reliability of palaeozoic palaeomagnetic poles and apwp of gondwanaland abstract a review of reliable palaeomagnetic data from gondwana palaeozoic rocks supports the apparent polar wander path apwp initially proposed by morel and irving this path or versions of it has recently gained favour with a number of groups the apwp suggests that during the mid palaeozoic gondwanaland was very mobile no palaeomagnetic pole position s has been yet reported that confirms the contentious segment of apwp from the late ordovician to the early silurian the apwp implies that the south pole moved rapidly from north africa in the ordovician to a position off southern south america by the silurian back to central africa by the early carboniferous and across gondwanaland to australia by the late carboniferous the distribution of palaeozoic tillites independently supports such a mobilistic gondwanaland the palaeomagnetic and tectonic evidence are compatible with the lachlan fold belt of australia having been in place since the mid devonian

generation of continental rifts basins and swells by lithosphere instabilities 1 continents may be affected simultaneously by rifting uplift volcanic activity and basin formation in several different locations suggesting a common driving mechanism that is intrinsic to continents we describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent we carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above dynamical behavior depends on three dimensionless numbers a rayleigh number for the unstable block a buoyancy number that scales the intrinsic density contrast to the thermal one and the aspect ratio of the block within the block instability develops in two different ways in an outer annulus and in an interior region in the outer annulus upwellings and downwellings take the form of periodically spaced radial spokes the interior region hosts the more familiar convective pattern of polygonal cells in geological conditions such instabilities should manifest themselves as linear rifts striking at a right angle to the continent ocean boundary and an array of domal uplifts volcanic swells and basins in the continental interior simple scaling laws for the dimensions and spacings of the convective structures are derived for the subcontinental lithospheric mantle these dimensions take values in the 500 1000 km range close to geological examples the large intrinsic buoyancy of archean lithospheric roots prevents this type of instability which explains why the widespread volcanic activity that currently affects western africa is confined to post archean domains

a carbon isotope reference curve for ca 1700 1575 ma mcarthur and mount isa basins northern australia abstract shallow marine paleo and mesoproterozoic sedimentary successions are widely distributed in several major basins across northern australia the successions are only gently deformed and their stratigraphy is relatively continuous thus offering an ideal opportunity to document secular variations in carbon isotopes marine carbonate intervals from two of these major basins the mcarthur and mount isa basins have been sampled to document secular variation in 13ccarb from approximately 1700 to 1575 ma in all cases the samples have been tied to a well established sequence stratigraphy which along with u pb shrimp zircon dates provides a time resolution of the order of 1 m y the data presented here thus provide the most comprehensive and best dated 13ccarb stratigraphy yet obtained from such ancient rocks diagenesis occurred early in the carbonate rocks from both basins with the result that fluid movements were restricted and primary carbon isotopic signatures were retained the 13ccarb values from both basins vary within a very narrow range around a mean of 0 6 with extreme values seldom lying further than 1 from the mean that is the curves are essentially flat the results of this study combined with earlier studies on younger rocks imply that the global ocean reached a state of equilibrium in the mid paleoproterozoic and remained stable for much of the following billion years current models of the ocean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required which in turn suggests that the availability of nutrients such as phosphorus was stable and low prolonged nutrient stability may therefore have exerted a major influence upon the evolution of the biosphere over this time interval

depositional controls on glacial facies associations in a basinal setting late proterozoic amadeus basin central australia abstract diamictites many established as being glacial in origin are globally distributed in late protezoic rocks their widespread occurrence is important in that it implies that significant climatic events occurred during the period between 900 and 600 ma however because of the age of the rocks correlations are poor and it is difficult to establish in a convincing manner whether these sediments were deposited synchronously during a major global ice age or if each occurrence of diamictite relates to local climatic or tectonic events since the biostratigraphy of these rocks is limited and only a small number of radiometric ages are available an alternative approach to correlation is necessary the most promising approach currently available appears to be the application of sequence stratigraphy and the careful evaluation of facies associations on a site by site basis before attempting global correlations late proterozoic diamictites occur in a number of intracratonic basins in a broad belt across the ancient australian craton two late proterozoic diamictite units both previously identified as glacial in origin are exposed in the intracratonic amadeus basin of central australia the areyonga formation the older of the two units forms the basis of this study the areyonga formation is contained within a major depositional sequence the areyonga sequence the bounds of which were determined by basin dynamics and eustatic sea level changes induced by an ice age of global dimensions during deposition of this sequence three major sedimentological systems were in operation along the northern margin of the basin glaciation dominated sedimentation in the south large volumes of sediment were deposited in a shallow marine setting whilst to the west braided streams supplied sediment to a major coastal plain and delta system all three systems operated within the same sea level and basinal constraints but each contributed sediment to the basin independently the glacial sediments of the areyonga sequence were deposited from a relatively small ice cap lying between the amadeus basin and the ngalia basin to the north sediments were deposited from two tongues of ice that extended about 100 km into the northeastern corner of the amadeus basin glacial sedimentation occurred in three stages determined by the initial fall and subsequent rise in eustatic sea level as the ice sheet grew and declined initially thick massive basal till units were deposited on an erosion surface intersected by subglacial and ice marginal channels they were then followed by thinner till units separated by channel like sandstone and conglomerate with associated boulder surfaces or concentrations that were deposited by an extensive subglacial drainage network finally the subglacial deposits grade into ice proximal and ice distal shallow marine sediments deposited in part by mass movement sedimentation appears to have been terminated by a major sea level fall in the ooraminna sub basin to the east rocks originally thought to have been deposited during a second glaciation appear to be part of a lowstand wedge formed during a sea level low following the areyonga glaciation a record of the second glaciation probably does not exist in the amadeus basin similarly diamictites in the inindia beds and originally correlated with the areyonga formation have been shown to belong to the non glacial lowstand wedge of the following sequence preservation of glacial sediments is most likely to occur in a basinal setting where depositional space is being actively created in the amadeus basin depositional space was created prior to deposition of the areyonga formation by subsidence following crustal extension that occurred approximately 150 m y previously the distribution of sequence boundaries and the succession of facies associations identified in the areyonga sequence provide the basis for a depositional model for glacial sedimentation during an ice age to be successful correlation of late proterozoic glacial sediments must be based on an understanding of these facies associations and their bounding depositional surfaces associations similar to those found in the amadeus basin have been documented in other proterozoic basins suggesting that the facies associations and relationships recorded in the areyonga sequence may form the basis for a generalized model for basinal sedimentation from an ice age of global significance if this is so it should be possible to determine if a sequence containing glacial sediments is part of a global climatic event or simply due to local conditions from a single site evaluation

piecewise linear model for large strain radial consolidation a piecewise linear model for radial consolidation called rcs1 is presented rcs1 accounts for vertical strain soil self weight hydraulic conductivity anisotropy radial and vertical flows soil

environmental interpretation and a sequence stratigraphic framework for the terminal proterozoic ediacara member within the rawnsley quartzite south australia abstract fossils of the ediacara biota are confined to preservational windows in the ediacara member of the rawnsley quartzite pound subgroup from the flinders ranges south australia the base of the ediacara member is a type 1 sequence boundary incised into the partly lithified chace quartzite member of the rawnsley quartzite the ediacara member and the upper half of the rawnsley quartzite comprise the rawnsley depositional sequence bounded above by the early cambrian uratanna depositional sequence the rawnsley sequence developed over an erosional surface cut into the chace quartzite member with some 250 m of relief southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone at lowstand local channels were filled by massive sand the highstand systems tract consists of several parasequence sets of laminated siltstone and channelized amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well bedded clean sandstone impressions of soft bodied ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward shoaling delta front environments between storm and fairweather wavebase the ediacara member is in places conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member distinctive microbial mat bound sand laminae described as petee structures are characteristic of the rawnsley quartzite above and below the ediacara member during deposition of the ediacara member colonization of the deeper substrates by microbial mat communities enabled preservation of the ediacara biota the erosional surface at the base of the ediacara member is one of at least four incision events recorded in the terminal proterozoic and early cambrian succession of the adelaide geosyncline the similarity of incision filling lithofacies of the rawnsley and uratanna sequences has in condensed sections led to confusion between the depositional sequences characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the proterozoic cambrian boundary

vertical drain consolidation with non darcian flow and void ratio dependent compressibility and permeability vertical drains increase the rate of consolidation in soft soils by facilitating faster dissipation of excess pore water pressure through short horizontal drainage paths this paper presents an analytical solution for non linear radial consolidation under equal strain conditions incorporating smear but ignoring well resistance three aspects of non linearity are considered a non darcian flow b a log linear void ratio stress relationship and b a log linear void ratio permeability relationship the analytical solution to non linear radial consolidation can explicitly capture the behaviour of both overconsolidated and normally consolidated soils for non linear material properties consolidation may be faster or slower when compared with the cases with constant material properties the difference depends on the compressibility permeability ratios cc ck and cr ck the preconsolidation pressure and the stress increase if cc ck 1 or cr ck 1 then the coefficient of consolidation increases as exc

timing the breakup of a proterozoic supercontinent evidence from australian intracratonic basins in the late proterozoic several broad shallow intracratonic depressions appeared across a vast area of central australia the basins which all contain shallow marine to fluvial successions appear to have been tenuously interconnected through much of their history analysis of their fill by means of tectonic subsidence curves suggests that they are the product of two separate and distinct periods of crustal extension one at about 900 ma and a second at about 600 ma these extensional episodes were probably the result of failed rifting events that almost fragmented the australian continent during the late proterozoic the second period of extension almost certainly relates to the breakup of a proterozoic supercontinent the results suggest that sediments preserved in the relatively protected environment of interior basins may provide a more subtle record of major tectonic events than continental margin sequences that were exposed to major continental interactions

tectonic climatic supercycle in the billion year plate tectonic eon permian pangean icehouse alternates with cretaceous dispersed continents greenhouse abstract the earth alternates in a supercycle 400 m y long from a single continent pangea and ocean panthalassa with an icehouse climate to many continents and oceans with a greenhouse climate the supercycle is driven by the heat that accumulates beneath the insulator of pangea and the greenhouse is made by the excess co2 vented from the mantle during the faster rate of plate activity in the current supercycle a since the mid carboniferous 320 ma the growth and decay of pangean heat is observed in globally synchronous cratonic sedimentary sequences that represent in ascending order 1 a stratigraphic gap 2 sagging 3 rifting 4 fast and 5 slow seafloor spreading stages 4 and 5 of the previous supercycle b are observed back to the start of the phanerozoic in the proterozoic earlier stages of b are postulated back to 720 ma and a third supercycle c to 1100 ma the big glaciations of the latest proterozoic 700 600 ma and late palaeozoic 320 260 ma are confined to the icehouse state during stages 1 3 the sturtian 800 ma latest ordovician 440 ma and quaternary 2 0 ma glaciations fall within greenhouse states during stage 5 of slow seafloor spreading the next icehouse glaciation is expected 80 ma hence the problem of long term past and future climate change is soluble by global studies urgently needed are 1 global stock takes of a the rate of granite emplacement which serves as a proxy via plate activity for the mantle source of atmospheric co2 and b carbonate sinks and other sedimentary indicators of co2 and 2 the extension of the sea level curve back past 570 ma to the start of the plate tectonic eon 1100 ma ago from such studies environmental baselines for past climatic states can be drawn for gauging present and future background variations

proterozoic evolution and tectonic setting of the northwest paterson orogen western australia abstract the proterozoic igneous deformation and metamorphic histories of the palaeoproterozoic rudall complex in the northwestern paterson orogen can be linked to those of the arunta inlier in central australia and in part with the capricorn orogen in central western australia the similarities in deformation and metamorphic histories for these widely separated regions indicate a palaeoproterozoic continent continent collisional event between the palaeoproterozoic west australian and north australian cratons between c 1830 and 1765 ma in the paterson orogen this palaeoproterozoic collisional event resulted in the yapungku orogeny which included thrust stacking of clastic sedimentary and volcanic rocks deposition of the protoliths for the c 1790 ma siliciclastic paragneiss succession contemporaneous with granitic intrusion and metamorphism up to granulite facies during this 65 million year period the arunta inlier and capricorn orogen were deformed metamorphosed at medium to high grades and intruded by granitoids during the strangways orogeny in the arunta inlier and the capricorn orogeny in the capricorn orogen the neoproterozoic tarcunyah throssell and lamil groups are clastic sedimentary sequences that were deposited after 1070 ma in the northwestern paterson orogen and deformed by the miles orogeny before 678 ma the miles orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults the orogeny may have been coincident with the c 750 720 ma areyonga tectonic movement affecting the arunta inlier and the lower neoproterozoic part of the amadeus basin in central australia at c 550 ma the paterson orogeny which is most likely equivalent to the petermann orogeny in the musgrave complex of central australia deformed the northwestern paterson orogen and was preceded by local intrusion of granites the similarities of styles and timing of deformation in the northwestern paterson orogen arunta inlier and capricorn orogen indicate that these three regions were probably linked during most of the proterozoic

consolidation of soil using vertical drains the use of vertical drains is examined critically in view of the fact that in half a century of use there have been a large number of occasions when vertical drains have failed to promote a more rapid consolidation of the soil under load the mode of operation of the drain is discussed and it is shown that an understanding of this has affected the design of the more recent band shaped drains the effects of smear and drain resistance are also discussed reference is made to a three dimensional finite difference computer program modelling the flow of excess pore water in the drain and the surrounding soil which is used to aid in the drain design specific cases which can also be analysed by published techniques have been used to check the validity of the model consideration is also given to the methods of site investigation which should be employed to obtain the appropriate soil parameters for design cases are cited where a lack of data on the soil parameters has restricted the design of the vertical dr

controls on the locus of intraplate deformation in central australia abstract the locus of intraplate deformation in central australia changed from the presently preserved southern margin of the amadeus basin during the late neoproterozoic to early phanerozoic petermann orogeny to the northern margin in the late phanerozoic alice springs orogeny immediately prior to each event the thickness of the amadeus basin sediments varied from as little as 1 5 km in the central parts of the basin to as much as 7 km in the vicinity of the presently preserved margins of the basin with the locus of deformation mimicking the locus of maximum sedimentary thicknesses at the onset of each orogenic event as such these orogenies represent extreme examples of basin inversion a process observed in many intracratonic basins worldwide we show that differential burial of the central australian proterozoic basement complexes beneath the amadeus basin is capable of producing variations in moho temperature of up to 30 c prior to the petermann orogeny and up to 110 c prior to the alice springs orogeny for a brace goetze model of lithospheric rheology the variations in moho temperature equate to variations in effective lithospheric strain rate of 1 4 orders of magnitude implying that variations in thickness of the sedimentary blanket may have played a primary role in localising phanerozoic intraplate deformation in central australia an appealing aspect of this model lies in its corollary that the removal of the sedimentary blanket during denudation of the orogen will be accompanied by dramatic cooling and strengthening of the lithosphere this may provide a plausible explanation for the long term persistence of the extraordinary gravity anomalies 150 mgal developed during these central australian intraplate orogenies

smnd mineral isochron ages of late proterozoic dyke swarms in australia evidence for two distinctive events of mafic magmatism and crustal extension two distinctive events of widespread dyke intrusion have been identified in central southern australia crystallization ages of the dykes were determined using sm nd mineral isochrons obtained by splitting clinopyroxene and plagioclase into different magnetic and density fractions respectively the stuart dyke swarm in the southern arunta inlier and the kulgera dyke swarm in the eastern musgrave inlier define essentially identical crystallization ages of 1076 33 and 1090 32 ma respectively marking the first episode of post orogenic mafic magmatism in the region two samples from the gairdner dyke swarm in the stuart shelf yield sm nd mineral isochron ages of 867 47 and 802 35 ma respectively whilst two samples from the amata suite in the central musgrave inlier 790 40 and 797 49 ma these four ages represent the second episode of mafic magmatism in central southern australia these ages allow new constraints to be placed on the timing of large scale crustal extension and formation of intracratic depositional basins in the region initial nd values of 7 8 to 6 6 for the stuart dykes and 0 7 for the kulgera dykes suggest they were derived from different mantle sources with the stuart dykes being produced from strongly enriched sources in contrast positive and relatively uniform initial nd values of 2 5 to 4 1 for the younger amata suite and gairdner dykes indicate that they were derived from relatively depleted and homogeneous mantle sources the 1076 33 ma sm nd age of the stuart dyke swarm is significantly older than its rb sr mineral isochron age of 897 9 ma previously reported suggesting mobility of rb and or sr and partial resetting of rb sr isotopic systematics this study demonstrates that reliable and relatively precise sm nd ages can be obtained for precambrian dykes despite the limited number of mineral phases available in the dykes

chemostratigraphy and palaeo environmental characterisation of the cambrian stratigraphy in the amadeus basin australia abstract the sedimentary succession of the central australian amadeus basin consists of neoproterozoic to carboniferous sedimentary rocks and contains shallow marine subtidal carbonates of middle to late series 2 to furongian cambrian age a combination of sequence stratigraphy geochemistry and mineralogy shows a transgressive 2nd order cyclicity deposited between 511 490 ma and a change from arid low energy to humid high energy depositional environments this is reflected in an initially evaporitic sequence with upward decreasing halite and anhydrite abundance and transition from oxygenated to anoxic conditions reflected by the fe mineral species change from hematite to pyrite during transgression sequence boundaries of several 3rd order cycles consisting of hst carbonate rocks and lst siltstones correlate with globally recognised sequence boundaries linked to the inferred eustatic sea level record for the upper two series of the cambrian system the carbon isotope record for this 1400 m thick succession in combination with biostratigraphic age correlation allowed the identification of the globally recognised steptoean positive carbon isotope excursion spice drumian carbon isotope excursion dice and redlichiid olenellid extinction carbon isotope excursion roece

ediacaran fossil preservation taphonomy and diagenesis of a discoid biota from the amadeus basin central australia abstract aspects of the preservation of the enigmatic soft bodied ediacara biota in relatively coarse grained siliciclastic sediments remain unresolved the taphonomic model proposed by gehling in 1999 is here validated through examination of previously unpublished discoid fossils from the shallow marine deltaic arumbera sandstone in the amadeus basin within the flinders style preservation the model predicts stalked frondose organisms attached by a discoid holdfast to a microbial mat encrusted sea bed with sedimentation as periodic storm event beds during a terminal neoproterozoic taphonomic window a pyrite sole veneer provided critical fossil rigidity during burial the sequence of sedimentation events presented here advances a slightly modified gehling model no direct evidence of pyrite is detected instead a thin haematite clay coating covers fossil hyporelief surfaces nevertheless an original pyrite sole veneer is indirectly validated by tracing its diagenetic history through to today s haematite detailed petrography demonstrates reducing conditions during eodiagenesis that remained anoxic mildly alkaline through mesodiagenesis thus the pyrite sole veneer survived deepest burial meteoric water penetrated the deep regolith during prolonged telodiagenesis creating oxic mildly acidic pore water so pyrite oxidised and hydrolised to haematite the applicability of gehling s model is higher where marine shelf sediments subsided continuously in shallower or eroded areas that experienced intermittent meteoric eodiagenesis diagenetic history was different and ediacaran fossil preservation unlikely diagenetic history therefore has crucial implications for ediacaran fossil preservation

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

did global tectonics drive early biosphere evolution carbon isotope record from 2 6 to 1 9 ga carbonates of western australian basins the 13 ccarb record of well preserved carbonates in outcrop and core is here examined from the 2 6 to 1 9 ga old basins of western australia these data which are constrained by a well defined stratigraphic and tectonic framework and by u pb zircon ages provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere in the latest archaean ca 2 6 ga the secular 13 ccarb curve is flat much like that seen in the later palaeoproterozoic basins of northern australia 1 8 ga this implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable in the early palaeoproterozoic beginning after 2 5 ga and continuing until at least 1 9 ga the 13 ccarb the curve is much more dynamic with significant positive and negative excursions including a major positive excursion 9 pdb close to 2 2 ga these excursions can be correlated with the lomagundi event identified in africa europe and north america previously published studies of the overlying meso to palaeoproterozoic bangemall basin and of 1 8 1 5 ga old basins in northern australia suggest that the 13 ccarb curve became relatively monotonic again after ca 1 8 ga and remained so for most of the following mesoproterozoic comparisons with data from other ancient cratons especially africa suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the neoproterozoic when the western australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins the data are consistent with an earth in which the carbon mass balance was in equilibrium the 13 ccarb curve began to oscillate following the onset of glaciation as the pilbara and yilgarn cratons began to converge during the capricorn orogeny suggesting periods of rapid carbon burial during continental dispersal however the major positive excursion is preserved in carbonates from back arc basins formed as the ocean closed and subduction began since similar tectonic processes can be recognised not only in northern australia but also on other early cratons it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development we explain this coincidence of carbon isotopic excursions

radial consolidation of clay using compressibility indices and varying horizontal permeability a system of vertical drains with surcharge load to accelerate consolidation by shortening the drainage path is one of the most popular methods of soft ground improvement the conventional radial consolidation theory including smear and well resistance has been commonly used to predict the behaviour of vertical drains in soft clay its mathematical formulation is based on the small strain theory and for a given stress range a constant volume compressibility mv and a constant coefficient of horizontal permeability kh are assumed however the value of mv varies along the consolidation curve over a wide range of applied pressure p in the same manner kh also changes with the void ratio e in this paper the writers have replaced mv with the compressibility indices cc and cr which define the slopes of the e log relationship moreover the variation of the horizontal permeability coefficient kh with the void ratio e during consolidation is represented by the e log kh relationship

radial consolidation with vertical drains and general time dependent loading a simple to use analytical solution for radial consolidation with vertical drains under equal strain conditions is presented in this paper this solution accounts for the effects of soil smear dra

the sichuan basin southwest china a late proterozoic sinian petroleum province abstract the sichuan basin in southwest china is a large intracratonic sedimentary basin which first subsided in the late proterozoic sinian the basin has had a long complex subsidence history that has continued through to the present day tectonic subsidence curves indicate that in its early history the basin developed as a complicated extensional basin whose subsidence and tectonic history was polyphase much of the subsidence was driven by pulses of extension followed by thermal relaxation of the lithosphere the sichuan basin is an important petroleum base for the people s republic of china because of the discovery and development of over 60 gas fields and 10 oil fields one of the fields is the large weiyuan gas field which is considered to be both sourced and reservoired in late proterozoic sedimentary rocks these rocks are the late sinian carbonates of the dengying formation which form part of a thick platform yangtze platform succession that was deposited from the sinian to the silurian the weiyuan gas field occurs in a large anticlinal structure 53 26 km 850 km2 of which 200 km2 is prospective for gas up until 1988 91 wells had been drilled in the field and 40 were in production production is approximately 800 000 m3 day 28 3 million cubic feet day with total production to 1987 being 13 6 billion m3 480 billion cubic feet total reserves in the weiyuan gas field are estimated at 30 40 billon m3 1060 1410 billion cubic feet the sinian and earliest cambrian succession in the sichuan basin is highly prospective for the discovery of additional reserves of natural gas

detrital zircon ages in neoproterozoic to ordovician siliciclastic rocks northeastern australia implications for the tectonic history of the east gondwana continental margin u pb detrital zircon ages in variably metamorphosed dominantly fine grained clastic successions are used in northeastern australia to identify two major successions along the east gondwana margin the older succession is of probable late neoproterozoic age and is considered part of a passive margin associated with rifting at c 600 ma most detrital zircons have ages in the range 1000 1300 ma and were probably derived from an extension of a late mesoproterozoic 1050 1200 ma orogenic belt from the central australian musgrave complex located 1500 km to the west no evidence has been found for 600 800 ma rifting of a rodinian supercontinent and therefore it is suggested that breakup must have occurred well outboard of the present early palaeozoic east gondwana margin the younger succession is of early palaeozoic age and contains the distinctive 500 600 ma detrital zircon signature that is widespread in east gondwana in addition to some samples with ages in the range 460 510 ma consistent with local igneous sources the younger succession is related to the active margin of gondwana that developed on the former passive margin in a back arc setting and the source of 510 600 ma zircons is considered to be a composite of rift related and back arc volcanic sources

sedimentary record of late neoproterozoic rifting in the nw tarim basin china abstract upper neoproterozoic sediments which have been exhumed in the nw tarim basin china comprise a 600 900 m thick non marine to marine synrift succession that reveals an early phase of extension this study presents new data from two measured and interpreted stratigraphic sections near aksu and wushi the synrift succession unconformably overlies high grade metamorphic basement and consists of the non marine sugaitebulake fm and the marine qegebulake fm the sugaitebulake fm is characterised by a basal alluvial fan conglomerate that passes upwards into fluvial and lacustrine facies sandstones and siltstones the stratigraphic position of the basal unconformity and the thickness of the basal conglomerate is locally variable which reflects the formation of small isolated depocentres during the onset of extension in addition to a pre existing topography the sugaitebulake fm also contains a series of concordant extrusive basalts which record episodic volcanism and together with the fluvial and lacustrine facies indicate the development of the rift system and the growth interaction and linkage of extensional faults the conformable qegebulake fm is characterised by foreshore to intertidal facies dolomites that contain abundant stromatolites representing a basin wide marine transgression and the rift climax stage this synrift sequence is shown to correlate with a similar succession in nw australia and corresponds to at least partial separation of the tarim block from east gondwana during the latest neoproterozoic to early palaeozoic

the efficiency of partially penetrating vertical drains the paper deals with the problem of consolidation of a clay layer in which vertical partially penetrating drains have been installed the influence of well resistance is considered in the analysis which is performed by means of the finite element method the drainage pattern for each drain is assumed to be axisymmetric diagrams of the average degree of consolidation versus time for some selected non dimensional parameters are presented these dia grams indicate for partially penetrating drains that the delay in the consolidation process within the zone penetrated by drains due to higher excess porewater pressure in the underlying undrained zone is noticeable only to a very limited height above the drain tips in the case studies presented the delay is negligible to a depth corresponding to at least 80 of the partially penetrating drain length l article concerne le probleme de la consolidation d une couche d argile dans laquelle on a installe des drains verticaux qui penetrent partiellement l inf

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

palaeomagnetic dating and tectonic significance of dolerite intrusions in the albany mobile belt western australia abstract dolerite dykes and sills in the mesoproterozoic albany mobile belt and neoproterozoic stirling range formation metasediments in southwestern australia provide evidence for extensional events between australia east antarctica and greater india paleomagnetic analysis shows that primary remanence was carried by single domain magnetite for samples from seven localities palaeomagnetic poles for the majority of sites fall close to the earliest cambrian part of the australian apparent polar wander path apwp the interpreted earliest cambrian dykes intrude both extensional dominant and shear fractures indicating nne ssw extension between the western australian and east antarctic shields and a horizontal ese wnw maximum compressive stress dyke emplacement appears to be contemporaneous with mafic volcanism in central and northern australia and igneous intrusion in once contiguous parts of the east antarctic shield these features are interpreted as being related to gondwanaland assembly and may represent a response to far field stresses resulting from the collision between east and west gondwanaland a second suite of nw striking mid carboniferous dykes is related to ne sw extension and rifting between greater india and the western australian shield during early stages in the formation of the perth basin in a sinistral transtensional regime along the southern western australian margin a palaeopole from the primary remanence in a nnw striking dolerite dyke falls on the poorly defined triassic to early cretaceous part of the australian apwp indicating a possible third phase of dyke emplacement regional tectonic constraints for rifting in the perth basin suggest that this youngest dyke most likely intruded during the triassic

apatite and monazite an effective duo to unravel superimposed fluid flow and deformation events in reactivated shear zones abstract mylonitic shear zones crosscutting homogenous granitoids can retain evidence of fluid driven metasomatic retrogression and reactivation however the relationships between fluid rock interaction retrogression deformation and mylonitisation and the timing thereof are often cryptically recorded this study focuses on the granulite facies boothby orthogneiss from the reynolds range central australia which contains a large scale mylonitic shear zone with an apparent record of structural inheritance fluid infiltration and reactivation the chosen site provides an ideal natural laboratory in which to investigate the timing of deformation associated fluid flow and mass transport u pb isotope analyses of monazite indicate an average pb recrystallization age of c 1560 ma demonstrating that the orthogneiss fabric developed during the mesoproterozoic chewings orogeny 1590 1550 ma structural mapping suggests that this shear zone represents a riedel branch of larger structures that were subsequently reactivated during the paleozoic alice springs orogeny 450 300 ma the timing of reactivation and fluid flow is constrained by u pb dating of apatite which is present as a stable u bearing mineral in both orthogneiss and mylonite modelling of apatite radiogenic pb retention ages accounting for a wide potential range in common pb compositions demonstrates at least some growth and or recrystallization at c 1500 ma and c 400 ma confirming apatite precipitation during alice springs shearing and the reactivation of chewings age structures in addition alice springs aged apatite is found along pre existing fabrics in the orthogneiss in the vicinity of the shear zone indicating pre kinematic fluid flow across the shear zone boundary and into country rock that was otherwise largely unaffected the combined datasets demonstrates that integrated apatite and monazite u pb geochronology is an effective method to unravel the record of superimposed fluid flow and deformation events this includes the detection of an inverse younging relationship where younger ages are preferentially recorded in the wall rock as rather than in the reactivated shear zone such effects are potentially common where deformation is driven by pre kinematic fluid rock interaction with subsequent deformation enhancing the removal of replacement assemblages in more deformed rocks and favouring their preservation in less deformed rocks

numerical modeling of vertical drains with smear and well resistance installed in soft clay this study describes the behavior of embankments stabilized with prefabricated vertical drains installed in soft clay foundations based on several fully instrumented case histories selected from thailand and malaysia a multidrain analysis is conducted based on an equivalent plane strain model developed by the authors the deformation of soft clay beneath an embankment i e along and away from the centerline is predicted and compared with the available field measurements the effects of both smear and well resistance are incorporated in the analytical formulation which is used in conjunction with the modified cam clay theory it is demonstrated that the installation of vertical drains significantly increases the rate of settlement improves the dissipation of pore water pressure and decreases the lateral deformation of the soft clay foundation to obtain better predictions especially of pore pressures and lateral displacements the equivalent discharge capacity of the drains needs to be modeled ap

the geological evolution of the reynolds range central australia evidence for three distinct structural metamorphic cycles the reynolds range is affected by three distinct structural metamorphic cycles di mi dii mii anddiii miii which represent the local expressions of a series of orogenic events that affected large sections of the arunta block central australia the tectonic cycles recognized in the reynolds range correlate extremely well with similar events in the nearby anmatjira range di mi took place around 1820 ma and only affected the nw reynolds range dii mii occurred between 1760 and 1650 ma and involved ne sw shortening with identical timing relationships between the deformations and metamorphism along the entire reynolds range dii commenced with the formation of a fabric sii1 preserved as inclusion trails in porphyroblasts sii1 formed early during a progressive event dii1 2 which involved the formation and tightening of upright nw se trending isoclinal folds a penetrative subvertical nw se trending fabric sii2 developed axial planar to these folds the general symmetry of dii2 microstructures suggests that fii2 folds were formed during progressive coaxial deformation under plane strain conditions dii3 structures are associated with conjugate sets of crenulation bands within the macroscopic crenulation bands large sections of earlier folds and fabrics are rotated to a near recumbent position the dominant set of crenulation bands is upright and trends ene wsw while the conjugate to this set trends nw se no penetrative fabric formed during dii3 dii4 is associated with a system of amphibolite grade ne dipping thrusts interconnected by numerous smaller shear zones the sense of movement is ne over sw during dii essentially penetrative coaxial deformation dii2 evolved into essentially localized non coaxial deformation dii3 dii4 dii deformation occurred concomitant with a low pressure granulite facies metamorphic event mii 1730 ma peak metamorphic sillimanite cordierite orthopyroxene ilmenite spinel garnet k feldspar quartz assemblages define sii2 oriented sillimanite biotite assemblages that formed early during dii3 and unoriented andalusite chlorite muscovite assemblages that formed late during dii3 indicate that cooling of mii was initially isobaric during dii4 thrusting staurolite garnet biotite quartz and biotite kyanite quartz assemblages defining the mylonitic fabric overprint the late dii3 andalusite assemblages dii4 shear movement is therefore accompanied by a slight rise in pressure dii4 shear zones gradually increase in grade eastward along the reynolds range this trend coincides with the variation in peak metamorphic grade attained during mii indicating that the rocks had not yet fully cooled when dii4 thrusting commenced diii miii is late devonian to carboniferous in age and caused minor differential movement 100 m and retrogression along dii4 shear zones

duration of high pressure metamorphism and cooling during the intraplate petermann orogeny abstract the ediacaran cambrian petermann orogeny central australia is an exceptional example of intraplate orogenesis it involved sub eclogite facies metamorphism and extreme basin inversion during the exhumation of musgrave province basement from beneath the formerly contiguous centralian superbasin sensitive high resolution ion microprobe shrimp u pb geochronology of zircon titanite and rutile along with ti in zircon thermometry from meta igneous samples have been used to determine the timing and duration of high pressure metamorphism and subsequent cooling associated with this orogenic event peak metamorphic temperatures of 720 760 c were attained at 544 7 ma u pb zircon subsequent cooling to 600 660 c by 521 ma occurred at a rate of 2 6 7 0 c myr 1 as recorded by the closure of pb diffusion in titanite further cooling to 585 560 c by 498 472 ma occurred at a rate of 0 9 4 8 c myr 1 as recorded by pb closure in rutile the duration of tectonism was long lived 40 myr across the central and western parts of the orogenic system and deformation occurred in a comparatively warm and weak portion of crust characterised by regional thermal gradients of 17 26 c km 1 this proposed duration of tectonism is much longer than that permitted by a shear heating mechanism which requires an exceptionally short duration of tectonism and additionally an overall cold lithosphere characterised by geothermal gradients of 9 c km 1

finite element modelling of vertical drains beneath embankments on soft ground a methodology is described for modelling vertical drains in plane strain finite element analyses of embankments on soft ground this includes the use of a suitably formulated drainage element to represent each drain criteria for matching the effect of a single drain in axisymmetric and plane strain analyses are developed on the basis of a widely accepted theory matching can be achieved by adjusting the geometry drain spacing and or the permeability of the soil the finite element analysis and matching procedure are validated by comparison with theoretical results the effects of both finite drain permeability well resistance and smear are modelled satisfactorily une methode est deacute acute crite pour simuler des drains verticaux dans une analyse aux elements finis a deformations planes de remblais sur sols mous ceci inclut l utilisation d un element de drainage formule de facon adequate pour representer chaque drain des criteres pour representer l effet d un drain unique dans les analyses axisy

intracratonic strike slip partitioned transpression and the formation and exhumation of eclogite facies rocks an example from the musgrave block central australia mountain belts developed in an intracratonic setting during the neoproterozoic in the center of australia mesoproterozoic granulite and amphibolite facies gneisses of the musgrave block were overprinted during the petermann orogeny 550 ma with zones of high strain concentrated along broadly east west trending shear zones these zones formed under eclogite facies t 650 c and p 12 kbars to greenschist facies conditions and may form part of a strike slip related crustal scale flower type structure granulite facies gneisses preserve ages older than 700 ma suggesting that these rocks did not experience temperatures greater than 350 c at 550 ma for these rocks not to have been thermally equilibrated to temperatures of 350 c at a depth of 12 kbars requires either an extraordinarily low geothermal gradient 9 c km 1 or if the geothermal gradient was more typical 20 c km 1 then transport to that depth and subsequent exhumation occurring in a short time interval 40 myr

sm nd and u pb zircon isotopic constraints on the provenance of sediments from the amadeus basin central australia evidence for ree fractionation abstract the amadeus basin of central australia is a late proterozoic to late palaeozoic ensialic depositional basin located between two proterozoic basement blocks of different nd crustal formation ages the older arunta block t dm nd 2 0 to 2 2 ga u pb zircon ages 1 5 1 9 ga to the north and the younger musgrave block t dm nd 1 7 to 1 9 ga u pb zircon ages 1 0 1 7 ga to the south initial nd isotopic compositions of the amadeus basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks indicating that the sediments are dominated by essentially two source components the older arunta block 147 sm 144 nd 0 114 and nd 0 20 4 and younger musgrave block 147 sm 144 nd 0 118 and nd 0 14 7 and or their equivalents stratigraphically higher sediments plot more closely to the evolutionary trajectory of the musgrave block this result together with the average provenance ages indicates the proportion of material from the musgrave block increases as the sediments become younger the sm nd isotopic data preclude a substantial involvement of archaean sources u pb zircon ion probe analyses of the late proterozoic heavitree quartzite and the late cambrian goyder formation are consistent with the sm nd isotopic constraints most of the zircons from the basal heavitree quartzite give concordant u pb zircon ages of 1500 to 1900 ma consistent with derivation from an arunta type source whilst zircons from the younger goyder sandstone give six discrete u pb zircon age groups of 511 20 615 15 960 109 1190 54 1633 24 and 1878 48 ma indicative of a dominantly musgrave type source with a minor contribution from the arunta block or reworked preexisting amadeus basin sediments the two youngest u pb zircon ages probably indicate some contribution from late proterozoic cambrian volcanics in central australia or a previously unrecognised younger local source in the basement blocks the 511 20 ma constrains the maximum depositional age of the sediment the amadeus basin sediments have a surprisingly wide range of 147sm 144nd ratios 0 077 0 136 irrespective of grain size and rock types in addition the calculated t dm nd values for samples from the same stratigraphic unit are well correlated with 147sm 144nd ratios with samples of different stratigraphic units forming sub parallel arrays these phenomena are interpreted in terms of ree fractionation and preferential sorting of preexisting ree rich phases during sedimentary recycling and deposition this process can lead to either an increase or a decrease in 147sm 144nd ratio of a sediment relative to its source and therefore an erroneous estimate of its t dm nd provenance age a theoretical model is developed which accounts for the observed correlation between t dm nd and 147sm 144nd ratios for suites of stratigraphically related sediments and allows a reliable estimate to be made of the provenance age of individual stratigraphic units assuming the average 147sm 144nd ratio of the source is known or can be estimated the provenance age for each stratigraphic unit of the amadeus basin has been calculated using this method the sm nd isotopic systematics in the amadeus basin sediments suggest that 1 1 ree fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered 2 2 the provenance of the amadeus basin sediments was controlled by local source regions and cannot be used to infer large scale continental averages hence some caution must be evoked in using sm nd isotopic constraints from restricted sampling for more generalised models of crustal evolution

mantle plume uplift in the sedimentary record origin of kilometre deep canyons within late neoproterozoic successions south australia in south australia canyons 200 250 km long are incised to depths of c 1 5 km in late neoproterozoic after c 590 ma marine deposits of the adelaide fold belt and canyons 90 km long and 700 m deep occur in correlative marine strata of the officer basin 1000 km to the nw in the adelaide fold belt the isotopic signature of limestone veneering canyon walls canyon sinuosity and sedimentary structures in canyon fill suggest that the canyons were cut subaerially and filled by shallow water marine deposits during coastal onlap eustacy cannot account for the kilometre scale change in sea level required for subaerial canyon erosion and chemostratigraphy and the lack of evaporites at that stratigraphic level in southern australia argue against a messinian style drawdown in each region uplift and crustal extension preceding or coeval with canyon incision are indicated by a regional unconformity turbidite or mass flow deposition and extensional faults these features and canyon incision may be explained by regional uplift above a rising mantle plume over a distance of 1000 km in southern australia flood basalt volcanism in the officer basin tentatively dated at 563 40 ma may have followed this uplift mantle plume uplift is expressed in the sedimentary record by emergent trends in facies regional thinning regional unconformities turbidites gravity slides normal faults and incised canyons and palaeovalleys

equal strain consolidation by vertical drains this paper shows the development of a series of closed form solutions of equal strain consolidation in the presence of a vertical drain with smear and well resistance using an approach that considers the effects of both the radial and vertical drainage in a fully coupled fashion solutions are obtained for the excess pore pressure and the degree of consolidation in the compressible soil subjected to a step or ramp loading situation the closed form solutions in the present paper may be evaluated in an electronic spreadsheet on a standard personal computer

consolidation by vertical drains a theory is described which can be used in designing a vertical drainage system this theory incorporates important parameters such as vertical discharge capacity remoulding effects during installation and filter resistance the importance of these parameters is widely recognized but very seldom taken into account in the design by engineers six well documented case records related to different drain types are briefly presented experience gained from these case records shows that in many cases even if the overall performance appears successful there are aspects of soil drain behaviour that cannot be explained on the basis of existing theories l article decrit une theorie que l on peut appliquer a la conception d un systeme de drainage vertical cette theorie renferme d importants parametres tels que la capacite de decharge verticale les effets du remaniement pendant l installation et la resistance de filtration l importance de ces parametres est reconnue par bien des ingenieurs mais ils en tien

tectonic subsidence of the early paleozoic passive continental margin in eastern california and southern nevada quantitative analysis of tectonic subsidence in cambrian and ordovician platform carbonates and associated strata exposed in the spring mountains nevada and the nopah funeral and inyo ranges california indicates that subsidence associated with this segment of the early paleozoic passive continental margin is exponential in form consistent with thermal contraction of the lithosphere following extension as in other parts of the north american cordillera continental separation in the southern great basin appears to have taken place between 590 and 545 ma these results are not sensitive to uncertainties in stratigraphic thickness biostratigraphic age control or paleobathymetry uncertainties in the cambrian time scale lead to predictable variations in the inferred time of onset of thermal subsidence but they have no effect on the inferred stratigraphic position of the rift to post rift transition a younger age for the base of the middle cambrian results in a younger inferred age of onset of thermal subsidence accompanied by greater rates of subsidence during the cambrian whereas a significantly older estimate of the onset of thermal subsidence can be obtained only if the base of the middle cambrian is substantially older than 540 ma a possibility that is inconsistent with available data results of the subsidence analysis are particularly significant because this is one of the few regions along the length of the north american cordillera where they can be compared directly to the geologic evidence for syn rift and post rift deposition basement involved faulting associated with the amargosa basin aulacogen ceased during deposition of the noonday dolomite which is thought to be older than 700 680 ma on the basis of stromatolites of late riphean affinity the overlying johnnie formation contains supposed vendian stromatolites younger than 700 680 ma if it is assumed that our results indicate the timing of the final rift to post rift transition then either the ages inferred from stromatolites are incorrect or the lithosphere was thinned regionally after deposition of the noonday the latter possibility is supported by limited geologic evidence for extension in latest proterozoic and early cambrian time the lack of appreciable physical evidence for crustal extension after deposition of the noonday however may imply that 1 a uniform extension model for lithospheric thinning is inappropriate for this part of the margin or that 2 some or all of the localities studied are continentward of the hinge zone and that the observed subsidence is exaggerated by flexural loading in a deeper basin to the west

relationships between deformation and basin evolution in the intracratonic amadeus basin central australia abstract tectonic subsidence curves based on the sedimentary record are used here to study basin evolution and to indicate deformational periods in the history of the intracratonic amadeus basin in central australia the basin developed during an initial period of extension at about 900 ma in the late proterozoic stage 1 was subjected to another period of extension at about proterozoic cambrian boundary time when there was also a shortening event on the southwest margin stage 2 and was shortened during the late devonian early carboniferous stage 3 stage 1 extension in the southwest and northwest corners of the basin was associated with rift basin sediments and volcanics previously regarded as basement to the amadeus succession in the metamorphic arunta complex on the northern margin of the basin a dolerite dyke swarm is considered to have been emplaced during this event extension in a n s direction associated with stage 2 occurred only in the northern part of the basin at the same time a major compressional event the petermann ranges orogeny was occurring along the southern margin in the north the sediment fill at this time firstly thickens rapidly towards the northern margin and then thins dramatically at the margin implying a half graben structure and the presence of a major bounding fault in the southwest the petermann ranges orogeny caused detachment of the sediments above the salt horizon in the bitter springs formation northward transport folding of the sediment pile and formation of major basement cored nappes during stage 3 a foreland basin associated with southward directed overthrust sheets of the alice springs orogeny formed in the northern part of the basin horizontal shortening was of the order of 50 100 km and hence the present basin margins are structural rather than depositional the structures that developed during this stage dominate the structural pattern of the basin

an analytical solution for consolidation with vertical drains under multi ramp loading various analytical solutions have been proposed for a unit cell consolidation with a vertical drain under surcharge loading these solutions involve different assumptions to address various aspects of consolidation there is a lack of generalised solution for analysing consolidation of soil assisted by a vertical drain under various loading conditions this paper presents a simplified solution for consolidation under multi ramp loading generalised governing equations of equal strain consolidation are solved simultaneous radial and vertical flow conditions as well as the combined effects of drain resistance and smear are taken into account fully an increase in total stress due to multi ramp loading is reasonably modelled as a function of both time and depth an analytical solution to calculate excess pore water pressure at any arbitrary point in soil is derived by using the method of separation of variables the conventional definition of the degree of consolidation is given in terms of the dissipatio

structural controls on metamorphism and syn tectonic magmatism the portuguese hercynian collision belt upper palaeozoic collisional movements along the ibero armorican arc were accommodated on oblique strike slip ductile shear zone systems in north central portugal these shear zones are coincident with long linear belts of high temperature low pressure metamorphism and s type granitic plutons a model is presented in which crustal thickening subsequent to collision was followed by anatexis at depths of 12 15 km this was achieved by a combination of i increased heat flow ii lowering of melting temperatures caused by fluxing of hydrous fluids and iii a heterogeneous assemblage of greywackes and pelites which encouraged local high ductility contrasts shear zones are proposed as the most important influence in controlling the site of development of such belts anatexis of metasedimentary crust and emplacement of the resulting s type plutons within these zones took place without an appeal to crustal rifting or an asthenospheric heat source