paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

elevated lateral stress in unlithified sediment midcontinent united states geotechnical and geophysical indicators for a tectonic origin abstract indirect and direct geotechnical measurements revealed the presence of high lateral earth pressure ko in shallow unlithified sediment at a site in the northernmost mississippi embayment region of the central united states results from pile load and pressuremeter tests showed maximum ko values greater than 10 however the complex geologic environment of the midcontinent made defining an origin for the anomalous ko based solely on these measurements equivocal although in situ sediment characteristics indicated that indirect tectonic or nontectonic geologic mechanisms that include transient overburden loads e g fluvial deposition erosion glacial advance retreat and dynamic shear loads e g earthquakes were not the dominant cause they were unable to provide indicators for a direct tectonic generation localized stresses induced anthropogenically by the geotechnical field tests were also considered but ruled out as the primary origin a high resolution shear wave sh reflection image of geologic structure in the immediate vicinity of the test site revealed compression style neotectonism and suggested that the elevated stress was a tectonic manifestation post paleozoic reflectors exhibit a tertiary structural inversion as evidenced by post cretaceous fault displacement and pronounced positive folds in the hanging wall of the interpreted faults the latest stratigraphic extent of the stress effects i e all measurements were in the late cretaceous to tertiary mcnairy formation as well as the relationship of stress orientation with the orientation of local structure and regional stress remain unknown these are the subjects of ongoing studies

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

eclogite facies rocks of the monotonous unit clue to variscan suture in the moldanubian zone bohemian massif abstract the formation and exhumation of variscan high to ultrahigh pressure metamorphic rocks from the moldanubian zone in the bohemian massif are the subject of controversy regarding their unclear relationship to possible subduction zones in this paper we present the results of a petrological study of eclogites that occur together with serpentinites within amphibolite facies gneisses in the moldanubian zone east of the tepla barrandian block more than 100 bodies of retrogressed eclogite and serpentinite follow an approximately 250 km long sw ne trending zone in the central part of the bohemian massif together with surrounding gneisses the eclogites share a medium to low pressure amphibolite facies metamorphism p t conditions estimated for the eclogite facies stage indicate a relatively low temperature geothermal gradient similar to those observed in the saxothuringian zone which occurs north and northwest of the tepla barrandian block the presence and distribution of eclogites and comparison of their p t and age data with hp uhpm rocks in other units in the bohemian massif allow us to constrain the variscan suture which straddles the se of the tepla barrandian block the existence of this suture in relation to available geotectonic models and its possible continuation through other allochthonous units along the european variscan belt are discussed

fission track evidence of the cenozoic thermal history of the idaho batholith abstract apatite and zircon fission track data are combined with published k ar biotite dates from the idaho batholith to outline the cenozoic thermal history of central idaho apatite apparent ages reveal that parts of the batholith have been at temperatures below 100 c for over 50 ma suggesting that much of the batholith has remained at shallow crustal levels throughout cenozoic time eocene plutonism and attendant hydrothermal circulation resulted in thermal resetting of apatite and zircon dates from near the plutons concordant zircon and biotite dates indicate that hydrothermal systems associated with eocene plutonism operated at shallow crustal levels so that cooling occurred rapidly as geothermal activity waned the calculated depth to the 105 c apatite annealing isotherm suggests that as much as 3 km of erosion has occurred in deep canyons during the last 10 ma presumably associated with final erosion of the batholith following the establishment of drainage at low elevation along the snake river plain

magnetic properties of granitic rocks from the southern half of the idaho batholith influences of hydrothermal alteration and implications for aeromagnetic interpretation the measured magnetic susceptibilities of rocks from more than 600 outcrops in the southern idaho batholith have a good spatial correspondence with the observed intensities of magnetic anomalies koenigsberger ratios are low 0 3 in 75 of cored samples indicating that remanent magnetizations are unimportant relative to induced magnetizations which are related to susceptibilities in aeromagnetic anomaly interpretations in this region extremely low susceptibilities 25 10 5 within the eastern and western parts of the batholith are the sources of positive aeromagnetic anomalies the associated lithologies include mesozoic tonalite and granodiorite as well as a diverse tertiary group of epizonal stocks and batholiths intense meteoric hydrothermal activity associated with the emplacement of these tertiary plutons caused a reduction in in some rocks although coarse igneous magnetite is surprisingly resistant to hydrothermal corrosion and oxidation however under conditions of lower water rock ratios a significant increase in commonly occurred in the leucocratic plutons generally reflecting growth of hydrothermal magnetite because the measured susceptibilities do not necessarily represent more than the uppermost part of the 14 to 20 km thick crust that gives rise to magnetic anomalies an apparent discordance between susceptibility measurements and susceptibilities inferred from aeromagnetic data can occur such discordance patterns occur near the margins of the batholith where granitic rocks are thin and also in hydrothermally altered regions where subsurface tertiary plutons are present magnetic models suggest that the idaho batholith thickens to 20 km in its east central part which is underlain by enormous tertiary intrusive bodies

palaeogeographic significance of mid silurian palaeomagnetic results from southern britain major revision of the apparent polar wander path for eastern avalonia summary wenlock c 430 ma lavas from the east mendips inlier southern briain are characterized by either single component magnetizations type 1 or a more complex system of three magnetization components type 2 during stepwise thermal demagnetization the type of demagnetization behaviour is related to changes in magnetic petrology within the lava pile the lower and intermediate unblocking temperature components of type 2 represent magnetic overprints of tertiary recent and hercynian mid carboniferous origin respectively the primary nature of the high unblocking temperature magnetization component types 1 and 2 dec 095 c inc 24 ag5 8 8 is demonstrated by positive agglomerate tests conversely an overlying upper old red sandstone sequence did not provide primary magnetizations the magnetic signature is governed by syn tectonic hercynian remagnetizations the primary silurian data demonstrate that the east mendips inlier experienced considerable clockwise rotation probably of the order of 80 prior to partial remagnetization during hercynian deformation and folding the silurian palaeopole vgp 13 n 271 e dp dm 5 9 cannot therefore be used for apparent polar wander path construction nevertheless the palaeolatitude estimate 13 f 5s suggests that southern britain eastern avalonia was at comparable mid silurian latitudes to baltica and northern britain 10 20 s both the iapetus ocean and tornquist sea therefore which separated southern britain from laurentia and baltica respectively in ordovician times were closed by wenlock time as ordovician and silurian palaeomagnetic data from baltica and eastern avalonia now record a comparable northwards translation from high southerly to near equatorial latitudes during this time we postulate pre wenlock collision of these blocks to form balonia baltica eastern avalonia a new apparent polar wander path for eastern avalonia constructed on the basis of this palaeogeographic model is presented

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

petrogenesis of the proterozoic athabasca basin northern saskatchewan canada and its relation to diagenesis hydrothermal uranium mineralization and paleohydrogeology abstract in the athabasca basin proterozoic sandstones unconformably overlie archean and aphebian rocks multiple fluid events involving isotopically and chemically distinct fluids which have migrated laterally for considerable distances and along fault zones have produced a paragenetically identifiable assemblage of clay silicate and oxide minerals in the basin and basement rocks altered sandstones and the underlying metasedimentary rocks proximal to fault zones host high grade u ni co unconformity type uranium ore deposits and contain an alteration assemblage of highly crystalline 2m 3t illite 1m kaolinite clinochlore and sudoite hematite dravite and euhedral quartz in the sandstones 18o values of coeval quartz hematite quartz illite quartz dravite illite hematite and kaolinite hematite fluid inclusion temperatures in diagenetic quartz and d values of 1m kaolinite 2m 3t illite dravite and in fluid inclusions in the quartz indicate formation from a basinal fluid that had d values of 60 10 and which underwent salinity temperature and 18o increases from 5 to 34 wt nacl 120 to 240 c and 5 to 3 respectively as a result of increasing burial depth and sustained water rock interactions with the basinal sediments over substantial time periods 18 o 16 o and d h ratios of sudoite and clinochlore in meta sedimentary basement rocks within the alteration haloes indicate formation from a basement derived fluid with 18o and d values of 4 4 and 30 10 respectively that was isotopically distinct from the basinal fluids 18o and d values of late kaolinites in reactivated fault zones indicate formation mainly from low temperature 50 25 c fluids having 18o and d values of 16 and 130 respectively similar to relatively modern meteoric waters in the area some kaolinites have 18o and d values which suggest they formed from meteoric waters at mid latitudes and have experienced varying degrees of oxygen and hydrogen isotope exchange with relatively modern meteoric waters in the re activated faults illite sudoite and dravite have d values that vary by 100 without concurrent changes to their 18o values indicating preferential hydrogen isotope exchange with relatively modern meteoric waters at low temperatures and under conditions of high integrated water rock ratios rb sr isochron and model ages of 1477 57 and 952 ma respectively for the crystalline 2m 3t illites indicate the timing of the high temperature basinal fluid events associated with fluid mixing rb sr and k ar ages on illites with the lowest d values are as young as 300 ma and indicate late stage resetting of their radiogenic isotope systematics as a result of substantial interactions with relatively modern meteoric waters the evolution of sr isotopic compositions of fluids in the basin and basement rocks as reflected by the 87 sr 86 sr ratios of chlorite illite and dravite indicates that mixing of two isotopically distinct fluids in the vicinity of faults was the process by which uranium was precipitated similar to that suggested by oxygen and hydrogen isotope systematics in the clays and silicate minerals nd ratios in the high temperature diagenetic and hydrothermal clay and silicate minerals and the cogenetic uranium ores suggest they all have formed from similar fluids which have interacted substantially with the basin sediments in general the isotopic chemical microthermometric and petrologic data indicate that the athabasca basin has had a protracted fluid history comparable to that documented for more modern sedimentary basins the data clearly show that the major sandstone aquifers have been affected by widespread lateral flow of diagenetic fluids over distances of hundreds of kilometers and also that these fluid migration paths have been modified by cross formational fluid flow near active fault zones

oxygen 18 oxygen 16 and d h studies of plutonic granitic and metamorphic rocks across the cordilleran batholiths of southern british columbia hydrogen and oxygen isotope ratios of 500 samples mainly from granitic plutons were measured along a 700 km e w traverse across the accreted terranes of southern british columbia latitudes 49 52 n despite the geological complexity and range of intrusive ages late triassic to tertiary and although there are steps in the isotopic values at some geologic boundaries e g across the strait of georgia two clear patterns emerge 1 the 18 o 16 o and d h ratios of the waters involved in hydrothermal interactions with the granitic rocks show a regular eastward trend of depletion in d and 18 o enormous areas were affected by the hydrothermal processes but the most intense alteration is localized along major north trending lineaments e g okanagan lake 2 independent of the hydrothermal effects the primary 18 o values of the granitic rocks also change systematically eastward from 7 0 to 8 5 in vancouver island reaching a minimum of 5 5 to 7 0 in the western and central coast plutonic complex then increasing progressively from the eastern coast batholith to the okanagan batholith and attaining a maximum of 10 0 to 12 0 in the nelson batholith two groups of samples are unique in their high d values the first group is represented by two geographically isolated batholiths guichon and thuya that were not affected by the tertiary meteoric hydrothermal systems and that have therefore preserved their early jurassic to triassic k ar ages the second group is represented by the jurassic plutons of vancouver island there the hydrothermal fluids were both d rich and 18 o rich 18 o 0 as evidenced by the fact that feldspars in the altered granites are enriched in 18 o relative to coexisting quartz both anomalies can be explained if these terranes were located closer to the equator and or in a maritime environment at the time of intrusive and hydrothermal activity in agreement with available paleomagnetic data excluding these anomalous areas two distinct ages of meteoric hydrothermal activity can be identified cretaceous in the west and early to mid tertiary in the east the isotopic trends in the rocks are similar to the present day patterns of meteoric waters in the region with one primary difference the paleowaters are enriched in d by about 20 compatible with a northward translation of these terranes a climatic change or both the similarities of the patterns suggest a topography similar to that of the present day a mountain chain along the coast during the early tertiary the whole rock 18 o values of the granitic rocks determined by extrapolation of the trends of 18 o quartz versus 18 o feldspar to correct for hydrothermal alteration effects suggest that the main part of the coast plutonic complex formed from magmas similar to those in oceanic island arcs derived from basaltic crust or from an upper mantle source toward the east an increasingly larger component of sedimentary rocks must be present in the source regions of the granitic magmas it may be significant that right at the edge of the north american craton there is a drop in the 18 o of the plutons from 11 down to 9 5 suggesting involvement of old granitic crust instead of sedimentary rocks these new data from southern british columbia confirm and amplify previous studies that have documented 1 the existence of widespread and pervasive hydrothermal effects throughout the north american cordillera and 2 regular west to east asymmetries in the amounts of different kinds of parent rocks in the source regions from which these mesozoic and cenozoic granitic magmas were derived

terrane wrecks coupled oroclines and paleomagnetic inclination anomalies abstract three major orogenic belts that are host to coupled oroclines terrane wrecks are likewise characterized by or directly associated with paleomagnetic inclination anomalies 1 the north american cordillera 2 the western european variscan belt and 3 the eastern european alpine system implied paleolatitudinal discrepancies in each orogen are of similar magnitude to the translation required by orogen parallel shortening during oroclinal formation the alaskan coupled oroclines accommodated 3000 km of orogen parallel shortening in the north american cordillera where primary remanent paleomagnetic inclinations in late cretaceous rocks require the bulk of the belt to have been translated 2000 to 3000 km northward relative to cratonic north america the iberian coupled oroclines accommodated 1100 km of orogen parallel shortening in the western european variscan belt within which mid paleozoic paleomagnetic inclinations are shallow with respect to stable gondwana the implied existence of a 3500 km wide paleotethys ocean between the variscan autochthon and the north gondwana margin is seemingly at odds with paleontological constraints the carpathian balkan oroclines accommodated 1000 km of orogen parallel shortening in the eastern alpine system directly north northeast of an eastern mediterranean region where primary remanent inclinations in late oligocene to miocene rocks require northward translation between 500 and 1000 km palinspastic restoration of the alaskan and carpathian balkan oroclines fully accounts for the translations recorded by the cordilleran and eastern mediterranean inclination anomalies respectively restoration of the iberian coupled oroclines confirms the existence of paleotethys but only accounts for one third of the paleolatitudinal discrepancy recorded by the variscan inclination anomaly fully resolving the variscan inclination anomaly requires 1 a paleomagnetic data set of greater temporal and spatial extent 2 refinement of the geometric and geological continuity of the variscan belt across western europe

a devonian palaeomagnetic pole for armorica summary the palaeomagnetic study of siluro devonian redbeds of the san pedro formation at cabrillanes asturias spain reveals a high temperature and pre tectonic magnetization determined by intersection of great circles as well as conventional averaging fisher this magnetization is carried by the haematitic pigment and is therefore of chemical and postdepositional origin the formation mean direction d 113 i 34 ag5 10 is interpreted as representing a record of a devonian palaeofield the corresponding pole after correction for the opening of the bay of biscay and the tectonic closure of the ibero armorican arc is in good agreement with contemporaneous european poles confirming that the arinorica plate had already joined the laurentia baltica assembly at that time

a stable isotope study of retrograde alteration in sw connemara ireland dalradian metamorphic rocks lower ordovician meta igneous rocks mgs and caledonian granites of the connemara complex in sw connemara all show intense retrograde alteration alteration primarily involves sericitization and saussuritization of plagioclase the alteration of biotite and hornblende to chlorite and the formation of secondary epidote the alteration is associated with sealed microcracks in all rocks and planes of secondary fluid inclusions in quartz where it occurs and was the result of a phase of fluid influx into these rocks in hand specimen k feldspar becomes progressively reddened with increasing alteration mineralogical alteration in the mgs and caledonian granites took place at temperatures 275 15 c and in the mgs pfluid is estimated to be 1 5 kbar during alteration the d values of alteration phases are 18 to 29 fluid inclusions 47 to 61 chlorites and 11 to 31 epidotes chlorite 18o values are 0 2 to 4 3 while 18o values for quartz k feldspar pairs show both positively sloped mgs and highly unusual negatively sloped caledonian granites arrays diverging from the normal magmatic field on a plot the stable isotope data show that the fluid that caused retrogression continued to be present in most rocks until temperatures fell to 200 140 c the retrograde fluid had d 20 to 30 in all lithologies but the fluid 18o varied both spatially and temporally within the range 4 to 7 the fo2 of the fluid that deposited the epidotes in the mgs varied with its 18o value with the most 18o depleted fluid being the most oxidizing the d values together with low 0 18o values for the retrograde fluid in some lithologies indicate that this fluid was of meteoric origin this meteoric fluid was probably responsible for the alteration in all lithologies during a single phase of fluid infiltration the variation in retrograde fluid 18o values is attributed to the effects of variable oxygen isotope shifting of this meteoric fluid by fluid rock interaction infiltration of meteoric fluid into this area was most likely accomplished by convection of pore fluids around the heat anomaly of the galway granite soon after intrusion at 400 ma however convective circulation of meteoric water and mineralogical alteration could possible have occurred considerably later

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

paleomagnetics of ringerike old red sandstone and related rocks southern norway implications for pre carboniferous separation of baltica and british terranes abstract paleomagnetic study of two stratigraphic sections of the upper silurian lower devonian ringerike sandstone oslo area norway indicate a straightforward multicomponent nrm for these rocks a lower temperature magnetization component gives a southwest and up reversed polarity in situ direction yields a negative fold test i e post folding at the 99 confidence level and is nearly identical to directions obtained from overlying permo carboniferous lavas a high temperature magnetization component exhibits both normal and reversed polarity directions passes a fold test at the 95 confidence level and is supported by a baked contact test which indicates a pre intrusion age of magnetization this characteristic high temperature component was successfully isolated in 79 samples from 19 sites within the ringerike succession results indicate a tilt corrected characteristic magnetic direction of declination 26 inclination 16 which is consistent with earlier work this direction corresponds to a time integrated paleopole position of 19 n latitude 164 longitude d p 4 8 d m 9 3 when compared to previous results from rocks of similar age from the british isles an approximately 20 latitudinal displacement between britain and baltica during late silurian early devonian time is suggested

paleomagnetic constraints on the assembly of the old red continent abstract a model is proposed on the basis of paleomagnetic and geological arguments for the assembly of the old red continent which in the devonian comprised north america the baltic shield and russian platform as well as a third element called armorica the latter is thought to have included most of hercynian europe and southern england and wales as well as possibly the avalonian terranes of northeastern north america and perhaps some parts of the atlas ranges and moroccan meseta the model envisions a late ordovician taconic collision of north america and armorica which are joined in the late silurian by the baltic shield and russian platform not until the late early carboniferous did gondwana join the old red continent in a collision responsible for a large internal megashear parallel to the caledonian orogenic belt as well as the hercynian alleghenian orogeny subduction of the large siluro devonian ocean between gondwana and the old red continent is held responsible for the mid devonian acadian orogeny

insights into hydrological regime of lake vostok from differential behavior of deuterium and oxygen 18 in accreted ice 1 we use isotopic data deuterium and oxygen 18 of the recently recovered deepest vostok ice core section down to 3650 m depth to study processes leading to the formation of lake ice and the hydrological regime of subglacial lake vostok the significant variability of the lake ice isotopic content implies fluctuations in physical conditions of ice formation mainly volume and or growth rate of frazil ice crystals as well as variations of the isotope composition of the freezing water the latter implies a poor mixing of the source waters glacier melt and hydrothermal water with the water of the main lake body poor mixing within the lake may have important consequences for the lake s chemical and gas balance and particularly for its microbiological content a poorly mixed lake may provide ecological niches where microbial life can hide from high oxygen concentrations likely typical for the lake we also show that the isotopic content of the main lake s input meltwater significantly differs from that of the output lake ice which can be explained by the contribution of an additional hydrothermal source this latter conclusion is supported by the observed noncovariant behavior of deuterium and oxygen 18 isotopes in the lake ice

oxygen isotope map of the giant metamorphic hydrothermal system around the northern part of the idaho batholith u s a abstract determinations of 18o values from 100 outcrops of belt supergroup wallace fm metasedimentary rocks in the idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange low grade argillites at large distances 60 km from the idaho batholith have high 18o values 15 compatible with their probable primary values pelitic rocks with anomalously low 18o values of 8 7 to 12 7 occur in the following zones 1 in a 5000 km2 zone of schist and gneiss peripheral to the idaho batholith generally coincident with high grade sillimanite bearing assemblages 2 in high grade metasedimentary roof pendants within the idaho batholith 3 peripheral to small cretaceous stocks and 4 within and near the scapolite bearing zone south west of st regis on 18o 18o plots data from coexisting minerals define trends with unit slopes indicating that the reductions in 18o occurred under high grade metamorphic conditions this metamorphism culminated in the emplacement of the idaho batholith probably as a consequence of profound crustal thickening associated with the cretaceous accretion of the wallowa seven devils arc terranes with north america the huge low 18o region is bounded by a steep 18o gradient 0 1 0 5 km that occurs in low grade rocks along and near the lewis and clark line well below the biotite isograd this boundary zone may be analogous to but is not nearly as sharp as those of meteoric hydrothermal systems in many regions the important ore deposits of the coeur d alene district are located in this peripheral zone suggesting that the metamorphic hydrothermal system may have been intimately involved in their formation in addition the metamorphic hydrotermal system in idaho is larger deeper and higher in temperature than typical meteoric hydrothermal systems and it involved fluids with much higher 18o values that were probably dominantly derived from formation waters accordingly this system produced rocks with 18o values similar to those of the idaho batholith and mineral assemblages that approach isotopic equilibrium under high temperature conditions

palaeomagnetism of the ordovician moulin de chateaupanne formation vendee western france summary a palaeomagnetic study of the moulin de chateaupanne formation reveals two characteristic components of magnetization the first one isolated by thermal treatment at intermediate temperatures is strongly resistant to chemical leaching its direction d 217 i 25 ag5 5 5 and the corresponding pole position lat 21 s long 320 e are typical of the hercynian overprints in the armorican massif the second component carried by fine grained haematite presents a steep downward direction d 228 i 81 ag5 6 pole position lat 34 n long 343 e already recognized in other ordovician rock units from the same area it is resistant to high temperature demagnetization treatment and is believed to be of chemical origin as assessed by a conglomerate test a mutual consistency test with the nearby thouars massif suggests that the tilt of the formation has a sedimentary origin and that the thouars massif has not suffered significant tilt during the hercynian orogenic events

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

upper ordovician palaeogeography of the bohemian massif implications for armorica summary the palaeogeographic position of armorica in the upper ordovician is still rather unclear due mainly to the lack of reliable palaeomagnetic data to help resolve this a palaeomagnetic study of caradocian and ashgillian sediments and volcanics of the barrandian basin in the central bohemian massif czech republic has been carried out a total of 29 sites were sampled including four from a diabase sill intruded into hirnantian age sediments thus providing a baked contact test three components of magnetization labelled a b and c have been resolved in addition to a low temperature component which corresponds to the present day direction of the local geomagnetic field d i 360 67 component a southerly and shallow before bedding correction is only sporadically present and corresponds to a remagnetization event of permo carboniferous age which is well known throughout hercynian europe component b present as stable endpoints in the sill and as a secondary component in the host rock sandstones yields an overall mean direction of 360 39 95 9 3 k 52 3 in situ and 355 21 95 9 3 k 52 3 after bedding correction component c identified as stable endpoints in sedimentary and volcanic rocks passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization the overall mean direction for c is 183 59 95 9 5 k 30 3 nine sites after bedding correction which corresponds to a palaeopole position of 80 s 360 e this translates into palaeolatitudes of 40 s for the area studied in latest ordovician times but indicates large amounts of rotation up to 170 of the bohemian massif before final consolidation of hercynian europe although such large amounts of rotation are difficult to accommodate they are consistent with palaeomagnetic results obtained from lower ordovician and upper silurian rocks elsewhere in the barrandian basin if the bohemian massif is interpreted as being an integral part of the armorican microplate then these results indicate that prior to caradoc times armorica rifted away from the northern margin of gondwana and that by ashgillian times it was located at intermediate palaeolatitudes

palaeomagnetic constraints upon the palaeogeographic position of the baltic shield in the ordovician the palaeomagnetic analysis of the ordovician limestones in southern scandinavia 75 sites has given the following results 1 in the southern part of the oslo region all ordovician sediments were remagnetized during the lower permian by a thermo chemical mechanism triggered by the emplacement of the permian volcanic and intrusive suite 2 in the northern part of the oslo region the upper caradoc mjosa limestone carries a high temperature component decl 53 incl 40 95 25 4 sites with corresponding palaeopole at 4 s 143 e 3 in southern sweden all lower ordovician orthoceras limestones sampled in dalarna narke scania vaster gotland oland and oster gotland display the same well defined southeast steep high temperature components leading to a palaeopole at 30 n 55 e a95 9 in areas with late palaeozoic volcanic activity scania vaster gotland this high temperature component is partially overprinted by a remagnetization associated with dolerite emplacement according to these results baltica was situated in temperate southern latitudes 50 s in the early ordovician and has drifted northwards at a latitudinal drift rate of 4 to 7 cm year to reach tropical latitudes 20 s by the late ordovician to late silurian this interpretation is in agreement with previous models based on the occurrence of ordovician reef structures and sheds new light on the early palaeozoic position of baltica relative to other continental blocks

new palaeomagnetic constraints on the position of central bohemia during early ordovician times summary apparently conflicting data within the palaeomagnetic database suggest that bohemia was at much shallower palaeolatitudes than the armorican and iberian massifs in ordovician times in order to resolve this controversy a palaeomagnetic study of arenig and llanvirn volcanic and sedimentary rocks from the barrandfan basin central bohemia has been carried out this basin comprises a sequence of unmetamorphosed lower palaeozoic rocks the main folding of which occurred in late devonian to early carboniferous times the samples collected l sites 153 samples were subjected to stepwise thermal and alternating field treatment eleven sites yielded reliable magnetic directions with analysis of the results using standard principal component and great circle analyses up to three directions of magnetiza tion can be identified in these rocks the first termed a is generally removed below 200 c although in some cases it persists up to 450 oc in the volcanic rocks it is approximately parallel to the present day earth s magnetic field in the study area dec inc 360 67 and is thus thought to be of recent origin the second direction identified termed b is isolated at intermediate blocking temperatures 150 350 c and yields an overall in situ mean direction of 195 8 k 22 3 a 95 13 1 seven sites these directions fail the fold test of mcfadden 1990 and correspond to a palaeopole position of 34 s 356 e this coincides with the late carboniferous sector of the european apparent polar wander path and thus the b direction is interpreted as being a secondary overprint of this age the highest blocking temperature direction termed c is identified at temperatures between 350 and 450 600 c it passes the fold test resulting in an overall mean direction of 312 83 k 21 9 a 95 14 6 six sites after bedding correction this direction is interpreted as being representative of the early ordovician palaeomagnetic field direction in the bohemian massif and yields a palaeopole position of 58 n 355 e this translates into palaeolatitudes of 76 s for bohemia thus demonstrating that the bohemian massif was at similar peri polar latitudes to the rest of armorica during early mid ordovician times and formed part of the northern margin of gondwana

palaeomagnetism of the ibero armorican arc and the hercynian orogeny in western europe van der voo1 has pointed out that further advances in the understanding of the hercynian orogeny require palaeomagnetic data from well studied orogenic areas we report here palaeomagnetic results of palaeozoic rocks taken in brittany portugal and spain in the ibero armorican arc iaa a large geological feature considered to be a key area for the hercynian orogeny in western europe they show evidence that the iaa formed in two stages extending results obtained in spain by ries et al 2 giving precise constraints for the former shape of the arc we can use pre hercynian palaeomagnetic directions found in spain and portugal to follow its evolution during palaeozoic times this megastructure linking brittany to iberia squeezed between gondwana and the northern continents during the carboniferous did break away from the former block during ordovician times

mesozoic alteration of permian volcanic rocks thuringer wald germany ar sr and o isotope evidence abstract besides older granites and gneisses the thuringer wald in central europe consists predominantly of altered permian volcanic and pyroclastic rocks the alteration history of these volcanic rocks was investigated by k ar 39 ar k 40 ar rb sr 18 o d and x ray studies on feldspar and biotite separates alteration is accompanied by increases in the triclinicity and 18 o values 14 to 19 of alkali feldspars an increase in the 18 o values of low potassium biotites 10 and in sericitized plagioclases 12 to 19 and also the generation of apparent mesozoic isotopic ages in contrast volcanic biotites with primary magmatic 18 o signatures still yield early permian ages and biotites of the granitic basement give carboniferous ages the observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines 18 o 5 at low temperatures 200 to 100 c during tectonic fracturing of the basement as a result sanidines were replaced by porous microclines which were subject to isotope exchange and ar loss feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes which may be due to their lower permeabilities plateau shaped 40 ar 39 ar age spectra of volcanic alkali feldspar indicate complete resetting of the k ar system whereas staircase spectra point to partial retention or repeated ar loss because the feldspars represent mixtures of microcline and sanidine remnants the apparent ages 260 to 160 ma represent maximum ages for the time of replacement rb sr feldspar model ages 180 ma or 230 to 170 ma respectively support the k ar data the minimal 40 ar 39 ar spectra ages are concordant with both k ar and 40 ar 39 ar sericite model ages 160 to 150 ma of sericitized plagioclases the model ages favour jurassic episodic fluid reactions with alkali feldspars and plagioclases in the thuringer wald

tectonic and paleogeographic interpretation of the paleomagnetism of variscan and pre variscan formations of the bohemian massif with special reference to the barrandian terrane abstract paleomagnetic data from variscan and pre variscan formations of the bohemian massif bm have been interpreted tectonically and paleogeographically evaluated with respect to the european paleomagnetic results the interpretation of data from the bm was preceded by the evaluation of the effect of the trans european suture zone tesz on the dispersion of the european paleomagnetic pole positions during the variscan orogeny stability of the european lithospheric plate is well documented by paleomagnetic results for the early permian to the quaternary rocks of the regions between the ural mountains and great britain north of the alpine tectonic belt for the late carboniferous and earlier times the stable plate is defined in the regions ne of the tesz in the east european craton only late carboniferous and older rocks from the regions sw of the tesz largely occupied by the west european variscides show tectonic deformations controlled by prominent horizontal rotations these rotations can be studied on a theoretical model simulating the distribution of pole positions controlled by rotation of rock formations about a vertical axis in the bm horizontal paleotectonic rotations are well documented for variscan and pre variscan rocks they reach several tens of degrees up to 180 in extreme cases of mostly clockwise sense the study of the tectonics and paleogeography of cambrian to devonian formations of the barrandian area which is considered a peri gondwana terrane with affinities to armorica may serve as a case history of the study of a terrane incorporated into a stable lithospheric plate here the barrandian terrane became a part of the european plate as a component of the emerging pangea supercontinent in the final phase of the variscan orogeny high almost peri polar paleolatitudes for the cambrian rocks of the barrandian area are within the limits of data scatter in agreement with the presumed position of gondwana and armorica rocks of ordovician age did not produce consistent results they will require further paleomagnetic and paleoclimatologic studies in relation to the data for the late cambrian and early silurian rocks of the barrandian area the analysis of data presented here shows that paleomagnetic declinations of variscan and pre variscan formations reflect paleotectonic rotations following the formation of the respective rocks including rotations induced by the variscan orogeny and rotation of the european plate to which the rocks became incorporated therefore paleomagnetic declination of none of the pre variscan formations can be identified with paleotectonic rotation either of the barrandian terrane or even of the whole bm magnitudes of horizontal paleotectonic rotations in the bm affected by variscan tectonism are notably similar to those in the western carpathians affected by alpine deformation senses of rotation are however different with clockwise rotation prevailing in the bm and counterclockwise rotations prevailing in the western carpathians recognition of the effect of horizontal rotations of rock formations if located in tectonically mobile belts and incorporated into a stable plate during their later history should be respected while deriving the apparent polar wandering path apwp

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

palaeomagnetic and rock magnetic study of devonian olistoliths set in early carboniferous flysch west sudetes poland summary the upper devonian greywacke mudstone and limestone olistoliths from the zdanow bardo mlynow zbm allochthonous complex of the gory bardzkie gb west sudetes have been sampled for palaeomagnetic investigations 46 hand samples and 30 drill cores from seven sites the random distribution of the olistolithic blocks within early carboniferous flysch provides an opportunity for the megaconglomerate test microscopic and rock magnetic studies showed that pyrrhotite and magnetite are the carriers of magnetic remanence thermal and af demagnetization revealed four components of characteristic remanence the c component found in one site only yields an in situ pole position of 19 s 340 e close to early carboniferous data for baltica the p component also found in one site yields an in situ pole position of 42 s 339 e which coincides with late permian poles for baltica the r component of moderate inclinations and northwesterly and northeasterly declinations found in all but one site is probably partial remagnetization of a devonian component during carboniferous and permian times the d component found in all sites shows significant dispersion of declination of site mean directions in geographical and stratigraphic coordinates but inclinations after tilt correction are consistent with inclinations expected for the middle late devonian calculated for the sudetes from the palaeopoles for baltica significant dispersion of declination is expected for primary magnetization of olistoliths distributed at random within flysch the fold test for inclination only applied to the site mean directions of the d component for the zbm complex is positive the d component is therefore interpreted as representative of the middle late devonian local field inclination the calculated palaeolatitudes 15 s 7 s indicate that the west sudetes were positioned in that time at the same latitude as the southwestern margin of baltica

changes in the hoar isotope composition of clays during retrograde alteration abstract k ar ages of illite alteration associated with middle proterozoic athabasca unconformity type u deposits in saskatchewan range from 414 to 1493 ma the k ar ages correlate with water contents and d values such that illites with young k ar ages have d values as low as 169 and water contents as high as 7 7 wt whereas illites with older ages have d values near 70 and water contents near 4 wt water extracted at 400 c from illites with low d values and high water contents has low d and 18 o values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration the alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the k ar ages and introduction of excess structural water in the form of interlamellar water the interlamellar water is enriched in 18 o by about 7 per mil relative to the water that physically surrounded the clay particle further alteration decreases the d value and increases the 18 o value of the illite by isotopic exchange between the mineral and the interlamellar water although the chemical compositions and xrd patterns of the altered illites indicate that no detectable smectite component is present in the samples the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed layer illite smectite by retrograde alteration of pure illite the wide variation of d values of chlorite and kaolinite from these u deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial the general association of altered clays with areas containing the highest concentrations of u is probably related to localized permeability within the ore zone

effects of regional uplift deformation and meteoric hydrothermal metamorphism on k ar ages of biotites in the southern half of the idaho batholith k ar ages of biotites from the southern half atlanta lobe of the idaho batholith are younger than the true emplacement ages of the rocks for mesozoic plutons with true ages probably greater than 100 m y the k ar ages range from 95 m y along the east and west margins of the batholith to 37 m y in the east central zone there is a conspicuous age gap between 46 and 53 m y however a north trending group of epizonal eocene plutons in the east central zone has k ar ages of 44 6 m y values which closely approximate the crystallization ages these eocene plutons which were emplaced at shallow levels and underwent rapid cooling are responsible for widespread 10 000 km2 meteoric hydrothermal alteration of the mesozoic country rocks partial chloritization and lowering of the k2o deuterium and 18o contents of the biotites accompanied the hydrothermal activity although the intensity of the hydrothermal effects is geographically correlated with the degree of lowering of the k ar ages the production of anomalously young k ar ages is attributed in part to regional uplift and doming in addition to the hydrothermal metamorphism in any given part of the atlanta lobe the k ar ages of the higher elevation samples become systematically older with increasing elevation this effect is observed in almost all of the biotites both those that have undergone intense hydrothermal alteration and those that show no evidence of any kind of hydrothermal effects the only exceptions are biotites that underwent complete ar loss such samples occur only at low elevations and within the contact metamorphic aureoles of the eocene plutons age versus elevation trend lines are linear with slopes that indicate uniform late cretaceous and early tertiary uplift 0 10 0 14 mm yr and cooling of the terrane the trend lines have almost identical slopes throughout the region but the positions of the trend lines shift systematically upward to higher elevations from west to east indicating that subsequent doming and faulting of the terrane modified the relative elevations of different subregions the zone where this more recent uplift is most prominent almost coincides with the axis of the eocene intrusions furthermore the k ar age versus elevation trend lines are systematically truncated both at low elevations and close to contacts with eocene plutons apparently because of a temperature increase due to widespread hydrothermal heating of the mesozoic rocks by the eocene batholiths in rocks located at present day elevations below about 1000 1500 m in the central part of the atlanta lobe biotites lost practically all their radiogenic ar during the eocene intrusive event

paleogeographic evolution of the united states pacific northwest during paleogene time paleogeographic reconstructions for oregon and washington during paleogene time illustrate a major transition from a dominantly compressional prior to middle eocene time to an extensional tectonic regime this transition resulted in the development of three phases of paleogene basin evolution in the united states pacific northwest during the initial phase basins formed along the continental margin during collision of oceanic islands sediments in these basins were derived from nearby orogenic highlands the second phase of basin development began in middle eocene time and consisted of rapid subsidence of individual basins that formed within a broad forearc region nonmarine basins that formed during this phase were caused by extension possibly associated with transcurrent faulting rapid sedimentation in both marine and nonmarine basins during this time consisted dominantly of sandstone derived from cretaceous plutonic sources far to the east the final stage of basin development was the modification

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

shear wave studies in glacial till within a high resolution shallow reflection survey program in saskatchewan canada s waves were produced using a single seismo electric blasting cap and were found to be distinguishable from surface wave phases the local glacial deposits have average velocities of 450 m s vp vs ratios average 3 6 in these sequences but they vary laterally according to the velocity analyses done in two boreholes drilled along the seismic line vertical resolution for s wave reflections are 0 75 m in the vertical seismic profiling vsp data and 1 5 m in the cdp data yet the s wave cdp results are still better than corresponding p wave data which had a vertical resolution of 2 6 m s wave anisotropy is inferred in the glacial deposits on the basis of particle motion analysis and interpretations of s wave splitting however the amount of observed splitting is small 2 6 ms over 5 10 m and could go undetected for seismic surveys with larger sampling intervals vsps indicate that s wave reflectivity is caused by

silurian paleogeography of armorica new paleomagnetic data from central bohemia due to the lack of reliable paleomagnetic data from armorica for mid ordovician to late devonian times the drift history for this tectonic element is poorly understood in order to help rectify this problem a paleomagnetic study of upper silurian volcanic intrusive and sedimentary rocks from the barrandian basin of the central bohemian massif czech republic has been carried out this basin comprises a sequence of unmetamorphosed lower paleozoic rocks the folding of which has an uppermost age limit of early carboniferous stepwise thermal demagnetization of the samples collected 182 samples from 21 sites and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization the first labelled a is in the direction of the present day local earth s magnetic field d i 360 67 and is thus interpreted as being of recent origin the other two directions termed b and c comprise the intermediate and high blocking temperature directions respectively the overall in situ mean direction obtained for b identified between 150 and 320 c is 195 8 d i k 18 5 95 13 2 eight sites it fails the fold test and yields an in situ paleo south pole position of 42 s 354 e this coincides with the late carboniferous segment of the apparent polar wander apw path for stable europe and is considered to be a secondary overprint of this age the third magnetization termed c is identified between 320 and 600 c gives an overall mean direction of 208 40 k 96 8 95 5 7 eight sites after bedding correction it passes fold and reversal tests on both local and regional levels and is interpreted as representative of the late silurian paleofield direction in the bohemian massif it yields a paleo pole position of 55 s 325 e assuming a normal polarity the inclination value obtained translates into paleolatitudes of 23 s for this part of armorica and implies a position adjacent to the southern margin of baltica this therefore indicates closure of the intervening ocean by late silurian times however adopting this polarity also implies large scale up to 140 anticlockwise rotations of the bohemian massif prior to deformation in the early carboniferous

failed silurian continental rifting at the nw margin of gondwana evidence from basaltic volcanism of the prague basin tepla barrandian unit bohemian massif the silurian volcanic rocks of the prague basin represent within plate transitional alkali to tholeiitic basalts which erupted in a continental rift setting through the thick cadomian crust of the tepla barrandian unit bohemian massif despite the variable often intense alteration resulting in post magmatic replacement of the basalt mass due to carbonatization the geochemical signatures of silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting the studied interval of silurian volcanic activity ranges from wenlock homerian 431 ma to late ludlow gorstian 425 ma with a distinct peak at the wenlock ludlow boundary 428 ma trace element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source wenlock basalts are similar to alkaline oib with depleted radiogenic nd signature compared to ludlow basalts which are rather tholeiitic emorb like with enriched radiogenic nd signature the correlation of petrogenetically significant trace element ratios with nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous possibly two component mantle source during the wenlock ludlow melting lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills the latter represent olivine rich cumulates of basaltic magmas of probably predominantly ludlow age meimechites with dolerites and to a lesser extent some lavas were subject to alteration due to wall rock fluid interaction the trigger for the wenlock to ludlow 431 425 ma extension and related volcanism in the prague basin is related to far field forces namely slab pull regime due to progressive closure of the iapetus ocean the main stage of the baltica laurentia collision then caused the prague basin rift failure at ca 425 ma that has never reached an oceanic stage

variations in geometric measures of topographic surfaces underlain by fractured granitic plutons silicate melts impose systematic fracture patterns on their hosts and their own subsolidus portions as they crystallize and cool to ambient conditions weathering of these plutonic bodies accentuates the fractures and produces fragmented topographic surfaces whose geometric measures exceed those of a euclidean surface because geometric measures of these surfaces are potentially indicative of the percolation properties of the fracture network contours and vertical sections were digitized from 1 250000 and 1 240000 scale maps and their statistical fractal dimensions d were computed regions underlain by granitic plutons with similar fragmentation patterns were found to haved values ranging from 1 15 to 1 28 within a single pluton this range of values is caused by noise and several geologic factors the values contain noise introduced by the map making and digitizing procedures that amounts tod 1 05 however all values lie well above this threshold and they correlate closely with local differences in weathering smallerd values derive from contours where accumulation of screen or glacial erosion has smoothed the surface whereas largerd values derive from contours along aretes and in unglaciated terrains in each of these regions largerd values occur locally where fractures are most frequent and or continuous larged values were also derived from regions underlain by host rocks that lie in the near field region of the stress field caused by the pluton the fractral values derived are apparently related to the magma hydrothermal event they can potentially be used to derive properties of the percolation networks that were active during the hydrothermal events

paleomagnetism of siluro devonian sequences ne spain the paleogeography of northeastern spain is addressed in a detailed paleomagnetic study of siluro devonian boundary sediments from the catalan coastal ranges ccr at santa creu d olorda barcelona and the eastern pyrenees at baen the sequences studied are all well dated biostratigraphically and have undergone only very minor deformation mainly during the late carboniferous and extremely low grades of metamorphism anchizone or less after stepwise thermal demagnetisation of samples and principal component analysis of the data up to two paleomagnetic directions can be clearly identified after removal of a present day magnetic overprint the first component labeled b is identified only in samples from the ccr with intermediate unblocking temperature spectra 300 400 c an overall mean direction of 187 03 semi angle of cone of 95 confidence 95 9 0 and precision parameter k 43 1 in situ and 195 03 95 24 4 and k 7 1 51 samples at seven sites after bedding correction is obtained for b the exact age of this secondary component is uncertain but when compared to published paleomagnetic data a late carboniferous to early triassic age could be inferred a stable endpoint direction labeled c can be identified between 400 and 600 c in samples from both regions for the ccr it is southwesterly directed with intermediate inclination values in situ and yields a mean direction of 208 48 95 12 8 and k 28 4 six sites after bedding correction it passes the fold test is considered to be primary in origin and is thus indicative of the siluro devonian paleofield direction in the ccr in the pyrenean samples the c direction is northwesterly directed in situ with a mean direction of 240 51 95 6 5 and k 86 5 seven sites after bedding correction although there is a clear improvement in grouping of these site mean directions this is not significant at the 95 level of confidence combining the site mean directions obtained for component c from both regions yields an overall mean direction of 224 51 95 8 5 and k 24 6 13 sites after bedding correction the improvement in grouping is significant at the 99 level of confidence at 90 unfolding reinforcing the interpretation that this is a primary magnetisation adopting the reversed polarity option the inclination value obtained translates into paleolatitudes of 30 s placing ne spain in the same latitudinal belt as the armorican massif and the barrandian terrane of the bohemian massif i e the armorican terrane assemblage in latest silurian earliest devonian times

effects of normal faulting on fluid flow in an ore producing hydrothermal system comstock lode nevada abstract normal faults in the comstock lode mining district nv usa are delineated by low 18o belts and strong isotopic gradients perpendicular to strike these faults perturb and also offset the regional pattern of 18o contours produced by pervasive hydrothermal alteration of the alta formation and have preserved the down dropped hanging wall rocks while exposing the footwall rocks to greater erosion fluid rock interaction was greatly enhanced within 200 m wide zones centered on the normal faults where permeability was increased and fluids were focused parallel to the fault plane the effects of displacement and permeability enhancement around the faults are investigated using oxygen isotope profiles through the comstock and occidental faults and across steeper secondary normal faults such as the coryell fault that may have formed after the principal faults were rotated to more shallow dips oxygen isotope profiles record both post hydrothermal displacement and syn deformational enhancement of fluid flow around the faults fluid focusing along normal faults perturbs the 18o values by increasing the water rock ratio this isotopic effect can be mathematically described for any distance x from the fault using the formula f a e bx 2 which is analogous to the fundamental solution for diffusion away from a plane source the maximum magnitude of 18o depletion a depends on the effects of permeability temperature mineralogy fluid isotopic composition and integrated fluid flux the constant b is related to the scale of exchange enhancement around the fault the characteristic width w 1 b of the fluid focusing effects is remarkably similar at 100 m for all three faults examined coupling this isotope exchange effect with a step function which accounts for fault displacement provides a good fit to the observed 18o fault profiles the 18o distribution around faults can be used to infer relationships and timing among the processes of structural deformation hydrothermal convection and ore deposition in the comstock district

shear wave birefringence measurements in faulted near surface sediment fluorspar area fault complex central united states abstract the location and geometry of seismogenic faults are fundamental to understanding the regional seismic hazard however surface evidence for active faults in the seismically active central united states is rare thus contributing to the uncertainty in the hazard estimation multicomponent s wave reflection energy was acquired near the northern boundary of the new madrid seismic zone across a well constrained n61 e oriented fault that extends above the top of paleozoic bedrock and into near surface quaternary sediment in order to evaluate the effectiveness of shear wave splitting measurements for determining azimuthal orientation of fault deformed unlithified sediment that lack a surface manifestation the data were analyzed within the shear wave window i e without wide angle distortion using orthogonal source receiver orientations normalized reflection energy spectra of the main and cross diagonal elements were rotated in 5 increments to determine fast s1 and slow s2 s wave directions associated with the azimuthally anisotropic inclusion the resultant natural coordinate system was n60 e and n30 w for s1 and s2 respectively the s1 orientation correlated with the established n61 e fault strike in addition there was an average dynamic mistie of 11 3 ms that suggests 3 6 azimuthal anisotropy between the top of bedrock and ground surface these results suggest that shear wave birefringence can be a useful tool for defining azimuthal orientation of neotectonic inclusions that lack surface or geomorphic expression as well as for areas that may have limited accessibility i e urban environments for deploying multiple widespread geophysical surveys

new early devonian paleomagnetic data from nw france paleogeography and implications for the armorican microplate hypothesis to constrain the early devonian paleogeography of the armorican massif and determine the tectonic relationships between the armorican and bohemian massifs the armorican microplate hypothesis a paleomagnetic study of lochkovian pragian sediments from the pointe de l armorique brittany nw france has been carried out after removal of a present day overprint a higher unblocking temperature component of magnetization is identified the b direction which yields an overall mean direction of 222 35 k 121 6 95 4 2 after bedding correction 11 sites no fold test is possible but the presence of a thin sill allowed sampling for a contact test the results of which are positive thus the b direction of magnetization is considered to be primary in origin and closely bracketed with the rock age the resulting paleopole 315 e 13 s corresponds to a paleolatitude of 19 s for the armorican massif placing it adjacent to the southern margin of baltica and avalonia this clearly demonstrates for the first time that this sector of the rheic ocean had closed by early devonian times and that the armorican massif was in its present day orientation with respect to baltica these results are significantly different to existing paleomagnetic data from bohemia which indicate large scale 140 rotation between the earliest and latest devonian and thus provide important information for the pre and syncollisional tectonic relationships of these massifs as it is unlikely that such large scale differential crustal rotations could have occurred in an intraplate setting it is proposed that the armorican and bohemian massifs did not form a single coherent microplate but belonged to a fairly loose mosaic of terranes prior to collision with avalonia thus the concept of an armorican microplate is no longer tenable and the term armorican terrane assemblage ata is proposed in reference to the gondwana derived paleozoic terranes now situated within the european variscan fold belt south of the rhenohercynian zone avalonia

shear wave splitting in quaternary sediments neotectonic implications in the central new madrid seismic zone determining the extent and location of surface near surface structural deformation in the new madrid seismic zone nmsz is very important for evaluating earthquake hazards a shallow shear wave splitting experiment located near the crest of the lake county uplift lcu in the central nmsz shows the presence of near surface azimuthal anisotropy believed to be associated with neotectonic deformation a shallow four component data set recorded using a hammer and mass source displayed abundant shallow reflection energy on records made with orthogonal source receiver orientations an indicator of shear wave splitting following rotation of the data matrix by 40 the s1 and s2 sections principal components of the data matrix were aligned with the natural coordinate system at orientations of n35 w and n55 e respectively a dynamic mis tie of 8 ms at a two way traveltime of 375 ms produced an average azimuthal anisotropy of 2 between the target reflector top of quaternary gravel at a depth of 35 m an

tectonic inferences from oxygen isotopes in volcano plutonic complexes of the india asia collision zone nw india the india asia collision zone in ladakh nw india is characterized by two major granitic batholiths the northern karakoram granitic complex and the southern ladakh granitic complex are separated by thrust bound belts of ophiolites flysch and calc alkaline volcanics khardung volcanics of late paleozoic to tertiary age oxygen isotope analyses of whole rock and coexisting mineral separates indicate that a the ladakh granitic complex belongs to an i type suite with 18o 5 6 to 6 1 and b the karakoram granitic complex comprises two contrasting suites of granitoids an older predominantly i type with 18o 6 2 to 7 7 and a younger s type with 18o 9 5 to 10 7 the khardung volcanics show wide variation in 18o values ranging from 0 75 to 10 7 indicating isotopic exchange with a meteoric hydrothermal convective system we suggest that the older i type suite of the karakoram batholith represents jurassic to early cretaceous arc magmatism along the bangong nujiang north saltoro suture this implies a continental collision between the lhasa shyok kohistan block and the asian plate prior to the tertiary collision between india and asia india asia collision resulted in the rejuvenation of the north saltoro suture with the emplacement of the younger anatectic granite of the karakoram batholith

palaeozoic palaeomagnetic studies in the welsh basin recent advances in the last two years new palaeomagnetic data fro m wale s have resulted i n radical revision of the ordovician palaeogeography of eastern avalonia part of the southern margin of the iapetus ocean combined with palaeozoic palaeomagnetic data from laurentia and gondwana these data suggest that eastern avalonia was a peri gondwanide high latitude continental fragment during at least part of ordovician time with a palaeolatitude of about 62 s and 51 s in arenig and llanvirn time respectively this implies a latitudinal width of the early ordovician iapetus ocean between eastern avalonia and laurentia of at least 30 geological evidence for the proximity of eastern avalonia and laurentia suggests that the intervening iapetus ocean closed during silurian time from late llandovery to early ludlow recent palaeolatitude data from the iapetus bordering continents are consistent wit h closure b y middle to late silurian time new pre acadian early devonian palaeomagnetic data from the old red sandstone places the welsh basin at about 17 s consistent with a palaeogeography in which laurentia baltica avalonia armorica and possibly gondwana wer e part o f a singl e supercontinent pervasiv e lat e carboniferous early permia n remagnetization affect s the welsh basin the remagnetization i s probably associated with fluids emanating from th e variscan thrust front w e do not observe remagnetization associate d with acadian orogen y and the remagnetizations which have been studied in more detail in north america appear to be a unique feature of the variscan hercynian alleghenian orogeny

paleozoic paleogeography of north america gondwana and intervening displaced terranes comparisons of paleomagnetism with paleoclimatology and biogeographical patterns new paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the silurian and devonian in this paper the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data the data from the three fields of paleomagnetism paleoclimatology and biogeography are generally in excellent agreement and an internally consistent paleogeographic evolutionary picture of the interactions between north america gondwana and intervening displaced terranes is emerging during the interval of the ordovician silurian and devonian north america stayed in equatorial paleoposition while rotating counter clockwise the northwest african part of gondwana was in high southerly latitudes during the late ordovician and was fringed by peri gondwanide terranes such as southern europe armorica and avalonian basement blocks now found in eastern newfoundland nova scotia the boston basin the appalachian piedmont and northern florida subsequently gondwana and the peri gondwanide terranes displayed rapid drift with respect to the pole this drift translates into the following pattern of movement for northwest africa during the latest ordovician early silurian this area moved rapidly northward from polar to subtropical latitudes followed by equally rapid southward motion from subtropical to intermediate about 50 s paleolatitudes during the late silurian middle devonian it is likely that significant east to west motion accompanied the latter shift in paleolatitudes with the caledonian acadian orogeny the result of silurian to early devonian convergence and collision between gondwana and north america this collision sandwiched several of the intervening displaced terranes between gondwana and north america subsequent to this collision gondwana was separated in the late devonian by a medium width ocean from north america and the avalonian and southern european blocks which were left behind adjacent to north america this new ocean closed during the carboniferous and the resulting convergence and collision were the cause of the hercynian alleghanian orogenic belt problems remaining for future research besides the further gathering of reliable paleopoles involve the uncertain pre devonian position of the southern british isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid late ordovician silurian apparent polar wander for gondwana