evolution of the eastern volcanic ridge of the canary islands based on new kar data the results of 64 new k ar age determinations together with 32 previously published ages show that after a period of erosion of the basal complex miocene volcanic activity started around 20 ma in fuerteventura and 15 ma in lanzarote forming a tabular succession of basaltic lavas and pyroclastics with a few salic dykes and plugs this series includes five separate volcanic edifices each one with its own eruptive history in fuerteventura several miocene eruptive cycles have been identified in the central edifice one around 20 17 ma followed by two others centred around 15 and 13 ma in the southern edifice the maximum of activity took place around 16 14 ma whereas in the northern one the main activity occurred between 14 and 12 ma in lanzarote a first cycle of activity took place in the southern edifice between 15 5 and 14 1 ma followed by another between 13 6 and 12 3 ma in the northern edifice three pulses occurred 10 2 8 3 6 6 5 3 and 3 9 3 8 ma an important temporal gap greater in fuerteventura than in lanzarote separates series i from the plio quaternary series ii iii and iv formed by multi vent basaltic emissions in fuerteventura the following eruptive cycles have been identified 5 2 9 2 4 1 8 1 7 0 8 0 4 and 0 1 ma in lanzarote the activity was fairly continuous from 2 7 ma to historic times with a maximum in the lower pleistocene eruptive rates in the series i edifices were on the average 0 1 0 01 km3 ka comparable but slightly smaller than in similar edifices in tenerife and la gomera but much lower than in gran canaria average post miocene eruptive rates were about 0 013 0 027 km3 ka in lanzarote and 0 003 0 007 km3 ka in fuerteventura all these volcanic edifices show a similar general sequence fissural eruptions erosion multi vent volcanism repeated at different periods in different parts of the eastern islands of the canaries the model of growth of the series i edifices is comparable to those in tenerife and la gomera long periods of activity sometimes greater than 6 m y with pulses separated by gaps however salic and intermediate differentiates frequent in tenerife and la gomera are very scarce in these islands the fuerteventura lanzarote ridge shows a decrease in volcanic activity with time and also a certain ssw nne polarity in the temporal development of volcanism

collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

shear zone structures in the oygarden area west norway abstract investigation of shear related sheath like folds from blastomylonitic gneisses in the oygarden complex norway shows that the folds have not initiated with fold axes systematically normal to or with another fixed angle to the lineation their short limbs do however pass from a period of thickening to final thinning as the folds close and the short limbs become inverted the formation of the folds appears to be controlled by local geometries within the shear zone and we suggest three fold generating mechanisms in one the mylonitic foliation curves ahead of a tectonic lens and starts to fold and in another mesoscopic shear bands with normal fault geometries cut subplanar mylonites and folds are generated by the accretion of material in the evolving gap between the hangingwall and the footwall the third mechanism involves perturbation of the main foliation by slip along an inclined s foliation so that shear folds may develop all the mechanisms involve a rotation of the mylonitic foliation into the compressional field of the instantaneous strain ellipse

comment on the treatment of residual water content in a consistent set of parametric models for the two phase flow of immiscible fluids in the subsurface by l luckner et al

ductile and brittle deformations in the northern snake range nevada abstract the northern snake range east central nevada is one of the metamorphic core complexes of the sevier hinterland within the range a major decollement separates an upper plate composed of brittlely deformed paleozoic sedimentary rocks mostly carbonates from a lower plate composed of metamorphic upper precambrian lower cambrian rocks intruded by gneissic granites a study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that the northern snake range decollement has been a zone of intense non coaxial e vergent shear and transport in a n115 e direction outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains of at least 20 the interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition but might result from two distinct phases of deformation ductile deformation in and below the decollement could have occurred before brittle deformation in the upper plate brittle faulting in the upper plate related to basin and range extension reactivated the upper surface of the ductile shear zone high topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent oligo miocene basins

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

the transition from single layer to foliation boudinage a dynamic modelling approach abstract foliation boudinage is a deflection of foliation in the vicinity of a central discontinuity in foliated rocks mostly filled with vein material it shows evidence for brittle deformation and void opening during ductile flow we used a two dimensional visco elastic spring model based on a discrete element approach to study the dynamic development of foliation boudinage and the behaviour of anisotropic visco elastic material deformed under pure shear conditions the anisotropies are set by defining rheological heterogeneities in the models with 1 a single layer in a weaker matrix 2 multi layers with different elastic properties and 3 random distributed micas rows of horizontally aligned elements with the same elastic properties the models show the nucleation and propagation of fractures during ductile deformation and flow perturbation and localization in the vicinity of fractures voids localize along extension fractures at intersections of conjugate shear fractures and in small pull apart structures along shear fractures which look identical to boudin necks in isolated stiff layers results show that strain localization is strong in models with lower elastic constants and anisotropy whereas it is distributed in the models with higher elastic constants and anisotropy in which asymmetric and complex geometries develop the models successfully show the development of and transitions between a single layer multi layer and foliation boudinage that are common structures in high strain zones

development of shear zone related lozenges in foliated rocks abstract tectonic lozenges are elongate bodies bounded by relatively more deformed rocks the focus of this study is on the 2 d structure of tectonic lozenges developed during ductile shear in rocks with a pre existing mechanical anisotropy on the basis of a detailed analysis of shear zones in foliated rocks from the cap de creus area variscan of the eastern pyrenees five mechanisms to explain the development of different types of lozenges in foliated rocks are suggested these mechanisms are explained on the basis of the orientation of the previous foliation relative to the bulk shearing direction it is shown that the prevailing mechanism does not majorly depend on the bulk kinematics but on the angular relationship between the pre existing foliation and the bulk kinematic axes and on shear zone interaction this has implications on the use of lozenge shapes in tectonic interpretations the fact that there is a wide range of initial orientations propagation modes and coalescence types implies that the final lozenge geometry is not univocally related neither to the type of strain nor to the kinematic regime

late to post collisional brittle ductile deformation in the himalayan orogen evidences from structural studies in the lesser himalayan crystallines kumaon himalaya india abstract the chiplakot crystalline belt ccb is a broad shear zone located within the meta sedimentary zone of the lesser himalaya evidence from mesostructures and microstructures reveals development of this shear zone in a compressional ductile tectonic regime during continental collision in the himalaya by top to sw shearing along ne dipping shear surfaces the shear surfaces are parallel to both the upper and basal thrust planes of the ccb a hitherto unrecognized late to post collisional brittle ductile tectonic regime comprised of two discrete phases is established from dynamic analysis of the kink bands extensional shear fractures foliation boudinage competent layer boudinage quartz veins and associated flanking folds and brittle normal and reverse faults in the ccb kink bands foliation parallel slip and general shear were major consequences of this late to post collisional brittle ductile tectonic episode and were associated with the intrusion of a swarm of quartz veins quartz veins associated with flanking folds are developed at high angles with respect to the mylonitic foliation and parallel to the axial plane of the kink bands throughout the ccb the geometry of the flanking folds along the quartz veins is a consequence of the fracturing and not vice versa the precise shape of the folds depends on the nature of general shearing along the wall of quartz veins and various orientations to the shear direction

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

paleobiogeography of scleractinian reef corals changing patterns during the oligocene miocene climatic transition in the mediterranean abstract during the oligocene miocene greenhouse to icehouse climatic transition the biogeography of reef corals or zooxanthellate like scleractinian corals was gradually changing from a pan tropical tethyan province in the eocene to three reef coral provinces of the western atlantic caribbean indo pacific and mediterranean our reefcoral database encompasses updated and homogenized data on paleoenvironmental and systematics of scleractinian corals occurring in the oligocene and miocene outcrops from circum mediterranean regions provided by most of relatively recently published data in the literature and by the study of published and unpublished collections of coral specimens from the same area including the important collections housed at the mnhn paris and our own collections as there is no validated direct criterion for the identification of the coral zooxanthellate symbiosis in the fossil record and considering the difficulty to use the biogeochemical approaches in the context of this study the subjectivity of the morphological criteria and the relative recent age of the fossil corals we are dealing with a uniformitarian approach has been used for inferring the symbiotic status of scleractinian genera in reefcoral among the 158 genera included in our database 93 can be considered as zooxanthellate and 10 have a doubtful zooxanthellate status this relatively exhaustive database was used to reconstruct the temporal and spatial distribution of scleractinian corals in the mediterranean during the oligocene miocene time in order to discuss the interplaying effects of the global cooling at that time the re organization of the tethyan realm resulting from the african arabian and eurasian plate collision and the emergence of the alpine chains driving the gradual northward movement of the whole region outside the tropical subtropical belt it is shown that the structure of the mediterranean z coral oligocene miocene paleobiodiversity was characterized by many geographically restricted genera with a moderate to short stratigraphical range and a few long ranging widespread genera a major consequence of this structure is that the extinction pattern has proceeded through the preferential extinction of rare occurrence genera through time the potential rapid long distance dispersal of most coral larvae compared to the size of the oligocene miocene mediterranean explains why no biogeographical subprovinces can be distinguished for the z coral fauna on a local scale ecological processes tend to sort coral taxa by limiting z coral development to geographically restricted and discontinuous areas this accounts for the large amount of geographically restricted taxa forming the mediterranean coral fauna the interaction of plate tectonics alpine orogenesis and climate at local to subregional scales exerts strong controls over the spatio temporal distribution of z coral assemblages within the circum mediterranean realm in particular we suggest that the richness and composition of the eastern atlantic coral fauna are indirectly related to the opening and closure of the eastern seaway connection with the indian ocean which controlled the e w circulation of surface waters and hence the westwards dispersal of pelagic larvae at the scale of the whole region the gradual regional climatic change produced by the northwards migration of the entire area superimposed on the global cooling appears in large part responsible for the extinction pattern of z corals through time in the mediterranean biogeographical province

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

analogue modelling of syntectonic leucosomes in migmatitic schists migmatites from the cap de creus tectonometamorphic belt display a wide variety of structures from those formed when the leucosomes were melt bearing to those developed during solid state deformation the observed field structures have been modelled by means of analogue experiments the materials used in the models are layered plasticine as a schist analogue and chocolate as analogue of the crystallizing leucosome a model for the development of syntectonic migmatites is proposed in which initial melt bearing patches preferentially formed within fertile pelitic layers progressively evolve towards lens shaped veins furthermore heterogeneous deformation of anisotropic metasediments facilitates formation of extensional sites for further melt accumulation and transport melt crystallization implies a rapid increase in effective viscosity of leucosomes producing a reversal in competence contrast with respect to the enclosing schists during the whole process deformation localizes around crystallizing veins giving rise to different and contrasting structures for melt bearing and for solid state stages

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

rhodalgal foramol facies in equatorial carbonates insights from miocene tectonic islands of the central atlantic abstract shallow water carbonates ranging in thickness from 250 to 500 m have been deposited during the miocene on the summit of transverse ridges flanking the romanche and vema transforms that offset the equatorial mid atlantic ridge these carbonate platforms are dominated by the same biota assemblages consisting mainly of perforate larger benthic foraminifera and red algae corals are present occasionally as minor components we discuss several factors i e temperature of sea water elevated co 2 levels hydrothermal brine fluxes trophic conditions biological induced precipitation processes water energy substratum stability or instability and platform geometry that could have played a role in favoring specific features that characterize our atlantic carbonates relative to other known examples from the equatorial realm we use the knowledge on the biota association of st paul rocks small islets from the equatorial atlantic the only modern example of oceanic tectonic islands to compare present day with ancient assemblages we propose that equatorial upwelling and humid conditions co 2 input and iron addition related to the mid ocean ridge water energy and substratum instability may have created the conditions suitable for the predominance of calcitic organisms over aragonitic corals in the miocene tectonic islands of equatorial atlantic moreover we do not exclude for the vema paleoislands the influence of the fresh water plumes from south american rivers

the emergence of volcanic oceanic islands on a slow moving plate the example of madeira island ne atlantic the transition from seamount to oceanic island typically involves surtseyan volcanism however the geological record at many islands in the ne atlantic all located within the slow moving nubian plate does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences this suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift eustatic changes or a combination of both and may not involve summit volcanism in this study we explore the consequences for island evolutionary models using madeira island portugal as a case study we have examined the geologic record at madeira using a combination of detailed fieldwork biostratigraphy and 40ar 39ar geochronology in order to document the mode timing and duration of edifice emergence above sea level our study confirms that madeira s subaerial shield volcano was built upon the eroded remains of an uplifted seamount with shallow marine sediments found between the two eruptive sequences and presently located at 320 430 m above sea level this study reveals that madeira emerged around 7 0 5 6 ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano basal intrusions are a likely uplift mechanism and their emplacement is possibly enhanced by the slow motion of the nubian plate relative to the source of partial melting alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes

granite emplacement by crustal boudinage example of the calmayo and el hongo plutons cordoba argentina this study deals with the structure and emplacement of the calmayo and el hongo trondhjemite plutons famatinian belt of cargentina it provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons new u pb geochronological data yield intrusion ages of 512 1 3 4 ma and 500 6 4 5 ma for the calmayo and el hongo plutons respectively the el hongo massif and the southern part of the calmayo trondhjemite preserve magmatic structures whereas the northern domain of calmayo shows the imprint of solid state deformation the main foliation in the country rocks outlines a boudin like pattern at the map scale and the granites are located along boudin necks suggesting that the emplacement of these tron dhjemite plutons was linked to large scale boudinage of the country rocks

miocene to pleistocene transatlantic dispersal of ceratoconcha coral dwelling barnacles and north atlantic island biogeography coral dwelling pyrgomatid barnacles subfamily ceratoconchinae were widely dispersed throughout the paratethys and mediterranean seas as well as the atlantic ocean during the neogene but today are limited to the western atlantic herein the paleobiogeographic origin and dispersal of the genus ceratoconcha is studied based on a combination of field taxonomic and literature studies the first confirmed appearances of ceratoconcha occur in lower miocene strata burdigalian with two closely related species on both sides of the atlantic in western france and florida fossils from the miocene of lanzarote in the canary islands and pleistocene of maio in the cape verde islands extend the known geographical and temporal range of the ceratoconcha barnacles in the eastern atlantic during the neogene dispersal of marine taxa was a two way process due to tectonic changes both influencing oceanic circulation and appearance and disappearance of oceanic islands during the early miocene gyre formation was weak and the atlantic ocean mid latitudes were warmer than today this resulted in increased hurricane activity and the expansion of hermatypic coral hosts farther north in the north atlantic normal ocean circulation transported barnacle larvae from east to west but currents generated by hurricanes may have transported them in the opposite direction towards the margins of the northeastern atlantic islands in between abetted barnacle contact and dispersal the temporal range for ceratoconcha is extended considerably in the eastern atlantic from the early pliocene to the pleistocene the hermatypic host corals of ceratoconcha suffered a severe decline in the eastern atlantic and the mediterranean after the miocene corals were present during the pliocene and pleistocene in the cape verde islands this suggests that the southernmost oceanic islands acted as a tropical refuge for host corals and their likely barnacle symbionts

cenozoic seawater sr ca evolution records of seawater chemistry help constrain temporal variations in geochemical processes that impact the global carbon cycle and climate through earth s history here we reconstruct cenozoic seawater sr ca sr casw using fossil conus and turritellid gastropod sr ca combined with an oxygen isotope paleotem perature record from the same samples the gastropod record suggests that sr casw was slightly higher in the eocene 11 4 3 mmol mol than today 8 54 mmol mol and remained relatively stable from the mid to late cenozoic we compare our gastropod cenozoic sr casw record with a published turritellid gas tropod sr casw record and other published biogenic benthic foraminifera fossil fish teeth and inorganic pre cipitate calcite veins sr casw records once the uncertainties with our gastropod derived sr casw are taken into account the sr casw record agrees reasonably well with biogenic sr casw records assuming a seawater ca history derived from marine evaporite inclusions all biogenic based sr casw reconstructions imply decreasing seawater sr through the cenozoic whereas the calcite vein sr casw reconstruction implies increasing sr through the cenozoic we apply a simple geochemical model to examine the implications of divergence among these seawater sr reconstructions and suggest that the interpretation and uncertainties associated with the gastropod and calcite vein proxies need to be revisited used in conjunction with records of carbonate depositional fluxes our favored seawater sr ca scenarios point to a significant increase in the proportion of aragonite versus calcite deposition in shelf sediments from the middle miocene coincident with the proliferation of coral reefs we propose that this occurred at least 10 million years after the seawater mg ca threshold was passed and was instead aided by declining levels of atmospheric carbon dioxide

late acadian alleghenian transpressional deformation evidence from asymmetric boudinage in the casco bay area coastal maine abstract asymmetric competent layer and foliation boudinage in the casco bay area of coastal maine formed during regional f 2 hinge parallel elongation and distributed dextral strike slip movement it consists of backward rotated oblique shear fracture boudinage and forward rotated extension fracture boudinage initially symmetric quartz filled extension fractures were intermittently developed as boudin partings orthogonal to the prominent ductile stretching lineation due to fault related fluctuations in fluid pressure and tectonic stresses modification by concurrent non coaxial deformation related to dextral strike slip movement resulted in the clockwise rotation sinistral slip and vein parallel elongation reflected in the forward rotated asymmetric boudinage discordant planar pegmatite veins and variably oriented boudinaged pegmatite sheets common in these exposures underwent a similar evolution the development of this asymmetric boudinage is attributed to progressive brittle ductile non coaxial deformation related to orogen parallel dextral transpression along the norumbega nonesuch river strike slip fault system during the later stages of the acadian orogeny and subsequent alleghenian deformation

rhodolith transport and immobilization on a volcanically active rocky shore middle miocene at cabeco das laranjas on ilheu de cima madeira archipelago portugal abstract extraordinary deposits of fossil rhodoliths occur at the cabeco das laranjas portuguese hill of the oranges in a small fault block at the northwest end of ilheu de cima off porto santo in the madeira archipelago stratigraphic repetitions of densely packed rhodolith beds up to 2 6 m thick are associated with a receding rocky shoreline and are interpreted as the result of hurricanes the initial storm deposit sits unconformably on basalt and eroded basalt boulders associated with tuff and volcaniclastic breccia approximately 90 000 rhodoliths of middle miocene age 14 15 ma are exposed on the upper surface of the initial deposit over a 450 m2 shelf exhumed from the hill s southeast side ranging in diameter from 3 cm to 20 cm many of the rhodoliths generated by crustose coralline red algae are now iron stained and resemble a mass of oranges in gross appearance sea stacks and large boulders rise through the thick basal rhodolith bed to form small catchment areas that held the deposit in place after the storm s passage the succeeding rhodolith deposits are variably separated by layers with mixed carbonate and volcanic sand pure volcanic lapilli and volcaniclastic tephra mixed with tuff showing swaley cross stratification three out of four rhodolith beds are truncated against the flank of the adjoining rocky shore only the youngest fourth rhodolith layer is fully exposed around the perimeter of the hill and can be shown to cross a basalt barrier that is traceable for 70 m in cross section as an erosional ramp dipping from 6 to 8 southeast the entire fossil rich sequence is capped by a basalt flow showing columnar disjunction based on thin section analysis three genera of coralline red algae are recognized in the basal rhodolith deposit sporolithon lithothamnion and neogoniolithon associated biodiversity is low represented by 16 kinds of marine invertebrates dominated by encrustations and borings on the rhodoliths and very few free body fossils the madeira region of the north atlantic may have been susceptible to major cyclonic storms immediately after the middle miocene climate optimum when a northward shift of the inter tropical convergence zone was stimulated by a steeper temperature gradient in the southern hemisphere related to expansion of continental glaciers on antarctica

relative hydraulic conductivity in the vadose zone from magnetic resonance sounding brooks corey parameterization of the capillary fringe the magnetic resonance sounding mrs method is used for noninvasive one dimensional assessment of aquifer structures i e for estimating the vertical water content and hydraulic conductivity distributions in the saturated zone so far mrs interpretation schemes for estimating the hydraulic conductivity as a function of the saturation degree in the vadose zone have not been developed in this study we developed a new inversion scheme for mrs to estimate the relative hydraulic conductivity krel which is the ratio of the unsaturated and saturated conductivities the new approach is based on the brooks corey parameterization bcp of the capillary fringe that is the water content distribution of the capillary fringe is considered to be equivalent to the water retention curve in equilibrium state the bcp inversion directly provides estimates of brooks corey parameters from mrs data which allows for the calculation of krel as a function of the saturation degree in doing so the water table must be gi

an outline of shear sense analysis in high grade rocks ductile shear zones are important in tectonic reconstructions as a source of information on the relative motion of large crustal blocks or plates in the geological past methods to interpret fabric in ductile shear zones were mostly developed for low grade rocks where overprinting relations are usually well preserved however high grade shear zones are common and dominate in many precambrian terrains high grade shear zones should be analysed in a different way from low grade zones the plane on which shear sense markers should be observed the vorticity profile plane is more difficult to find than in low grade shear zones the most reliable shear sense markers in high grade shear zones are shear bands mineral fish mantled porphyroclasts sigmoids and asymmetric boudins 2006 published by elsevier b v on behalf of international association for gondwana research

alkenone producers during late oligocene early miocene revisited this study investigates ancient alkenone producers among the late oligocene early miocene coccolithophores recorded at deep sea drilling project dsdp site 516 contrary to common assumptions reticulofenestra was not the most important alkenone producer throughout the studied time interval the comparison between coccolith species specific absolute abundances and alkenone contents in the same sedimentary samples shows that cyclicargolithus abundances explain 40 of the total variance of alkenone concentration and that the species cyclicargolithus floridanus was a major alkenone producer although other related taxa may have also contributed to the alkenone production at dsdp site 516 the distribution of the different alkenone isomers mec37 2 etc38 2 and mec38 2 remained unchanged across distinct changes in species composition suggesting similar diunsaturated alkenone compositions within the noelaerhabdaceae family during the late oligocene early miocene however the overall larger cell size of cyclicargolithus may have implications for the alkenone based reconstruction of past partial pressure of co2 our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods 1 85 ma predating the first occurrence of contemporary alkenone producers i e emiliania huxleyi and gephyrocapsa oceanica

large scale foliation boudinage in gneisses abstract large scale extensional structures in gneisses symmetric and asymmetric foliation boudinage c type shears are common in crustal scale shear zones and may constitute shear criteria as well as being indicators of high strain

review on symmetric structures in ductile shear zones symmetric structures in ductile shear zones range widely in shapes and geneses matrix rheology its flow pattern its competency contrast with the clast degree of slip of the clast shear intensity and its variation across shear zone and deformation temperature and degree of confinement of clast in shear zones affects independently the degree of symmetry of objects kinematic vorticity number is one of the parameters that govern tail geometry across clasts for example symmetric and nearly straight tails develop if the clast matrix system underwent dominantly a pure shear compression prolonged deformation and concomitant recrystallization can significantly change the degree of symmetry of clasts angular relation between two shear zones or between a shear zone and anisotropy determines fundamentally the degree of symmetry of lozenges symmetry of boudinaged clasts too depends on competency contrast between the matrix and clast in some cases and on the degrees of slip of inter boudin surfaces and pure shear parasitic folds and post tectonic veins are usually symmetric

the canary record of the evolution of the north atlantic pliocene new 40ar 39ar ages and some notable palaeontological evidence abstract two new 40ar 39ar ages and previously published k ar ages of basaltic pillow lava flows are coeval with closely related fossiliferous marine layers allowing us to establish the beginning 5 8 5 0 4 8 ma at ajui fuerteventura island and 4 8 0 03 ma 2 at tamaraceite and a middle stage 4 20 0 18 ma 2 at la esfinge in gran canaria island of early pliocene marine deposits in the canary islands here the presence of tropicopolitan fossils megaselachus megalodon janthina typica suggests the influence of a possible central american circumtropical current during the pliocene and in the north atlantic basin the presence of both janthina typica and the argonauta argo also indicate the occurrence of tropical storms in the area of the canary islands around that time in addition the pillow lava flow at la esfinge overlies a submarine pyroclastic layer containing ostrea offreti that was preserved in its life position a fossil from the terminal miocene and commonly found on the south eastern iberian peninsula at la cruz on fuerteventura island a dune formed above marine deposits lies beneath a palaeosol and a basaltic lava flow that was k ar dated at 2 9 ma near el marmol in gran canaria the corresponding palaeosol lies between lava flows k ar dated at 3 ma based on these observations we argue in this paper that the first appearance of the canary current the eastern boundary current of the north atlantic sub tropical gyre occurred between 4 2 ma and 2 9 ma ago this was followed by a major sea level drop of about 40 m and the formation of a first fossil bearing palaeosol which indicate that major climate change that took place around 2 95 2 82 ma

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

finite strain determination of gneiss application of fry s method to porphyroid in the southern massif central france abstract we present an attempt to apply fry s method of finite strain ellipsoid determination to deformed gneissic rocks several finite strain ellipsoids have been measured on different porphyroid gneiss samples from the cevennes area southern massif central france the ellipsoids are characterized by a k parameter close to 1 plane strain or k 1 constriction field the method appears to be powerful and we show that in some cases effects of a pretectonic fabric can be eliminated from these results and microstructural data a strain path can be inferred

dispersal in the ordovician speciation patterns and paleobiogeographic analyses of brachiopods and trilobites abstract the middle to late ordovician was a time of profound biotic diversification paleoecological change and major climate shifts yet studies examining speciation mechanisms and drivers of dispersal are lacking in this study we use bayesian phylogenetics and maximum likelihood analyses in the r package biogeobears to reanalyze ten published data matrices of brachiopods and trilobites and produce time calibrated species level phylogenetic hypotheses with estimated biogeographic histories recovered speciation and biogeographic patterns were examined within four time slices to test for changes in speciation type across major tectonic and paleoclimatic events statistical model comparison showed that biogeographic models that incorporate long distance founder event speciation best fit the data for most clades which indicates that this speciation type along with vicariance and traditional dispersal were important for paleozoic benthic invertebrates speciation by dispersal was common throughout the study interval but notably elevated during times of climate change vicariance events occurred synchronously among brachiopod and trilobite lineages indicating that tectonic climate and ocean processes affected benthic and planktotrophic larvae similarly middle ordovician inter oceanic dispersal in trilobite lineages was influenced by surface currents along with volcanic island arcs acting as stepping stones between areas indicating most trilobite species may have had a planktic protaspid stage these factors also influenced brachiopod dispersal across oceanic basins among laurentia avalonia and baltica these results indicate that gyre spin up and intensification of surface currents were important dispersal mechanisms during this time within laurentia surface currents hurricane tracks and upwelling zones controlled dispersal among basins increased speciation during the middle ordovician provides support for climatic facilitators for diversification during the great ordovician biodiversification event similarly increased speciation in laurentian brachiopod lineages during the hirnantian indicates that some taxa experienced speciation in relation to major climate changes overall this study demonstrates the substantial power and potential for likelihood based methods for elucidating biotic patterns during the history of life

late oligocene initiation of the antarctic circumpolar current evidence from the south pacific the antarctic circumpolar current acc is a key feature of the southern ocean its development may have helped cool antarctica and initiate southern hemisphere glaciation the deep circulation of the acc must have been established after both the tasman gateway between antarctica and australia and the drake passage between south america and antarctica opened however estimates for acc initiation range over 20 m y from the middle eocene to early miocene a new piston core of upper oligocene to holocene sediments from the south pacific has allowed us to delimit the formation of the acc to the late oligocene ca 25 23 ma upper oligocene current worked sediments and a hiatus to the upper miocene result from the beginning of the modern acc flow i e when strong currents and mixing throughout the water column were established previously published nd isotope data date the first intrusion of pacific water into the atlantic much earlier the discrepancy with our results can be reconciled by the different methods measuring different flow regimes tracer methods such as nd are sensitive to relatively small and shallow incursions of water whereas pelagic erosional regimes require vigorous deep flow

evaluating the relative air permeability of porous media from their water retention curves accurate modeling of water and air flow in porous media requires the definition of the relevant hydraulic properties namely the water retention curve wrc and the relative hydraulic conductivity function rhc as well as the definition of the relative air permeability function rap capitalizing on the approach developed previously to represent the rhc a new model allowing the prediction of rap based on information resulting from the wrc is proposed the power value a in the model is a decreasing exponential function of the coefficient of variation characterizing the pore size distribution of the porous medium and derived from its wrc the model was calibrated using data from 22 disturbed and undisturbed soil samples and was validated using data from eight soil types ranging from quartz sand to silty clay loam the proposed model provided accurate prediction of the soil rap and performed in some cases sandy loam and silty clay loam soils better than available alternative models

geometry of torn boudin an indicator of relative viscosity abstract the present study determines the role of viscosity on the development of rectangular torn boudin and its various types defined by the curvature of their exterior and face margins numerical modeling was performed with the help of finite element method considering maxwell visco elastic materials in commercial code ansys seven different viscosities were used and interchanged among the boudin inter boudin and matrix materials to understand the effect of viscosity ratios specifically of relative viscosity of inter boudin material results show that the viscosity of inter boudin material has significant control on the shape of torn boudins apart from the viscosity ratio of boudin to matrix material bone shaped boudin develops only when the inter boudin is more competent than boudin and it becomes more prominent when matrix is also competent than boudin but incompetent than inter boudin when boudins are stiffer than inter boudin barrel shaped and fish head boudins with concave faces develop exterior or face margins remain almost straight when boudin is relatively rigid compared to its surrounding matrix materials or when there is no or very little viscosity contrast between boudin and inter boudin material even in case of large boudin matrix viscosity contrast therefore the relative viscosity among the boudin inter boudin and matrix materials can be estimated qualitatively by studying the shape of boudin in the field

circum mediterranean oligo miocene biogeographic evolution the gastropods point of view based on studies in iran qom basin esfahan sirjan basin zagros mountains in turkey mut and sivas basins in the mesohellenic basin and in northeastern egypt a new palaeobiogeographic concept for the oligocene and miocene in the circum mediterranean area with special emphasis on the distribution patterns of gastropod faunas is presented a very strict biogeographic terminology is proposed to avoid the common fusion of geographic faunistic and geodynamic terms in the palaeontologists jargon our main interest focuses on the affinities and the faunistic interactions between the early mediterranean and the early indo pacific regions during the oligo miocene especially the role of the early indo pacific faunas as the mythic centre of origination of early neogene european mollusc faunas as often vaguely indicated in the literature is critically evaluated this leads to the establishment of a mediterranean iranian province and a western indian eastern african province as palaeobiogeographic subunits of the western tethys region during the oligocene due to major geodynamic and related biogeographic changes in the early miocene the western tethys region no longer existed and was replaced by the proto mediterranean atlantic region for the area of the paratethys the palaeobiogeographic unit danubian province which includes the protocaspian subprovince is proposed furthermore the underrated transatlantic communication of the oligo miocene nearshore mollusc faunas is demonstrated altogether this paper is a contribution to the discussion on the biogeographic concepts classification and nomenclature in palaeontology as initiated by westermann palaeogeogr palaeoclimatol palaeobiogeogr 158 2000 1 13 163 2000 49 68 2002 elsevier science b v all rights reserved

boudinage in nature and experiment abstract deformation of rocks produces structures at all scales that are in many cases periodic folding or boudinage with variable amplitude and wavelength here we focus on boudinage a process of primordial importance for tectonics in the present study we carried out measurements of natural boudins and experimentally tested the effects of two variables on boudinage layer thickness and compression rate the models were made of a competent layer mostly brittle as in nature of either elastic soft paper or viscoelastoplastic clay material embedded in a ductile matrix of linear viscous silicone putty the competent layer lied with its greatest surface normal to the principal shortening axis and greatest length parallel to the principal stretching axis the model was then subjected to pure shear at constant piston velocity and variable competent layer thickness model 1 or at different piston velocity and constant layer thickness model 2 the results of model 1 show an exponential dependence of boudin width on competent layer thickness in disagreement with data from the studied natural occurrence this indicates that variables other than competent layer thickness are hidden in the linear relationship obtained for the natural boudinage the results of model 2 show that the higher the velocity the smaller the boudin width following a power law with exponent very similar to that of analytical predictions the studied natural boudinage occasionally occurs in two orthogonal directions this chocolate tablet boudinage can be the result of two successive stages of deformation buckling followed by stretching of competent sandstone layers or buckling followed by rotation of reverse limbs into the extensional field of simple shear

unsaturated hydraulic conductivity function based on a soil fragmentation process we present a new two parameter expression for relative hydraulic conductivity rhc of partially saturated soils it is based on the premise of assouline et al 1998 that soil structures evolve from a uniform random fragmentation process this assumption allows us to derive hydraulic properties of soils water retention curves and unsaturated hydraulic conductivity from primary properties such as pore geometry and soil structure we tested our rhc expression against different soil types and found that it fits data better than the widely used models of brooks and corey 1964 and van genuchten 1980

a post deformational post peak metamorphic timing for mineralization at the archaean big bell gold deposit western australia abstract previous workers were unable to find any suitable criteria to time gold mineralization at big bell mine an enigmatic low grade large tonnage deposit hosted in an upper amphibolite facies greenstone belt sequence in the murchison province of the archaean yilgarn craton western australia earlier studies suggested that the gold mineralization occurred by hydrothermal alteration either synchronous with volcanism or prior to peak metamorphism the following conclusions have been drawn from a detailed meso and microstructural analysis of the mine sequence and greenstone belt hosting the orebody 1 1 prior to mineralizaton the big bell area underwent five phases of deformation and m1 regional metamorphism to the staurolite andalusite zone of the amphibolite facies for metasediments 2 2 granite emplacement post dated d5 folding causing local m2 thermal metamorphism this peaked at the ms qtz breakdown reaction for pelites on or close to the and sil transition but below the onset of partial melting characterized by the narrow non degenerate divariant field involving seven kfmash phases qtz ms bt crd kfs als h2o this hornfelsing consumed cross cut and post dated s5 cleavage prograde qtz and sil veins replaced this hornfels assemblage and s5 cleavage and were caused by volatile escape at depth and hydraulic fracturing coincident with a reduction in bulk rock permeability resulting from post deformational hornfelsing 3 3 retrograde m3 assemblages and two stages of gold related mineralization cross cut and replaced all previous structures the retrograde gold bearing assemblages are both underformed stage 1 mineralization is typically found with widespread retrograde muscovite plates and consists of the following mineral associations py po ccp ilm mag au tur sch rt spn zrn ms bt ep czo qtz the second stage of mineralization is characterized by pervasive sericitic alteration that is locally developed to an extent where all pre existing mineralogies including quartz are totally destroyed within sericitic domains the stage 2 mineral associations are apy sbt sb mo sp au py po ilm mag ccp sericite chl carbonate cal ep czo qtz 4 4 gold mineralization at big bell was post deformational and post peak metamorphic unlike many archaean gold deposits there are no obvious shear zones controlling the formation of the big bell gold deposit at the structural level exposed either on the surface or in drill core big bell mine was in a low strain domain during the formation of late stage sinistral sub vertical d6 shear zones that are typically concentrated at the granitoid greenstone contacts the retrograde character of the mineralization has been deduced from detailed meso and microstructural observations and demonstrates the emplacement of this gold deposit was very late in the overall orogenic history of the murchison province

modeling the miocene climatic optimum ocean circulation 1 ocean circulation is investigated using the community climate system model 3 ccsm3 forced with early to middle miocene 20 14 ma topography bathymetry vegetation and modern co2 significant bottom water formation is modeled in the weddell sea along with intermediate north component water formation in the north atlantic this is attributed primarily to stronger and weaker than modern convective preconditioning in the weddell and labrador seas respectively global meridional overturning and gyre circulation is weaker in the miocene due to weaker midlatitude westerlies in the southern hemisphere caused by lowering of the meridional surface temperature gradient in addition to regional influences on convection subsurface temperatures in the miocene are significantly higher in the far north atlantic greenland norwegian seas and arctic basin compared to the present ocean heat transport is symmetrical about the equator and resembles that simulated for late cretaceous and early cenozoic climates suggesting the northern hemisphere dominated ocean heat transport active today developed after the middle miocene simulated deep water warming in the miocene is more than an order of magnitude lower than indicated by proxies this discrepancy is not reconciled by higher co2 due to the persistence of sea ice at sites of deep water formation this suggests that either the ccsm3 is insufficiently sensitive to miocene boundary conditions greater greenhouse forcing existed than is currently reconstructed or that proxy records of warming are exaggerated given the diversity of global miocene proxy records and their near unanimous estimate of a significantly warmer earth the first two options are more likely

evidence for age and evolution of corner seamounts and great meteor seamount chain from multibeam bathymetry the comer seamounts in the western north atlantic and great meteor seamount chain in the eastern north atlantic are thought to progress in age from late cretaceous through late cenozoic they both presumably formed by volcanism above the new england hotspot when first the north american plate and then the mid atlantic ridge axis and african plate moved over the hotspot high resolution multibeam bathymetry of the seamounts shows geomorphic features such as guyots terraces and a base level plateau cruiser plateau that we interpret to have formed at sea level we have backtracked these features to sea level along the north atlantic crustal age depth curve in order to estimate their ages the derived age pattern of volcanism indicates formation of the comer seamounts at ca 80 ma to 76 ma with migration of the mid atlantic ridge plate boundary over the hotspot and formation of the cruiser plateau about 76 ma seamount ages suggest that subsequent volcanism on the african plate moved first northward in the late cretaceous to early cenozoic plato tyro and atlantis seamount groups then southward to great meteor seamount in the late cenozoic recurrent volcanism appears to have occurred at some seamounts up to 20 30 m y after their initial passage over the hotspot it would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away

regional anomalies of sediment thickness basement depth and isostatic crustal thickness in the north atlantic ocean abstract we calculate the anomalous basement topography for the north atlantic ocean from 30 to 70 n latitude and from 0 to 70 w longitude at a resolution of roughly 6 6 km using grids of total sediment thickness and observed and predicted sea floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age plotting this residual topography for various plate reconstructions during opening of the north atlantic we delineate consistent patterns of basement highs related to variations in hotspot related volcanism in addition to iceland and the azores we recognize three centers of excess volcanism at the mid atlantic ridge the milne seamounts and azores biscay rise 75 40 ma the southeast newfoundland ridge and madeira tore rise 130 110 ma and the east and west thulean rises 60 50 ma the duration of volcanic activity ranges from 8 to 10 m y thulean rises to 60 m y iceland and thus it appears that both long and short lived hotspots coexist even in relatively close proximity in contrast during the period 110 60 ma we observe little excess volcanism during either continental breakup or seafloor spreading we estimate isostatic crustal thickness from the anomalous basement depths after first removing dynamic effects created by mantle flow maximum thicknesses of volcanic features from 30 km beneath the greenland iceland faeroe ridge to 15 km beneath the azores biscay rise are broadly consistent with seismic data and predictions of decompression melting widths of volcanic features indicate that thickening primarily occurs within 100 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity i e east greenland and the hatton rockall bank we observe conjugate structures south of greenland and edoras bank where excess volcanism appears to have extended beyond the margin proper and into oceanic crust similar conjugate features appear in the labrador sea south of davis strait finally we identify anomalous oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non isostatic effects within the mantle

origin and geochemical evolution of the madeira tore rise eastern north atlantic the madeira tore rise located 700 km off the nw african coast forms a prominent ridge in the east atlantic the age and origin of the rise are controversial this study presents major and trace element sr nd pb hf isotope and 40ar 39ar age determinations from volcanic rocks dredged from different sites along the rise in addition isotopic compositions of rock samples from great meteor seamount in the central atlantic are presented the new radiometric and paleontologically constrained ages identify two major episodes of volcanism the first is the base of the rise circa 80 to 95 ma and the second is seamounts on the rise 0 5 16 ma it is proposed that interaction of the canary hot spot with the mid atlantic spreading center formed the deep basement of the madeira tore rise and the j anomaly ridge west of the atlantic spreading center in the mid cretaceous age and geochemical data and plate tectonic reconstructions suggest however that the recovered late cretaceous volcanic rocks represent late stage volcanism from the time when the madeira tore rise was still close to the canary hot spot long after moving away from the influence of the canary hot spot the madeira tore rise was overprinted by late cenozoic volcanism miocene to pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the azores gibraltar fracture zone african eurasian plate boundary the geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere possibly originating from the cretaceous canary plume alternatively these late cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic eclogitic material in the upper mantle isotopically more depleted pliocene to pleistocene volcanism at the southern end of the madeira tore rise may be related to the nearby madeira hot spot

miocene ocean circulation inferred from marine carbon cycle modeling combined with benthic isotope records 1 in a modeling sensitivity study we investigate the evolution of the ocean circulation and of marine carbon isotope 13c records during the miocene about 23 5 million years ago for this purpose we ran an ocean circulation carbon cycle model of intermediate complexity large scale geostrophic hamburg ocean carbon cycle model version 2s exploring various seaway configurations our investigations confirm that the central american seaway played a decisive role in the history of the miocene ocean circulation in simulations with a deep central american seaway depth range 1 3 km typical for the early to middle miocene deep water production in the north atlantic is absent or weak while the meridional overturning circulation is dominated by water mass formation in the southern ocean deep water formation in the north atlantic begins when the central american seaway shoals to a few hundreds of meters which is typical for the late miocene our results do not support ideas that the mid miocene closing of the eastern tethys contributed to antarctic glaciation on the other hand we find some water exchange between the indian ocean and the atlantic via the eastern tethys during the early miocene our model results for the atlantic meridional overturning circulation and for atlantic 13c during the late miocene are largely independent from depth variations of the greenland scotland ridge to a large extent the evolution of miocene deep sea 13c records can be explained with large scale ocean circulation changes our model data comparison for the middle and early miocene suggests that during the early neogene the seaway effect on benthic 13c may have been superimposed by further factors such as climate regime shifts and or terrestrial carbon cycle changes

steep structure formation in the okiep copper district south africa bulk inhomogeneous shortening of a high grade metamorphic granite gneiss sequence abstract subhorizontal gneissosities and lithologies of the 1200 1000 ma granulite facies granite gneiss terrane of the okiep copper district south africa are cut by narrow discontinuous e trending cusp like and or monoclinal structures in which the regional gneissosity has been rotated to subvertical attitudes closely associated migmatization and charnockitization indicates that these steep structures formed during high grade metamorphic conditions in this paper we present a model which relates the deformational style strain and orientation of the steep structures to bulk inhomogeneous shortening the development of steep structures can be described as a progression from initial folding via explosive fold amplification to the progressive obliteration of folds by a subvertical e trending intensely developed transposition fabric this fabric is parallel to the axial planes of regional scale open folds shortening in the structures is mainly accommodated by a vertical material extrusion resulting in the commonly upward pointing steep structure geometries and volume loss the strain compatibility with surrounding rocks is maintained by bounding shear zones the progressive development of steep structures illustrates the close relationship between buckle folding and the formation of internal induced anisotropies during deformation of the high grade metamorphic granitic gneisses the apparent discrepancy between large scale open folding and simultaneously developed steep structures recording large finite shortening strains is interpreted to be the result of a large component of internal layer parallel shortening and strain partitioning

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

miocene pliocene rocky shores on sao nicolau cape verde islands contrasting windward and leeward biofacies on a volcanically active oceanic island abstract north atlantic islands in the cape verde archipelago off the coast of west africa commonly feature an elongated n s shape in which reduced northern coasts and longer eastern shores absorb the brunt of wave activity and long shore currents generated by prevailing north east trade winds located in the middle windward islands sao nicolau is unusual in profile with an elongated e w configuration that offers a broad target against high energy wind driven waves conversely the south shore of sao nicolau provides relatively wide shelter in a leeward setting reconstruction of the proto island prior to the onset of the main eruptive stage during the late miocene at 5 1 ma reveals a moderately smaller island with essentially the same e w orientation this study combines previous data with results from a detailed stratigraphic log based on upper miocene limestone deposits on the island s south flank for comparison with stratigraphic profiles of upper miocene limestone from the island s northeast quarter logs from a pliocene sandy limestone outcropping on the south central coast of sao nicolau give added context to the diversity of marine invertebrates including branching coral colonies and delicate ramose bryozoans that found shelter in a leeward setting whole rhodoliths contribute the main fabric of carbonates deposited against rocky shores on the northern exposed side of the miocene island whereas only traces of worn rhodoliths and rhodolith sand occur as in finer miocene grainstone on the island s southern protected side miocene and pliocene carbonate deposits were terminated by submarine flows on an actively growing volcanic island the passage zone from submarine to subaerial flows on the island s flanks makes a useful meter stick to gauge absolute water depth at the moment of local extinction by volcanic activity

large scale deformational systems in the south polar layered deposits promethei lingula mars soft sediment and deep seated gravitational slope deformations mechanisms abstract the present study is the first attempt at a detailed structural and kinematic analysis of large scale deformational systems observed in the south polar layered deposits splds in the promethei lingula pl margins mars by systematically collecting attitude data referable to previously unknown deformational structures and defining the cross cut relationships of the structures we reconstructed a deformational history consisting of two superimposed well defined stages the first stage is dominated by large scale strike slip and transtensional faults arranged into conjugate systems and delimiting shear zones that show a wide range of subsidiary structures including normal and reverse faults drag folds boudins s c tectonites and sub horizontal interstratal shear planes marked by sygmoidal boudins other typical structures referable to this event are ductile folds locally true convolute folds and lobes ball and pillow structures affecting certain marker beds of the succession we suggest that the structural assemblage might be the expression of a shallow soft sediment tectonics that possibly occurred during warm periods of the south pole climate the second stage seems to affect the weaker and in certain cases pre deformed stratigraphic levels of the spld succession this stage is mainly characterized by extensional deformations caused by gravity the consequence of the deformations is the nucleation of deep seated gravitational slope deformations dsgsds marked by typical morphostructures such as scarps trenches and bulging basal contractant zones these phenomena were never observed within an ice cap according to terrestrial modeling these slow collapses were caused by 1 the presence of detachment levels i e subhorizontal bedding planes along which the ice sheet margins can slide and 2 the development of listric faults within the glacial mass which merge with sub horizontal shear planes in the subsurface the presence of complex deformational systems in the spld necessarily implies that a large scale dynamics of the ice sheet occurred in the past the relatively fast internal creep and basal internal sliding inferable from the structure assemblage can be due to partial melting of the ice possibly caused by climatic changes in the promethei lingula region in this manner we believe that climate heating which according to the literature is likely caused by orbital variations softened some of the spld layers triggering or accelerating the ice sheet s outward movement the evidence of a marked disharmonic deformational style through the spld succession suggests the possibility of local periodic compositional variations in the sequence

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

two dimensional numerical modeling of fracturing and shear band development in glacier fronts in this contribution we present a two dimensional numerical model of a deforming glacier front the model is based on a hybrid lattice spring network approach where particles in the model can deform in a volume conservative visco elastic manner but at the same time they can be compressed elastically and fracture by discrete failure we restrict ourselves to a simple setting where the glacier sits on a frictionless slope that dips with 5 10 the ice block is fixed on one side and has a free surface on the other the glacier varies in viscosity and can flow at the base whereas it is brittle at the top results show that the head of the glacier is unstable failure happens as a combination of extension fractures crevasses at the top surface of the glacier and shear fractures that are dipping toward the glacier head once the shear fractures intersect with the free side wall of the glacier a triangular ice block is carving from the glacier head during successive flow of the glacier the failure is stepping backwards into the glacier and large shear planes develop that connect the sliding ice at the base with crevasses at the top variations of overall viscosity of the glacier indicate that higher viscosities and thus a more brittle glacier lead to larger spacing of shear surfaces and thus to larger ice blocks that are carving from the head of the glacier in addition the geometry of the deformation structures within the glacier does not vary significantly with the height of the ice indicating that larger glaciers carve larger blocks a higher tilt of the ground surface however leads to tighter spacing of shear surfaces and a more pronounced crevasse development this indicates that glacier heads that lie on steeper slopes will carve smaller blocks than glacier heads that lie on shallower slopes failure and carving of ice from the model glaciers is a combination of early developing closely spaced extension fractures crevasses and later developing wider spaced and more localized shear fractures or shear zones

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values