exhumed by rapid erosion and climatically forced crustal flow7 8 magnetotelluric data can be used to image subsurface electrical resistivity a parameter sensitive to the presence of interconnected fluids in the host rock matrix even at low volume fractions here we present magnetotelluric data from the tibetan himalayan orogen from 77 e to 92 e which show that low resistivity interpreted as a partially molten layer is present along at least 1 000 km of the southern margin of the tibetan plateau the inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in southern tibet

tectonic evolution of the late archaean to early proterozoic mount bruce megasequence set western australia the mount bruce megasequence set formerly the mount bruce supergroup was deposited on the pilbara craton during the late archaean and early proterozoic it comprises two megasequences the chichester range megasequence and the overlying hamersley range megasequence each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of phanerozoic style tectonics the basal nullagine supersequence which comprises mostly subaerial basalt and terrigenous sedimentary rock was deposited during an episode of wnw ese directed crustal extension that was initiated at about 2770 ma the overlying mount jope supersequence which comprises mostly basalt and tuff was deposited during the subsequent development of a wnw ese trending rift before about 2690 ma mudrock carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 ma marra mamba supersequence package were deposited on the ensuing divergent craton margin an unconformity or condensed succession covering a time span of as much as 130 m y separates the top of the chichester range megasequence from the hamersley range megasequence during which time an oceanic island arc is interpreted to have collided with the southern pilbara craton margin subsequently the subducting oceanic plate flipped from southwest dipping to northeast dipping and convergence resulted in the formation of a marginal subduction related orogen with associated arc and backarc volcanism iron formation and distal arc derived tuffs of the brockman supersequence package were deposited on a siliciclastic starved platform in a neutral continental backarc setting at around 2470 ma the overlying circa 2440 ma woongarra supersequence was also deposited in a backarc continental setting and comprises iron formation felsic tuffs and probably coeval mafic rocks the latter two rocks types were derived from a backarc magmatic belt shortening of the southern pilbara craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the turee creek supersequence were deposited and ultimately resulted in a continent continent collision the mount bruce megasequence set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen

uk onshore oil exploration 1930 1964 abstract the earliest attempts to find oil in britain dating from 1918 1922 based on anticlines flanking the pennines were largely unsuccessful renewal of exploration in the 1930s aimed at a broader range of prospects met with much criticism but went ahead after nationalization of the unknown and undiscovered oil resources 1934 the south of england ranked as first priority but the initial drilling of the new campaign carried out in 1935 1937 yielded only minor quantities of oil and gas attention was transferred in 1938 to upper palaeozoic prospects in the midlands and north resulting in small gas and oil discoveries in scotland and yorkshire and discovery of a series of commerical oilfields in the upper carboniferous of nottinghamshire in the 1950s the first commercial discovery in the jurassic was made at kimmeridge in dorset further carboniferous discoveries were made in nottinghamshire and lincolnshire and a series of fresh objectives defined by seismic reflection were drilled in the mesozoic basin of southern england this phase of exploration was terminated at the end of 1964 by adverse fiscal changes continuation of effective exploration operations remained uneconomic until the oil price rise in 1973

internal differentiation of rare element pegmatites effects of boron phosphorus and fluorine lithium rich rare element pegmatites are characterized by high concentrations of b p and f the interactions of these components with h2o and rare alkalis lower liquidas and solidus temperatures enhance silicate liquid h2o miscibility and control partitioning and concentration of group i elements and higher field strength cations boron f and perhaps p may form peralkaline na and li species that promote early saturation in mica quartz activities of f p and especially b are largely unbuffered throughout crystallization concentration of these components through fractional crystallization involving muscovite generates a peralkaline na aluminosilicate rich melt or vapor from which albitites rich in tourmaline phosphates f rich micas beryl zircon and nb ta sn oxides crystallize phase equilibrium experiments with peraluminous b p f rich rhyolite obsidian macusanite simulate many features of rare element pegmatites especially at h2o undersaturated conditions

tectonic setting and basin evolution of the bangemall supergroup in the northwestern capricorn orogen abstract the mesoproterozoic bangemall supergroup is the youngest depositional element within the capricorn orogen and comprises 4 to 10 km of fine grained siliciclastic and carbonate sedimentary rocks this succession is intruded by two generations of dolerite sills with ages of 1465 and 1070 ma field studies and geochronological data support a two fold subdivision of the bangemall supergroup into the c 1620 1465 ma edmund group and the c 1400 1070 ma collier group the principal syndepositional structural elements are parallel to the trend of major structures in the underlying ashburton fold belt and gascoyne complex these structures include the talga fault the faulted southwestern limb of the wanna syncline the lyons river fault and the northwest trending fault system that bounds the mangaroon and ti tree synclines the bangemall supergroup can be divided into six depositional packages defined by basal unconformities or major marine flooding surfaces packages 1 4 make up the edmund basin and consist of carbonate and siliciclastic shelf to basinal facies that are strongly controlled by synsedimentary movement along northwest trending basement faults source areas for the siliciclastic rocks were to the northwest northeast and southeast during deposition of packages 5 and 6 which form the collier basin the influence of the basement faults was less marked and the basin was filled with deltaic to deep marine siliciclastic facies that were derived from the northwest and southeast the bangemall supergroup has been frequently compared with proterozoic successions in northern australia but these successions are either too old or imprecisely dated to make meaningful correlations in addition the west and north australian cratons may not have been joined until the assembly of rodinia in the late mesoproterozoic

magnetotelluric constraints on the tectonic setting of grenville aged orogenesis in central australia abstract magnetotelluric mt data were collected over a 480 km long profile in central australia extending from the mesoarchaean palaeoproterozoic gawler craton into the mesoproterozoic neoproterozoic musgrave province the two regions have contrasting geological histories and many reconstruction models of proterozoic australia propose that they were juxtaposed during the mesoproterozoic despite the significant differences in geological background between the gawler craton and the musgrave province the mt data did not image an electrical structure or any change in electrical character between them mt models suggest that southern central australia is electrically separated into two parts the first comprising the northernmost gawler craton and the musgrave province has a 5 km thick layer of moderate resistivity overlying highly resistive crust the second comprising the central northern gawler craton displays moderate resistivity to the base of the crust the lithospheric mantle underlying both electrical regions is uniformly highly resistive our results suggest that the musgrave province and gawler craton are more closely related than is currently recognized we propose that the gawler craton forms basement to the musgrave province and that the grenvillian musgrave orogeny was an intracratonic reworking event that overprinted the older basement

deformational and stratigraphic influences on mineralization in se ireland the evidence that has accumulated to date to support the thesis that deformational and stratigraphic controls are reflected in the occurrence and siting of metal deposits in se ireland is evaluated in the case of an important tungsten deposit stratigraphic influences can be seen in the geochemical and mineralogical development of wallrocks prior to crystallization of granite hosted mineralization tourmalinites confined to a definable stratigraphic interval are associated with pegmatite hosted lithium deposits though the stratigraphic control is important the tungsten and lithium deposits are in addition confined to the trace of a major deformation zone

exploring the effect of lithium on pegmatitic textures an experimental study the effect of lithium li on the development of pegmatitic textures was evaluated experimentally at h 2 o saturated and h 2 o undersaturated conditions at 500 mpa pressure and temperatures ranging from 400 to 800 c the addition of 3700 ppm li to a starting material of common granitic composition lake county obsidian reduces the crystallization and melting temperatures by approximately 50 70 and 200 c respectively and promotes the formation of quartz feldspar graphic and granophyric intergrowths the presence of li in the hydrous granitic system lowers significantly the degree of undercooling needed for the development of graphic and granophyric intergrowths to 85 110 c in comparison with the li free systems where approximately 200 c of undercooling is necessary the feldspar crystals developed in li bearing samples can incorporate from 150 to 250 ppm li assume skeletal and spherulitic morphologies and exhibit one order of magnitude faster growth rates than crystals in li free samples the results of this study attest to the effectiveness of li as a fluxing agent and highlight the crucial role it plays in the development of pegmatitic textures leading to important implications for the conditions of formation of li bearing granitic pegmatites

evidence for lithium aluminosilicate supersaturation of pegmatite forming melts new experimental data on the solubility of lithium li at spodumene lialsi2o6 and petalite lialsi4o10 saturation at 500 mpa and 550 750 c reveal evidence for lithium supersaturation of pegmatite forming melts before the formation of li aluminosilicates the degree of li enrichment in granitic melts can reach 11 000 ppm above the saturation value before the crystallization of li aluminosilicate minerals at lower temperatures comparison of the experimental results with the spodumene rich moblan pegmatite quebec is consistent with extreme li enrichment of the pegmatite forming melt prior to emplacement which cannot be explained with equilibrium crystallization of li aluminosilicates from a common granitic melt the results of this study support the model of disequilibrium fractional crystallization through liquidus undercooling as the most plausible mechanism for the generation of such li rich ore resources

geological history of the early proterozoic paraburdoo hinge zone western australia abstract palinspastic profiles across the paraburdoo hinge zone which forms the southwestern boundary to the hamersley shelf indicate the existence of a fore trough adjacent to the ashburton trough on the edge of the hamersley shelf the fore trough existed during turee creek group and lower wyloo group times of deposition and is characterized by unconformities or shallow water facies on its margins the distribution of mature iron ores in the hamersley group suggests a relationship of this type of ore body to the fore trough margins continual uplift or a gradual migration northeastwards of a geanticline is suggested for the ashburton trough region from post hamersley group times to pre ashburton formation times of deposition

seismic structure of the crust and uppermost mantle of the capricorn and paterson orogens and adjacent cratons western australia from passive seismic transects abstract the seismic structure of the proterozoic capricorn and paterson orogens and adjacent archaean yilgarn and pilbara cratons western australia is determined using a passive seismic approach we use recordings of distant earthquakes made along two transects of 3 component broadband stations the stations were deployed for approximately 1 year mid 2006 2007 during which time 70 earthquakes were recorded at each station with a suitable signal to noise ratio for receiver function analysis and hence the s wavespeed profiles of the crust and uppermost mantle beneath each recording station are determined we investigate the deep crustal constraints on terrane boundary locations the patterns of seismic discontinuities in the crust and the variations in the depth and character of the moho this broad scale information regarding the present day crustal architecture and hence the crustal evolution of western australia complements previous surface geological and other geophysical studies western australia is an exceptionally large well preserved region of ancient crust and hence this work also adds to the body of knowledge regarding proterozoic orogenic processes in general the new passive seismic work shows a region of double crust or upper mantle discontinuities beneath the glenburgh terrane adjacent to the northwest yilgarn craton the upper crust of the orogens is always layered whereas the cratons have a simple upper crust both the capricorn and paterson orogens are characterised by deeper moho discontinuities with a lesser wavespeed contrast than the very sharp discontinuity observed beneath the adjacent pilbara and yilgarn cratons this is consistent with the weaker orogenic crust of the capricorn and paterson orogens accommodating most of the horizontal deformation during assembly and reworking of the west australian craton while the pilbara and yilgarn cratons acted as rigid crustal blocks

source of the zhawulong granitic pegmatite type lithium deposit in the songpan ganze orogenic belt western sichuan china constrants from sr nd hf isotopes and petrochemistry abstract the zhawulong ore field hosts lots of rare element pegmatite dikes and is an important producing area for lithium resources in the songpan ganze orogenic belt sgob east tibet this paper aims to evaluate the sources and ore forming processes for granite and pegmatite in the zhawulong ore field as well as the petrogenetic links between these rocks using petrography whole rock major and trace element analysis and sr nd hf isotope analysis in this ore field pegmatites are common as dike swarms that emanate from the external and or internal contact zone of the zhawulong granitic pluton and strata of the triassic xikang group showing close spatial relationships measured values of k rb vs zr hf and cs contents indicate geochemical affinities between granite and pegmatite which suggests that the pegmatite was generated by processes involving high fractional crystallization from the residual parental magma of the host granite additionally the granite have high 87sr 86sr i values 0 711064 to 0 716105 negative end t values 9 6 to 9 1 and ehf t values 32 6 to 2 0 and ancient nd and hf model ages 1 2 3 1 ga which indicate a continental crustal source metasedimentary rocks of the xikang group are rich in li 95 ppm rb 124 ppm nb 15 ppm and cs 17 ppm and their rare earth element ree patterns show moderate enrichment in light rees and a negative eu anomaly eu eu 0 64 revealing characteristics similar to those of granite the coeval nature and close spatial relationships between the zhawulong granite and other syn orogenic s type granites indicate that the formation of this granite was related to crustal melting the petrology and geochemical signatures of the zhawulong rare metal bearing granite pegmatite imply that it was generated by the partial melting of metasedimentary rocks during the late triassic orogeny assembly and was further enriched in li through processes involving high fractional crystallization

lithium pegmatites in the leinster granite southeast ireland abstract vein like pegmatites aplites and granitic country rocks from the se region of the leinster granite se ireland were studied petrographically and geochemically the results are reported and amongst other data numerous mineral analyses of spodumene muscovite lithian muscovite and feldspar are given the pegmatites consist of quartz alkali feldspar and small amounts of muscovite they contain trace quantities of oxide minerals of nb and ta the pegmatites in the se region are of special interest because they include up to 60 wt of spodumene li 2 o contents over 4 wt greisenized li pegmatites show partial or complete replacement of spodumene and microcline by new lithian muscovite up to 60 wt in the rocks with high concentrations of li rb cs and f albite in the original pegmatites was changed into cleavelandite the leinster granite is particularly rich in alkalies and proves to be a granite granodiorite according to the calculated position in the streckeisen diagram aplites which are frequently associated with pegmatites consist mainly of finegrained albite and subordinate quartz the relatively rare occurrence of li pegmatites in the leinster granite is probably due to rather low concentrations of water in the magma conditions which prevented the formation of larger quantities of a water saturated rest melt which is usually rich in li rb cs be etc

controls on chemical evolution and rare element enrichment in crystallising albite spodumene pegmatite and wallrocks constraints from mineral chemistry abstract internal differentiation and consequent geochemical evolution in pegmatites are significant processes in the development of economically viable deposits of metal bearing minerals albite spodumene pegmatites which represent important resources of li and ta worldwide challenge the general rules of pegmatite petrogenesis as these are nearly homogeneous bodies with little or no intrusion scale pegmatite zonation bulk intrusion concentrations of li are in the uppermost range obtained by magmatic enrichment experiments around 2 wt li2o and extensive volumes of saccharoidal or platy albite are present in leinster southeast ireland weakly zoned to homogeneous albitised spodumene pegmatites and their wallrocks were studied to compare mineral chemistry variations and understand the internal evolution of pegmatites characteristics linked to the poor development of zonation and links between internal evolution and pegmatite wallrock interactions leinster pegmatites present mineralogical textural and geochemical characteristics coherent with li saturation and possibly supersaturation prior to crystallisation weak border to centre zonation in the thickest bodies can be attributed to geochemically evolved initial melt likely leading to nearly contemporaneous crystallisation throughout the intrusion and resulting in limited internal geochemical fractionation increased abundance of minerals bearing highly incompatible elements e g columbite group minerals and cassiterite and network modifiers e g phosphates in albitite indicates it is a fractionation product from pegmatite crystallisation enrichment in incompatible elements b li rb cs and f in spodumene pegmatite exocontacts in different country rock types suggests unmixing of a hydrous fluid from the residual melt after the crystallisation of main pegmatitic assemblages and that the h2o rich component was mobilised into country rocks

the late palaeozoic gondwana glaciation the glacial deposits of gondwana are today spread over one whole hemisphere on the re assembled gondwana continents they still occupy an area exceeding that of the pleistocene glaciation the onset of the glaciation in the different areas has not yet been dated satisfactorily the earliest lower and middle carboniferous glacial beds occur in the andean belt and in eastern australia where high mountains may have existed at that time during stephanian sakmarian time when the pole wandered over antarctica large ice sheets reached sealevel in all the major depositories palaeotopographical reconstructions allow the conclusion that some of the ice centres were supported by uplands which reached altitudes of up to 1 500 m above sealevel depending on their palaeotopographical positions the glacigenic sediments exhibit the full facies range to be expected between glaciated uplands and glaciomarine environments there are indications that ice may have flowed from africa into the parana basin and from antarctica into the great karoo basin and into australia at least 12 advances and retreats have been recognized in the parana basin closer to the ice centres the record is less complete the retreats were probably more of an interstadial than an interglacial nature there is some evidence that the final deglaciation proceeded in stages from south america over africa to antarctica the ice centres were fed by moist air from the pacific ocean and the tethys

a buried granite batholith beneath the east midland shelf of the southern north sea basin published bgs gravity data of the uk southern north sea basin show the presence of a gravity low trending to the ese across the northern half of quadrant 47 two simplified geological cross sections have been constructed across this feature with sedimentary structure based upon well information and reflection seismic data gravity modelling of these sections demonstrates that the gravity anomaly is unlikely to be produced by the presence of thick low density sediments consequently the presence of a large buried granite batholith is postulated the buoyant influence of such a granite may have exerted considerable tectonic control during the development of the southern north sea basin in particular its presence may help to explain the development of the east midland shelf the associated complex fault movements along the flamborough head and dowsing fault zones and the areal distribution and thickness of the rotliegendes sand

the geology of the rough gas field summary the rough gas field was discovered in 1968 it lies in the united kingdom continental shelf blocks 47 3d and 47 8b and produces from the permian rotliegendes sandstone in a faulted anticlinal structure initial reserves were 384 bcf the field was on production until 1980 when the british gas corporation began developing rough as a gas storage facility to meet peak season demand in the mid to late 1980 s three main facies associations are recognised from core studies of the rotliegendes reservoir aeolian sands proximal fluvial sands and conglomerates and distal sheet flood sands the vertical arrangement of these associations within the rotliegendes sequence define three sedimentary units which represent semi arid alluvial fan progradation and abandonment these units can also be recognised on electric logs and can be correlated across the field reservoir porosity and permeability is largely controlled by the texture and mineralogy of the sedimentary facies with a concomitant diagenetic influence the geological model of facies distribution and trends in reservoir quality together with well test data has been used to optimise future development locations and update reservoir simulation of field behaviour in the storage mode

reconnaissance dating of events recorded in the southern part of the capricorn orogen abstract zircon geochronology confirms the errabiddy shear zone as a major structural boundary separating two distinct geological terranes within the southern part of the palaeoproterozoic capricorn orogen of western australia rocks on the south side of the errabiddy shear zone represent parts of the archaean yilgarn craton that were variably reworked during the c 2 0 ga glenburgh and c 1 8 ga capricorn orogenies rocks on the north side represent an exotic terrane the glenburgh terrane comprising c 2500 ma and possibly older gneisses which evidently record tectonothermal activity c 2390 ma and which together with quartzitic metasedimentary units deposited after c 2270 ma collided with the yilgarn craton during the glenburgh orogeny regional deformation high grade metamorphism and voluminous felsic plutonism occurred throughout the complex in association with both the glenburgh and capricorn orogenies

the location and evolution of the tectonic boundary between the paleoproterozoic jiao liao ji belt and the longgang block northeast china abstract in this paper we discuss the location nature and evolution of the tectonic boundary between the jiao liao ji belt and the longgang block northeast china the boundary in terms of paleogeography which involves a study of the liaohe group or tectonics employing non seismic geophysical data e g gravity data and magnetotelluric soundings existing geological data and the differences in physical properties between the jiao liao ji belt and the longgang block we clarified the spatial distribution of the tectonic boundary and characterized the geological bodies at different depths on either side of the boundary we analyzed the most recently updated bouguer gravity anomaly gravity magnetic and electrical profile data as well as comprehensive data on the physical properties of the rocks in the research area thus providing information on the geological nature of the boundary at deep levels the results indicate that during the early period of formation of the jiao liao ji belt a basin controlling boundary fault developed that constituted the original tectonic boundary between the jiao liao ji belt and the longgang block this fault extended from shuangtaling to sandaoling lian shanguan dawanggou balidianzi and erhulai to guaimozi following a roughly east west trend most previous studies have considered that the liaohe group which extends from benxi city to benxi county and huanren county is the paleo geographic boundary of the jiao liao ji belt

giant calcite concretions in aeolian dune sandstones sedimentological and architectural controls on diagenetic heterogeneity mid cretaceous iberian desert system spain aeolian dune sandstones of the iberian erg system cretaceous spain host giant calcite concretions that constitute heterogeneities of diagenetic origin within a potential aeolian reservoir the giant calcite concretions developed in large scale aeolian dune foresets at the transition between aeolian dune toeset and damp interdune elements and in medium scale superimposed aeolian dune sets the chemical composition of the giant concretions is very homogeneous they formed during early burial by lowmg calcite precipitation frommeteoric pore waters carbonate componentswith yellow orange luminescence form the nuclei of the poikilotopic calcite cement these cements postdate earlier diagenetic features characterized by earlymechanical compaction fe oxide cements and clay rims around windblown quartz grains resulting from the redistribution of aeolian dust over the grain surfaces the intergranular volume igv in friable aeolian sandstone ranges from 7 3 to 15 3 whereas in cemented aeolian sandstone it is 18 6 to 25 3 the giant calcite concretions developed during early diagenesis under the influence of meteoric waters associated with the groundwater flow of the desert basin although local e g activity of fluid flow through extensional faults and or other regional controls e g variations of the phreatic level associated with a variable water influx to the erg system and varying sea level could have favoured the local development of giant calcite concretions the spatial distribution pattern of carbonate grains and the main bounding surfaces determined the spatial distribution of the concretions in particular the geometry of the giant calcite concretions is closely associated with main bounding aeolian surfaces thus interdune superimposition and reactivation surfaces exerted a control on the concretion geometries ranging fromflat and tabular ones e g bounded by interdunes towedge shaped concretions at the dune foresets e g bounded by superimposition and reactivation surfaces determining the spatial distribution of the heterogeneities of diagenetic origin in the aeolian reservoir

electrical evidence of continental accretion steeply dipping crustal scale conductivity contrast 1 many geophysical images of ancient orogenic belts utilise seismic reflection data but increasingly magnetotelluric mt data have been shown to provide insights into orogenic architecture not evident in seismic surveys particularly through their ability to image steep structures a 140km long mt survey carried out in palaeoproterozoic crust in central australia has imaged a crustal scale conductivity contrast across a steeply dipping boundary the surface location of this contrast coincides with the redbank thrust zone previously shown from seismic reflection and gravity data to be a moderately dipping thick skinned fault that produces one of the largest continental gravity gradients known on earth the steeply dipping conductivity interface is interpreted to be a strike slip boundary separating the north australian craton from a younger accreted terrane this result is therefore a first order geophysical image of a proterozoic accretion system in australia the steep boundary is not evident in the seismic data and highlights the utility of mt in defining fundamental crustal boundaries

the northern margin of the capricorn orogen western australia an example of an early proterozoic collision zone abstract the early proterozoic 2200 1600 ma capricorn orogen is a major zone of deformation metamorphism and granite emplacement between the archaean yilgarn and pilbara cratons of northwest australia capricorn orogen structures are recognized along the southern margin of the pilbara craton deforming the archaean granite greenstone terrain of the sylvania inlier together with rocks of the hamersley ashburton and blair basins in the sylvania inlier deformation is characterized by large scale shear zones interpreted as nne directed thrusts in the hamersley basin orogenic deformation forms the ophthalmia fold belt and can be divided into two groups of structures in the southwest large scale dome and basin folds pre ashburton basin are present in the southeast folds and faults are part of a younger syn ashburton basin n directed foreland fold and thrust belt directly linked to deformation in the sylvania inlier the deformed ashburton basin and adjacent parts of the hamersley basin form the ashburton fold belt two periods of deformation are recognized early recumbent folding d1a post ashburton basin pre blair basin and later dextral wrench faulting and associated folding d2a syn or post blair basin capricorn orogen structures are interpreted as the result of an oblique continent continent collision between the pilbara and yilgarn cratons along the southern pilbara margin collision first occurred in the southeast and migrated northwest with the ashburton basin evolving as a foreland basin late stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins

identifying the lithospheric structure of a precambrian orogen using magnetotellurics the capricorn orogen western australia abstract a 300 km long magnetotelluric mt survey was carried out across the capricorn orogen in western australia the capricorn orogen includes reworked crust of the archaean yilgarn and pilbara cratons and the allochthonous latest archaean to palaeoproterozoic glenburgh terrane in the southern gascoyne complex and the palaeoproterozoic northern gascoyne complex the survey aimed to increase our understanding of the juxtaposition of these crustal elements by obtaining information about their electrical structure with depth phase tensor analysis of the data showed that 2d mt inversion could only be carried out for stations in the centre of the profile crossing the gascoyne complex the resulting model showed that there is no electrical distinction between the glenburgh terrane and the northern gascoyne complex suggesting that the glenburgh terrane may form basement to the whole of the gascoyne complex 3d forward modelling was carried out for the entire survey line incorporating the basement blocks as well as the surrounding ocean and sedimentary basins station data are most closely reproduced by a model in which the margin between the glenburgh terrane and the yilgarn craton the errabiddy shear zone dips south with crust of the glenburgh terrane wedged beneath the northern yilgarn craton forward modelling results also suggest that the main boundary between the gascoyne complex and the pilbara craton to the north is a steeply dipping structure beneath the edmund and collier basins that may correlate with the talga fault

magnetotelluric data from the southeastern capricorn orogen western australia an example of widespread out of quadrant phase responses associated with strong 3 d resistivity contrasts

lower rotliegend fluvio lacustrine sequences in the saar nahe basin the rotliegend beds of the middle of the kusel group to the middle of the lebach group in the saar nahe basin are characterized by sedimentary sequences which developed in streams deltas and lakes

new constraints on the seismic structure of west australia evidence for terrane stabilization prior to the assembly of an ancient continent we present a new near comprehensive survey of the variations in seismic structure across the west australian craton at the scale of the main terrane groups analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region we found the best fi tting structure by modeling the conversions from p to s wave motion the receiver function that take place as the seismic energy travels upward through the lithosphere such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited and they provide an effective alternative to active source seismic techniques for deep crustal targets the seismic structure is consistent within several of the individual archean terranes most notably the pilbara murchison and southern cross these terranes are underlain by lower crust of low seismic velocity and show a sharp seismic moho the structure shows signifi cant contrasts between neighboring terranes thus major tectonic units have a velocity profi le that is a signature of that terrane or terrane group we infer that the seismic structure of the archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the west australian craton

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

the cretaceous opening of the northeast atlantic abstract a continuous rift system from the rockall trough through the faeroe shetland channel and the more and voring basins up to the tromso and bear island basins is inferred to have developed during the cretaceous oceanic crust was generated in its southern part but its width presumably decreases toward the northeast and probably ends in the more basin this rift spreading system requires a clockwise rotation of greenland and the rockall plateau during the cretaceous around a rotation pole at its northern end at about 74 n 21 e north of the pole of rotation crustal shortening must have occurred this compression is interpreted to have taken place on west spitsbergen the orogeny there is reinterpreted as 1 1 a cretaceous folding phase 2 2 a phase of overthrusting at the paleocene eocene boundary and 3 3 an extensional faulting phase during the oligocene this plate tectonic model can resolve a series of problems in the northeast atlantic region the connection of the caledonian fronts of scotland and greenland can be easily established the northern ends of the west shetland basin and the north sea graben the unusual depth of the more and voring basins the marked obliqueness of structural trends between the northeast greenland shelf and the norwegian shelf the fact that the western hammerfest basin was intersected by a younger north south trending graben which formed the deep tromso basin all these enigmatic observations can be explained by the cretaceous rift system

development of n w europe s southern permian gas basin summary the southern permian basin contains over 4 1 10 12 m 3 of recoverable non associated gas most of which is found in early permian rotliegend sandstones the most important source of this gas is carboniferous coal deposited in a proto tethys foredeep basin where the coals were overlain by rotliegend desert sands and by late permian zechstein halite the scene was set for creating important reservoirs for gas deformation of the reservoir and seal to create a trap and the generation and migration of carboniferous gas resulted from the structural history of the area which was shaped in part by underlying crustal blocks and zones of crustal weakness much of this history was related to events most clearly expressed beyond the limits of n w europe in the north atlantic and tethys 1 e w tension which gave rise to the oblique slip induced subsidence of many sub basins within the southern permian basin caused the consecutive creation of the viking central graben system and rockall trough and ended with the crustal spreading of the atlantic ocean and 2 the early mesozoic opening of tethys its cretaceous closure and ensuing alpine orogeny which induced inversion in many areas of the southern permian basin that are now gas bearing

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

neoproterozoic 40ar 39ar mica ages mark the termination of a billion years of intraplate reworking in the capricorn orogen western australia abstract the tectonic history of the proterozoic capricorn orogen western australia records complex intraplate reworking lasting nearly one billion years although the paleo mesoproterozoic reworking history is well defined in the crystalline basement of the gascoyne province at the western end of the orogen the younger reactivation history remains unclear four reworking events affected the orogen at 1820 1770 ma 1680 1620 ma 1320 1170 ma and 1030 900 ma these events were succeeded by a breakout in predominantly dextral strike slip reactivation of major shear zones across the gascoyne province currently the age of this reactivation is constrained by only one date of c 570 ma from a single shear zone but field relationships imply that some of the shear zones must be older than a suite of c 755 ma dolerite dykes in order to constrain the age of fault and shear zone reactivation we obtained new 40ar 39ar dates for mica and in situ shrimp u pb dates for xenotime within shear zones our results when combined with previously published data show that reactivation occurred between 920 and 830 ma these dates overlap with the youngest reworking event the 1030 900 ma edmundian orogeny furthermore neoproterozoic u pb phosphate ages are known from the bounding cratons and faulting within the adjacent mesoproterozoic sedimentary basins suggest this event is of regional significance in contrast to previous suggestions that this neoproterozoic reactivation was the result of a collision from the west we propose that it reflects north south compression that caused dextral strike slip fault reactivation in the north and exhumation of the southern part of the orogen

wave propagation modelling of induced earthquakes at the groningen gas production site gas extraction from the groningen natural gas field situated in the netherlands frequently induces earthquakes in the reservoir that cause damage to buildings and pose a safety hazard and a nuisance to the local population due to the dependence of the national heating infrastructure on groningen gas the short term mitigation measures are mostly limited to a combination of spatiotemporal redistribution of gas production and strengthening measures for buildings all options become more effective with a better understanding of both source processes and seismic wave propagation detailed wave propagation simulations improve both the inference of source processes from observed ground motions and the forecast of ground motions as input for hazard studies and seismic network design the velocity structure at the groningen site is relatively complex including both deep high velocity and shallow low velocity deposits showing significant thickness variations over relatively small spatial extents we performed a detailed 3 d wave propagation modelling study for an induced earthquake in the groningen natural gas field using the spectral element method we considered an earthquake that nucleated along a normal fault with local magnitude of ml 3 we created a dense mesh with element size varying from 12 to 96 m such that with a dominant source frequency of 7 hz waveforms with a frequency content up to 10 hz could be accurately modelled the velocity density model is constructed using a 3 d geological model of the area including both deep high velocity salt deposits overlying the source region and shallow low velocity sediments present in a deep and narrow tunnel valley the results show that the 3 d density velocity model in the groningen area clearly plays a large role in the wave propagation and resulting surface ground motions and causes significant lateral variations in site response the highvelocity salt deposits have a dispersive effect on the radiated wavefield reducing the seismic energy reaching the surface near the epicentre in turn the presence of low velocity tunnel valley deposits can locally cause a significant increase in peak ground acceleration here we study induced seismicity on a local scale and use specfem3d to conduct full waveform simulations and show how local velocity variations can affect seismic records the author s 2018

early carboniferous extension in east avalonia 350 my record of lithospheric memory abstract despite reactivations during variscan and alpine orogenies and the opening of the northern atlantic avalonia is one of the few regions where the initial mid paleozoic basin structure is still recognisable largely intact and well studied its kinematics and dynamics remain however largely unknown in particular in view of the successive late paleozoic to recent evolution of the basin consequently the importance of the mid palaeozoic tectonic evolution and structural controls are often overlooked and poorly understood in basin studies in this paper we reassess the importance of mid palaeozoic tectonics on subsequent sedimentary basin evolution of north western europe to this end we analyse the dynamics of early variscan extension in avalonia based on the integration and re evaluation of available geophysical and geological data from lithosphere to basin scales based on a revised crustal map of the thor suture zone we present a new paleo tectonic reconstruction and tectonic scenario for the devonian carboniferous rifting these findings are key for a better understanding of long lived tectonic segmentation and post rifting deformation phases our findings indicate that the structural grain of many crustal fault dominated sedimentary basin structures such as the north sea central graben were created in the early carboniferous consequently the main basement structuration of northwest europe was completed before the variscan orogeny and successive post variscan extension and inversion phases reactivated the existing basement structures without creating major new fault groups incorporation of the paleozoic structural grain allows for a consistent tectonic framework for the mesozoic contributing to fundamental understanding of basin evolution from our tectonic framework analysis avalonia therefore stands out as a fine example of long lived lithosphere memory spanning over 350 my of structural control in geodynamic evolution

tectonic model of the limpopo belt constraints from magnetotelluric data a b s t r a c t despite many years of work a convincing evolutionary model for the limpopo belt and its geometri cal relation to the surrounding cratons is still elusive this is partly due to the complex nature of the crust and upper mantle structure the significance of anatectic events and multiple high grade meta morphic overprints we use deep probing magnetotelluric data acquired along three profiles crossing the kaapvaal craton and the limpopo belt to investigate the crust and upper mantle lithospheric struc ture between these two tectonic blocks the 20 30 km wide composite sunnyside palala tshipise shear zone is imaged in depth for the first time as a sub vertical conductive structure that marks a fundamental tectonic divide interpreted here to represent a collisional suture between the kaapvaal and zimbabwe cratons the upper crust in the kaapvaal craton and the south marginal zone comprises resistive granit oids and granite greenstone lithologies integrating the magnetotelluric seismic and metamorphic data we propose a new tectonic model that involves the collision of the kaapvaal and zimbabwe cratons ca 2 6 ga resulting in high grade granulite limpopo lithologies this evolutionary path does not require a separate terrane status for each of the limpopo zones as has been previously suggested

magnetotelluric evidence for a deep crustal mineralizing system beneath the olympic dam iron oxide copper gold deposit southern australia the iron oxide copper gold olympic dam deposit situated along the margin of the proterozoic gawler craton south australia is the world s largest uranium deposit and sixth largest copper deposit it also contains significant reserves of gold silver and rare earth elements gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings to delineate crustal structures that may constrain mineral system fluid pathways coincident deep crustal seismic and magnetotelluric mt transects were obtained along a 220 km section that crosses olympic dam and the major crustal boundaries in this paper we present results from 58 long period 10 104 s mt sites with site spacing of 5 10 km a two dimensional inversion of mt data from 33 sites to a depth of 100 km shows four notable features 1 sedimentary cover sequences with low resistivity 1000 m archean crustal core from a more conductive crust and mantle to the north typically 500 m 3 to the north of olympic dam the upper middle crust to 20 km is quite resistive 1000 m but the lower crust is much more conductive 100 m and 4 beneath olympic dam we image a low resistivity region 100 m throughout the crust coincident with a seismically transparent region we argue that the cause of the low resistivity and low reflectivity region beneath olympic dam may be due to the upward movement of co2 bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries simultaneously annihilating acoustic impedance boundaries the source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with proterozoic crustal deformation

grenvillian aged orogenesis in the palaeoproterozoic gascoyne complex western australia 1030 950 ma reworking of the proterozoic capricorn orogen in situ shrimp u pb geochronology of monazite and xenotime in pelitic schists from the central gascoyne complex western australia shows that greenschist to amphibolite facies metamorphism occurred between c 1030 and c 990 ma monazite from an undeformed rare element pegmatite from the same belt gives a 207pb 206pb age of c 950 ma suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism metamorphism in the central gascoyne complex was previously interpreted as barrovian largely based on the identification of kyanite in peak metamorphic assemblages and has been attributed to intense crustal shortening and substantial tectonic thickening during palaeoproterozoic continent continent collision however the stable al2sio5 polymorph has been identified in this study as andalusite rather than kyanite and the prograde assemblages of staurolite garnet andalusite biotite muscovite quartz indicate temperatures of 500 550 c and pressures of 3 4 kbar these data show that the palaeoproterozoic gascoyne complex underwent an episode of grenvillian aged intracontinental reworking concentrated in a nw se striking corridor during the edmundian orogeny until now the edmundian orogeny was thought to have involved only reactivation of structures in the gascoyne complex along with deformation and very low to low grade metamorphism of mesoproterozoic cover rocks some time between 1070 and 755 ma however we suggest that it involved regional amphibolite facies metamorphism and deformation granite magmatism and pegmatite intrusion between c 1030 and c 950 ma therefore the capricorn orogen experienced a major phase of tectonic reworking c 600 myr later than previously recognized our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events

a new paleoproterozoic tectonic history of the eastern capricorn orogen western australia revealed by u pb zircon dating of micro tuffs abstract the earaheedy basin is one of several paleoproterozoic siliciclastic basins in the eastern part of the capricorn orogen in western australia in the absence of an adequate chronostratigraphic framework multiple conflicting models have been proposed for the tectonic history of the basin we present in situ shrimp u pb zircon dates of 1971 11 ma 1954 5 ma and 1949 10 ma from three thin tuff horizons micro tuffs in the yelma formation at the base of the basin succession these micro tuffs are only slightly younger than the 1990 6 ma imbin porphyry which has been interpreted as a basement inlier our new dates combined with field mapping suggest that the imbin porphyry is a volcanic subvolcanic complex within the basin micro tuffs in the yelma formation are more than 50 m y older than c 1890 ma micro tuffs within the overlying frere formation deposition of the yelma formation was coeval with continent continent collision farther west in the orogen marked by the 2005 1950 ma glenburgh orogeny and the formation is possibly correlative with peripheral foreland deposits of the padbury group the frere formation was deposited in shallow marine conditions within an intracratonic basin about 60 m y after the glenburgh orogeny the frere formation is unconformably or disconformably overlain by the miningarra group the youngest unit of which is the mulgarra sandstone this sandstone was deposited after c 1865 ma and derived either from exposed frere formation or from a now buried source within the capricorn orogen that was uplifted by inception of the 1820 1770 ma capricorn orogeny therefore the earaheedy basin comprises at least three distinct unconformity bound successions deposited in contrasting tectonic settings