biostratigraphic and palaeoenvironmental controls on the trilobite associations from the lower ordovician fezouata shale of the central anti atlas morocco abstract trilobites are a major component of both biomineralized and soft bodied assemblages of the fezouata biota central anti atlas morocco trilobite taxa of the fezouata shale tremadocian floian in the zagora area are typical of lower ordovician communities from shallow shelf environments and show strong affinities with early ordovician assemblages from the montagne noire southern france dramatic variations in both taxonomic composition and taphonomic attributes together with the sedimentological facies changes allow us to discriminate the palaeoenvironmental versus stratigraphic control on trilobite assemblages three communities are described along an onshore offshore profile namely the pilekiid bavarillid agerina and raphiophorid biofacies additionally three stratigraphic assemblages are identified assemblage 1 is confined to the late tremadocian assemblage 2 ranges from late tremadocian to middle floian and assemblage 3 is restricted to the floian the higher diversity of some levels is explained by storm induced inputs of debris from taxa living in adjacent environments additionally control of relative seal level variation on some peculiarities of the trilobite fauna specific endemicity lack of some expected taxa is evidenced in the fezouata shale finally temporary dysoxic events are highlighted by preliminary geochemical analyses probably allowing episodic exceptional preservation

a condensed lower to middle ordovician graptolite succession at matane quebec canada the highly condensed lower to middle ordovician deep water succession from the cap des rosiers group at matane bears three conspicuous graptolite faunas of late tremadoc to mid darriwilian age the tremadoc fauna belongs to the aorograptus victoriae zone in the castlemainian isograptus victoriae maximodivergens zone fauna the earliest pseudisograptus species pseudisograptus initialis n sp appears together with the ancestral arienigraptus gracilis a darriwilian fauna with holmograptus spinosus and bergstroemograptus crawfordi represents the youngest pre nemagraptus gracilis zone fauna described from the quebec appalachians

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

mechanism of rifting considerations based on results of physical modelling and on geological and geophysical data abstract on the basis of results of physical modelling and an analysis of the relevant geological and geophysical data the mechanism and regularity of rifting are studied as a rule rifting develops in extensive and long living hot regions characterized by a reduced thickness of lithosphere rifts develop due to regional extension of the lithosphere in the hot regions and are zones of localization of deformation neckings the style of deformation and faulting in a necking are studied experimentally the plastic layers of low strength in the continental crust play an essential role in rifting those places favourable for the localization of plastic deformation are related to the parameters expressiveness of these layers in various parts of a hot region the development of a necking rifting is accompanied by replacement of the lower crust by high strength mantle ultrabasic material when there is an interruption of the extension this might cause an effective strengthening of the lithosphere in the rift zones and results in jumps when tension is renewed modelling has indicated that rifts can form angular junctions and branching in hot spots and older faulting zones various generations of rifts can converge and intersect in a hot spot in contrast to this cold lithospheric zones of higher strength are detoured by rifts basic magmatism including plateau basalts is often present in such zones

early palaeozoic palaeobiogeography and palaeoecology of stylophoran echinoderms abstract stylophorans cornutes mitrates represent one of the most diverse classes of cambro ordovician echinoderms they were free living benthic non radiate forms closely related to asterozoans and crinoids taphonomic sedimentological and palaeosynecological data provide useful information on key aspects of stylophoran palaeoecology such a combined approach suggests that the rarity of stylophorans in proximal environments above storm wave base was probably original and does not exclusively result from the possession of a loosely articulated polyplated calcitic test conversely stylophorans were relatively abundant in deeper settings below storm wave base where they frequently occur in low diversity dense beds several tens to hundreds of individuals per square metre such assemblages were either largely dominated by stylophorans only middle cambrian darriwilian or by the association of both stylophorans and ophiuroids latest darriwilian silurian comparison with ecological requirements of similar modern and fossil dense populations of ophiuroids suggests that stylophoran beds occurred in well oxygenated oligotrophic waters with low predatory pressure and that sea temperature played a key role in the palaeobiogeographic distribution of stylophorans seven main stylophoran biofacies are identified and described the associated environments and tropical distribution of primitive heavily plated forms suggest that the initial radiation of the class probably took place in warm to temperate shallow settings earliest middle cambrian late cambrian however from this primitive stock stylophorans radiated several times towards deeper and or colder environments possibly because of the lower predation pressure in such settings cothurnocystid cornutes in the middle cambrian mitrates in the late cambrian hanusiids in the tremadocian as a consequence the fact that most cambro ordovician cornutes and mitrates were psychrospheric i e cold water adapted probably explains their relatively wide palaeobiogeographic distribution in high palaeolatitudes cambro ordovician stylophorans were abundant and diverse in shallow to deep environments but they were generally restricted to deep settings in low palaeolatitudes the great caradocian transgression favoured the extension of stylophorans from circum polar regions e g western margin of gondwana towards external settings at lower palaeolatitudes avalonia baltica and eastern margin of laurentia where one group of mitrates underwent a major radiation anomalocystitids and extended towards shallower and warmer palaeoenvironments in contrast all silurian stylophoran assemblages known so far are rare poorly diversified and restricted to shallow warm to temperate environments in tropical regions this distribution partly results from 1 the probable eradication of psychrospheric forms e g cornutes because of the prolonged anoxia of deep environments in the late hirnantian early silurian of west gondwana 2 the radiation of some mitrates anomalocystitids in shallow tropical settings in the late ordovician and 3 of strong taphonomic and or sampling biases numerous ghost lineages

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

the habberley formation youngest tremadoc in the welsh borderlands the habberley formation overlies the shineton shale formation in the shelve inlier shropshire and includes the youngest tremadoc strata known from the welsh basin it probably represents deposition during the interval cut out by the sub arenig unconformity in north wales and removed by the sub caradoc unconformity in the wrekin district there is no angular unconformity between tremadoc and arenig in the shelve inlier the habberley formation is likely to correlate in part with the hunnebergian stage of scandinavia it was deposited probably in a largely stagnant basin and records a shallowing upwards sequence prior to the arenig transgression of the stiperstones quartzite a sparse trilobite fauna proves the presence of the angelina sedg vickii biozone and a succeeding interval is characterized by asaphellus cf graffi the trilobites are briefly described they include the first undistorted material of angelina sedgwvickii the brachiopod lingulella bella walcott hitherto known only from eastern newfoundland is described by a w a rushton

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

dynamic models of continental rift valley systems abstract in experimental scale models simulating the major continental rift valley systems rifting is caused by diapiric uprise of viscous material induced by a centrifugal body force this force plays the same role as gravity in nature the scale models show that bending alone as a mechanism of rifting is unrealistic rather the rift structures form as a result of the combined effect of gentle bending and lateral stretching of the crust the models also show that graben structures could form at any stage of development of an ascending and laterally spreading diapiric body depending upon the density and viscosity contrast of the body and its surroundings and the strength of the model crust the close resemblance in structure of the models with natural rift systems suggests a close similarity of dynamics and driving forces

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

lower ordovician ibexian trilobites from the tribes hill formation central mohawk valley new york state the early ordovician tribes hill formation of east central new york state is a sequence of peritidal to subtidal carbonates and minor shales that rests disconformably on late cambrian carbonates and is in turn succeeded disconformably by middle ordovician strata more than 800 trilobites from 24 collections are assigned to six species bellefontia gyracantha raymond clelandia parabola cleland hystricurus ellipticus cleland hystricurus cf hystricurus oculilunatus ross symphysurina convexa cleland and symphysurina cf symphysurina woosteri ulrich two distinct biofacies are present the bellefontia biofacies in subtidal shales with thin storm generated bioclastic interbeds and the gastropod rostroconch biofacies in shallow carbonate bank lithofacies the trilobites of the tribes hill formation are assigned to a single informal biostratigraphic unit the clelandia parabola fauna which is correlative with trilobite zone b of the garden city formation of utah and with the bellefontia frank

hot regions and the origin of marginal basins in the western pacific abstract volcanic provinces showing no genetic relationship to plate boundaries may be classified into two categories hot spots which are a few hundred kilometers across and may be nearly fixed in position with respect to deep mantle and hot regions which are a few thousand kilometers across and can migrate more freely the mantle upwellings which presumably give rise to them may originate at different depths by different causes it is observed that the marginal basins older than 15 ma of the western pacific become progressively younger northward with the exception of the south fiji basin this leads to the hypothesis that the spreading of marginal basins was caused by the mantle upwelling associated with a migrating hot region and that a hot region which started from the australia region in the late cretaceous migrated northward successively creating the tasman coral sea caroline south china parece vela shikoku and japan basins through paleocene eocene oligocene and early miocene time this hot region presently underlies northern china mongolia and adjacent areas another hot region is assumed to have caused the spreading of the south fiji basin and all the currently spreading basins in the southwestern pacific spreading of individual marginal basins occurred during the short period when a hot region was passing the pertinent area the main merit of this hypothesis is that it can explain the relationship between the location and age as well as the short duration of spreading of western pacific marginal basins marginal basins are formed commonly though not always in the overriding plates near subduction zones because in such a setting a spreading marginal basin is readily accommodated either by an oceanward movement of the nearby subduction zone or by a continentward movement and subduction of the overriding oceanic plate there are a few marginal basins that were formed in other tectonic settings and accommodated by some other mechanism of compensative subduction presumably the splitting of the supercontinent pangea was also caused mainly by the effect of the mantle upwelling associated with the same hot region that later migrated northward creating marginal basins in the western pacific the main basis for this presumption is the fact that the splitting of pangea was completed in the australia region in middle cretaceous time whereas the spreading of marginal basins in the western pacific began in the same australia region in the late cretaceous immediately following

cenozoic extensional tectonics in china abstract this paper gives a brief review of the cenozoic extensional structures of china during the cenozoic eastern china was in a rifted setting where divergent plate motion and extensional structures were dominant subsidence has occurred initially in response to attenuation of the crust and was enhanced later by subsidence due to thermal contraction in western china particularly in the qinghai tibetan plateau where rifting occurs in an orogenic setting in which convergent plate movements are dominant the rifting is associated with continental collision and lateral escape and large intra continental transform boundaries extensional structures in china may be grouped into eleven graben systems in eastern china and four in western china they occur in different styles linear rift valley basin and range structures and pull apart basins associated with strike slip faults the geometric characteristics kinematics of extension and the mechanism of continental extension in china are discussed the cenozoic extrusion structural model proposed by tapponnier et al 1982 elegantly accounts for the gross tectonics and the formation of the graben systems of western china however in considering the overall strain pattern during the extension of the early tertiary basin and range region in eastern china a large scale simple shear kinematic model may be preferable to the pure shear rift model

the early palaeozoic of the north atlantic region as a test case for the use of fossils in continental reconstruction abstract fossils have an important contribution to make in the reconstruction of early palaeozoic palaeogeography the north atlantic regions include well known early palaeozoic fossil faunas which should relate to the dispositions of continental plates during the period however different versions of ordovician palaeogeography based on fossils are described in the literature and in this paper we evaluate the influences which produce these discrepancies the most reliable results are obtained by combining analysis of faunal similarity especially shelf faunas with mapping the distribution of those biofacies which relate to former deep water sites planktic organisms may not be reliable palaeocontinent discriminants cladistic analyses of different groups of organisms living at the same early ordovician time point to the usefulness of trilobite data for palaeogeographic reconstruction such data indicate a three fold division of the north atlantic region into baltica laurentia and gondwana from the cambrian to at least the mid ordovician paris and robardet 1990 denied the existence of a tornquist s sea separating baltica from gondwana in the early ordovician it is considered that their different reconstruction was because the planktic organisms on which they based their maps were not affected by this important oceanic separation to the same extent as shelly benthos on the other hand there is no evidence of deeper water biofacies along the supposed line of rheic ocean separation of avalonia from gondwana until the late ordovician however there is some evidence of deeper biofacies along the nantes sardic line new term extending from southern armorica to sardinia

a revision of ordovician series and stages from the historical type area this paper is a revision of the standard chronostratigraphy of the ordovician of the historical type area in england and wales the revision is a response to the need for more precise definitions of series and stages especially for practical international correlation in some cases this has entailed moving horizons for series bases away from those in the classical sections the scheme will be used as the standard for a forthcoming revision of the correlation chart of the british ordovician system the ordovician is divided into five series tremadoc arenig llanvirn caradoc and ashgill the llanvirn is extended to include part of the classical llandeilo series which is included within it as a stage where stage subdivisions of standard series have not yet been defined as in the tremadoc they are proposed in this paper however bancroft s fine stadial divisions of the caradoc which have been criticized for their brevity and local utility are combined into four new stages which will have wider application the former caradoc subdivisions are retained as substages for the purposes of regional correlation

deep seismic reflection evidence for the role of extension in the evolution of continental crust summary deep seismic reflection profiling as well as geologic studies indicate that extensional basins are a common feature of continental crust the wide spread occurrence of extensional basins combined with published models on the effects of extension on lower crustal rocks suggests that extensional processes play an important role in the evolution of continental cmst extension is now recognized to have been the last major tectonic event to affect approximately 50 of the united states this observation and the preservation of extensional features in areas which have experienced subsequent episodes of compression suggests that extensional processes may lead to a strengthening of continental crust in areas of active extension such as the western united states seismic and petrologic data as well as theoretical modeling of heat flow data suggests that the lower crust may be predominantly intrusive igneous material emplaced during extension and that the present mohorovicic discontinuity formed during extension although the interpretation of the various data is somewhat speculative we suggest that the volume of continental material in some areas has doubled as a result of extension thus extension may result in the addition of a significant amount of new material to the continents

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

the central european graben system rifting imitated by clay modelling abstract central europe is traversed by an active graben belt between the north sea and the mediterranean along with a lateral rift rift offset between some segments along these offsets connecting the individual segments specific patterns have been observed which may be physiographically compared with transform faults a series of clay experiments were carried out to understand rhinegraben type graben formation especially the transform like features were modelled to learn more about the kinematics of transform faulting in the continental domain it has been attempted to approach natural structural and lithological boundary conditions of graben formation by varying the experimental conditions in such a manner that experimental and in situ conditions resemble in an optimal way the experiments are related to the paleogene extensional stage of the evolution of the rift system and do not consider the pliocene to recent kinematics of shear rifting

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

a trilobite fauna from the highest shineton shales in shropshire and the correlation of the latest tremadoc a rich fauna dominated by trilobites and calcified chordates has been collected in shropshire from the arenaceous beds the highest member of the tremadoc shineton shale formation and hitherto regarded as poorly fossiliferous this fauna shows that shelf conditions persisted longer in shropshire than has been supposed it is likely that even younger tremadoc is cut out at the unconformity below the caradoc in the shineton inlier the correlation of the later tremadoc is reviewed and the nomenclature of british tremadoc biozones is revised the trilobites described here are a mixture of previously known and new forms the name shumardia conophrys salopiensis callaway is revived for british material traditionally assigned to the scandinavian species shumardia pusilla sars from which it is distinct the new taxa litagnostus meniscus sp nov apatokephalus sarculum sp nov and skljarella cracens sp nov are described the type species of asaphellus a homfrayi and leptoplastides l salteri are redescribed and geragnostus callavei pseudokainella impar and parapilekia sp are recorded new information on the ontogeny of s c salopiensis and a homfrayi is given litagnostus and skljarella are recorded from the british isles for the first time

the cenozoic tectonic evolution of the great north china two types of rifting and crustal necking in the great north china and their tectonic implications abstract the great north china underwent widespread continental rifting during cenozoic time that caused extension and necking of the crust in this area and formed four major rift systems in this paper we provide geological and geophysical evidence that rifting and extension in the western part of the great north china differs from that in its eastern part in age and nature the space time relationships show that the origins of the western rift group of the great north china are probably related to the tibet plateau tectonics whereas those of the eastern rift group are related to the subduction events in the western margin of the pacific plate furthermore the quaternary tectonic regime in the eastern part of the great north china differs from the neogene tectonic pattern and shows relations with western rifts we speculate that during the quaternary the processes that caused rifting in the west have spread further east and now affect the eastern part of the great north china

terminal cambrian and lowest ordovician succession of mexican west gondwana biotas and sequence stratigraphy of the tinu formation the tinu formation of oaxaca state is the only fossiliferous lower palaeozoic unit between the laurentian platform in northwest mexico and gondwanan successions in andean south america the tinu traditionally has been referred to the lower ordovician tremadoc and regarded as having a provincially mixed fauna with laurentian avalonian and gondwanan elements bio and lithostratigraphic re evaluation demonstrates that the tinu is a gondwanan passive margin succession it includes a lower thin to 16 m condensed uppermost cambrian yudachica member new the yudachica nonconformably overlies middle proterozoic basement as a result of very high late late cambrian eustatic levels the yudachica changes from storm dominated but slightly dysoxic shelf facies fossil hash limestone and shale in the south to an upper slope facies with debris flows 50 km to the north three biostratigraphically distinct depositional sequences comprise the yudachica the yudachica has gondwanan aspect trilobites with low diversity conodonts characteristic of unrestricted marine temperate facies the upper tinu or rio salinas member new is a lower ordovician tremadoc depositional sequence that records strong early but not earliest tremadoc eustatic rise marked by graptolite and olenid bearing dysoxic mudstones higher strata shoal upward into shell hash limestones and proximal tempestite sandstones with upper lower tremadocian unrestricted marine temperate conodonts new taxa include orminskia rexroadae landing gen et sp nov from the cordylodus andresi zone this euconodont is related to hyaline coniform genera best known from ordovician tropical platform successions cornuodus clarkei landing sp nov resembles the coeval upper lower tremadoc tropical species scalpellodus longipinnatus ji barnes

river channel changes adjustments of equilibrium a method for predicting river channel s adjustments of equilibrium is presented and illustrated by examples this method is based upon the quantitative relationships among the variables of water di

a formation and evolution of overlapping spreading centers constrained on the basis of physical modelling abstract physical modelling and geodynamic analysis have been carried out concerning the formation and evolution of overlapping spreading centers oscs with fast spreading rates crust and underlying magma chamber were modelled by solidified and melted respectively a composite system composed of mineral oils solid hydrocarbons and surface active substances regularities of the model crust quasi brittle failure under tension were investigated obtained results permit to conclude that there are two classes of oscs small ones with widths d d 4 6 km differing in structure formation and mechanism of their evolution small oscs have a short life of the order of 10 3 10 4 years controlled by periodic jumps of spreading centers tectonic cycles large oscs exist for millions of years and represent quasi stationary structures a formation and evolution model of oscs is proposed on the basis of the experimental results

first discovery of early palaeozoic bathysiphon foraminifera test structure and habitat of a living fossil the giant agglutinated foraminiferan bathysiphon sars previously triassic recent occurs in much older sedimentary rock early ordovician late early tremadocian of avalonia the genus extends back to c 485 ma based on its discovery in platform mudstone of the chesley drive group in cape breton island nova scotia elongate up to 60 mm epibenthic bathysiphon tubes occur in wave rippled green grey mudstone with a low diversity probably dysoxic fauna the mudstone is coeval with and lithologically similar to the shineton formation in shropshire and the welsh borderlands scanning microscopy of the bathysiphon walls shows imbricated mica grains that parallel the long axis of the tests the lumen has a melange of packed sediment grains some of which are spherical structures of siliciclastic mud studded with tetrahedral pyrite crystals a felt like agglutinated test a lumen packed with spherical structures probable stercomata and the domal ends of some specimens are consistent with modern bathysiphon this report is the first time that cytoplasmic activity and stercomata formation have been used to refer fossil protists to a modern group bathysiphon differs from the cambrian foraminiferan platysolenites pander which has an open lumen without stercomata but support a comparable sediment deposit feeding niche bathysiphon is truly a living fossil with a mode of test construction cytoplasmic activity that formed stercomata and a niche unchanged for almost 500 million years foraminiferans have not been found prior to the cambrian period and the early cambrian appearance of agglutinated foraminiferans is part of the radiation of phanerozoic communities

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

palaeoecological aspects of the diversification of echinoderms in the lower ordovician of central anti atlas morocco abstract echinoderms are one of the major components of benthic faunas in the lower ordovician sequence near zagora central anti atlas morocco the fezouata shale tremadocian late floian has yielded numerous exquisitely preserved echinoderm assemblages ranging through several stratigraphic levels and palaeoenvironmental conditions these associations offer a unique opportunity to document both evolutionary and palaeoecological aspects of echinoderm diversification in high latitude siliciclastic dominated western gondwana sediments where rapid in situ burials facilitated excellent faunal census conditions lower shoreface deposits of the fezouata shale provide the most complete record of successive echinoderm faunas in late tremadocian times these relatively shallow shelf deposits show the progressive replacement of low diversity opportunistic cambrian like dwarfed communities dominated by cornute stylophorans in unhospitable dysoxic environmental conditions by higher diversity benthic assemblages more typical of the great ordovician biodiversification event and dominated by blastozoans on well oxygenated sea floors the turnover of the ordovician radiation was apparently slightly delayed in more proximal settings eocrinoid meadows persisted in shallower environmental conditions up to the middle floian in the late floian they were replaced by diploporite dominated communities typical of later ordovician high latitude peri gondwanan faunas from a palaeobiogeographic point of view low diversity assemblages display relatively strong affinities with cosmopolitan late cambrian echinoderm faunas whereas high diversity communities are dominated by peri gondwanan taxa

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

theoretical and experimental studies of mantle upwelling penetrative magmatism and development of rifts in continental and oceanic crusts abstract the relationship between mantle perturbation penetrative magmatism crustal uplift and the processes of rifting and rift propagation was studied quasi scale model experiments and theoretical analyses by boussinesq s three function method were used three dimensional stress analyses around a rising mantle intrusion and semiquantitative model experiments on mantle upwelling and magma penetration into brittle and brittle ductile lithospheric materials indicate the origin of ridges rifts and rift basin structures these structures are commonly initiated by broad domal or plateau uplift the magnitude and nature of the uplift fissure or rift development branching and pattern s are controlled primarily by flexural rigidity magnitude of stress development due to mantle intrusion and magma penetration and the degree of preferred orientation of weakness anisotropy in the lithosphere continued magma penetration into the crust produces en echelon fissures vertical and lateral dike propagation graben and graben and horst structures with step or block faulting this is followed by axial subsidence fault opening and periodic volcanism with continued crustal extension stretching and slow spreading mantle and magma penetration into the crust are shown to be important in developing in situ lateral compressive stress adjacent to and around the active rift zones transverse fissures dike swarms volcanoes and anticlines between two rifts as in iceland result from such lateral compressive stress this stress is also important in the development of marginal fissures and faults as in fast spreading centers time progressive model experiments on magma formation and magma penetration from the asthenosphere lithosphere boundary show the importance of magma chambers these chambers cause thinning and stretching of the composite lithosphere vertical and lateral propagation of dikes and dike swarms and initiate rifts with or without significant uplift this study also suggests that multiple dike wedging with rapid extension fracturing can generate excess magma pressure and push force and may contribute seismicity to spreading centers