plate and intraplate processes of hercynian europe during the late paleozoic abstract it is speculated that until late carboniferous time the region of hercynian europe was occupied by an elongated island arc system underlain by a segment of continental crust in the upper carboniferous two subduction zones are assumed to have extended from the north and south beneath hercynian europe an extensive zone of hot partially molten upper mantle lay above and between these and diapiric uprise of portions of this material led to separation of mafic magmas widespread partial melting in the lower and middle crust high temperature low pressure metamorphism in crustal rocks and regional uplift and extension of the crust as indicated by intermontane troughs and their associated volcanic rocks in visean to westphalian time hercynian europe collided with both the large neighbouring plates north america europe and africa during these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of hercynian europe after the collision processes had been terminated processes of upper mantle activity continued causing further crustal uplift and even enhanced crustal extension for several tens of million years into the lower permian decline of the upper mantle activity beneath hercynian europe is indicated by crustal subsidence and formation of a peneplain in permian time followed by the upper permian transgression of both the zechstein sea and the tethys sea which mark the end of the hercynian geodynamic cycle

multicomponent magnetization in the helmsdale granite north scotland geotectonic implication abstract a palaeomagnetic study of the helmsdale granite u pb 420 m y k ar 400 m y northeast scotland has revealed a multicomponent remanence dominated by two characteristic axes of magnetization the suggested oldest of these magnetizations the direction of which is nearly horizontal and directed n s is thought to have been acquired in upper silurian lower devonian times the existence of this shallow direction of magnetization discounts a recent hypothesis of a ca 2000 km sinistral offset along the great glen fault the second component of magnetization appears to be partly carried by haematite that apparently formed through disintegration of biotite and or plagioclase this secondary magnetization has a direction that can be associated with a permian early mesozoic age similar overprinted magnetizations are characteristic features also in the devonian sedimentary sequences north of helmsdale

proterozoic tectonic evolution and late svecokarelian plate deformation of the central baltic shield the continental crust of the central baltic shield evolved by accretion towards the west during the svecokarelian orogeny 1700 2200 ma ago the following features are consistent with a plate tectonic mechanism involving subduction of oceanic crust below an archean craton in the east flysch sediments with serpentinite masses and pillow lavas linear high grade metamorphic zones island arc type volcanic belts and late tectonic batholiths with porphyry type cu mo deposits semi consolidated new crust was affected by late svecokarelian deformation dn after 1850 ma nne trending folds with crenulation cleavage were overprinted on older structures together with associated nw trending ductile transcurrent shear zones that curve the fn folds into gentle s and z shapes the late tectonic batholiths intruded partly at the same time as and partly after the dn deformation

brittle deformation events at the western border of the bohemian massif germany investigations of brittle deformation structures present within the crystalline rocks of the bavarian oberpfalz reveal a complex late to post variscan crustal evolution upper carboniferous mainly westphalian granites were emplaced into semibrittle to brittle rocks of the zev zone of erbendorf vohenstraus and the egz erbendorf greenschist unit respectively from both the alignment of the granites and the direction of granite related tension gashes a north east south west extension must be assumed for the period of magmatic activities apart from the granite intrusions rapid crustal uplift about 1 5 km my led to an increase in the geothermal gradient from 40 c km late variscan post granitic the increased geothermal gradient persisted during the succeeding reverse faulting which results from late carboniferous probably stephanian east west and northeast south west compression although not evidenced directly in the area considered strike slip faults seem to have played an important part during the late variscan crustal evolution particularly in the early permian the strike slip events indicate further crustal shortening and indentation under north south compression a similar indentation was present in cretaceous time after a weak phase of early cretaceous reverse faulting which results from north south compression strike slip faults formed under north west south east and north south compression all these faults in particular the strike slip faults seem to be related to the cretaceous and lowermost tertiary convergence of the alpine carpathian orogeny a late stage of crustal extension characterized by a radial stress tensor 2 3 is indicated through high angle normal faults which probably formed during the subsidence of the adjacent neogene eger graben

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

the variscan structural and metamorphic evolution of the eastern southalpine basement the basement of the southern alps northern italy belongs to the southernmost part of the european variscan mountain belt in contrast to other areas of the alps the post variscan metamorphic and tectonic overprint is weak and therefore permits the unravelling of the variscan tectonometamorphic evolution of this region overprinting criteria and the mapping of the penetrative structural elements s2 and l2 allow three deformational events to be distinguished major variscan deformation d2 in the southern alps commenced in the carboniferous and lasted until the end of the late carboniferous tectonic movement during d2 was north directed and was accompanied and followed by greenschist facies metamorphism peak metamorphic conditions reached 450 550 c and 5 6 5 kbar in the brixen area and decreased in a southeasterly direction tectonic movement during d2 followed subduction of the plankogel terrane and collision of the noric terrane northernmost part of gondwana with laurasia and is interpreted to result from late orogenic extensional collapse of the variscan mountain chain extensional deformation caused exhumation of the basement rocks as evidenced by greenschist facies basement pebbles which show no substantial retrograde overprint in late carboniferous early permian conglomerates

tracing crustal evolution in the nw iberian peninsula through the rbsr and upb systematics of palaeozoic granitoids a review abstract the abundant granitoids in the palaeozoic basement of the northwestern iberian peninsula include 470 440 ma old suites early middle ordovician now present as orthogneisses and several variscan suites the age data presently available suggest that the latter have been generated in three separate time intervals 330 320 ma ago late visean early namurian 310 300 ma ago westphalian and 290 280 ma ago late stephanian autunian rb sr whole rock and u pb zircon systematics along with field and chemical evidence testify to the generation of the early and late palaeozoic granitoids from lower palaeozoic and precambrian maximum age 1 7 1 6 ga sedimentary sequences respectively the source rocks contained zircons derived from a late archaean early proterozoic continental crust that probably underlies the whole of western europe although for the greater part modified beyond recognition by younger crustal reworking

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

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

lower palaeozoic convergent plate margin volcanism on bomlo sw norway and its bearing on the tectonic environments of the norwegian caledonides the island of bomlo presents a geological section through a caledonian convergent plate margin the lykling ophiolite and the overlying ensimatic are sequence of the geitung unit were emplaced onto the baltic shield probably in arenig times after a period of deep erosion the basin and range type volcanics of the siggjo complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism possibly due to the successful subduction of a spreading ridge at the top of the mainly sedimentary ashgill llandovery vikafjord group the basin and range type volcanism resumed this was probably preceded immediately by the calc alkaline subaerial volcanism at the base of the langevag group which passes stratigraphically upwards into basin fill lithologies such as submarine volcaniclastics cherts turbidites and submarine greenstones of chemistry compatible with the early stages of ensialic back arc basin development the stratigraphy and volcanism are discussed in context with recent views on the rest of the norwegian caledonides

a model for the crustal evolution of southern scotland in the late silurian southern scotland was a mature accretionary active margin of sumatra type on the southeastern edge of north america while england was on a passive margin on the opposite side of the iapetus ocean southward subduction below england during the middle ordovician produced a magmatic arc which now lies close to the iapetus suture and thus argues for a missing forearc terrane tectonic erosion or strike slip removal may be responsible for some of this missing terrane to the north of the lake district but we now suggest that underthrusting below southern scotland may also be an important cause if the ordovician forearc terrane of northern england is now below the southern uplands we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the iapetus suture velocity structure from the lispb study sr u pb isotopic signatures and geophysical attributes of the c 400 ma granites the under thrusting model still leaves room for considerable modification of both margins by large strike slip faults seismic refraction profiles of modern accretionary active margins sumatra sw japan allow us to propose a likely velocity profile for late silurian southern scotland we choose a mesozoic forearc truncated by strike slip faulting the queen charlotte islands margin of the ne pacific as an analogue for the late silurian southern iapetus margin following previous suggestions of highly oblique and slow convergence during the final stages of closure of iapetus we combine the two profiles by under thrusting the southern margin to produce a model which involves minimal shortening in the collision zone the crustal profile resulting from such a collision would be very similar to the modern southern scotland profile both in terms of the scale of the tectono stratigraphic terranes involved and of the resulting crustal velocity structure the cessation of accretion after the mid wenlock may be related to impingement of the english continental crust against the scottish trench subsequent uplift of the southern uplands may be connected with the appearance of coarse detritus in the lake district during the silurian

the rand granite in the southern schwarzwald and its geodynamic significance in the variscan belt of sw germany the rand granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the moldanubian zone at the southern margin of the central schwarzwald gneiss complex it is a largely mylonitized elongated body and is thrust over the badenweiler lenzkirch zone forming a nappe with shear zones along its northern and southern boundaries it comprises meta granites meta trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions nappe thrusting which also affected the south schwarzwald gneiss complex occurred in visean time during high temperature low pressure metamorphism kinematic indicators in the mylonites document e to ese directed nappe transport highly transpressive relative to the trend of the nappe boundaries and the foliation the trondhjemites formed at 351 5 4 ma predating the variscan ht metamorphism they have initial end values of 6 6 to 6 7 and relatively low initial 87sr 86sr ratios 0 7042 to 0 7063 indicating a predominant mantle origin the granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 ma suggested by u pb zircon model ages they are different from the trondhjemites with low initial end values 4 7 to 3 3 and higher initial 87sr 86sr ratios 0 7068 0 7077 indicating that large part of the rand granite originated from anatexis of continental crust internal structure of zircons from the rand granite reveals mixing of magmas derived from both mantle and crust sources these new data support an interpretation that the rand granite developed along an active continental margin and therefore represents a possible root of a variscan magmatic arc

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

an analytical solution and case study of groundwater head response to dual tide in an island leaky confined aquifer 1 this paper presents an analytical solution of groundwater head response to dual tide fluctuation in the transect of an island leaky aquifer system comprising a confined aquifer and its overlying semipermeable confining layer both layers terminate at the coastlines on two sides of the island solution analysis indicates that the tidal waves from the two sides of the island transect interfere at the middle of the island and the interference decreases to zero as the horizontal length of the aquifer increases to infinity the leakage of the overlying confining layer enhances the landward attenuation of the tidal head fluctuation and shortens the time lag between the head and tide fluctuations the solution agreed well with the observations in eight piezometers in garden island on the continental shelf of western australia reported by trefry and bekele 2004

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

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

eocene oligocene tectonics and kinematics of the rhine saone continental transform zone eastern france a tectonic analysis of brittle deformaton was performed in the rhine saone transform in order to decipher and understand the deformation processes in a typical continental hansfer zone cenozoic major and minor fault patterns are described and brittle structures are interpreted in terms of paleostress orientations the eocene n s compression related to africa eurasia convergence is marked mainly by strike slip faults it was followed by a major phase of crustal extension mainly oligocene in age related to the west european rifting this extensional phase is expressed by normal faults tension gashes and extensional strike slip faults which are especially abundant in the transform zone these two events are found to be quite different in terms of distribution of horizontal shess trajectories the trend of ol axes is very homogeneous for the n s compression whereas o3 orientations associated with the major extension vary from e w in most of the rift system to i v se within the transform zone finally the mio pliocene tectonic emplacement of the jura fold and thrust belt is marked in the field by strike slip faults associated with a well defined trend of ol oriented w iiw e se to nw se the preexisting basement fracture pattern between the grabens was also considered in order to estimate which faults were likely to have been reactivated during the oligocene transform kinematics and the theoretical shear motion on these inherited fault surfaces when submitted to e w extension was calculated left laterauextensional reactivation of a preexisting 60 n trending fault system in the brittle upper crust is likely to have accommodated the shear strain arising between the rift segments in response to crustal stretching two independent numerical modelings based on distinct element and finite element analyses respectively enabled us to verify and to refine the hypothesis of transform kinematics related to e w extension and inducing regional perturbations of extensional stress trajectories the overall mechanism and the transtensional kinematics of the rhine saone transform zone are thus tightly conshained the inherited crustal anisotropy the large scale perturbations of extensional stress trajectories and the development of distributed brittle deformation within the sedimentary cover are found to play a major role in the tectonic evolution of continental rift transform systems

magma and sediment i emplacement mechanism of late carboniferous tholeiite sills in northern britain president s anniversary address 1981 like other great tholeiite sheets of the world the midland valley sill and whin sill were emplaced in thick accumulations of sediments which at the time of intrusion were not much older nor had been orogenically deformed however the carboniferous sediments of northern britain had been deposited under the influence of continuous differential subsidence so that they were already disposed in syn sedimentary basins and swells when intrusion began by drawing palinspastic contours of the sills relative to a late westphalian marine band approximating to the surface at the time of emplacement it is established that the dolerites mainly follow bedding planes dipping at angles of as much as 5 down to the bottoms of the basins isopachs of the sills indicate a further relationship for the intrusions are thickest at the bottoms of the basins these facts are not explained by any intrusion mechanism previously postulated though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step and stair transgression of sediment bedding it is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0 5 and 1 0 km below the contemporary surface thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up dip under pressure of head the sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow

comment on extending the radiocarbon calibration beyond 26 000 years before present using fossil corals by t c chiu r g fairbanks r a mortlock a l bloom quaternary science reviews 24 2005 1797 1808

paleomagnetic evidence for post devonian displacement of the avalon platform newfoundland the possibility that the avalon platform where the avalonian lithotectonic belt is best developed was involved in late paleozoic displacement was tested by paleomagnetic study of red sandstones of the upper devonian terrenceville formation of eastern newfoundland two magnetization directions were identified by thermal demagnetization analysis of 60 oriented samples from 10 sites a high blocking temperature thermally discrete a component of normal and reversed polarity and an intermediate blocking temperature thermally distributed b component of reversed polarity the b component d 185 9 i 3 3 a95 7 2 for n 8 sites is interpreted as a postfolding secondary magnetization and gives a paleomagnetic pole position latitude 43 6 n longitude 117 1 e near early to late permian paleopoles for north america the a component d 181 6 i 28 0 a95 10 1 for n 9 sites is interpreted as the characteristic magnetization possibly dating from near the time of deposition of the terrence ville formation the corresponding paleomagnetic pole position latitude 27 4 n longitude 123 5 e falls within a group of late devonian early carboniferous paleopoles obtained from the acadia displaced terrain encompassing the coastal areas of new england and the canadian maritimes which form another part of the avalonian belt these paleopoles are systematically offset by 15 to 20 in latitude from coeval pole positions obtained from cratonic north america thus the avalonian belt of the northern appalachians which is thought to represent a remnant of a precambrian and early paleozoic microcontinent on tectonostratigraphic considerations appears to correspond to a late paleozoic displaced terrain on the basis of paleomagnetic evidence

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

geochemical evidence for progressive rift related early palaeozoic volcanism in the western sudetes the volcanigenic rocks of the kaczawa mts western sudetes poland in the eastern variscides show changing geological and geochemical evolution during early palaeozoic time the lower part of the succession cambrian ordovician has three components 1 shallow marine to subaerial metabasalts associated with limestones and volcaniclastics the lavas are dominantly of a transitional tholeiitic alkaline type and their trace element patterns typically represent a rift related environment they pass laterally and upwards into more depleted basalts resembling enriched morb with nd isotopic characteristics indicating contamination by continental crust 2 interlayered rhyodacitic lavas and volcaniclastics which show negative nd values suggesting formation of the original magma by crustal melting 3 an overlying alkaline bimodal suite of lavas and volcaniclastic rocks as well as alkaline metabasites of shallow intrusive character the geochemistry of the latter resembles oceanic island volcanics but they may well have been emplaced in the same evolving rift environment the upper part of the kaczawa succession ordovician silurian in age comprises a thick and monotonous sequence of deep marine pillowed and massive metabasalts associated with black shales and cherts these lavas exhibit morb trace element characteristics with minor evidence of crustal contamination during this stage of rifting true oceanic crust probably formed it is thus suggested that the studied part of the kaczawa succession developed in a progressively evolving rift initially within an ensialic environment and finally reaching the stage of a basin underlain by oceanic type crust together with similar cambrian ordovician volcanic sedimentary associations widely distributed in western europe from portugal through france and germany they represent a record of the early palaeozoic rifting in the northern periphery of gondwana

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

paleomagnetism of the navan zn pb deposit ireland for the past quarter century the navan mine has exploited europe s premier zn pb deposit its genesis is controversial in part because it has not proved amenable to radiometric dating the ore is epigenetic and is hosted in lower carboniferous platform carbonates of the early courceyan navan group 351 4 ma and of the late chadian early arundian boulder conglomerate 338 1 ma paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating field and thermal step demagnetization plus isothermal remanence analyses paleomagnetic fold tests on navan group ore and arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding further a contact test using a tertiary dike that cut ore was positive meaning that the remanence predates intrusion of the dike both the ore and host rocks retain a stable characteristic remanence direction in single to pseudosingle domain magnetite at d 196 0 i 6 0 n 21 95 2 6 k 147 that defines a pole position of 25 6 w 31 8 s p 1 3 m 2 6 comparison of the pole to the rotated laurentian and unrotated eastern avalonian apparent polar wander paths yields a late arundian to early asbian age of 333 4 ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks the result implies that the ore mineralization event followed deposition and tilting of the overlying arundian holkerian lucan formation at navan

trends in the postsvecokarelian development of the baltic shield the postsvecokarelian development of the baltic shield shows a parallel development with tension and dolerite intrusions in the core zone and granite intrusions compression and crustal shortening in the south western margin a crustforming event with calk alkalic granitoid intrusions which with time moves westwards is followed by remelting and intrusion of alkali intermediate granites and metamorphism the south western margin of the shield probably was a stable ocean continent border zone for a very long time in spite of several attempts no conclusive testable model for the development can be put forward today

mise en evidence par le paleomagnetisme d une importante rotation antihoraire des vosges meridionales entre le viseen terminal et le westphalien superieur the volcano sedimentary formations from the southern vosges are subdivided in two main series a lower visean series characterized by a volcanism of spilite keratophyre type and an upper visean series which includes a normal volcanic association of shoshonitic tendency paleomagnetic study of 50 sites sampled in both series but mostly in the upper one yields three types of directions of characteristic magnetizations the first type corresponds to tertiary and quaternary remagnetizations with low apparent blocking temperatures 350 500 c titano maghemites the second group is formed by remagnetizations which have taken place during late carboniferous early permian times and which show high blocking temperatures of magnetite and mostly titano haematites the mean direction is d 16 i 7 95 9 for 13 sites 43 n 165 e the last group is represented by primary magnetizations of latest visean age and post sudetic remagnetizations with blocking temperatures of magnetite and haematite the mean direction d 323 i 17 95 9 for 18 sites 25 n 228 e deviates from about 60 from the theoretical direction calculated with the early carboniferous european pole position this deviation is interpreted as resulting from a counterclockwise rotation of the southern vosges between late visean and westphalian times one consequence may be the formation of the variscan v due to the anticlockwise rotation of the eastern branch of the chain the northwesterly directions show a variation of the inclinations which may indicate that the rotation was preceded by a relatively significant drift of the vosges to the north

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

a finite element model of the stress field in strike slip basins implications for the permian tectonics of the southern alps italy abstract a set of finite element models have been used to simulate pull apart basin formation related to a releasing overstep along two right strike slip faults the modelling results have shown the stress field at a releasing fault overstep with respect to strike slip transtensional and transpressional boundary constraints based on the assumption that normal faults strike parallel to the maximum horizontal stress we have predicted the orientation of potential normal faults within the overstep zone linear elastic rheology and plane stress conditions were assumed and four kinds of models with different overlap separation ratios 0 0 5 1 and 2 between the strike slip faults were analysed displacement boundary conditions were applied to the upper left hand and lower right hand corners of the models in order to simulate a right strike slip movement outward and inward displacement components were applied respectively to reproduce transtensional and transpressional conditions all the models mostly show an extensional stress field i e with at least one extensional principal stress axis large zones inside the overstep are affected by general extension both principal axes are extensional and their size increases with both an increase in the overlap separation ratio and with changes from transpressional to transtensional conditions the areas of maximum extension are located near the ends of the fault segments inside the overlap area it is fairly probable that the genesis of the extensional faults that bound the pull apart basins observed throughout the world occurs in these maximum extensional areas and that the extensional faults are parallel to the maximum compression stress axis trajectories the angle between the potential normal faults and the strike slip faults is strongly dependent on the overall boundary conditions i e pure strike slip transtension and transpression of the model in contrast the elastic rock properties and the overlap separation ratio of the faults have a minor control the angle between the normal and strike slip faults can vary from a few degrees in transtensional conditions to 100 120 in transpressional conditions inside the overlap zone an area of subsidence occurs the axis of which is coincident with the junction of the strike slip fault tips where it is also at a maximum i e in the same zones of maximum extension a comparison of the modelling results with the structural setting of the collio and pramollo permo carboniferous pull apart basins southern alps italy indicates that the collio basin developed under strike slip conditions while the pramollo basin was formed under transtensional conditions

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

suspect terranes and accretionary history of the appalachian orogen the north american connection or the grenvillian continental crust that separated from eastern north america during the initiation of the appalachian orogenic cycle is poorly defined within the present orogen a variety of terranes occur outboard of the north american miogeocline all structurally uncoupled and therefore suspect stratigraphic analysis indicates that appalachian accretion progressed from the miogeocline outward the boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes later boundaries between eastern terranes are steep mylonitic zones and brittle faults accretionary events defined by stratigraphic analysis correspond to times of major deformation plutonism and metamorphism in the history of the orogen

combined thrusting and wrenching in the ibero armorican arc a corner effect during continental collision this paper is an attempt to reconcile strain pattern and deformation history in an overall arcuate structure with continental collision processes geological data are first presented for the early history 400 ma of the variscan orogenic belt in france principal stretch trajectories are on a large scale more or less parallel with the trend of the belt they became strongly oblique to this trend near the basal surfaces of the nappes a kinematic model combining two orthogonal shear components is proposed to explain such a strain pattern it is concluded that the observed strain trajectories result from a combination of a southward thrusting and a sinistral wrenching this conclusion is in good agreement with previous conclusions gained by several local microtectonic studies the geodynamic significance of combined thrusting and wrenching at the scale of the ibero armorican arc is discussed and a progressive deformation model is proposed

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

the quartz diorites of limousin elemental and isotopic evidence for devono carboniferous subduction in the hercynian belt of the french massif central a chain of late devonian early carboniferous intrusions on the western margin of the french massif central the limousin tonalite belt consists of quartz diorites which are unusual among french hercynian granitoids in their low silica contents 53 68 wt sio2 and abundant hornblende they represent the earliest and most primitive plutonism post dating the main phase of hercynian metamorphism and have chemical characteristics strongly resembling andesitic lavas of calc alkaline affinity they differ from other massif central hercynian granitoids in their low 87sr86sr360 0 7047 0 7059 and high nd360 0 7 to 1 8 their initial pb composition was rather homogeneous 206pb204pb360 18 15 18 38 207pb204pb360 15 57 15 62 208pb204pb360 38 06 38 26 and their 207pb204pbi values are significantly lower than those of other hercynian granitoids the limousin quartz diorites are considered to have originated from an enriched mantle with characteristics modified by subduction of oceanic crust and terrigenous sediments the primitive quartz dioritic magmas were then contaminated in the lower continental crust by an afc process involving acid metaigneous or metasedimentary lithologies the occurrence of such plutons in the earliest phase of hercynian plutonism in the massif central is probably related to minor subduction at an active continental margin

palaeomagnetism of some lithological units from the lower palaeozoic of the vendee france abstract results of the first palaeomagnetic study carried out in the west vendean region at the southern edge of the armorican massif are presented the lithological units sampled range from middle to upper cambrian and silurian after alternating field demagnetization and thermal cleaning la lombardiere basaltic rocks of silurian age yielded only one residual component of magnetization d 214 i 11 after tilt correction the corresponding palaeopole is located at 28 s 321 e la meilleraie sandstone and quartzite of lower ordovician age which are equivalent to the armorican sandstone also contain one single residual component of magnetization d 244 i 5 after demagnetization and tilt correction the corresponding palaeopole is 15 s 293 e finally quartzites and rhyolites of middle to upper cambrian age are characterized by two and possibly three components of magnetization the mean direction of the main residual component after demagnetization and tilt correction is d 319 i 9 the corresponding palaeopole position is 40 n 222 e the apparent path wander of the poles obtained in the vendee is compared with the one of stable europe western and gondwana a convergence of the poles from the vendee and stable europe is observed from the beginning of ordovician time and a reasonable similarity at silurian time this study partly fills the gap of palaeomagnetic data for the period upper cambrian silurian for the armorican plate as early as upper cambrian the vendee was an integral part if the armorican plate the plate tectonics scenario proposed in this paper is the following accelerated separation of the armorican block from gondwana at middle upper cambrian time motion of the armorican block towards stable europe at ordovician time and proximity of armorican plate to the northwest european craton at silurian time if this scenario is right the nature of the hercynian orogen is more likely the result of a collision the armorican plate acting as a buffer zone palaeomagnetic studies of the vendean bocage rock sequence are considered important for future work

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

the association of continental crust rocks with ophiolites in the northern apennines italy implications for the continent ocean transition in the western tethys abstract the late cretaceous sedimentary melanges from the external liguride units of the northern apennines include large slide blocks of ophiolites and lower and upper continental crust rocks representative of a continent ocean transition between the internal liguride oceanic domain and the thinned continental margin of the adria plate the slide blocks preserve a record of the long lived history of rifting which led to opening of the jurassic western tethys basin the external liguride ophiolites consist of 1 undepleted spinel peridoties partly re equilibrated under plagioclase facies conditions which were interpreted as unroofed subcontinental mantle 2 rare gabbroic rocks mainly troctolite to olivine bearing gabbro probably crystallised from n morb magmas and 3 basalts with n to t morb affinity covered by late callovian early oxfordian radiolarian cherts both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re equilibration to plagioclase facies conditions the slide blocks of lower continental crust are composed of gabbro derived mafic granulites and felsic granulites the latter include quartzo feldspathic granulites and rare quartz poor to quartz free charnockitic rocks in both mafic and felsic granulites granulite facies re equilibration was followed by a retrograde metamorphic evolution to amphibolite greenschist and subgreenschist facies conditions retrogression is commonly accompanied by deformations progressively changing from plastic to brittle the upper crustal rocks occurring as slide blocks consist of hercynian granitoids with orogenic affinity mainly biotite bearing granodiorites and peraluminous two mica leucogranites locally the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts the granitoids underwent polyphase brittle deformations under subgreenschist facies conditions which predated the basalt emplacement the tectono metamorphic evolution recorded by the slide blocks of the external liguride units started in the late carboniferous early permian about 290 ma with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites the associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro derived granulites mafic and felsic granulites subsequently underwent tectonic exhumation in permo triassic times as testified by the development of granulite to amphibolite facies ductile shear zones the granulites were finally exhumed to shallow levels probably in association with the subcontinental mantle in late triassic middle jurassic times the latter period was most likely characterized by extensive brittle faulting at shallow crustal levels thus giving rise to extensional allochthons formed by stretched slices of granitoids the western tethys opening is finally testified by the basalt intrusion and effusion in the late jurassic followed by deep sea pelagic sedimentation the external liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent ocean boundary this reconstruction fits well with the available data on the present day continental margins derived from passive lithosphere stretching