the composite north american craton superior province deep crustal structure and mantle plume model of neoarchaean evolution abstract this paper aims to synthesize geological and geophysical data on superior province and in general archean crust of the composite north america craton relying on experience with deep geological and geophysical studies of composite east european craton by onset of the neoarchean archean provinces of the north american craton were parts of the single stable continent intensely reworked in the neoarchean crustal rocks were combined into a round oval domain wherein distribution of geophysical structural and metamorphic parameters displays concentric zoning the central zone occupies hudson bay basin located immediately above the lithospheric keel the internal zone is mainly composed of granulite facies plutonic volcanic and sedimentary rocks the external zone encompasses southern part of the superior province together with hearne and rae provinces this paper presents 3d crustal models of southern superior province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along lithoprobe geotransects and of the eastern abitibi belt based on higher resolution reflection seismic images neoarchean evolution of the north american craton represents plate tectonic processes initiated by a superplume it is only one of series of similar phenomena recorded 2 75 ga ago in a number of regions the most important features repeated to a certain degree in tectonic units of this type are 1 synchronous formation between 2 79 and 2 58 ga 2 frequently repeated combination of high grade granulite and high temperature amphibolite facies and low or moderate grade greenschist and epidote amphibolite facies metamorphic rocks 3 lower crustal granulite basaltic layer reflectivity zone displayed in seismic images of crust formed or transformed at the final stage of endogenic activity

tectonic history of the lunksoos composite terrane in the maine appalachians several ordovician volcanic terranes are preserved between the paleozoic north american margin and the avalon terrane in the northern appalachians of maine they have been interpreted as remnants of an island arc on the basis of their current position in the appalachians or as ocean islands near the margin of avalon on the basis of paleontologic data a study of one of these terranes the lunksoos terrane in northern maine was undertaken through a combination of paleomagnetic and structural analyses in order to understand its tectonic evolution paleomagnetic study of the pre caradoeian pre late ordovician stacyville volcanics revealed three components of magnetization the final magnetization removed d 90 i 36 95 11 8 is interpreted to be primary and yields a paleolatitude of approximately 20 s however a late ordovician remagnetization cannot be excluded the stacyville volcanics now overlie a cambrian and lower ordovician sequence the grand pitch and shin brook formations that is believed to have been deposited at the southern side of iapetus possibly near the margin of avalon during the middle and late ordovieian avalon was at paleolatitude s higher than 40 s while the volcanics were either formed or remagnetized at a paleolatitude of 20 s the pre caradocian stacyville volcanics therefore could not have been deposited directly on the grand pitch shin brook sequence but must have been tectonically emplaced the rockabema diorite which intrudes the entire cambro ordovician sequence constrains the timing of this emplacement to be pre late early silurian these relationships indicate that the lunksoos area is a composite terrane within the maine appalachians the tectonic emplacement scenario is unnecessary if the grand pitch and shin brook formations were not located on the south side of iapetus during the ordovician this alternative however poses conflicts with the interpretation of faunal affinities

numerical modelling of strata movement at footwall induced by underground mining abstract during metal mining the deformation and failure of rock mass are very important factors that dominate rock movement and zoning numerical modelling provides a feasible way to study the deformation and failure of rock mass in this study a numerical method of udec was adopted to study strata movement mechanism influenced by nww trending joints at the footwall at chengchao iron mine and the mohr coulomb model and the coulomb slip model with residual strength were adopted as the constitutive models of rock masses and joints respectively the whole strata movement process was simulated and the strata movement mechanism at footwall and zoning phenomenon was clarified in the chimney caving development stage gravity is the driving force and surface deformations are mainly subsidence deformation of the surface above the ore body in the post chimney deformation stage horizontal tectonic stress is the driving force and surface deformation and subsidence are dominated by horizontal deformation of the hanging wall and footwall in the mining area after the level 395 0 m is mined out the deformation characteristics of deep surrounding rock were revealed by means of numerical modelling the maximum failure depth of the cantilever beam and the spatial distribution of deep surrounding rock zones can be determined at last the influencing boundary of surface deformation expands rapidly in a linear way with the excavation step and the expanding velocity of the horizontal deformation is greater than that of the subsidence in the post chimney deformation stage

geochemical evidence for the geotectonic setting of early proterozoic metavolcanic sequences in lapland systematic geochemical study of early proterozoic metavolcanic rocks from the northern part of the baltic shield supports the concept that a complex mosaic of continental masses volcanic arcs and marginal basins were welded into one cratonic massif during the svecokarelian orogeny 1900 1750 ma in the north in finnmark and northern norrbotten tholeiitic metabasalts together with thick shallow water clastic metasediments form the main part of the karelian supracrustal sequences which in turn overlie archaean basement the trace element signatures of these metabasalts are comparable to those of post palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust it is not however possible to determine by geochemistry alone whether the continental rift basin in question lay in an intraplate or back arc setting metabasalts with a volcanic arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the karelian continent is approached in the skellefte field an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water norrland geosyncline which may have been fully oceanic in character subduction magmatism continued during the svecokarelian orogeny giving rise to synorogenic metavolcanic suites such as the kiruna porphyries the evidence for marked spatial variation of magmatic activity in the early proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the archaean we believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics and present a plate tectonic model for the early proterozoic development of the baltic shield

tectonics of the kola collision suture and adjacent archaean and early proterozoic terrains in the northeastern region of the baltic shield as preparation for the deep seismic and other geophysical experiments along the polar profile which transects the granulite belt and the kola collision suture structural field work has been performed in northernmost finland and norway and published geological information including data from the neighbouring soviet territory of the kola peninsula have been compiled and reinterpreted based on these studies and a classification according to crustal and structural ages the northeastern region of the baltic shield is divided into six major tectonic units these units are separated and outlined by important low angle ductile shear or thrust zones of late archaean to early proterozoic age the lateral extension of these units into soviet territory and their involvement in large scale crustal deformation structures are described using the view down the plunge method a generalised tectonic cross section that predicts the crustal structures along the polar profile is compiled and the structures around the kola deep drill hole are reinterpreted the kola suture belt through parts of which the kola deep bore hole has been drilled is considered to represent a ca 1900 ma old arc continent and continent continent collision suture it divides the northeastern shield region into two major crustal compartments a northern compartment comprising the murmansk and sorvaranger units and a southern compartment including the inari unit the granulite belt and the tanaelv belt as well as the more southernly situated south lapland karelia craton of the karelian province of the svecokarelian fold belt the kola suture belt is outlined by a 2 40 km wide and ca 500 km long crustal belt composed of 1 1 early proterozoic ca 2400 2000 ma old metavolcanic and metasedimentary sequences which originally formed part of the attenuated margin of the northern archaean compartment and 2 2 the remains of a ca 2000 1900 ma old predominantly andesitic island arc terrain this island arc terrain was built up above a sw plunging subduction zone initiated ca 2000 ma ago in the southern part of a newly formed oceanic domain the kola ocean due to continued subduction and complete consumption of this ocean the northern passive margin deposits and the island arc terrain were brought into tectonic juxtaposition and during the final arc continent and continent continent collision they were overthrusted onto the northern archaean continent along its southern boundary the kola suture belt is tectonically overlain by the archaean rocks of the inari unit this unit was derived from a microcontinent split from the southern compartment the depositional basin of the protoliths of the granulite belt being formed to the south of the microcontinent the inari microcontinent appears to have wedged out towards the southeast as the continuation of the granulite belt north of the white sea is in direct tectonic contact with the kola suture belt the granulite belt is composed of high grade paragneisses and minor amounts of meta igneous rocks the paragneisses formed from thick turbidite and mass flow deposits lain down in a back arc basin south of the inari microcontinent a thermal anomaly beneath the partly oceanic basement of the back arc basin is believed to have contributed to the ca 2000 1900 ma old granulite facies metamorphism of the granulite assemblages granulite facies conditions still prevailed when the inari microcontinent overrode the granulites and when the granulite belt as such was formed and was overthrusted for at least 100 km towards the southwest in conjunction with the latter event the rocks of the basement of the basin also became involved in thrust movements these now form the tanaelv belt which shows gradational tectonic contacts towards underlying cover and basement rocks of the south lapland karelia craton although not all parts of this craton were affected by the svecokarelian deformation it is considered to belong to the karelian province of the svecokarelian fold belt a ca 1900 1800 ma old episode of wrench faulting and the intrusion of 1790 1770 ma old post kinematic granites concluded the svecokarelian evolution of the northeastern shield region

sm nd and u pb dating of early proterozoic mafic felsic volcanism in northernmost sweden a sm nd mineral isochron of 1932 45 ma on a kiruna greenstone is defined by greenschist facies metamorphic minerals apparently older than the regional svecokarelian metamorphism 1880 ma however this age agrees within error with u pb zircon ages of 1909 1617 ma on quartz bearing prophyries which overlie but are in part intercalated with basic volcanics if these basic volcanics following previous studies are considered to be contemporary with the kiruna greenstones the present data suggest that eruption of both basic and acid volcanics and associated hydrothermal metamorphism took place over a short time interval close to 1 9 ga the greenstone sample is depleted in light rare earths and has high initial 145nd 144nd indicating a source in which such depletion has existed for a long time in contrast an albite diabase sample with a somewhat younger mineral age 1 80 0 08 ga is light rare earth enriched and has a correspondingly low initial nd isotopic composition

paleomagnetism of the upper silurian and lower devonian carbonates of new york state evidence for secondary magnetizations residing in magnetite abstract paleomagnetic directions for the upper silurian and lower devonian carbonates of the helderberg escarpment new york state differ from expected late silurian and early devonian directions for cratonic north america the mean direction d 165 i 10 paleopole at 50 n 129 e is similar to late carboniferous and early permian results negative fold tests and a lack of reversals suggest that the magnetization is secondary however low coercivities low blocking temperatures the thermomagnetic curves tc near 570 c and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite if a detrital origin of these magnetites is assumed the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks however no evidence for such thermal resetting is seen in the alteration of conodonts more likely perhaps is a chemical or thermochemical origin of the remanence this would require the magnetites to be authigenic

gis based assessment of risk due to salt mining activities at tuzla bosnia and herzegovina ground subsidence triggered by salt mining from deposits located beneath the city of tuzla bosnia and herzegovina is one of the major dangers acting on a very densely urbanized area since 1950 when the salt deposit exploitation by means of boreholes began as demonstrated in this paper subsidence induced several hazard factors such as severe ground deformations the arising of deep and superficial fractures and a very fast water table rise connected with the brine extraction now affecting several districts the above mentioned factors have been quantified by the use of geomatic methodologies including field surveys and analysis of geographical data in order to estimate the historical sinking rates authors processed the large and never before processed amount of topographical data collected during two periods from 1956 to 1991 and from 1992 to 2003 with only poor data collected afterward traditional surveys were completely and definitively stopped the analysis reveals a cumulative subsidence as high as 12 m during the whole period causing damage to buildings and infrastructures within an area that includes a large portion of the historical town at present almost entirely destroyed modern sinking rates have been monitored with static gps whereas the presence of superficial fractures monitored with kinematic gps factors related to the presence of deep fractures and water table rise have been evaluated by curvature analysis techniques and piezometric data respectively finally hazard factors have been combined in a risk map using the gis geographical information system map algebra capabilities and a simple multicriteria decision analysis mda in order to do that a vulnerability map has been derived on the basis of information reported on a couple of recently sensed high resolution satellite imageries the final risk arisen from the combination of single hazard factors and vulnerability map highlights critical scenarios and unsuspected threatening that are under consideration by the local decision makers and urban planners in particular as highlighted in the final risk map the present day water table rise triggered by the decrease in brine pumping is seriously posing a threat to a portion of the city which is not the most involved in ground deformations

ordovician paleomagnetic results from the st george and table head carbonates of western newfoundland we have studied 85 limestone and dolostone samples from 21 sites of the st george and table head groups lower and middle ordovician on port au port peninsula humber zone their gentle 15 tilt is probably devonian a steep viscous component dominant in the natural remanence is removed by thermal and less efficiently alternating field demagnetization in about 60 of all samples the thermal treatment yielded a southeasterly reverse component a which typically remained blocked to 450 500 c and then decayed to noise mean a component directions calculated from zijderveld plots and referred to bedding are d i 148 5 33 6 for n 9 st george sites and d i 150 0 i 40 7 for n 5 table head sites corresponding antipoles are 17 5 n 152 3 e dp dm 2 8 4 9 and 13 4 n 149 2 e dp dm 2 9 4 7 respectively we interpret these results as representative of the geomagnetic field in the early and mid ordovician in 14 samples mainly from the older st george strata thermal

absolute dating of dedolomitization by means of paleomagnetic techniques paleomagnetic analysis in conjunction with petrographic studies is an approach whereby absolute dates can be placed on diagenetic events hematite is associated with dedolomite in the lower ordovician kindblade formation in south central oklahoma and is interpreted as a by product of the dedolomitization of ferroan dolomite paleomagnetic results date the time of dedolomitization as late pennsylvanian early permian dedolomitization was probably caused by oxidizing fluids with high calcium contents that migrated through the kindblade formation following uplift of the arbuckle mountains

hangingwall surface subsidence at the kiirunavaara mine sweden large scale surface subsidence has been experienced at the kiirunavaara mine since sublevel caving was implemented as a mining method surface disturbances are affecting part of the city of kiruna the railway and the power station continuous and discontinuous subsidences characterize the hangingwall deformation which is periodically monitored using surveying techniques and mapping of surface cracks a historic review of subsidence prognoses has been carried out and the results compared with the actual condition of the hangingwall the review showed discrepancies between different prognoses in addition limit equilibrium analyses indicated that break angles flatten while the mining depth increases however this tendency is not clear in the field where break angles show a large dispersion of values by using surveying data two different analyses have been performed the time dependent movements of the hangingwall have been described using time displacement curves and strain analysis has been performed for different sections of the hangingwall three different stages of the time displacement behavior have been identified and described it has been concluded that extension strain can reach values which may damage civil structures before surface crack can be observed

geochronology of an archaean tonalitic gneiss dome in northern finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone archaean gneiss greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late archaean greenstone assemblages were deposited on older tonalitic crust

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

paleomagnetic and petrographic investigation of the taum sauk limestone southeast missouri paleomagnetic results from 17 sites in one quarry of the cambrian dresbachian stage taum sauk limestone member bonneterre formation reveal two components of magnetization on the basis of thermal demagnetization coercivity spectrum analysis and petrographic evidence both components reside in hematite sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria the first group consists of massive pink limestones and contains a component with declination 270 inclination 11 corresponding to a cambrian pole at 175 6 e 3 6 s the second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite the sites are dominated by the second component of magnetization which has declination 147 and inclination 10 and corresponds to a reversed pole position at 133 1 e 37 3 n a significant fraction of this component which is interpreted to be carboniferous in age resides in hematite intergrown with calcite in the burrows petrographic relationships indicate that the calcite and hematite were acquired before dolomite which places a maximum age on dolomitization the third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions with closely overlapping blocking temperature spectra the presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units

investigation on the distribution characteristics of ground cracks in the chengchao iron mine china in this study a comprehensive assessment of the distribution characteristics of ground cracks in the chengchao iron mine was conducted by field investigation and monitoring in general ground cracks are initially distributed around collapse pits and subsequently extend outwards centered on the mined out area greater ground deformation leads to more developed ground cracks ground cracks can be divided into cut through crack deep crack and shallow crack based on their downward extension depths furthermore the formation of ground cracks is controlled by both underground mining and geological structures ground cracks can be divided into mining induced crack and tectonic crack based on their main influencing factors moreover many types of ground cracks are caused by different failure modes including tensile failure shear failure and mixed failure ground cracks can be divided into vertical shear crack vertical shear and horizontal tensile crack horizontal tensile crack horizontal shear crack and mixed crack based on their formation mechanisms in particular the movement line and the break line defined as e 2 mm m and 6 mm m respectively can be reasonably regarded as disturbance boundary and crack boundary in a metal mine the disturbance zone caused by underground mining can be divided into collapse zone break zone and movement zone based on the size of ground deformation and the characteristics of ground cracks increases in ground crack width and length exert different degrees of stress release on the rock masses outside and inside the crack thus increasing their deformation difference in addition the extension of ground cracks is related to the ground topography and the strength and integrity of the rock masses and soil cracks on hilltop are more developed than those on flat land and cracks will initially appear in rock mass and soil rather than on a concrete structure due to the lower strain resistance of the rock mass and soil however concrete structures are more sensitive to cracking than rock mass and soil because of the better integrity of concrete structures

major and transition trace element abundances in the khondalite suite of the granulite belt of lapland fennoscandia evidence for an early proterozoic flysch belt abstract the granulite belt of lapland comprises two main units the more extensive unit the khondalite suite is formed of approximately equal proportions of sillimanite garnet gneisses and garnet gneisses which include impersistent layers of quartz rich gneisses calc silicate rocks and metavolcanics the less extensive unit the charnockite complex corresponds to several tholeiitic and calc alkaline series each of them ranging from gabbro to tonalite an attempt has been made to elucidate the primary features of the khondalite suite and its depositional environment major and transition trace element variations suggest that it has an overall sedimentary parentage and that it corresponded to a graywackeshale sequence turbidites original clastic sediments were mainly derived from an eroded continental crust and in lesser amount from contemporary volcanism the geochemical trends for alkalies seem to indicate that this crust was relatively silicic and that it had undergone previous granite forming events taking into consideration the geological setting the geometry and the facies of these sedimentary formations we propose to reinterpret the granulite belt of lapland as a flysch belt located at a plate junction and pre dating a continent continent collision with subducted oceanic crust

the palaeomagnetism of mesoproterozoic eriksfjord group red beds south greenland multiphase remagnetization during the gardar and grenville episodes the eriksfjord group comprises 3000 m of lavas and sediments rapidly deposited in a rift which developed within an andean type batholith in juxtaposition to the southern margin of the laurentian shield in south greenland at ca 1300 ma the lavas have been shown to preserve a detailed record of the geomagnetic field at the time of eruption incorporating normal reversed and transitional directions this study has examined the magnetic properties of the intervening red sediments they are found to possess a diagenetic remanence imparted by mediating fluids at later times the impact of diagenesis is stratigraphically controlled the base of the rift infill has magnetizations partially resident in magnetite which are either unstable to thermal cleaning or record a single polarity b magnetization d i 284 67 31 samples 95 5 5 palaeopole at 244 1 e 47 5 n dp dm 7 5 9 1 this corresponds in polarity and closely in direction to remanence observed in mid gardar gabbro giant dykes and dyke swarms emplaced along the axis of the rift system at ca 1160 ma the causative diagenetic magnetite appears to have grown from hydrothermal systems motivated by this magmatism in a sealed reservoir setting within the lower part of the rift infill the ilimaussaq alkaline igneous complex was emplaced into the southern extension of the rift at ca 1130 ma and possesses a dual polarity magnetization d i 327 81 95 6 4 10 sites eriksfjord lavas within the thermal aureole are overprinted to varying degrees by comparable magnetizations with steep inclinations the mean pole position 283 e 71 n dp dm 12 12 lies near the apex of an apparent polar wander loop incorporating the gardar track ca 1300 1140 ma and the keweenawan track ca 1115 1050 ma magnetizations in the eriksfjord sedimentary succession have not been significantly reset by emplacement of the ilimaussaq complex but higher levels of the rift infill are dominated by an a magnetization d i 305 34 95 4 3 57 samples palaeopole at 202 1 e 32 4 n dp dm 2 8 4 9 resident in haematite the pole position does not correspond with any part of the gardar track but does correlate with the return keweenawan track at ca 1090 ma close to the time of grenville orogenesis along the bordering southeastern margin of the laurentian shield this remanence is attributed to diagenesis during extensional tectonism linked to the collapse of the grenville orogen formerly sited 100 200 km to the south

late paleozoic remagnetization of the trenton limestone samples of the middle ordovician trenton limestone were collected from 8 sites in quebec ontario and new york state for paleomagnetic study stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53 0 n 126 9 e k 97 7 a 95 2 7 this pole is not at all close to other ordovician poles from cratonic north america but is near late carboniferous and early permian ones indicating that the trenton was remagnetized in the late paleozoic an earlier paleomagnetic investigation of the trenton gave a pole that did not appear to be late paleozoic in age however we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is fe3o4 that does not contain ti or other elements in solid solution that would indicate a detrital source the magnetite and its associated remanence appear to have a diagenetic origin

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

late paleozoic to early triassic multiple roll back and oroclinal bending of the mongolia collage in central asia abstract the architecture and mechanics of an orogen can be understood in terms of a system of collages that are characterized by a complex assemblage of multiple components but the fundamental paleogeographic framework and the tectonic relationships between the different components are often insufficiently defined because of unavailable data the central asian orogenic belt caob provides an ideal opportunity to address the fundamental framework of paleogeography and tectonic relationships between the diverse and many components in this huge collage in this paper we review several lines of available evidence which enable us to propose a new tectonic model of huge roll back in the formation of the accretionary tectonics of the mongolian collage in central asia in the early paleozoic the mongolia collage comprised the southern siberian and the tuva mongol oroclines the siberia craton and the mongolia collage jointly formed a giant tadpole shape within the paleo asian and panthanlassic oceans its head siberia was to the south and the tail tuva mongol to the northwest the structures and tectonic zonation of the mongolia collage are characteristic of multiple arcs which have been separately described in detail in different segments southwards from the southern siberia east sayan west sayan gorny altai chara via the lake zone junggar tianshan gobi altai beishan alxa to the manlay hegenshan baolidao solonker segments almost all segments underwent early paleozoic to permian or even triassic frontal subduction and accretion while rifting in the late carboniferous to permian or triassic occurred in the outward oceanward westward advancing mongolian collage therefore we suggest that a huge complex roll back active from the carboniferous to permian or even to late triassic facilitated the formation of the mongolian collage the outward multiple roll back process was compatible and almost coeval with the start of the tuva mongol orocline and rotation of the siberian craton as confirmed by paleomagnetic and structural data during the roll back processes an archipelago paleogeography was formed behind the frontal subduction and accretion in which independent arcs or terranes were amalgamated or collided to form composite arcs or terranes either simultaneously or at slightly different times the roll back process was affected by the collision of the kazakhstan collage along the chara and karamay zones in the early permian the collision of the tarim craton along the south tianshan zone in the early permian the collision of the dunhuang block along the liuyuan zone in the early permian triassic the collision of the alxa block along the qugan qulu zone in the permian and the collision of the north china craton along the solonker zone in the middle late triassic the tectonic styles and architecture of accretionary orogenic belts like the caob are characterized both by the amalgamation of multiple terranes and by oroclinal bending the systematic anatomy of the multiple roll back processes and their interactions with the adjacent collages shed light on the evolving orogenic architecture and the crustal accretionary history of orogens

use of an integrated finite discrete element method discrete fracture network approach to characterize surface subsidence associated with sub level caving abstract the lkab kiruna sub level cave mine located in kiruna sweden is one of the most well studied sub level cave mines due to proximity of the city to the hanging wall the mine has developed a comprehensive surface deformation monitoring program which is perhaps unparalleled in the mining industry this surface monitoring scheme provides an excellent data source with which to constrain numerical modelling in this study a finite discrete element modelling approach coupled with a discrete fracture network fdem dfn is utilized to analyse the kiruna hanging wall surface subsidence with an emphasis on investigating the influence of discontinuity persistence and spacing the fdem dfn data interpretation uses a variety of novel approaches including time displacement hanging wall deformation characterization numerical inverse velocity analysis and virtual hanging wall inclinometer simulation to improve our understanding of the extent and mechanism of hanging wall failure with mine advance the simulated displacements correspond closely to the actual field data illustrating the ability of the proposed approach to reproduce sub level cave behaviour three distinct time displacement phases namely regressive progressive and steady state are observed in the fdem dfn models the paper also reviews the potential application of the inverse velocity method and virtual inclinometers in characterizing sub surface brittle failure associated with sub level caving

empirical investigation and characterization of surface subsidence related to block cave mining abstract for guidance on relationships between caving depth and surface subsidence a comprehensive database was developed after an exhaustive search of published data from cave mining operations from around the world the distribution of data was found to largely focus on caving angles and macro deformations very little empirical data exists on the extent and magnitudes of smaller surface displacements the data clearly show that caving induced surface deformations tend to be discontinuous and asymmetric due to large movements around the cave controlled by geologic structures rock mass heterogeneity and topographic effects the data also show that as undercut depth increases for a given extraction volume the magnitude and extent of the caved zone on surface decreases however numerical modeling indicates that this is only the case for macro deformations and the extent of smaller displacements actually increases as a function of undercut depth the results presented caution against relying on existing empirical design charts for estimates of caving induced subsidence where small strain subsidence is of concern as the data being relied upon does not properly extrapolate beyond the macro deformations i e caving angles that make up the majority of the observations the findings also suggest that the extent and magnitudes of subsidence may be underestimated if the analysis adopted neglects the influence of geological structures and assumes symmetrical surface displacements above the undercut

is the cape smith belt northern quebec a klippe the cape smith belt is one of the most interesting and controversial of the proposed geosutures in the canadian shield a new tectonic model is presented in which the mafic ultramafic thrust sheets of the belt constitute a klippe 20 000 km2 in area separated from underlying basement of superior province and its thin autochthonous cover by a continuous decollement exposed along the north margin and the plunging eastern end of the belt thrusting is directed southward and the entire stack is folded into a regional antiform north of the belt and synform the belt itself it is proposed that the decollement is rooted 30 90 km north of the belt in a zone paralleling sugluk inlet across which there is a major positive deflection in the bouguer gravity field an abrupt switch from broad north south to narrow east west magnetic anomalies and a change in metamorphic grade from amphibolite to granulite facies from south to north in the model this zone is a north dipping ductile shear zone juxtaposing cru

two stage felsic volcanism in the lower proterozoic upper aillik group labrador canada its relationship to syn and post kinematic plutonism

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

determination of actual limit angles to the surface and their comparison with the empirical values in the upper silesian basin czech republic abstract this study deals with the determination of the actual limit angles in the long term formed subsidence basin in the czech part of the upper silesian basin the remaining part lays in poland here there was the most extensive underground exploitation of black coal in the north east of the czech republic a limit angle is absolutely decisive in the determination of deep mining influences in relation to the current and future utilisation of this territory the two csm and paskov mine localities on the edge of the subsidence basin were selected for this study since they could not have been affected by mining in neighbouring mines on the grounds of the long term levelling monitoring of points on the surface which started prior to mining the horizontal distance between the mined out area and the undisturbed ground surface was identified the impact of limit angle on the surface was calculated by means of the horizontal distance and the established mining depth such identified actual limit angles reached the values of 36 at the csm mine and 25 for the paskov mine the calculated values are significantly lower when compared with the currently applied empirical values of 57 for the csm mine and 56 for the paskov mine this means that the extent of the undermining effects is unambiguously wider by 611 m at the csm mine and by 883 m at the paskov mine this study thus establishes the existence of a significantly vaster area influenced by coal deep mining than previously expected the identified facts lead to the conclusion that regular revisions and follow up updates of the empirically determined parameters are recommended

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

spatial and temporal diversity of early proterozoic volcanic sequences comparisons between the baltic and laurentian shields abstract early proterozoic volcanic supracrustal sequences in the baltic shield exhibit considerable diversity in their geochemical composition sedimentary and metallogenetic associations the earliest volcanic sequences are calc alkaline associations whose age and mode of origin is poorly understood at present tholeiitic metabasalts erupted at about 2 2 2 0 ga form a significant component of lapponian and jatulian supracrustal sequences in the northern part of the shield these lavas were erupted in marine basins and rifts within archaean cratonic lithosphere ultramafic lavas are present but appear to be spatially restricted to certain supracrustal belts marginal to possible crustal suture zones e g the karasjok belt tholeiitic metabasalts of slightly younger age 1 96 ga form part of an ophiolitic association at jormua in north central finland other kalevian suites of similar age at outokumpu and in northern finland may have formed in marginal basins adjacent to a proterozoic ocean the svecofennian domain of central sweden and southern finland locally contains primitive oceanic arc like terranes representing a juvenile addition to the proterozoic crust most of the svecofennian complex is however composed of turbiditic sediments felsic calc alkaline volcanics dated at 1 92 1 87 ga and plutonic suites which reflect sedimentary and igneous reprocessing of the juvenile arc terranes the porphyry volcanics of norrbotten were subaerially erupted between 1 9 and 1 87 ga they reflect maturation of the svecofennian arc system to an andinotype continental margin terrestrial calc alkaline volcanism continued until 1 6 ga in the transscandinavian granite porphyry belt all the volcanic components of the classic wilson cycle have now been recognised in the baltic shield the early proterozoic history of which is thus readily explicable in terms of plate tectonic concepts in the laurentian shield tholeiitic metabasalts are associated with ultramafic lavas in the cape smith belt and elsewhere along the northern margin of the superior craton the ophiolitic association has recently been described from the cape smith belt the trans hudson and svecofennian belts are complex accretional orogens with comparable magmatic and tectonic histories pre grenville reconstructions suggest that the continuation of the transscandinavian granite porphyry belt is to be found in labrador and possibly in the southern part of the concealed mid continent region in the usa the numerous similarities between the two shields suggest they underwent a parallel and possible juxtaposed evolution in early proterozoic time

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

the correlation between the palaeoproterozoic orogens and granulite belts in the baltic shield and north america craton a suggested model of palaeoproterozoic plate tectonics abstract the earth s archaean history was completed with the formation of the first supercontinent pangaea 0 at ca 2 5 ga ago the main tectonic units of the younger palaeoproterozoic crust in the north atlantic realm include the collision and accretional orogens and in some cases the granulite thrust nappe belts three cycles of geodynamic activities are known in palaeoproterozoic at 2 5 2 33 ga 2 25 1 9 ga and 1 9 1 82 ga not only the starting and final events of the palaeoproterozoic megacycle with the corresponding extension and compression stages but also each cycle of evolution of the collision orogens including both pechenga imandra varzuga trans hudson cape smith new quebec and other suture belts and lapland snowbird queen maud granulite thrust nappe belts and of accretional orogens svecofennian penokean wopmay are strongly correlatable the palaeoproterozoic plate tectonic evolution appears unique in the absence of 1 general dispersal of the archaean supercontinent and 2 manifestation of subduction related magmatism in the accretional orogens along the craton margins of the north atlantic realm during 2 45 to 1 93 ga i e for about 600 ma an exception from the above generalization is the reported intrusion of calc alkaline granitoid complex cutting the lower part of the huron supergroup along the passive margin of the superior craton during 2 39 2 33 ga it is presumed that during palaeoproterozoic the oceanic crust in the oceanic hemisphere underwent rapid recycling that was accompanied by unusually slow wilson type evolution within the supercontinent that was probably nowhere disrupted in full rapid recycling of oceanic crust appears possible if the mid ocean ridges mors are presumed to lie above the sea level this might have prevented hydration of oceanic lithosphere which inhibited evolution of calc alkaline magmas this particular model of global tectonics during the palaeoproterozoic may be styled as microocean tectonics to distinguish it from the archaean microplate tectonics and the phanerozoic plate tectonics

new palaeomagnetic petrographic and 40ar 39ar data to test palaeogeographic reconstructions of caledonide svalbard new palaeomagnetic and petrographic data are presented from cambrian rocks of sw svalbard to test for the first time palaeozoic reconstructions of the major terranes of svalbard in the course of thermal demagnetization three chrm characteristic remanent magnetization components were identified which were labelled hornl hornm and hornh respectively on the basis of their different unblocking temperatures the hornm magnetization is related to the late ordovician silurian formation of the synmetamorphic s 1 foliation the hornm palaeopole 18 5 359 d p d m 5 8 11 4 plat 6 n matches exactly the silurian sectors of the baltica laurentia apparent polar wander paths after the closure of iapetus 455 415 ma the 450 ma 40 ar 39 ar age determination from mica ages obtained from the broad zone of mylonites along the billefjorden fault zone which separates the central and eastern terranes also suggests that the two terranes were eventually amalgamated by 450 ma the hornm vgp also lies very near the palaeopole derived from the middle proterozoic rocks of the eastern terrane ny friesland metamorphosed during caledonian time suggesting its close proximity to the study area central terrane the present study has shown that at least two of the major terranes of svalbard as defined by previous authors occupied similar geographical locations by silurian time and the previously proposed large scale late devonian left lateral displacements are not supported

a paleomagnetic investigation of the lower ordovician st george group port au port peninsula newfoundland implications for the iapetus ocean and evidence for late paleozoic remagnetization abstract to obtain an early ordovician paleomagnetic pole position for north america and to determine the paleolatitude of its margin of the lapetus ocean prior to the taconic orogeny a study of the lower st george group tremadocian arenigian of western newfoundland was undertaken analysis of 84 oriented core samples from six localities on port au port peninsula indicates that the nrm of the st george rocks is the resultant of three distinct paleomagnetic directions the first is a steep downward component commonly observed in the low temperature range of demagnetization room temperature 250 c in virtually all samples that roughly corresponds to the present day axial dipole field direction in situ d 350 i 68 the second is a stable component with southerly declinations and shallow intermediate negative inclinations d i in situ 176 20 d i tilt corrected 173 5 respectively usually seen in the range of 450 600 c and found in approximately 46 of the specimens the third is a stable southeasterly shallow intermediate positive component d i insitu 161 16 d i tilt correcied 163 34 respectively generally unblocked within the 350 550 c or 450 600 c range which was observed in 40 of the specimens these components of remanence are herein referred to as the type 1 type 2 and type 3 components respectively the remaining 14 of the specimens were found to be either directionally unstable to thermal demagnetization or totally overprinted by the type 1 component the type 2 component is interpreted to represent a post folding late paleozoic remagnetization possibly associated with the alleghanian orogeny on the other hand the type 3 component is interpreted as an early ordovician possibly pre taconic remanence and in tilt corrected coordinates yields a paleolatitude for this region of the north american craton of 19 s 6

mining induced ground deformation in tectonic stress metal mines a case study abstract in situ monitoring data of ground deformation in the western area of chenchao iron mine were collected over eight years results were analysed to conduct a preliminary investigation into the ground movement induced by underground mining 1 in the northeastern area the ratios of horizontal to vertical displacement increase gradually and the horizontal displacement may be as much as an order of magnitude larger than the vertical displacement at a large distance which induces regional horizontal movement curves of the ratio of horizontal to vertical displacement with respect to time can be categorised as fluctuation fallback convergence and increase types 2 in the mine road stack site and southern area around the high voltage tower ground deformation can be divided into two stages stable deformation and rapid deformation while in the southeastern area of the transport tunnel service shaft and road tunnel it can be divided into three stages stable deformation rapid deformation and subsequent stable deformation three categories are found in the time displacement curves folding s and linear types 3 the ground deformation of the footwall underwent a significant change in january 2010 because the large horizontal tectonic stress in the mine was released at that time surrounding rock masses were subjected to toppling deformation along the major structural plane in the northeastern area while tensile deformation occurred perpendicular to the major structural plane in the northern area

progressive failure of hanging wall and footwall kiirunavaara mine sweden abstract this paper presents an evaluation of the progressive failure of the hanging wall and the footwall at the kiirunavaara mine in northern sweden the failure of the hanging wall and footwall is a direct result of sublevel caving operations and has caused significant surface and subsurface disturbances on the hanging wall these disturbances include large scale subsidence surface cracking and shear displacements the footwall has experienced shear displacements and failure of underground openings using mine data failure mechanisms describing the progressive failure of the hanging wall and footwall were developed these data indicate that the primary failure mechanism is shear failure with toppling and wedge failures being present as secondary mechanisms shear failure occurs along pre existing geologic structures as well as through intact rock the data also showed that the hanging wall and footwall rock mass failure is closely linked to mining activity movements of caved waste rock and the presence of geologic structures an analytical model is presented to provide a simple tool for evaluating the extent of rock mass failure in the hanging wall and footwall while also accounting for interactions of caved waste rock the analytical model is unique in that it combines a limit equilibrium approach for the hanging wall and footwall with a coulomb type surface traction force to represent the caved rock the developments presented in this paper can be used to further the understanding of large scale rock mass behavior related to underground mining the failure mechanisms presented may also be applicable to open pit slope stability

paleomagnetism of the clam bank formation and paleolatitude estimates for western newfoundland n 16 sites k 25 3 a 7 5 with a corresponding paleopole at 23 2 n 145 0 e dp dm 4 s0 8 2 component b with d 172 9 i 5 7 n 35 specimens k 10 2 a 6 4 with a corresponding paleopole at 38 2 n 130 1 e dp dm 3 2 6 4 component c with d 350 4 i 69 8 n 33 specimens k 8 9 a9 8 9 a pre mesozoic origin of the a and b components is indicated by the presence of normal and reversed components in specific sites by the lack of correspondence between the a and b paleopoles and the mesozoic and later pole positions from the appalachians and the north american craton and by agreement with paleozoic poles from the region the a component was probably acquired immediately after deformation during the acadian orogeny the b component is probably a chemical remanence that was acquired during permo carboniferous kiaman time the c component is of recent origin probably acquired in the present earth s field paleomagnetic data from western newfoundland are used in a localized setting to construct a paleopole sequence and to estimate paleolatitudes for western newfoundland during the paleozoic keeping in mind the paucity of data for siluro devonian age from this region western newfoundland seems to have been at its southernmost position at the end of the ordovician and to have occupied equatorial latitudes during the permo carboniferous the paleolatitude trend suggests that this block which is part of the north american craton moved in a southerly direction during the early paleozoic and in a northerly direction during the middle and late paleozoic

early cambrian continental reconstructions climatically related sedimentary facies and faunal distributions have been combined with palaeomagnetic data to make provisional reconstructions of the early cambrian world laurentia baltica and siberia appear to have formed a continental group which rifted apart from each other prior to 600 ma the consolidation of much of gondwana probably occurred in the pan african orogeny well before the cambrian though the assembly of several east asian terranes is still speculative the archaeocyathan reefs of siberia southern europe and morocco with their bigotinid trilobite fauna suggest that these areas were adjacent to each other and at low latitudes avalonia had close faunal links with western gondwana but lacked bigotinids and archaeocyathans and may have been situated off west africa and florida during the early cambrian there was a northward migration of laurentia towards the equator as its separation from baltica and siberia increased and a southward movement of gondwana

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

limestones of western newfoundland that magnetized before devonian folding but after middle ordovician lithification a positive fold test and a negative conglomerate test help determine when and how stable remanence was acquired in the middle ordovician table head group limestones of the port au port peninsula of newfoundland the limestones magnetized after lithification and incorporation as clasts into a middle ordovician breccia hence the limestones do not carry a detrital or other primary remanence despite their very low conodont colour alteration index of 1 the remanence may be thermoviscous or diagenetic and was acquired before devonian folding this suggests the need for caution in interpreting paleomagnetic results from other early paleozoic limestones whose remanence resides in magnetite of blocking temperature lower than 400 c

lateral variations in supracrustal geology within the swedish part of the southern svecokarelian volcanic belt abstract the southern volcanic belt of the proterozoic svecokarelian orogen exhibits significant lateral changes of lithology and stratigraphy in the swedish part of the belt metavolcanites dominate the stratigraphic column to the west whereas they form only minor interbeds in the predominant metasediments to the east the metavolcanites of the western area are almost exclusively rhyolitic while the eastern area also contains mafic and intermediate rocks mg rich quartz mica schists which are believed to have been formed by synvolcanic alteration at depth appear to be restricted to the western area these features are compatible with an area of rhyolitic volcanic centres in the west supplying tephra into a sedimentary basin in the east this episode of volcanism thus appears to represent an interlude in the deposition of sediments it has been suggested that this rhyolitic volcanism originated from a continental crust which was activated by extensional tectonics

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

tectonic styles of early proterozoic ore deposition in the fennoscandian shield the early proterozoic metallogenic events of the fennoscandian shield are connected with two major stages of crustal evolution the karelian continental development 2 5 2 1 ga and the svecofennian orogeny 2 0 1 75 ga the karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on archaean crust a tensional regime probably associated with a mantle plume and partial melting of the upper mantle culminated in the generation of an ene wsw trending rift zone which was intruded by tholeiitic magma 2 44 ga ago some 15 layered igneous bodies hosting cr pge ni cu and fe v ti deposits are known in the kemi suhanko and koillismaa belts these mafic ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km nw se and nnw sse trending dolerite dykes intruded the rifted and attenuated archaean crust 2 25 2 0 ga ago preceding ocean opening the coeval ne sw trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen the failed arm of a triple armed rift system oceanic crust is preserved in 1 96 ga old ophiolite complexes the cu co zn ni and cr contents of the outokumpu type deposits derive from hydrothermal leaching of oceanic crust and were discharged together with chemogenic cherts and carbonates from ascending hydrothermal fluids on the seafloor above fissure vents in the northern part of the shield rifting and probably ocean opening resulted in the formation of the layered tholeiitic sedimentary complex of the petsenga ni cu district intruded by mineralized mafic ultramafic magma the age of this event is poorly constrained the basin was subsequently closed and continental collision of the kola suture was accompanied by southward obduction of the lapland granulite belt on the archaean crust during the svecofennian orogeny the divergent continental margin was converted into an active margin early subduction of oceanic crust produced oceanic island arcs intruded by 1 93 1 9 ga old tonalites and trondhjemites subsequent subduction of oceanic crust generated 1 9 1 87 ga old continental island arcs associated with massive sulphide cu zn pb ag au deposits the island arcs were accreted to the archaean crust in a concave belt containing the ore districts of kiruna skellefte vihanti pyhasalmi aijala orijarvi and bergslagen bimodal volcanic suites with felsic calc alkaline and mafic tholeiitic volcanites are attributed to rifting of the overriding plate above subduction zones ni cu deposits in synorogenic 1 9 1 87 ga old mafic ultramafic intrusions occur in linear fault controlled belts at the margin of the archaean crust the generation of ni bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone magma differentiation and magma upwelling along transcurrent faults on the continental margin sulphur was probably assimilated from the supracrustal wall rocks 1 89 1 87 ga old early svecofennian granitoids are associated with porphyry type cu mo au deposits widespread late svecofennian s type 1 84 1 75 ga granitic magmatism generated minor mo u w sn and ree deposits emphasis is laid on the importance of geotectonic models as a basis for further exploration in the fennoscandian shield

accretion tectonism in the proterozoic svecokarelides of the baltic shield the duality of early middle proterozoic terranes in the baltic shield reflects fundamental differences in history between coeval mobile belts and the manner of their interaction the karelides formed on a continent ocean transcurrent margin in which offsets developed as arcs and back arc basins on stretched continental crust at least one back arc basin was obducted onto the continental margin preserved as an allochthonous terrane in the karelian nappe pile remnant arc fragments form accreted exotic terrane in pohjanmaa the svecofennides developed as a succession of island arcs while the site of subduction moved south and successive extinct arcs were accreted onto the archean craton arc terrane impingement affected deformation style in the karelides which changed from one of major horizontal translation to one dominated by major dextral wrench faults analogies can be drawn with the phanerozoic evolution of the northeast pacific margin

reversed polarity characteristic magnetizations in the albian contessa section umbrian apennines italy implications for the existence of a mid cretaceous mixed polarity interval paleomagnetic and paleontologic data from the valle della contessa umbrian apennines italy span a 14 m y gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization chrm in limestones and marls of the albian marne a fucoidi rock magnetic data indicate that the reversed chrm is carried by hematite having high unblocking temperatures 625 690 c hematite with these unblocking temperatures also carries a normal polarity chrm in other intervals a potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component carried by magnetite and hematite is isolated at lower unblocking temperatures 300 600 c bulk magnetic properties vary throughout the measured section in the portion containing most of the reversed chrm intervals hematite is the dominant carrier of natural remanent magnetization while magnetite is the dominant carrier stratigraphically above and below these changes correspond approximately with lithology the zone of dominant hematite magnetization is marked by reddish bands pigmentary hematite which can be correlated throughout umbria and are thought to record changes in the oxidation state of the seafloor during deposition two models may account for the observed magnetization directions the reversed magnetizations may record a remagnetization which occurred at least 18 m y after deposition while the beds were still flat lying in reversed polarity chron 33r 83 79 ma or later if the reversed chrm represents a remagnetization the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals alternatively the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid albian 107 104 ma and may record unrecognized intervals of reversed geomagnetic field polarity milankovitch like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals using these estimates two of the potential reversed polarity intervals are of sufficient duration 100 kyr to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys a primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the biticinella breggiensis and ticinella primula foraminiferal zones and within the prediscosphaera cretacea nannofossil zone on a worldwide basis

early svecofennian stratigraphy of southern and central norrland sweden and the possible existence of an archaean basement west of the svecokarelides abstract the stratigraphy of supracrustal sequences of seven different areas in southern and central norrland sweden are compared all the sequences belong to the svecofennian part of the early proterozoic svecokarelian fold belt it is concluded that quartzites and meta arkoses are usually found at high stratigraphic levels they are sparse or absent in the north but are more abundant in southern norrland this may suggest that a granitic archaean or early proterozoic basement was exposed to erosion somewhere southwest of the regions under consideration future research should aim at clarifying the age and extent of this basement and the provenance of the various metasedimentary rocks

enriched komatiitic basalts from newton township ontario their genesis by crustal contamination of depleted komatiite magma lree enriched komatiitic basalts are commonly found in association with lree depleted komatiites this association is found in a sequence of late archaean lavas from newton township ontario the komatiitic lavas at newton township differ from most late archaean examples in that both the komatiites and the komatiitic basalts are depleted in al and in the hree the close association and distinctive al and hree depletions of the two lava types strongly suggest a common origin despite their contrasting lree patterns a model is proposed whereby the lree depleted komatiites represent the parental magma to the lree enriched komatiitic basalts the two being linked by a combination of crystal fractionation and crustal assimilation the composition of the contaminant is estimated by comparing the lree enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry the contaminant composition coincides closely with taylor mclennan s estimate of the composition of the archaean upper crust it is concluded that lree enriched komatiitic basalts can be produced from lree depleted komatiite parent magmas by combined assimilation and fractionation and that such a process best explains the geochemistry and nd isotopic features of most komatiitic basalts

provenance of early proterozoic and archaean metasediments in finland a smnd isotopic study abstract sm nd isotopic analyses of early proterozoic and archaean clastic metasediments from finland are presented they have been used to calculate crustal residence ages and to constrain the sedimentary provenance the mean crustal residence age t dm for the svecofennian metagreywackes in the tampere schist belt is 2 22 ga which is about 0 3 ga in excess of the assumed stratigraphic age this indicates that the bulk of the sediments were eroded from newly formed crust their major source was most likely svecofennian orogenic material mixed with a small amount of archaean derived detritus the precursors of many granitoids in the svecofennian terrain also had similar short crustal residence times the kalevian metasediments which were deposited 2 05 1 9 ga ago on the edge of the archaean craton in eastern and northern finland have a mean t dm of 2 41 ga which is 0 4 0 5 ga more than the inferred stratigraphic age hence their source was on average older than that of the svecofennian metasediments investigated but definitely not purely archaean neither could crust similar to the bulk of the 1 9 ga old svecofennian terrain have been the only source various sources contributed at least locally but the bulk of the sedimentary material probably had a common provenance major sources consistent with existing isotopic data could have been 2 1 ga old jatulian rocks the c 2 0 ga old granulite belt in northern finland and or the svecokarelian early orogenic arc terrain mixed with archaean derived nd the method and data do not allow a distinction to be made between the possible alternatives the t dm model ages for two analysed archaean metasediments are slightly over 3 ga which is about 0 3 ga in excess of their assumed stratigraphic age thus providing no evidence for crust much older than 3 ga in this area

curvature analysis as a tool for subsidence related risk zones identification in the city of tuzla bih abstract shallow miocene salt deposits located below the city of tuzla bih have been exploited during the last 60 years by means of wells extracting salt saturated groundwater brines the massive extraction activities have led to severe subsidence accommodated by collapse and strain localization surface topography and geomorphology have been influenced and modified by several faults and fractures a series of sequential topographical survey data collected during the last 50 years have been used to obtain the total subsidence envelope surface in order to analyze the salt dissolution related morphological development of the tuzla topography and to identify the location of faults and fractures in the deformed area we applied curvature analysis to the subsidence surface by means of two different analytical methods the directional two dimensional curvature and the analytic gaussian curvature the comparison of the curvature maps with those of surface fractures visible in the area shows a good spatial agreement between the directional curvature and fracture intensity whereas the subsurface normal faults are more evident in the gaussian curvature maps

hydrothermal alteration as a control of regional geochemistry and ore formation in the central baltic shield in a geochemical study of the proterozoic volcano sedimentary bergslagen ore province in central sweden more than 1000 eruptive rocks were analyzed for main and trace elements the volcanic rocks form a bimodal suite dominated by rhyolites but deficient in andesites igneous geochemical variation bears little relevance to the occurrence and distribution of ores which mostly appear related to secondary alteration patterns hydrothermal metasomatic alteration occurs on a regional scale and has affected markedly more than half of the 732 analyzed macroscopically fresh porphyritic volcanic rocks metasomatism produced extreme na and k differentiation and fe mg si relationships incompatible with igneous variation patterns enrichment in na and mg and leaching of ore forming metals are prominent in the lower parts of the volcanic pile whereas meta volcanics enriched in k and volcanogenic sediments associated with ores and marbles dominate the upper levels the metasomatic alteration of the volcanic rocks and the formation of most ores are two aspects of a comprehensive process of hydrothermal convection which circulated sea water extracted the ore forming elements from the lower levels of the volcanic pile and enriched them at or close to the top of the volcanic sequence

volcanic and geochemical stratigraphy of the nussir group of arctic norway an early proterozoic greenstone suite the nussir group of west finnmark is a suite of metabasaltic lavas and tuffs about 1 8 km thick associated with continental and shallow marine sediments this assemblage is typical of karelian early proterozoic supracrustal sequences throughout northern norway and northernmost sweden primary textures are well preserved and permit the establishment of a volcanic stratigraphy for the nussir group the krokvatn formation is dominated by mafic aquagene tuffs while the overlying svartfjell formation is composed mainly of submarine pillow basalt petrographic evidence for the petrogenesis of the metabasalts is poor because of greenschist facies metamorphism during the svecokarelian orogeny c 1840 ma the earliest lavas have the most primitive compositions but subsequent flows exhibit strong enrichment in fe ti and incompatible elements as a consequence of extensive crystal fractionation the tholeiitic nature of the suite is confirmed by the trace element data the later lavas are more uniform and less evolved and are considered to represent steady state compositions produced by a periodically replenished tapped open system magma chamber the field relationships lithological and geochemical evidence suggest that the nussir group lavas were erupted in a submarine rift basin within continental crust they form part of an extensive tholeiitic magmatic province which occupied much of northern norway sweden and finland in the early proterozoic the tectonic environment of this province is poorly constrained by the geochemical data continental plateau basalt provinces such as the columbia river and karoo provide the closest phanerozoic geochemical analogues

general geology and evolutionary history of the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt in the northeastern baltic shield abstract the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt is situated in the northeastern part of the baltic shield and is discontinuously developed over a length of ca 1000 km it comprises five main zones which are separated by longitudinal and a number of transverse syndepositional faults each zone contains a number of lithostratigraphic units at the rank of groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16 000 m the sequence comprises a series of sedimentary volcanic rhythms usually separated by stratigraphic breaks marked by palaeo weathering the groups are separated by major disconformities which may mark record rift inversion or orogenic episodes at least seven distinctive lithostratigraphic markers occur in the belt including two komatiitic volcanic series separated by immature conglomerates two red coloured proto evaporitic formations separated by the alkaline volcanics and two different black shale formations perhaps the main characteristic of the belt is the apparently long history of sedimentary volcanic development at the same site spanning a time period of 2500 to 1800 ma a three stage rift development model is considered 1 an intracontinental rift stage 2500 2100 ma separated by the major inversion 2400 2330 ma and comparable with present day afar triangle and east african rifts 2 a transitional from intracontinental to to intercontinental rift stage 2100 1970 ma with possible short lived spreading about 1990 1970 ma 3 a collision related intercontinental rift stage 1970 1800 ma followed by the svecofenian orogeny 1800 1700 ma the observed distribution of platinum group elements pge mineralization is related to layered gabbro norite complexes fedorova and panskie tundry gora general skaya intrusions dated at 2470 ma which are apparently co magmatic and coeval with the first komatiitic volcanic series seidorechka formation found in the eastern part of the belt only the major chromite occurences are connected with a layered gabbro norite intrusion imandra lopoloth which is co magmatic and coeval between 2440 2330 ma with the second komatiitic volcanic series rusinga polisarka akhmalahti and malbekk formations the ni cu deposits of the belt are connected with a differentiated gabbro werhlite complex which is co magmatic and coeval ca 1970 ma with the specific fe and ti rich ferropicrites with komatiitic affinities

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

paleomagnetism of the moreton s harbour group northeastern newfoundland appalachians evidence for an early ordovician island arc near the laurentian margin of iapetus paleomagnetic results have been obtained from mafic volcanic units in the upper part of the moreton s harbour group which is part of an accreted island arc terrane that is preserved in the notre dame bay subzone of the central mobile belt of the newfoundland appalachians detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization carried by magnetite at a large number of sites in pillow basalts and coeval basaltic dikes the intrusives contain both polarities and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes and by a positive structural test involving a correction for block rotations strike correction the overall mean direction for the characteristic component after tectonic tilt and strike correction flows and intrusives d 171 i 22 k 22 6 95 6 5 pole 29 n 135 e flows d 166 i 22 k 33 2 95 6 5 corresponds to an early ordovician paleolatitude for the arc of 11 s which is indistinguishable from the expected paleolatitude of the north american margin this implies that the arc formed at or near the margin of the craton in contrast the avalon block which formed the other margin of iapetus was widely separated from the arc and the craton at this time the early ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton subsequent convergence between the arc and laurentia in middle ordovician time resulted in closure of this narrow ocean basin and obduction of the back arc basin oceanic crust onto the ancient margin of the craton thus giving rise to the taconic orogenic pulse in the newfoundland appalachians

the depositional evolution of the svecofennian supracrustal sequence in finland and sweden abstract on the basis of six new zircon ages from metavolcanic rocks together with other isotopic geophysical and geological data the chronological and tectono stratigraphic evolution of the early proterozoic svecofennian supracrustal rocks in finland and sweden are discussed these data suggest that a nw se displacement from the bothnian bay to lake ladoga and a sinking of the southwestern block was initiated 1950 2000 ma ago accumulation of pelitic sediments and greywackes on the subsiding archaean basement continued until about 1860 ma in south central and northern sweden shallow submarine and subaerial volcanics were extruded 1880 1890 ma ago the thick sedimentary piles and volcanic rocks were subsequently intruded by plutonic rocks forming large batholiths 1860 1890 ma ago

early proterozoic ensialic spreading subsidence evidence from the garpenberg enclave central sweden abstract the early proterozoic metavolcanic complex in the garpenberg enclave of central sweden displays bimodal basaltic rhyolitic volcanism reflecting alternating compressional and tensional tectonic regimes the acid metavolcanics show a continuous calcalkaline differentiation trend geochemically unrelated to the associated interbedded metabasalts a dacitic horizon indicates mixing of the two end member magmas during a compressional stage bimodal volcanism is characteristic of the early proterozoic supracrustals leptite formation throughout the central swedish province of the baltic shield the paucity of ocean floor rocks and high pressure metamorphism together with the lack of abundant andesites suggest that this province does not represent an island arc rather the combination of an ensialic edifice with alternating compressional and tensional tectonic regimes suggests andinotype subduction coupled spreading subsidence within the margin of a continental plate

a methodology based on strain analysis for identifying potential discontinuous deformation zones in sublevel caving mines abstract underground mining operations using the sublevel caving method often result in discontinuous ground deformations involving surface cracks determining the limit of the discontinuous deformation zone is important for the relocation of neighbouring residential areas mining infrastructure and the planning of underground mining operations the limit of the discontinuous deformation zone is characterised by the outermost surface crack which is a tensile crack when it occurs and is correlated with the horizontal ground tensile strain in this study a mathematical procedure combining the horizontal displacement as periodically monitored by total station and gps multiquadric radial basis function mq rbf interpolation and continuum mechanics is provided to calculate the strain state at any point on the ground then the relationship between a new tensile crack and horizontal ground tensile strain is studied to determine the criterion representing the presence of a new tensile crack this means that when the criterion at any point on the ground is satisfied the point will have the potential to crack based on this the potential discontinuous deformation zone where surface cracks may occur can be identified potential break angles at representative sections can then be determined a case study is presented to illustrate the results given by the above methodology the result shows that when major principal horizontal ground strain e1 at any point on the ground reaches the potential cracking major principal strain pcmps 6 mm m in this case study the point will have the potential to crack subsequently the area within the outermost contour of the pcmps can be defined as the potential discontinuous deformation zone moreover the approximate matching of the limits of potential discontinuous deformation zones and surface cracks verifies the reliability of the mathematical procedure and the rationality of the pcmps in identifying the discontinuous deformation zone additionally in this case study the potential break angles at two respective sections are significantly different indicating that the break angles should be further studied and care taken in design of works in these two zones respectively

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

proterozoic palaeomagne tism and single continent plate tectonics summary proterozoic 2700 570 ma palaeomagnetic poles from the major shields conform to a single apparent polar wander path on a contiguous reconstruction which brings into alignment both with each other and with the long axis of the primary continental crust tectonic and magmatic features ranging from late archaean 2900 ma to late proterozoic 1100 ma age this discovery implies that the continental lithosphere behaved on a megatectonic scale as a single coherent plate during this era subject to internal deformation but not breaking up in a brittle fashion until late precambrian times peripheral parts of the supercontinent were redistributed over a short interval at ca 1100 ma but the continental crust was not dismembered and widely dispersed until 570 530 ma this latter episode correlates with the proterozoic phanerozoic transition and defines the onset of multi continent plate tectonics the proterozoic apw record comprises nine large loops with periods ranging from 270 to 110ma and averaging 200ma between 2150 and 1000 ma the loops had a similar signature and their resultant effect was to return the continental lithosphere repeatedly to a stable position with its centre at the geographic pole probably by the goldreich toomre mechanism this periodicity correlates with theoretical and experimental studies of mantle convection predicting avalanche type behaviour due to build up of instability in the surface boundary layer with cycles of 50 200 ma the consolidation of the continental lithosphere defines several phases in the history of mantle convection 1 between 3500 and 2800 ma the sialic scum resulting from differentiation of a thickened and subducting lithosphere was accumulated into the primary body by a large scale mantle convection system 2 this operated simultaneously with a subcontinental lithosphere small scale 50 100 km convection system showing a transition from bimodal to cylindrical roll mode represented by the later greenstone belts 2900 2200 ma and straight belts ca 2800 2400 ma 3 a transition to larger scale episodic subcontinental convection is represented by the proterozoic mobile belts 2200 1100 ma 4 a modification of the major suboceanic lithosphere convection is suggested by reorganization of the peripheral shields at 1100 ma and 5 a subdivision of this major system resulted in continental dispersal during lower cambrian times

early silurian paleopole for redbeds and volcanics of the king george iv lake area newfoundland redbeds and felsic and mafic volcanics of the king george iv lake area of the dunnage zone in southwestern newfoundland have been studied paleomagnetically after unfolding both hematite and magnetite carry a consistent direction of magnetization d 29 6 i 1 0 k 19 3 95 11 3 corresponding to a near equitorial paleolatitude 0 5 n 6 and yielding a paleopole at 84 6 e 35 9 n dm 11 3 dp 5 6 a positive fold test shows that the remanence was acquired before devonian deformation a positive conglomerate test and a reversal of the magnetization in a continuous redbed sequence demonstrates that the remanence was acquired at the time of formation which has been assigned to the early silurian on the basis of a u pb zircon date of 431 5 ma no other undisputed primary or pre fold remanence has been reported from atlantic canada for the late ordovician silurian period therefore from paleomagnetic data it is not yet possible to estimate the amount of post silurian movement within

implications of new upb zircon chronology to early proterozoic crustal accretion in northern sweden abstract u pb ages ranging from 1 89 to 1 78 ga are reported for zircon populations formed during major magmatic events in the proterozoic development of northern sweden the studied area includes parts of the continental and the marine svecokarelian domains of the baltic shield the ages record crustal accretion and reactivation off the margin of an exposed archaean craton and show that magmatism was broadly coeval throughout the entire north swedish portion of the svecokarelian province irrespective of regions involved a detailed chronostratigraphy is presented for the skellefte district of northern sweden and comparisons are made with the evolution in other parts of the svecokarelian province two main magmatic phases commencing at 1 89 and 1 79 ga ago are recognized the first represents large scale volcanism and plutonism which in the skellefte district was characterized by the introduction of mainly mantle derived magmas or by magmas formed from rocks with short crustal residence time in the vicinity of the archaean craton some distance to the north the crust generating processes included the mobilization of archaean sources and appear to have continued until c 1 85 ga ago a relatively long period of consolidation and denudation separates this magmatic phase from the one at c 1 79 ga ago the generation of these later migmatite associated granites in the svecokarelian province coincided in time and was possibly associated with those processes which gave rise to the initial plutonism in the neighbouring transscandinavian granite porphyry belt