geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

using airborne lidar sensing technology and aerial orthoimages to unravel roman water supply systems and gold works in nw spain eria valley leon lidar technology based on earth s surface scanning allows the compilation of high resolution digital terrain models recently used in archaeological works for the discovery and description of heritage features such as ancient human made structures and building ruins it is particularly effective in areas with dense vegetation or difficult access where surveying becomes complicated among its main advantages over other traditional methods of archaeological research it highlights the possibility of controlling display parameters which in turn facilitates data analysis and interpretation in this study we present an integrated lidar and aerial photography data analysis in order to obtain a detailed map of ancient mining works within a small sector of the roman mining district in northwestern spain the presence of gold deposits led to intensive extractive work during the first century b p t although many of the mining activities were focused on the northwestern area of las medulas and omanas large deposits were also found along the duerna and eria river valleys our results complement previous work carried out in the eria area valderia providing new insights into the hydraulic engineering techniques and the geometry of the main roman exploitations this work highlights the scope of the ancient mining works and their impact on the landscape which are much greater and more important than previously thought

the hercynian crust in northwestern spain a seismic survey abstract in summer 1982 a network of crustal seismic profiles was established in nw spain with shotpoints at sea and on land some profiles were designed to capture the general features of the crust whereas others were aimed at resolving the detailed structure of specific geological features the interpretation of three profiles aimed at the most important features of the earth s crust in northwest spain is discussed here the crust consists of three layers with compressional velocities of 6 0 6 1 km s 6 2 6 3 km s and 6 7 6 9 km s the thicknesses of these layers vary laterally the average values under the central part of galicia being 13 km 8 km and 10 km the fine structure of the lower crust has still to be determined so far lateral variations in the velocity structure of the lower crust from a constant velocity layer to a gradient layer have been detected along the main profile from ne to sw over a distance of 250 km the moho discontinuity is reached at 30 32 km in the central part of the surveyed area this decreases to 23 27 km to the north and to the west on the basis of pmp amplitudes the moho seems to be rather sharp the pn velocity is close to 8 3 km s which is slightly above the normal value elsewhere in hercynian europe the crustal structure is remarkably similar to other hercynian domains particularly brittany the massif central and the central iberian massif

a simple method of determining sand shale ratios from seismic analysis of growth faults an example from upper oligocene to lower miocene niger delta deposits a plot of fault throw vs depth is a simple geometric tool that graphically represents strata thickness variations in growth fault and growth fold settings and has previously been used to infer fault kinematics in this paper we use it as a prediction tool for lithological change employing only seismic data if growth faulting is a continuous process intervals of shale deposition are recorded by unthickened units while intervals of sand fill in topographic lows the throw vs depth plot easily allows depiction of unthickened and thickened sedimentary intervals from even rough seismic records and can therefore be used to predict sand shale ratios the method is here applied to a growth fault in the niger delta that affects oligocene to lower miocene deltaic deposits most shale intervals are identified and the sand shale ratios are predicted we suggest that the method can be a valuable tool in oil exploration

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

emplacement of the cuera and picos de europa imbricate system at the core of the iberian armorican arc cantabrian zone north spain new precisions concerning the timing of arc closure the final stages of the variscan orogeny mostly carboniferous in the western european variscan belt involved the development of the iberian armorican arc which is cored by the cantabrian zone nw iberia the cantabrian zone is the foreland of the western european variscan belt and it is interpreted to record the waning stages of the closure of the rheic ocean the distalmost tectonic units within the cantabrian zone the cuera unit and the picos de europa province were the last tectonic units emplaced at the core of the western european variscan belt orocline together they form an imbricate system and associated wedge top basins that are key to understanding the development of the orocline the emplacement of the cuera unit and the picos de europa province occurred in the latest pennsylvanian between moscovian and gzhelian times new detailed mapping together with stratigraphic sedimentologic and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the cantabrian variscan foreland fold and thrust belt most probably during the oroclinal bending of the sw european variscan belt that formed the iberian armorican arc the n s directed imbricate system was characterized by a shallow dip of the topographic surface 150 15 km and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying gondwana lithosphere and may reflect the closure of the iberian armorican arc

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

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

two dimensional geostatistical modeling and prediction of the fracture system in the albala granitic pluton sw iberian massif spain abstract we use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two dimensions 2d in the mina ratones area located in the albala granitic pluton sw iberian massif the fracture index fi is a quantitative estimate of the fracture density in discrete domains to validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1 1000 the resulting grids expressed as pixel maps describe the continuous value of the fi in 2d for the whole mina ratones area based on the modelled distribution of the fi and their correlation with mapped faults we distinguish two structural domains in the studied area elongated bands of fracture zones with high fi values and romboidal blocks located between them with low fi values the separation between both domains is gradual though a threshold value of the fi that separate both structural domains is not clearly defined the fracture zones generally present fi 1 and the individualized blocks fi

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

p wave velocity depth distribution in the hercynian crust of northwest spain abstract the results of a seismic refraction wide angle reflection survey in the hercynian of galicia northwest spain are presented two out of the five seismic refraction profiles obtained in 1982 are discussed in detail and the results are integrated with those already published to give a three dimensional view of the crust the crust is structured in three layers of varying thickness resulting in a moho depth of 26 27 km near the atlantic ocean and 32 km in central galicia the velocity in both the lower crust and the upper mantle are higher in central galicia 6 7 6 9 8 3 km s 1 than close to the atlantic coast 6 6 6 8 8 0 km s 1 the obtained results show remarkable similarities with those reported for other areas of the hercynian crust in western europe

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

relationships between syn orogenic sedimentation and nappe emplacement in the hinterland of the variscan belt in nw iberia deduced from detrital zircons flysch type syn orogenic deposits of carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the variscan belt of nw iberia new u pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry la icp ms are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting the age populations of four syn orogenic formations are compared with those of the pre orogenic sequence in the autochthon and parautochthon representing the gondwanan passive margin and in the allochthon formed by peri gondwanan and oceanic terranes in addition a new structural study has been carried out to understand the relationships between the syn orogenic deposits and the development of variscan structures the aims are to identify the sources of sediments and to establish the relationship between variscan structural evolution and syn orogenic sedimentation development of a forebulge outwards from the allochthonous front deduced from the structural study suggests the existence of depocentres that hosted the syn orogenic sediments together with the trend shown by the more recent zircons in each formation that are younger towards the external zones the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement the zircon age populations point to the allochthon as the main source of detritus for the syn orogenic basins with perhaps a limited participation of the parautochthon and autochthon in the younger formations

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

the ibero armorican arc a collision effect against an irregular continent the ibero armorican arc is the main variscan macrostructure in western europe although it was recognized in the 1920s its genesis is still debatable and the proposed models for its generation questionable however even if some doubts persist it seems that most agree on the general geodynamic evolution of this virgation in both branches in the middle to late devonian iberia was deformed by a sinistral transpressive regime while in the northern branch thrusting events were predominant in the carboniferous a dextral transpression begins to predominate in the armorican branch while the southern branch was deformed by southward thrusting in an attempt to correlate these events we propose that during the late devonian a cantabrian indentor moved northward producing the oblique closure of the southern part of the rheic ocean and an almost orthogonal closure in central europe in the carboniferous the collision with the irregular margin of laurasia induced a rotation of the indentor the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one

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

superposed hercynian and cadomian orogenic cycles in the ossa morena zone and related areas of the iberian massif polyphase ductile deformation and metamorphism in the ossa morena zone of the iberian massif are related to a complex evolution including two major tectonothermal episodes of cadomian late proterozoic to early cambrian and hercynian age middle to late paleozoic petrological structural and geochronological data attest to a number of tectonic episodes in the 620 480 ma interval the model presented for this orogeny comprises stages of volcanic arc generation backarc extension tectonic inversion crustal thickening and cratonization in an andean type continental margin the hercynian cycle began in the early paleozoic with episodes of continental rifting orogenic events occurred in the 390 300 ma interval and affected both the cadomian basement which was heterogeneously reworked and overprinted and the paleozoic cover hercynian regional metamorphism was generally of low grade high pressure assemblages and thermal domes developed locally and were related to subduction and late extensional events the geodynamic model proposed for the cadomian and hercynian orogenies and the correlation with comparable areas from pre mesozoic massifs elsewhere in western europe account for the geodynamic scenarios that drove the fate of cadomian terranes

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

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

investigating the kinematics of local thrust sheet rotation in the limb of an orocline a paleomagnetic and structural analysis of the esla tectonic unit cantabrian asturian arc nw iberia the esla tectonic unit lies along the southern boundary of the cantabrian asturian arc a highly curved foreland fold thrust belt that was deformed during the final amalgamation of the pangea supercontinent previous structural and paleomagnetic analyses of the cantabrian asturian arc suggest a two stage tectonic history in which an originally linear belt was bent into its present configuration creating an orocline the esla tectonic unit is a particularly complex region due to the interaction of rotating thrust sheets from the southern limb of the arc and the southward directed thrusts of the picos de europa tectonic domain during late stage north south shortening and oroclinal bending these structural interactions resulted in intense modification of early phase thin skinned tectonic structures that were previously affected by a deeper out of sequence antiformal stack that passively deformed the early thrust stack a total of 75 paleomagnetic sites were collected from the portilla and santa lucia formations two carbonate passive margin reef platform units from the middle devonian similar to other regions of the cantabrian asturian arc esla unit samples carry a secondary remanent magnetization that was acquired after initial thrusting and folding of variscan deformation in the late carboniferous protracted deformation during late stage oroclinal bending caused reactivation of existing thrust sheets that include the esla and younger corniero and valbuena thrusts when combined with existing structural data and interpretations these data indicate that the present day sinuosity of the esla unit is the consequence of both secondary rotation of originally linear features in the western esla exposures e g frontal thrusts and secondary modification and tightening of originally curvilinear features in the eastern esla exposures e g hanging wall lateral oblique ramps differences in structural style between the esla and other tectonic units of the arc highlight the complex kinematics of oroclinal bending which at the orogen scale buckled an originally linear north south in present day coordinates trending cantabrian asturian thrust belt during the final stages of pangea amalgamation

thrust tectonics deep structure and crustal subduction in the alps and himalayas the structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution deep structure and amounts of plate convergence after the initial contact between two continental masses balanced cross sections have been constructed parallel to the local displacement directions across the western alps and western himalayas and demonstrate very large amounts of crustal shortening above intra crustal detachments to achieve a balance substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands a model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the alpine and himalayan hinterlands patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale the development of crust mantle detachments being of crucial importance such a profile is proposed for the himalayas suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism in the alps however a steeper thrust profile apparently developed emplacing mantle onto crust causing wholescale eclogite metamorphism in the underlying franco swiss crust the resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the po plain region

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

magnetotelluric signature of the western cantabrian mountains the magnetotelluric signature of the alpine part of the cantabrian mountains was investigated in a n s magnetotelluric profile from the duero basin to the cantabrian shoreline a two dimensional conductivity model was obtained from the joint inversion of tm te modes and the geomagnetic transfer functions a shallow conductor associated with the sediments of the duero basin and three north dipping conductors in the upper and middle crust were detected of the latter one is interpreted as the sole detachment of the cantabrian mountains whereas the others correlate with the cabuerniga and san vicente de la barquera faults this geometry suggests that the cantabrian mountains comprise uplifted variscan basement a comparison with a seismic reflection profile shows a good correlation between these conductors and seismic features the lower crust is characterised by high conductivities

reconstitution of east facing variscan nappes between the grand banks of newfoundland and spain abstract a sequence of west dipping deep seismic reflectors recorded in the eastern part of the grand banks of newfoundland are aligned after accounting for the closure of the north atlantic with hercynian west dipping thrusts mapped in northern spain the roots of these variscan structures are now isolated at depth on the grand banks from the main orogeny mapped in europe in map view the grand banks roots describe the same flexure as the variscan cantabrian front in spain but exhibit gentler curvature the hercynian ibero armorican arc structure between spain and brittany corresponds to the rejuvenation of an arcuate precambrian structure the precambrian remnants of which are now identified on the grand banks the variscan tightening of this arc was contemporaneous with the collision between laurentia and gondwana in the carboniferous

deep weathering of granitic rocks a case of tunnelling in nw spain abstract this paper highlights a case of extensive development of saprolith from granitic bedrock in nw spain the weathering front at the study area extends to more than 250 m in depth the drilling of a railway tunnel madrid galicia high speed line through the bedrock and saprolith resulted in significant technical issues and economic losses cross sections revealing the 3d complexity of saprolith development are mainly based on fieldwork boreholes laboratory tests and the excavation works saprock is always observed overlying unweathered igneous bedrock two saprolite facies one clay rich and the other clay poor overlie the saprock the recognition is based on several criteria these deposits gradually thicken from the boundaries between the granitoid and metasedimentary roof pendants factors controlling saprolith development are discussed and the chemical and mineralogical changes and trends that occur are described variations in some geotechnical properties and the loss of quality of the rock masses from an engineering perspective are also quantified

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

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

lithospheric structure of the mid norwegian margin comparison between the more and voring margins the lithospheric structure across the more margin has been studied by integrating regional elevation gravity geoid and heat flow data together with available seismic and geological data our results show that the moho depth varies from 42 km beneath the caledonides to 20 15 km beneath the basin and to 13 16 km in the oceanic domain the lithosphere thins in a step like manner from 160 km inland to 60 80 km in the oceanic domain a comparison with the voring margin reveals that both margins have similar crustal and lithospheric structures major dissimilarities are 1 the volume of underplating beneath the ocean continent boundary 2 the lithospheric thickness beneath the caledonian front 3 the lithospheric thickness beneath the distal parts of the margins the heat transfer across the voring transform margin the secular variation of lithospheric thickness with age and the jump of the aegir spreading ridge are responsible for these differences

sequence of thrusting and origin of culminations in the northern and central oman mountains abstract detailed mapping and structural analysis of three large scale culminations sumeini and asjudi half windows and haybi hawasina window in the oman mountains shows a considerably more complex history of deformation than a simple foreland or downward sequence of thrust development early thrusting processes tended to create a regular stacking order of imbricate slices and major thrust sheets complying with the rules of thrust propagation assembled progressively downwards and forwards in the direction of translation out of sequence thrusts can also be demonstrated in places by truncation of footwall structures folds imbricate slices etc gross strain differences between thrust sheets downward facing structures in footwall units and elimination of thrust sheets beneath late stage thrusts frequently cut up section through the previously assembled stack putting previously younger lower thrust sheets over previously older higher ones many of the culminations in the northern and central oman mountains were formed by ramping associated with this late stage leap frog rethrusting event

the origin of the variscan upper allochthons in the ortegal complex northwestern iberia sm nd isotopic constraints on the closure of the rheic ocean northwestern iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the variscan orogeny by the late paleozoic collision between laurussia and gondwana are exposed lower allochthons contain ordovician ophiolite known as the lower ophiolite and are structurally overlain by devonian ophiolite upper ophiolite which are in turn structurally overlain by allochthons containing high grade metamorphic rocks with continental affinities and late cambrian early ordovician protolith ages known as the upper units geochemical and sm nd isotopic data from the upper ophiolite and the structurally overlying upper units exposed in the ortegal complex of galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages the general isotopic characteristics of each assemblage are similar to allochthons in other variscan complexes in nw iberia suggesting that the allocht

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