low to medium pressure variscan metamorphism in galicia nw spain evolution of a kyanite bearing synform and associated bounding antiformal domains

geometry and regional significance of neoproterozoic cadomian structures of the narcea antiform nw spain the narcea antiform is located at the hinterland to foreland transition of the variscan belt in the nw of the iberian massif it consists of an antiformal stack of fault propagation folds that developed in the hanging wall of allochthonous thrust sheets during the first variscan deformation event in the palaeozoic rocks the neoproterozoic narcea slates formation cropping out in the core of the narcea antiform records a cadomian deformation event that formed large asymmetric nw vergent folds with related axial planar cleavage developed in the lower greenschist facies these folds are truncated by the lower cambrian angular unconformity lower cambrian conglomerates contain pebbles with predepositional microstructures that are widespread in the underlying neoproterozoic rocks the narcea antiform provides valuable information concerning major tectonic cadomian events the large scale near recumbent cadomian folds developed between 560 and 540 ma top to the nw kinematics can be deduced for the cadomian plate convergence transverse to the variscan convergence in present coordinates it was followed shortly afterwards by large displacements along the northern gondwana margin that resulted in its accretion alongside the essentially undeformed neoproterozoic rocks of the central iberian zone before early cambrian times

role of environmental change in rock boring echinoid trace fossils abstract well preserved groove borings related to the boring activity of regular sea urchins are preserved on neogene rocks associated with rocky palaeoshores in a volcanic oceanic island santa maria azores archipelago and a continental margin foz da fonte central west portugal the new trace fossil consists of a series of relatively deep grooves with sinuous pathway and with a fairly uniform width along its trajectory a series of forward oriented concave markings occur frequently inside the grooves reflecting the movement direction of the producer the trace fossil is designated as ericichnus igen n with two new ichnospecies ericichnus bromleyi isp n and ericichnus asgaardi isp n ericichnus igen n could have been produced by regular echinoids similar to the extant echinometrid echinoid echinometra lucunter linnaeus 1758 for e bromleyi isp n and similar to the echinid paracentrotus lividus lamarck 1816 for e asgaardi isp n ethologically the compound trace fossil ericichnus igen n circolites may correspond to an agrichnion pascichnion and or domichnion our study suggests that the new ichnospecies e bromleyi may be used as a palaeoenvironmental tool indicating the boring activity of termophile rock boring echinoids in the fossil record

tracers of uplift and subsidence in the cape verde archipelago abstract ocean islands are subject to vertical movements through their lifetime and these movements9 origins are poorly understood tracking island freeboard provides insight into the mechanisms that cause these movements the cape verde archipelago is the result of hotspot volcanism ranging from 26 ma to the present and constitutes a structural pile that has preserved a record of the palaeo positions of mean sea level these palaeo markers both sedimentary and volcanic are datable and can be used to track the island9s history of vertical movements when used in conjunction with a eustatic curve we have studied the volcanostratigraphy of cape verde and mapped the palaeo marker positions and features and we provide the sea level height information that can be estimated through them evidence exists for differential uplift histories throughout the archipelago with substantial uplift in some of the islands synchronous with volcanism other islands in contrast exhibit evidence for stability or complex uplift subsidence histories supplementary material detailed island geology information is available at http www geolsoc org uk sup18390

estudio sismico de la corteza iberica norte 3 3 a seismic image of the variscan crust in the hinterland of the nw iberian massif an offshore vertical incidence reflection seismic study with simultaneous on land wide angle recording has been conducted as part of the estudio sismico de la corteza iberica norte escin project

production and off bank transport of carbonate sediment black rock southwest little bahama bank abstract surficial sediments between the intertidal zone and 600 m were studied with the objectives of characterizing their source dispersal pathways and sites of accumulation bioerosion is a major source of sediment in the intertidal and shallow subtidal environments in situ enclosure experiments showed that the urchin echinometra lucunter produced 667 mg sediment cm 2 yr 1 and an associated diverse rock infauna produced an additional 183 mg cm 2 yr 1 microborers caused erosion of 25 mg cm 2 yr 1 and feeding activity by the chiton acanthopleura granulata caused additional erosion of 10 mg cm 2 yr 1 sequential weighing of tethered limestone plates showed an abrasion rate of 5 5 mg cm 2 yr 1 due chiefly to impacts on the plate edges surficial sediments between the intertidal zone and the top of the bank margin wall at 30 m are gravel sand mixtures containing 0 2 mud molluscs and halimeda plates are the main constituents of gravel fractions sand fractions are composed of bioerosion produced carbonate rock fragments 1 70 skeletal material from halimeda 3 63 forams 1 33 molluscs 6 15 and coral 0 6 gravel and sand are transported as bedload in chutes across the narrow rocky shelf chutes end at 45 m and sediment traps indicate an average flux of 5 4 kg chute 1 yr 1 equivalent to 945 kg km 1 yr 1 moving to deep water a dense population of halimeda living on the wall between 30 70 m contributes gravel and sand to sediment on wall ledges at the base of the steep wall 65 a ramp slopes into the northwest providence channel between 120 300 m the ramp 30 slope is a zone by passed by coarse and fine sediment at 300 m the slope decreases to an average of 13 and an apron shaped body of mud is accumulating the apron extends parallel to the bank margin and wall and reaches depths 600 m traps showed that sediment flux to the ramp is 0 12 kg m 2 yr 1 which is 2 of the particle flux overtopping the bank margin surficial sediment of the ramp is mostly mud 70 which is primarily of shallow water origin some form of bottom current transporting gravel and sand mostly halimeda and abraded coral heads has cut gullies into the ramp this bottom flow is probably episodic and turbulent enough to suspend and supply halimeda sand to sediment traps on the ramp at 463 m

a 2 5d interpretation of the eastern galicia magnetic anomaly northwestern spain geodynamical implications abstract the eastern galicia magnetic anomaly egma shows intensities up to 190 nt and is one of the major features of the magnetic field in northwestern spain taking into account the seismic refraction and surface geological data a 2 5d model of the magnetic properties of the crust in the area of the egma has been carried out models show that the anomaly can be produced by a middle to lower crustal layer a few kilometers thick with magnetic susceptibility of approximately 0 045 si emplaced with a ramp flat like geometry in higher crustal levels and giving rise to horizontal displacements of about 50 km on the basal eastern galicia thrust this structure is in close agreement with surface geology since the culmination of the lugo dome and the extensional viveiro fault can be interpreted in relation with the displacement along the eastern galicia thrust the southern termination of the egma can be related to the termination of the anomalous layer on a lateral hanging wall ramp of this thrust rocks in the magnetic layer are interpreted as mainly lower crust mafic and or ultramafic rocks in the northern sector at least part of the anomaly must be attributed to ultramafic hercynian intrusives cropping out in the vicinity of the viveiro fault

organism substrate interactions and astrobiology potential models and methods abstract organism substrate interactions and their products biogenic structures are important biosignatures on earth this study discusses the application of ichnology the study of organism substrate interactions to the search for present and past life beyond earth three main questions are addressed 1 why to look for biogenic structures i e traces and ichnofabrics beyond earth 2 what biogenic structures to expect on other planets moons and asteroids 3 how to study extraterrestrial biogenic structures review of terrestrial evidence highlights a set of properties that make traces and ichnofabrics important for the search of potential extraterrestrial life trace fossils preserve the activity of soft bodied organisms biogenic structures are resilient to processes that obliterate other biosignatures i e mechanical and chemical degradation diagenesis tectonism and metamorphism traces are very visible biosignatures traces indicate environment and behaviour traces can be universal biosignatures i e biosignatures ideally suited for detecting any type of life a model of organism substrate interactions beyond earth is here proposed expected extraterrestrial traces are those that manifest behaviours that allow to maintain homeostasis excavations meandering traces and biodeposition structures most of the existing rovers and orbiters provide basic instruments for searching these traces it is here suggested that the search for extraterrestrial biogenic structures by rovers would also benefit from artificial adjustable lighting gpr lidar and drilling equipment with optical televiewer in this study open access databases of rover and orbiter imagery have been searched for traces and ichnofabrics but no unquestionable evidence of biogenic structures beyond earth has been found besides those produced by humans this sounds along the lines of the famous fermi paradox if the universe is teeming with aliens where are their traces results of this search show that habitable environments are not the only place to look for biogenic structures non habitable environments such as moons without atmosphere can favour the preservation of shallow tier traces the better preservation potential of traces compared to other biosignatures greatly widens the issue of planetary protection including the interaction between astronauts or vehicles and the substrate may produce disturbances although this study highlights a new direction of study with the tools and concepts of ichnology dialogue between the astrobiological and ichnological communities is needed to use its full potential and possibly answer one of the major questions of science does life exist beyond earth

continuous deformation ductile thrusting and backfolding of cover and basement in the sierra de guadarrama hercynian orogen of central spain abstract hercynian deformation has affected two different crustal ensembles a crystalline basement and a metasedimentary cover in the sierra de guadarrama the rheological basement is a medium to high grade metamorphic complex composed mostly of granitic orthogneisses with minor amounts of sediments the cover consists of a thick sequence of low to high grade metasediments with minor gneissic bodies ranging in age from late proterozoic to devonian the deformation in the cover was analysed mainly in the eastern part of the sierra de guadarrama whereas the crystalline basement was analysed in the segovia area to the west both cover and basement deformation started with a stage d 1 of continuous penetrative deformation the deformational regime has a strong component of subhorizontal simple shear with east vergence a second phase d 2 of shear deformation gave rise to a broad subhorizontal shear zone and a thrust in the cover and to narrow ductile shear zones in the basement the transition between the two first phases of deformation seems to have been gradual the third phase d 3 is a generalized backfolding episode which induced an intense and penetrative strain in broad areas of both ensembles finally large open folds with steep axial surfaces developed in two mutually perpendicular systems giving rise to dome and basin interference patterns and some subvertical ductile shear zones and faults were also created the structural evolution of the basement and cover points to a thicker skinned model of deformation giving rise to an important crustal thickening a salient feature of the model arose during the second phase when a thick detachment level developed in the lower part of the cover the generalized existence of this level may account for the scarcity of basement outcrops in the spanish hercynian belt

magnetic fabric and 3d geometry of the hombreiro sta eulaliapluton implications for the variscan structures of eastern galicia nw spain abstract in this paper the internal structure and 3d geometry of the hombreiro sta eulalia granitic pluton hercynian belt nwspain are described characterization of the internal structure was performed from surface data based on microstructural analysis and anisotropy of magnetic susceptibility studies the geometrical features of the pluton morphology at depth were derived from gravity data the pluton is made up of paramagnetic granites and leucogranites k values ranging between 1 1 and 13 3 10 5 si with well developed magnetic fabrics anisotropy degree p 1 05 for 78 of the sampling localities the results obtained enable us to depict a flat lying magnetic structure with associated n120 e trending magnetic lineations the gravity survey corroborates the subhorizontal laminar geometry of the pluton which has an estimated thickness of 2 km and an almost planar floor the 3d morphology is elongate at depth parallel to the linear internal fabric this would imply that magma emplacement took place along this preferential direction whereas the deepest zones of the pluton trend wnw ese elongate parallel to regional 1 it is almost rootless we present data showing that the hombreiro sta eulalia pluton emplacement was closely related spatially and chronologically along with other granitoid plutons from the lugo dome with two major structures of eastern galicia the basal shear zone of the mondonedo nappe and the extensional vivero fault

the vertical structure of toga coare convection part i radar echo distributions radar data collected by the 5 cm mit radar which was deployed aboard the r v vickers during the intensive observing period of the tropical ocean global atmosphere coupled ocean atmosphere response experiment are partitioned into convective and stratiform cartesian grid columns the vertical structure of convective echo is examined through the use of two variables echo top height and the height of the 30 dbz reflectivity contour the first of these variables has traditionally been used to describe the vertical structure characteristics of convection and the second has recently been linked to internal microphysical properties and lightning histograms of the relative frequency of convective only echo top heights and 30 dbz contour heights were constructed for the three cruises of the vickers with each cruise experiencing different phases of the intraseasonal oscillation iso cruise 1 which was dominated by the convectively inactive phase of the iso was characterized by the highest frequency of shallow convection based on echo top heights whereas cruise 2 which was dominated by a particularly well defined passage of the convective phase of the iso exhibited the tallest echo top heights cruise 3 convection was influenced by moderate westerly surface winds characteristic of postwesterly wind burst conditions and convection was of intermediate height when viewed as a function of internal vertical structure i e 30 db z contour height the frequency distributions vary less from cruise to cruise with cruises 1 and 2 having nearly identical distributions of convective 30 dbz contour heights furthermore when the contribution to convective rainfall is examined as a function of 30 dbz contour height it is seen that relatively more rain fell from vertically intense convection i e convection with tall 30 dbz contours during cruises 1 and 3 than during cruise 2 instantaneous correlations between rainfall rate and radar echo height were highly scattered about a mean value of about 0 55 whereas rainfall rate and 30 dbz contour height correlations peaked at about 0 8 and exhibited much less scatter

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

opposite p t t paths of hercynian metamorphism between the upper units of the cabo ortegal complex and their substratum northwest of the iberian massif abstract in the cabo ortegal region two metamorphic groups can be distinguished which have followed opposite p t paths in their hercynian metamorphism the upper group comprises the three uppermost units of the allochthonous complex with mafic and related lithologies of cabo ortegal the lower metamorphic group includes the two basal units of the complex and the lithological sequence outcropping in the ollo de sapo anticlinorium the lithologies of the upper group were affected by high p and medium high t eo hercynian metamorphism during a subduction episode related to the onset of the hercynian orogenic cycle as a result of such metamorphism the basic rocks were transformed into b type eclogites and also into high p mafic granulites the advance of the hercynian collision produced the progressive uplift of the subducted lithologies and finally their emplacement as large thrust sheets over the lower metamorphic group this final evolution is clearly reflected in the p t t paths of the high p lithologies which show pronounced decompressions with only minor cooling the evolution of the lower metamorphic group does not reflect high p events the characteristic regime is a barrovian type medium p metamorphism showing p t t paths with a progressive compression during heating at least partially the burial of the rock association was due to the overthrusting of a thick allochthonous pile this is represented by the upper metamorphic group

variations in precipitating convective feature populations with itcz width in the pacific ocean abstractmany recent studies have aimed to better understand changes in the characteristics of the intertropical convergence zone itcz including itcz location width and precipitation intensity

the guitiriz granite variscan belt of northern spain extension controlled emplacement of magma during tectonic escape abstract the ollo de sapo domain of the northern part of the variscan belt of spain contains precambrian and ordovician metamorphic rocks intruded by the guitiriz granite the domain is bounded by two n s transcurrent shear zones detailed structural and anisotropy of magnetic susceptibility studies of the guitiriz pluton reveal a syntectonic emplacement relative to movement on the two n s trending bounding shear zones characterized by 1 development of n s trending structural and magnetic fabrics 2 concordant structures in granites and country rocks and 3 development of shear zones along the east and west granite margins gravity data show two roots trending 110 e extending down to 4 5 km from an otherwise thin

low p metamorphism following a barrovian type evolution complex tectonic controls for a common transition as deduced in the mondonedo thrust sheet nw iberian massif the mondonedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low and medium p metamorphic domains this unit underwent an initial medium p event during the initial stages of variscan convergence related to crustal thickening subsequently the thrust sheet evolved to a low p baric type of metamorphism related to syn convergence thinning and exhumation its footwall cropping out in two tectonic windows registered a different evolution with a low p history that evolved from low to high t under a high geothermal gradient several different p t paths of the mondon edo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area following the initial crustal thickening two main syn convergence extensional shear zones developed one of them occurs in the hangingwall whereas the other affects the footwall unit both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet and their activity is viewed as a consequence of the need for gravitational re equilibration within the orogenic wedge the most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low p metamorphism is essentially a late phenomenon and is linked to late orogenic tectonic activity in the mondon edo thrust sheet our conclusions indicate that low p metamorphism may also develop during convergence and that this may occur in at least two cases one is tectonic denudation of an allochthonous unit during its emplacement and the other thinning and extension at the footwall unit of an advancing thrust sheet as a consequence the low p evolution may show different characteristics in different units of an orogenic nappe pile

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

crustal thickening and attenuation as revealed by regional fold interference patterns ciudad rodrigo basement area salamanca spain abstract the structure of the ciudad rodrigo area iberian massif central iberian zone has been revisited in order to integrate new geological data with recent models of the evolution of the iberian massif detailed mapping of fold structures along with a compilation of field data have been used to constrain the geometry and relative timing of ductile deformation events in this section of the hinterland of the variscan belt the structural evolution shows in the first place the development of a regional train of overturned folds with associated axial planar foliation d 1 towards the lower structural levels the deflection of the fold limbs and a subhorizontal crenulation cleavage depict the upper structural boundary of a superimposed low angle shear zone d 2 which extends at least to the deepest parts of the basement exposed in the study area the amplification and rotation of d 1 folds about a horizontal axis also occurred within this shear zone the flat lying character of the d 2 structures accounts for the attenuation of the previously thickened crust which developed following gravity gradients during thermal re equilibration subsequent deformation led to the formation of two orthogonal sets of upright folds d 3 representing a new shift between crustal thinning and crustal thickening in the region

what darwin did not see pleistocene fossil assemblages on a high energy coast at ponta das bicudas santiago cape verde islands funding for fieldwork on santiago island in june 2011 was provided under grant cgl2010 15372 bte from the spanish ministry of science and innovation to project leader eduardo mayoral university of huelva financial support to a santos came from the spanish ministry of science and technology juan de la cierva subprogram ref jci 2008 2431 additional support by the junta de andalucia spanish government to the research group rnm276 is also acknowledged partial funding to j ledesma vazquez on this project came from the programma integral de fortalecimiento institucional 2010 we thank christopher k pham department of oceanography and fisheries university of the azores portugal for help with identification of the fossil barnacles and ricardo ramalho institut fur geophysik westphalishe wilhelms universitat germany for discussions about bioerosion by sea urchins on basalt surfaces

a tectonic model for the metamorphic evolution of the basal gneiss complex western south norway a review of currently available information relevant to the basal gneiss complex bgc of western south norway combined with the authors own observations leads to the following conclusions 1 most of the bgc consists of proterozoic crystalline rocks and probably subordinate lower palaeozoic cover 2 the last major deformation of these rocks was during the caledonian orogeny and involved large scale thrusting recumbent folding and doming the structural development of the bgc is closely tied in with that of the caledonian allochthon 3 the whole eclogite bearing part of the bgc has suffered a high pressure metamorphism with conditions of between 550 c 12 5 kbar sunnfjord and about 750 c 20 kbar more og romsdal at the metamorphic climax 4 this metamorphism was of caledonian age probably rather early in the caledonian tectonic history of the bgc and is considered to have been a rather transient event by setting these conclusions in a framework provided by geophysical evidence for the deep structure of the crust in southern norway we have constructed a geotectonic model to explain the recorded metamorphic history of the bgc it is suggested that considerable crustal thickening was caused by imbrication of the baltic plate margin during continental collision with the greenland plate this resulted in high pressure metamorphism in the resulting nappe stack progradation of the suture caused underthrusting of the baltic foreland below the eclogite bearing terrain causing it to emerge at the earth s surface aided by tectonic stripping and erosion application of isostacy equations to the model shows that eclogites can be formed by in situ metamorphism in crustal rocks and reappear at the land surface above a normal thickness of crust in a single orogenic episode of approximately 65 70 ma duration

magnetic fabric and deformation in the navia alto sil slate belt northwestern spain abstract ries and schackleton ries a c schackleton r m 1976 patterns of strain variation in arcuate fold belts philos trans r soc london a 283 281 288 discussed the distribution of finite strain in the ibero armorican arc and suggested that the arc was produced by tectonic bending of a straight or gently curved fold belt later investigations by others revealed different strain patterns in the fold thrust belt and so its evolution continues to be controversial this study focuses on the strain pattern in the navia alto sil slate belt the low field anisotropy of magnetic susceptibility ams was measured in ordovician black slates and cambrian to lower ordovician quartzites sampled around the arcuate belt the magnetic fabric is controlled primarily by the slaty cleavage in both lithologies the ams was also measured with a high field torsion magnetometer and the results indicate that paramagnetic minerals are responsible for the magnetic fabric in the slates and diamagnetic and paramagnetic minerals in the quartzites strain markers i e pressure shadows around pyrite grains and rutile distribution are found in the slates and the orientation of the principal axes of strain ellipsoids agrees with those of the ams ellipsoid a systematic variation in the orientation of the strain ellipsoid and anisotropy ellipsoid suggests that there is strain heterogeneity along the slate belt and that the arc did not form by tectonic bending

cloud to ground lightning observations from toga coare selected results and lightning location algorithms abstract recently observations of electrified oceanic convection and associated cloud to ground cg lightning were obtained over the tropical western pacific ocean during toga coare tropical ocean global atmosphere coupled ocean atmosphere response experiment during coare observations of convection were made using a variety of instrument platforms including ship and airborne doppler radars an advanced lightning direction finder aldf network and a shipborne inverted electric field mill this study focuses on data collected by the coare aldf network fusion of those data with observations and the methods used to calculate accurate cg return stroke locations analysis of cg lightning data and doppler radar data indicates that lightning producing oceanic convection is characterized by deep vertically developed convective cells with radar reflectivities exceeding 30 dbz above the height of the 10 c level in several cases a peak in cg frequency occurred coincident with the descent of precipitation

large extensional structures developed during emplacement of a crystalline thrust sheet the mondonedo nappe nw spain the mondonedo nappe is a crystalline thrust sheet characterized by large recumbent folds regional intermediate pressure metamorphism synkinematic intrusion of granitoids during nappe emplacement and an extensional ductile shear zone developed within the nappe during thrusting a large tectonic window permits the study of the footwall unit revealing another extensional shear zone contemporaneous with thrusting and a low pressure metamorphic evolution in contrast to that of the hanging wall unit the two main extensional shear zones produced e w extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend furthermore subordinate n s longitudinal extension was accommodated by normal faults in the footwall and some of these faults were used as lateral ramps in late stages of thrusting the role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate convergence but characterized by large scale rheological heterogeneities within it deep seated viscous flow triggered by heat accumulation seems to account for the horizontal stretching and probable tapering of the orogenic wedge which was induced by gravitational instabilities due to partial melting and underplating by buoyant continental crust

a new source of isotopically heavy sulphur in lode gold deposits of nw spain a batch volatilization process in the conversion of pyrite to pyrrhotite by contact metamorphism the conversion of pyrite to pyrrhotite is identified in this study as a new source of isotopically heavy sulphur in thermal domes contact metamorphism of a sequence of pyrite rich shales by the penedela blind intrusion nw spain causes the conversion of diagenetic pyrite to pyrrhotite at a temperature of 150 c this process occurred in a closed system with total consumption of the diagenetic pyrite and production of h2s rich fluids the h2s liberated in this unidirectional reaction shows a relatively heavy 34s in agreement with a batch volatilization process the mixture of this h2s rich fluids with derived sulphur from lower cambrian carbonates produced gold bearing pyrite at a temperature of 340 c the contribution from these two different sulphur sources was 86 and 14 respectively the metamorphic fluids migrated through permeable zones such as fractures and shear zones producing a lode gold deposit

two bioerosion ichnofacies produced by early and late burial associated with sea level change in bioerosion as in trace fossils as a whole deeply emplaced structures have greater survival value than shallow structures that is to say tiering the relative depth to which rasping etching and boring organisms penetrate their substrate is of paramount importance for the preservation potential of individual trace fossils an entobia ichnofacies is established for trace fossil assemblages dominated by deep tier borings and arising from long term bioerosion such as occurs on sediment free submarine cliffs or hardgrounds a gnathichnus ichnofacies comprises assemblages containing all tiers including superficial sculptures produced by radulation that have very little preservation potential such assemblages occur in short term bioerosion situations as on shell surfaces and hardgrounds buried early by sedimentation

three dimensional kinematic and microphysical evolution of florida cumulonimbus part ii frequency distributions of vertical velocity reflectivity and differential reflectivity abstract high resolution radar data collected in florida during the convection and precipitation electrification experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages a statistical technique is employed to examine the evolving properties of the ensemble small scale variability of radar reflectivity vertical velocity and differential reflectivity over the entire storm differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm s lifetime dual doppler radar data show that throughout the storm s lifetime both updrafts and down drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation size particles thus the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertic

comparing the convective structure and microphysics in two sahelian mesoscale convective systems radar observations and crm simulations two mesoscale convective systems mcss observed during the african monsoon multidisciplinary analyses amma experimentaresimulatedusingthethree dimensional 3d goddardcumulusensemble model this studywasundertaken todeterminethe performance ofthe cloud resolving modelin representing distinct convective and microphysical differences between the two mcss over a tropical continental location simulations are performed using 1 km horizontal grid spacing a lower limit on current embedded cloudresolving models within a global multiscale modeling framework simulated system convective structure and microphysics are compared to radar observations using contoured frequency by altitude diagrams cfads calculated ice and water mass and identified hydrometeor variables vertical distributions of ice hydrometeors indicate underestimation at the mid and upper levels partially due to the inability of the model to produce adequate system heights the abundance of high reflectivity values below and near the melting level in the simulation led to a broadening of the cfad distributions observed vertical reflectivity profiles show thathighreflectivityis presentat greaterheights thanthe simulations produced thoughtto bea resultof using a single moment microphysics scheme relative trends in the population of simulated hydrometeors are in agreement with observations though a secondary convective burst is not well represented despite these biases the radar observed differences between the two cases are noticeable in the simulations as well suggesting that the model has some skill in capturing observed differences between the two mcss

a review of the arcuate structures in the iberian variscides constraints and genetic models abstract in the iberian variscides several first order arcuate structures have been considered in spite of being highly studied their characterization formation mechanisms and even existence is still debatable the main ibero armorican arc iaa is essentially defined by a predominant nw se trend in the iberian branch and an e w trend in the brittany one however in northern spain it presents a 180 rotation sometimes known as the cantabrian arc ca the relation between both arcs is controversial being considered either as a single arc due to one tectonic event or as the result of a polyphasic process according to the last assumption there is a later arcuate structure ca overlapping a previous major one iaa whatever the models they must be able to explain the presence of a variscan sinistral transpression in iberia and a dextral one in armorica and a deformation spanning from the devonian to the upper carboniferous another arcuate structure in continuity with the ca the central iberian arc cia was recently proposed mainly based upon on magnetic anomalies geometry of major folds and ordovician paleocurrents the critical review of the structural stratigraphic and geophysical data supports both the iaa and the ca but as independent structures however the presence of a cia is highly questionable and could not be supported the complex strain pattern of the iaa and the ca could be explained by a devonian carboniferous polyphasic indentation of a gondwana promontory in this model the ca is essentially a thin skinned arc while the iaa has a more complex and longer evolution that has led to a thick skinned first order structure nevertheless both arcs are essentially the result of a lithospheric bending process during the iberian variscides

palaeoecology of the pleistocene mis 5 5 outcrops of santa maria island azores in a complex oceanic tectonic setting abstract santa maria is the oldest island of the azores archipelago and the only one that possesses late miocene early pliocene and pleistocene fossils two pleistocene outcrops lagoinhas and prainha from the marine isotopic substage 5 5 mis 5 5 are known from santa maria island we studied the palaeoecology of the lagoinhas fossil assemblages and we also provide detailed lithological sections from both outcrops allowing for more detailed palaeoecological reconstruction ten bulk samples of approximately 1 kg each were collected from the sand facies at lagoinhas most of the gastropod taxa represented in these samples occur on the present azorean shores the exceptions being conus sp gibbula cf umbilicalis polynices lacteus and trachypolia nodulosa which have locally disappeared from the azorean malacofauna during the course of the last glaciation most of the fossil gastropods were epifaunal and crawlers about half of them were herbivores about one quarter was carnivores and only a few less than 10 were detritivores or parasites the preferred type of substrate was rocky substrate and or algae the palaeoecological evidence that results from the analysis of the fossiliferous beds the bioerosion structures the coralline algal and mollusc assemblages and their bathymetrical zonation suggests a marine environment depositional setting from shallow infralittoral shoreface to foreshore beach a single highstand episode including minor oscillations is apparent during mis 5 5 in the azores

conical folding in the core of an orocline a geometric analysis from the cantabrian arc variscan belt of nw iberia abstract the cantabrian arc of the variscan belt has been recently defined as a true orocline constraining kinematics and deformation timing this curved sector of the orogenic belt is characterized by two different fold sets 1 one runs parallel to the outcrops of the main thrusts and describes a horseshoe shape concave towards the east and 2 another is radial to the arc a detailed geometric study of the fold interference patterns in the cantabrian arc revealed the conical nature of the folds belonging to the radial set these conical folds developed with different geometrical characteristics semi apical angles and axis attitudes depending on the initial orientation and geometry of the folded surfaces they are interpreted to result from a vertical axis rotation during oroclinal buckling of the variscan belt in nw iberia this study of conical folds in the cantabrian arc highlights that conical folds in curved orogenic arcs are a powerful tool for establishing the sequence of tectonic events because interference patterns due to vertical axis secondary differential rotations provide unique geometrical characteristics observed in the cantabrian arc that can be extrapolated to other oroclines

hercynian blueschist metamorphism in north portugal tectonothermal implications aegirine jadeite clinopyroxene 60 mol jadeite locally occurs within blueschists of the lower allochthon exposed in the tras os montes region of northern portugal peak conditions attained during blueschist facies metamorphism are estimated to have been c 420 c and 11 kbar porphyroblastic white mica paragonite phengite within the blueschist assemblage records a 36ar 40ar versus 39ar 40ar isotope correlation age of 329 4 1 6 ma in view of the relatively low t nature of the metamorphism the c 330 ma age is interpreted to date closely the high p recrystallization this tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex upper allochthon ophiolite nappe onto iberian protoliths of the lower allochthon during terminal stages of the hercynian orogeny

the polygenetic kahouanne seamounts in the northern lesser antilles island arc evidence for large scale volcanic island subsidence abstract remotely operated vehicle rov explorations of the kahouanne seamounts located 25 km se of montserrat in the northern lesser antilles island arc have discovered the occurrence of honeycomb scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level mbsl these features combined with the flat topped morphology of the seamounts the occurrence of shallow water carbonates rhodoliths benthic foraminifera and the presence of oxidized highly vesicular volcanic fragments suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth the subsidence is likely to have been in response to a combination of 1 graben development southeast of the island of montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the lesser antilles volcanic arc and 2 regional subsidence inferred from studies of nearby carbonate platforms 40ar 39ar dating of basalt andesite lavas from the central seamount indicates active volcanism around 5 4 ma shallow water carbonates recovered from an apparent wave cut terrace at 600 mbsl in the same area were deposited approximately 3 ma based on sr isotope stratigraphy indicating significant subsidence of the complex since that time the total subsidence is in line with displacements in the kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of guadeloupe and martinique implying that intra arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc

lower paleozoic extensional tectonics in the limit between the west asturian leonese and central iberian zones of the variscan fold belt in nw spain the limit between the west asturian leonese and central iberian zones in the nw of the iberian peninsula is described in the area of caurel truchas provinces of orense and leon from the distribution of the sedimentary formations and the variscan structures it is inferred that the transition between the two zones was a fault system which was active during at least the ordovician and lower silurian the faults were supposedly normal though they had probably some strike slip component and gave rise to a half graben in which a syn rift sequence was deposited the latter underwent inversion at the onset of the variscan tectonism producing a ramp anticline syncline pair that forms the more important folds in the area the varying geometry of these folds is interpreted partly as being due to the existence of previous en echelon folds related to the strike slip component of the faults and partly as a result of the variable intensity of the shear strain undergone during the first variscan phase

anatomy of the mondonedo nappe basal shear zone nw spain this paper analyses a major shear zone from the iberian hercynian belt which forms the basal thrust of the mondonedo nappe the shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle ductile deformation in greenschists facies folds in the shear zone are asymmetric very tight 1c or similar class and frequently developing sheath geometries the sheath folds originated by non coaxial flow superimposed on earlier irregularities the fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non mylonitic tectonites c axis fabrics vary across the shear zone but show a dominant monoclinic symmetry the blastomylonitic rocks include the fabrics representing the highest temperatures the main foliation of the schists results from flattening of an earlier foliation recording occasional microfolds the use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen

linear response of a stratified tropical atmosphere to convective forcing abstract the three dimensional response of the tropical atmosphere to an isolated heat source is investigated using a primitive equation model linearized about a resting basic state on an equatorial plane the model equations are solved by applying vertical and horizontal normal mode transforms the thermal forcing was chosen to simulate the convection which occurs over tropical south america during the southern hemisphere summer the vertical dependence of the forcing is such that the heating is zero at the top and bottom boundaries with a variable level of maximum beating the model results with steady forcing are compared with the average circulation over tropical south america for a 19 day period in the southern hemisphere summer of 1979 and it is shown that the model reproduces many aspects of the observed circulation the partition of the energy of the steady state response between the vertical modes is calculated and it is shown that the internal mode with an equivalent depth of 208 m is domina

drop size distributions and radar observations of convective and stratiform rain over the equatorial indian and west pacific oceans abstracttwo dimensional video disdrometer 2dvd data were analyzed from two equatorial indian gan and west pacific ocean manus islands where precipitation is primarily organized by the intertropical convergence zone and the madden julian oscillation mjo the 18 3 5 months of 2dvd data from manus gan island show that 1 the two sites have similar drop size distribution dsd spectra of liquid water content median diameter rain rate r radar reflectivity z normalized gamma number concentration nw and other integral rain parameters 2 there is a robust nw based separation between convective c and stratiform s dsds at both sites that produces consistent separation in other parameter spaces the 2dvd data indicate an equatorial maritime average c s rainfall accumulation fraction frequency of 81 19 41 59 at these locations it is hypothesized that convective fraction and frequency estimates are slightly higher than previous radar based studies because the ubiquitous weak shallow convec

a radar study of convective cells in mesoscale systems in gate part i vertical profile statistics and comparison with hurricanes abstract this is part i of a two paper describing the structure of the vertical profile of radar reflectivity in convective cells which are of part of mesoscale convective systems in gate earlier work has established that characteristic mean vertical velocities in such convective clouds in gate were rather weak 3 5 m s 1 the microphysical implications of the weak updrafts have been proposed to include the rarity of large particles above the freezing level as a working hypothesis for these papers it is proposed that cells with weak updrafts have characteristic vertical reflectivity profiles exhibiting modest reflectivities at low levels and decreasing rapidly with height above the freezing level vertical radar reflectivity profiles are compiled from 296 convective cells having at least 40 dbz surface reflectivity and it is found that the profiles are consistent with the above reasoning the mean profile is of modest strength 45 dbz echo at the surface and a 20 dbz echo top of 8 2 km and the refle

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

the vilalba gold district a new discovery in the variscan terranes of the nw of spain a geologic fluid inclusion and stable isotope study abstract this work defines the vilalba gold district a new gold region in the nw of spain comprising three mineralized areas castro de rei valina azumara and arcos and two different types of mineralization 1 w au skarn in castro de rei and 2 au as ag pb zn cu sb vein disseminated mineralization in valina azumara and arcos mineralization is mainly hosted by impure limestone and black slates lower cambrian and spatially related to variscan and late variscan structures rhyolite dikes occur in arcos 299 6 ma and castro de rei both consisting of peraluminous high potassium and calc alkaline rocks these dikes represent the fractionated expression of unexposed post tectonic granitoids located at depth the castro de rei area exhibits a contact aureole with biotite cordierite and calc silicate hornfels prograde skarn consists of garnet pyroxene quartz wollastonite and scheelite the retrograde alteration took place in two stages and mainly consists of amphibole epidote zoisite quartz calcite chlorite and sulfides gold mineralization in the skarn occurs as invisible gold in sulfoarsenides and as electrum related to bi te s minerals the mineralization of valina azumara and arcos comprises two stages the first consists of as rich pyrite and arsenopyrite with invisible gold the second stage mainly consists of base metal sulfides and ag pb zn cu sb sulfosalts hydrothermal fluids involved in the skarn formation are low salinity up to 6 6 wt nacl eq aqueous carbonic fluids h2o co2 ch4 n2 evolving into aqueous fluids during the last stage of gold mineralization stable isotope geochemistry indicates the importance of a magmatic fluid in the early stages 18osmow from 7 5 to 11 34scdt from 2 7 to 5 9 that evolved through interaction with host rocks and mixing with metamorphic and or meteoric fluids the 34scdt values 7 8 10 9 of second retrograde stage sulfides suggest the incorporation of 34s enriched from host rocks in valina azumara and arcos the fluid inclusion and isotope studies indicate similar hydrothermal fluids to those associated with the second retrograde skarn stage taking into account the fluid inclusion study and mineral geothermometry an attempt was made to determine the p t conditions of ore formation the prograde skarn of castro de rei formed at temperatures and pressures ranging from 520 to 560 c and from 1 7 to 2 5 kb the mineralization of valina azumara formed at minimum p t conditions of 1 7 kb and 355 c the estimated temperatures for the arcos mineralization are similar to those of valina azumara up to 383 c but at lower pressures we propose the model of intrusion related gold systems irgss to explain the mineralizations of the vilalba gold district in this model the castro de rei skarn and the valina azumara mineralization represent a proximal and a distal deposit respectively relative to an unexposed granitoid situated at depth we also consider arcos as a distal deposit in relation to another unexposed granitoid located further south in the district

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

structural and metamorphic evidence of local extension along the vivero fault coeval with bulk crustal shortening in the variscan chain nw spain abstract the vivero fault is a w dipping n s striking ductile shear zone separating the ollo de sapo antiform in its western hangingwall and the lugo dome in its eastern footwall two stages of deformation f 1 and f 2 produced nearly coaxial folds with sub horizontal axes a crenulation cleavage s 2 transposes an older s 1 three sets of shear bands in the hangingwall define a pervasive fabric consistent with an e w bulk shortening perpendicular to a composite s 1 2 foliation and nne stretching parallel to l 2 the vivero fault zone is marked by a mylonitic foliation with a steeply nw plunging stretching lineation and extensional crenulation cleavage ecc indicating normal slip in the vicinity of the fault sub horizontal nne trending f 3 folds with a crenulation cleavage s 3 deform earlier formed fabrics including a mylonitic foliation pressure temperature conditions obtained from mineral assemblages on both sides of the vivero fault yield a minimum throw of 5 5 km andalusite bearing pelite in the hangingwall was infolded by an f 2 synform into the kyanite field at 450 500 c the eastern edge of these rocks was later accreted to the footwall and heated to andalusite staurolite conditions at 600 c slip on the vivero and valdovino faults is kinematically related east west shortening during f 2 involved folding and sinistral strike slip on the valdovino fault which induced local extension along the newly generated vivero fault synkinematic emplacement of granitoids along the vivero fault is favoured by extension coeval slip on both faults took place during the later stages of f 2 folding geometrical constraints caused northwards escape of the crustal block bounded by the valdovino and vivero faults recorded by nne stretching defined by l 2

observed structure of mesoscale convective systems and implications for large scale heating mesoscale convective cloud systems that produce large amounts of rain in mid latitudes and most of the rain in the tropics consist of a combination of convective and stratiform cloud the convective regions contain numerous deep cells that are often but not always arranged in lines the stratiform region is an outgrowth of the convective ensemble it lies adjacent to the convective region and is seeded by ice particles detrained from convective towers sometimes it lies to the rear of a propagating convective line while on other occasions it surrounds the convection the heating of the large scale environment by a mesoscale convective system is affected by both the convective and stratiform regions although processes such as melting and radiation are important the net heating by a system is dominated by condensation and evaporation associated with vertical air motions this paper reviews recent observational evidence regarding the mean vertical motion profiles in the convective and stratiform regions of mesoscale convective systems and the implications of these mean motions for the vertical distribution of heating of the large scale environment in both the convective and the stratiform regions vertical motions have been determined by various techniques including composites of rawinsonde and aircraft wind data single and dual doppler precipitation radar analyses and wind profiler observations in stratiform regions these data consistently show mean vertical velocity that is upward in the upper troposphere and downward in the lower troposphere the level separating upward from downward motion is located from 0 to 2 km above the 0 c level depending on location within the stratiform region diagnostic calculations show that these vertical motion profiles imply heating of the upper troposphere and cooling of lower levels by stratiform region processes data on vertical motions in the convective regions are less consistent from case to case these data sometimes indicate that the mean vertical velocity in convective regions is maximum in the lower troposphere in other cases the data indicate a maximum in the high troposphere diagnostic calculations show that heating profiles diagnosed from these two types of profile are quite different the first having a maximum of heating in the lower troposphere while the second has a maximum in the middle troposphere although it is difficult to determine whether or not the differences in estimates arise from different types of observations analysis methods or sampling strategies it seems likely that they stem from differences in the large scale environment of the different mesoscale systems the ubiquitous occurrence of stratiform regions in mesoscale convective systems and hurricanes together with their consistent heating profiles which systematically concentrate heating in upper levels while cooling lower levels are a major consideration in evaluating the interaction of mesoscale systems with the large scale environment however the consistency of the stratiform profiles from case to case indicates that the variability of net convective plus stratiform heating profiles from case to case lies primarily in the variation of the convective region profiles from one case to another it is suggested that future work should focus on the convective region vertical profiles of vertical motion and heating and on the large scale environmental factors that may control the variation of these profiles from case to case

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

compressional granite emplacement model structural and magnetic study of the trives massif nw spain abstract the trives granodiorite is a syntectonically emplaced pluton belonging to the great manzaneda batholith which has intruded into the gneisses of the ollo de sapo formation northern part of the iberian variscan belt during carboniferous times combined metamorphic petrofabric structural and anisotropy of magnetic susceptibility studies reveal that interactions between regional metamorphism deformation and magmatism played the main role in strain partitioning in and around the emplacing trives pluton low dipping s c structures carrying n140 e trending stretching lineations exist in both the roof pendants which contain sillimanite bearing gneisses and in the subjacent granodiorites which are located in the central areas of the trives pluton whereas subvertical shear zones but with the same stretching lineation orientations are found along the pluton margins at deeper levels of the trives granodiorite there are fabrics which are related to the crystallization of magma at the emplacement level in these rocks magnetic foliations are mainly steeply dipping and magnetic lineations are n140 e trending the anisotropy of magnetic susceptibility data here is consistent with field observed macroscopic structures samples showing solid state deformation display magnetic foliations and lineations subparallel to c planes and their macroscopic stretching lineations respectively the foliations and lineations of magmatic origin are characterized by lower degrees of anisotropy structural maps of magmatic and solid state deformation structures suggest that the emplacement of the trives pluton was controlled by d 3 deformational structures n140 e trending transcurrent shear zones and crustal detachments which developed in response to a ne sw compressional stress field

kinematics of a kilometric recumbent fold the courel syncline iberian massif nw spain the courel recumbent syncline is one of the best examples of a regional scale recumbent fold outcropping in the nw iberian massif a strain microfabric and minor fold analysis of this structure has been integrated in an evolutionary model that begins with an episode of layer shortening buckling and body rotation associated with deformation dominated by simple shear subsequently the fold was flattened by dominant irrotational strain with maximum shortening perpendicular to the axial surface and maximum stretching parallel to the fold axis this occurred during the first phase of the variscan deformation and gave rise to a fold with an axial surface dipping moderately towards the hinterland ssw the recumbent character of the fold was increased during the third phase of the variscan deformation which produced a large scale open structure with a subhorizontal limb

trypanites ichnofacies palaeoenvironmental and tectonic implications a case study from the miocene disconformity at foz da fonte lower tagus basin portugal abstract a well preserved bioeroded surface occurs at the unconformity separating cretaceous limestones and lower miocene sediments outcropping on the western coast of the peninsula of setubal central west portugal the ichnoassemblage present in this bioeroded surface is herein assigned to the trypanites ichnofacies the preservation characteristics of the borings reflect several episodes of encrustation boring and physical erosion the erosional truncation of bioerosive structures and the predominant preservation of the largest borings gastrochaenolites isp in the ichnocoenoses are herein related with repeated phases of bioerosion and physical abrasion occurred during an early miocene transgressive pulse the recognition of this bioeroded transgressive surface also allowed confirming the presence at that time of an emergent topographic relief related to salt domes formed earlier probably already during palaeogene times

granite laccolith emplacement in the iberian arc ams and gravity study of the la tojiza pluton nw spain a laccolithic geometry is inferred from geological and geophysical data for the la tojiza granitic body emplaced in an extensional regime nearly circular in outcrop this 16 km diameter pluton one of the so called post tectonic granodioritic plutons of galicia is located in the northern variscan arc of iberia within the mondonedo nappe western asturian leonese zone from anisotropy of magnetic susceptibility measurements and extensive microstructural data the magnetic fabric of this pluton has been inferred to correlate with the magmatic fabric magnetic foliations strike ne sw and dip gently se magnetic lineations trend nw se and plunge gently se gravity data suggest that the pluton has the shape of a sheet with a maximum preserved thickness of 1 km and a root zone located at its eastern border this pluton is reinterpreted as late kinematic and emplaced during an extensional tectonic regime related to the ductile vivero normal fault it is suggested that the emplacement of most post tectonic plutons of this part of the variscan orogenic belt may have been controlled by the late extensional collapse of the iberian massif

small scale structures formed during progressive shortening and subsequent collapse in the navia alto sil slate belt hercynian fold belt nw spain abstract a prominent unit of the hercynian orogen in north western spain is the navia alto sil slate belt which is formed predominantly by llanvirn caradocian slates lower upper ordovician from a structural point of view the belt forms an infolded syncline between two uplifted areas the narcea antiform and the mondonedo fold nappe both uplifted areas are hangingwall antiforms the thrusts below these structures ramp downwards towards the hinterland and flatten towards the foreland the first deformation events were controlled by low angle shears which at the cover level produced recumbent and asymmetric folds associated with a slaty cleavage in the west and the detachment of the cantabrian zone in the frontal part of the orogen eastwards of the narcea antiform these structures were modified as deformation progressed and two generations of steep dipping crenulation cleavages were locally formed these crenulations occur in belts separated by others in which the slaty cleavage remains undisturbed the youngest structures are flat lying crenulations small chevron folds and kink bands which develop where the slaty cleavage has not been previously crenulated the flat lying small scale structures end in general against the bands with vertical crenulations although in some cases they cross the boundary the alternation of belts with vertical and horizontal crenulations is found in all scales from the kilometre to the microscopic scale the flat lying structures indicate near vertical shortening and together with a few w dipping normal faults can be regarded as the result of orogenic collapse

bioerosion of intertidal limestone co clare eire 1 paracentrotus lividus abstract the boring sea urchin paracentrotus lividus is widely distributed in the mid and lower intertidal zones on carboniferous limestone in co clare eire erosion rates have been estimated from the measurement of excavation depth and the counting of growth rings on the test shell the rates ranged from 0 25 1 5 cm a 1 on an exposed shore to 0 1 0 cm a 1 on a sheltered site larger older individuals were commoner on the exposed site the distribution activities and geomorphological significance of the organisms are discussed together with the validity of the procedures adopted to estimate erosion rates it is clear that paracentrotus is a major factor in pool deepening in the mid intertidal zone and in weakening the rock mass in the lower intertidal especially when paracentrotus erosion rates are compared with general surface lowering rates of 0 2 mm a 1

granite emplacement during contemporary shortening and normal faulting structural and magnetic study of the veiga massif nw spain abstract the veiga massif belongs to the calc alkaline series of hercynian granitic rocks of the ibero armorican arc the veiga granodiorite intruded during the upper carboniferous into the core of the wnw ese n verging ollo de sapo antiform formed by precambrian and palaeozoic metasediments internal fabrics show that magma intrusion was contemporary with shortening measurements of feldspars orientations and anisotropy of magnetic susceptibility ams throughout the granite are consistent and indicate a foliation striking wnw ese parallel to folding with a constant dip of 75 85 n the zonation of bulk low field susceptibility is related to mineral content and indicates a more basic composition at the southern and western borders the difference in elevation between outcrops more than 600 m allows us to infer the three dimensional attitude of granite fabrics throughout the massif syn magmatic fabric folds are preserved in the inner part of the igneous body the highest degree of magnetic anisotropy is observed in areas located near the bottom and top of the intrusion at the scale of the massif foliation is convergent toward the bottom of the intrusion along a line located at its northern border where the magma source is interpreted to be located in the western border of the massif the presence of c and s structures indicates that magma cooling was coeval with movement of the chandoiro fault a n s striking normal fault with a n290e hanging wall displacement direction these results indicate that emplacement of the veiga granite is coeval with nne ssw shortening and with an wnw ese extension direction parallel to the trend of the late folds

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

setting new constrains on the age of crustal scale extensional shear zone vivero fault implications for the evolution of variscan orogeny in the iberian massif the vivero fault is crustal scale extensional shear zone parallel to the variscan orogen in the iberian massif belt with an associated dip slip movement toward the hinterland to constrain the timing of the extension accommodated by this structure we performed zircon u pb la icp ms geochronology in several deformed plutons some of them emplaced syntectonically the different crystallization ages obtained indicate that the fault was active at least between 303 2 and 287 3 ma implying a minimum tectonic activity of 16 5 ma along the fault the onset of the faulting is established to have occurred later than 314 2 ma the geochronological data confirm that the vivero fault postdates the main variscan deformation events in the nw of the iberian massif and that the extension direction of the late carboniferous early permian crustal scale extensional shear zones along the ibero armorican arc was consistently perpendicular to the general arcuate trend of the belt in sw europe

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

structural and kinematic analysis of the corredoiras detachment evidence for early variscan synconvergent extension in the ordenes complex nw spain an important detachment is described in the allochthonous ordenes complex in the nw iberian massif and its meaning is related to the kinematics of contemporaneous convergent structures the corredoiras detachment cd separates a hangingwall unit characterised by a medium pressure metamorphic gradient from a footwall high pressure and high temperature unit and an underlying ophiolitic unit an associated ductile shear zone nearly 2000 m thick developed in the lower part of the hangingwall unit where the corredoiras orthogneiss a lower ordovician metagranite was progressively transformed into augengneisses mylonitic and ultramylonitic gneisses the attitude of the stretching and mineral lineation in the mylonites varies due to late refolding at map scale but the sense of movement can be estimated being roughly top to the se according to crosscutting relationships the cd developed subsequent to the thrusting of the high pressure high temperature unit onto the ophiolitic unit and prior to younger extensional detachments upright folding and strike slip tectonics the geometric relationships of the cd with the previous structures in the footwall unit the subtractive character of the metamorphic gap between its hangingwall and footwall and the available isotopic data suggest that the cd is an early variscan ductile extensional detachment the movement of which was roughly simultaneous with the onset of thrusting of the allochthonous complexes over their relative autochthon

persististrombus coronatus mollusca strombidae in the lower pliocene of santa maria island azores ne atlantic paleoecology paleoclimatology and paleobiogeographic implications abstract the family strombidae is one of the twenty three families and subfamilies of gastropods associated with tropical environmental conditions and therefore useful as a biogeographical and paleoclimatic proxy today the strombid genus persististrombus is represented in the ne atlantic by a single species restricted to the tropical mauritanian senegalese province this work reports the occurrence of persististrombus coronatus from the lower pliocene of santa maria island in the azores archipelago based on this occurrence and on the mio pliocene fossil record of the ne atlantic oceanic islands paleoclimatological considerations are discussed which allow for the first time to include the azores and the other macaronesian islands in a wider context of the ne atlantic paleobiogeographical molluscan provinces late miocene to present day molluscan biogeographic units ranging in latitude from 60 n down to 17 s are here defined and the boundaries of the proposed climatic zones are outlined we suggest that during the upper miocene lower pliocene the paleoclimate at santa maria island was drastically different from that seen at those latitudes today with mean annual sea surface temperatures ssts about 3 7 c to 6 3 c higher than the present day 20 6 c and with mean monthly ssts ranging from 20 c to 28 c with six months with mean ssts over 24 c conditions typical of a tropical setting

structural metamorphic and magmatic history of the mondonedo nappe hercynian belt nw spain abstract the mondonedo nappe is one of the main units in the internal zone of the hercynian belt in the iberian peninsula it consists of low to medium grade metasediments late precambrian and palaeozoic in age folded by large d 1 e verging recumbent structures and thrusted several tens of km towards the east during a d 2 deformation episode at the base of the thrust sheet a shear zone of up to 3 km thick was developed granitoid bodies which were intruded after d 1 and before or during d 2 were deformed in this basal zone the geometry of d 1 and d 2 structures and that of the isograds are described as well as the relationships between metamorphism and deformation a structural evolution is inferred which shows an episode of generalized ductile deformation d 1 followed by another in which the deformation tended to concentrate in a subhorizontal ductile shear zone d 2 and culminated with the thrusting along an important fracture the shear zone originated in a thermally softened level which according to metamorphic data was at temperatures in the range 550 600 c the structural metamorphic and magmatic features of the mondonedo nappe suggest a relationship with a crustal scale overthrusting of the continental mass to which the nappe belongs this could be a consequence of the collision against another continental mass to the west

the late variscan ht lp metamorphic event in nw and central iberia relationships to crustal thickening extension orocline development and crustal evolution

kinematic and thermodynamic characteristics of the flow over the western pacific warm pool during toga coare abstract rawinsonde and satellite infrared radiation ir data from the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western pacific warm pool infrared data for the 4 mo intensive observing period iop and vertical motion fields indicate that the intensity of convection height of maximum upward motion and sst all increased from west 140 e to east across the coare domain iop mean ir data show a double itcz intertropical convergence zone structure north and south of the intensive flux array ifa centered at 2 s 156 e although marked variability in the patterns occurred on a month to month basis three prominent westerly wind bursts occurred over the ifa during the 4 mo iop in association with the intraseasonal oscillations isos strong upward motion usually occurred 1 3 weeks prior to the peak low level westerlies subsidence dominated when

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

multiscale variability of deep convection in relation to large scale circulation in toga coare deep convection over the indo pacific oceanic warm pool in the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare occurred in cloud clusters which grouped together in regions favoring their occurrence these large groups of cloud clusters produced large scale regions of satellite observed cold cloud top temperature this paper investigates the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm pool region to the scale of individual cloud clusters and their relationship to the large scale circulation and sea surface temperature sst the dominant convective variability was associated with the intraseasonal oscillation iso a large eastward propagating ensemble of cloud clusters marked the iso s progress the meridional structure of the iso was strongly affected by the seasonal cycle with a southward shift from the northern hemisphere in october november to the southern hemisphere in january february the sst had an intraseasonal signal in lagged quadrature with the cold cloudiness and rainfall in coare the sst increased decreased during the convectively suppressed active phases of the iso enhanced low level westerly winds occurred toward the later stages of the enhanced convection periods of the iso though not always centered at the equator the strongest westerlies tended to be located between two synoptic scale cyclonic gyres which were often not symmetric about the equator in the low level wind field this asymmetry in the anomalous equatorial low level westerlies may have different implications for the oceanic response in the coupled atmosphere ocean system than those centered on the equator the cyclonic gyres contained highly concentrated deep convection and in four cases the gyres developed into tropical cyclones within the envelope marking the convectively active phase of the iso cloud clusters were frequently concentrated into westward propagating disturbances with a local periodicity of similar to 2 days these 2 day disturbances have been identified in earlier spectral studies and appear to be related to westward propagating inertio gravity waves in coare they typically contained numerous cloud clusters which underwent a distinct diurnal cycle most of the cloud clusters embedded in the 2 day disturbances moved westward though some were stationary and a few moved eastward a cloud duster tracking program identified groups of clusters lime dusters that exhibited continuity in time and space in the most convectively active period of the iso the tracking program identified almost the entire iso cloud ensemble as a long lasting trackable superconvective system this observation indicates the lack of a distinct scale separation between convection and large scale disturbances during the most intense convective periods in coare

coastal evolution on volcanic oceanic islands a complex interplay between volcanism erosion sedimentation sea level change and biogenic production abstract the growth and decay of oceanic hotspot volcanoes are intrinsically related to a competition between volcanic construction and erosive destruction and coastlines are at the forefront of such confrontation in this paper we review the several mechanisms that interact and contribute to the development of coastlines on oceanic island volcanoes and how these processes evolve throughout the islands lifetime volcanic constructional processes dominate during the emergent island and subaerial shield building stages during the emergent island stage surtseyan activity prevails and hydroclastic and pyroclastic structures form these structures are generally ephemeral because they can be rapidly obliterated by marine erosion with the onset of the subaerial shield building stage coastal evolution is essentially characterized by rapid but intermittent lateral growth through the formation of lava deltas largely expanding the coastlines until they typically reach their maximum extension with the post shield quiescence in volcanic activity destructive processes gradually take over and coastlines retreat adopting a more prominent profile mass wasting and marine and fluvial erosion reshape the landscape and if conditions are favorable biogenic processes assume a prominent role post erosional volcanic activity may temporarily reverse the balance by renewing coastline expansion but islands inexorably enter in a long battle for survival above sea level reef growth and or uplift may also prolong the island s lifetime above the waves the ultimate fate of most islands however is to be drowned through subsidence and or truncation by marine erosion

seismic signature of variscan and alpine tectonics in nw iberia crustal structure of the cantabrian mountains and duero basin the seismic structure of the crust of the nw iberian peninsula along the cantabrian mountains and their southern foreland the duero basin is investigated from the first data set available from refraction and wide angle reflection profiles the velocity depth distributions obtained along e w and n s transects evidence lateral variations in crustal structure that can be associated to variscan and alpine tectonic episodes the western part of the range toward the variscan hinterland zones as well as the duero basin below the sediments show a crustal structure similar to that of the variscan belt elsewere in europe the total crustal thickness is 30 32 km and three main layers upper middle and lower crust with average velocities 6 0 6 25 and 6 8 km s are resolved some paleozoic structures such as the basal detachment of the cantabrian zone are expressed in the seismic models the crust appears to vary eastward in the external zones the most outstanding feature is a crustal root beneath the highest cantabrian summits where the moho is found at 47 km depth a prominent alpine reworking of the crust in the eastern part of the cantabrian zone is thus revealed and the n s seismic transects delineate a geometry similar to the one found across the central pyrenees in the etude continentale et oceanique par reflexion et refraction sismique ecors profile revealing the importance of the alpine deformations in the northern part of the iberian plate

structure hydrothermal alteration and composition of the rubiales pb zn orebody lugo spain genetic model the rubiales pb zn ore deposit northwestern spain is situated in the westasturian leonese zone according to the division of the hercynian chain in the iberian peninsula julivert et al 1972 the orebody is placed in a subvertical shear zone developed at the eastern limb of the baralla syncline within the middle and upper members of the lower cambrian transition series the deposit is a vertical lenticular mass with a n30 w direction its length is about 1200 m in a n s direction by 600 m wide and has an average thickness of 30 m

the vertical profile of radar reflectivity of convective cells a strong indicator of storm intensity and lightning probability abstract reflectivity data from doppler radars are used to construct vertical profiles of radar reflectivity vprr of convective cells in mesoscale convective systems mcss in three different environmental regimes the national center for atmospheric research cp 3 and cp 4 radars are used to calculate median vprr for mcss in the oklahoma kansas preliminary regional experiment for storm central in 1985 the national oceanic and atmospheric administration tropical ocean global atmosphere radar in darwin australia is used to calculate vprr for mcss observed both in oceanic monsoon regimes and in continental break period regimes during the wet seasons of 1987 88 and 1988 89 the midlatitude and tropical continental vprrs both exhibit maximum reflectivity somewhat above the surface and have a gradual decrease in reflectivity with height above the freezing level in sharp contrast the tropical oceanic profile has a maximum reflectivity at the lowest level and a very rapid decrease in reflectivity with he

oroclines of the variscan orogen of iberia paleocurrent analysis and paleogeographic implications abstract coupled structural and paleomagnetic analyses have shown that the northern iberian bend of the variscan orogen referred to as the cantabrian orocline developed by vertical axis rotation of an originally linear orogen however palinspastic restoration of the orocline has proven difficult owing to 1 an unusually great orogenic width of over 700 km and 2 exposure of shallow water strata of the gondwanan margin in the northern and southern portions of the orogen we present paleocurrent data from lower ordovician shallow marine clastic sedimentary rocks across the variscan of northern and central iberia collected to constrain palinspastic restoration of the orogen paleocurrent data were collected from over 50 sites and include cross bed foresets ripple crests and casts as well as rare ball and pillow structures syn sedimentary slump folds and incised channels paleocurrent directions fan around the cantabrian orocline are consistently oriented at a high angle to structural strike and yield a consistent offshore direction outward from the oroclinal core similarly changes in structural strike and paleocurrent direction across central iberia imply the presence of a second more southerly orocline the central iberian orocline that is continuous with but convex in the opposite direction of the cantabrian orocline together the cantabrian and central iberian oroclines define an s shaped pair of continental scale buckle folds palinspastic restoration of the oroclines yields a linear continental margin 1500 km long characterized by consistent offshore paleoflow to the west defining a westerly oceanic domain presumably the rheic ocean and an easterly landward direction presumably gondwana recognition of the southern orocline explains the unusual width of the orogen the geometry of aeromagnetic anomalies attributable to variscan rocks and is consistent with available structural data paleomagnetic declination data and the distribution of correlative paleozoic and older rock sequences including shallow water strata of the gondwanan margin the s geometry of the oroclines is similar to the z geometry of the alaskan oroclines of the north america cordillera suggesting that as in the cordillera the iberian oroclines may record a significant margin parallel translation event

potential field constraints on the deep structure of the lugo gneiss dome nw spain abstract the lugo gneiss dome in the nw iberian massif spain is a variscan structure developed during late stages of orogenic collapse crustal extension was mainly accomplished by two kilometre scale conjugate extensional shear zones and by the late development of the dome and a huge normal fault these structures overprint previous contractional recumbent folds and a thrust fault the lugo dome and its southward continuation the sanabria dome are the site of the conspicuous eastern galicia magnetic anomaly egma a n s band 50 km wide and 190 km long with a maximum amplitude of 190 nt integrated potential field modelling of the egma and its corresponding gravity signature have been carried out aided by constraints provided by the measurement of c 900 magnetic susceptibilities and by previous geophysical data mainly seismic refraction and reflection profiles results suggest that a large volume of low density migmatites and associated inhomogeneous granites are the main source of the magnetic anomaly small massifs of basic and ultrabasic rocks inside the migmatites and high susceptibility iron ore bodies sparsely distributed in low grade middle ordovician slates are also thought to contribute to the anomaly but to a minor extent although otherwise similar to other gneiss domes the lugo dome is accompanied by a striking magnetic anomaly whose origin is discussed in terms of the tectonic evolution of this structure and the provenance of the magnetite bearing migmatites and inhomogeneous granites that core it

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma