cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain a discussion on the age of the main tectonometamorphic events

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

geochronological constraints on the evolution of a suture the ossa morena central iberian contact variscan belt south west iberian peninsula one of the main tectonic boundaries of the variscan belt in the iberian peninsula is the ossa morena central iberian contact this contact is marked by a highly deformed unit central unit which recorded an initial high pressure high temperature metamorphic evolution rb sr whole rock isotopic data from three gneissic bodies cropping out in the central unit yield two late proterozoic ages 690 134 and 632 103 ma and an early palaeozoic age 495 13 ma which we interpret as protolith ages the two late proterozoic orthogneisses show initial 87sr 86sr ratios typical of mantle derived materials or those with significant mantle participation 87sr 86sr 0 709 these new radiometric data together with ages previously published and the structural evolution of the central unit lead to the conclusions that 1 there are magmatic protoliths of late proterozoic and early palaeozoic ages 2 the metamorphic evolution of this area including the high pressure event belongs to the variscan orogenic cycle 3 the deformations observed affect the rocks of the entire central unit accordingly they are post ordovician i e variscan and 4 consequently the ossa morena central iberian contact is interpreted here as a variscan suture

geochronology and geochemistry of the pola de allande granitoids northern spain their bearing on the cadomian avalonian evolution of northwest iberia the pola de allande pre variscan tonalite granodiorite plutons are located in the narcea antiform at the boundary zone between the cantabrian and west asturian leonese zones of the iberian variscan belt these granitoids were intruded into a neoproterozoic siliciclastic sedimentary sequence with subordinate volcanic intercalations and were subsequently overprinted by variscan thrust related shear deformation u pb laser ablation inductively coupled plasma mass spectrometry icp ms dating of zircons from two plutons yielded concordant ages of intrusion of 605 10 and 580 15 ma to the authors knowledge this is the first reported u pb cadomian avalonian age for igneous rocks in this section of the iberian variscan belt these intrusions are coeval with the main episode of dominantly calc alkaline magmatic activity related to cadomian avalonian subduction major and trace element composition of the granitoids is characteristic of i type high k calc alkaline granitoids generated in continental arc se

sedimentation rates off sw africa since the late miocene deciphered from spectral analyses of borehole and gra bulk density profiles odp sites 1081 1084 abstract sedimentation rates srs off sw africa were calculated by performing spectral analyses in the depth domain on borehole and gamma ray attenuation gra bulk density data from odp sites 1081 1084 inversion and integration of srs versus depth from spectral analysis yielded detailed sr profiles in the time domain our technique allowed the detection of excursions in calculated srs that not only often differed from those established through coarse scaled biostratigraphic data but also revealed a greater regional variability in the sediment accumulation over time high resolution bulk density data exhibited distinct periodicity in the waveband of milankovitch cycles precession at 19 23 kyr obliquity at 41 kyr eccentricity at 100 kyr the pronounced milankovitch cyclicity suggests that climate variability and trends in srs along the benguela current system bcs were responding to insolation patterns during the past 6 myr we find relatively low srs when evolutive amplitude spectra are dominated by obliquity and eccentricity periods in contrast significant sr peaks at all sites often occur when strong precessional amplitudes coexist with obliquity and eccentricity cycles episodes of high srs often coincide with peaks in organic carbon mass accumulation rates mar c org and reduced sea surface temperature sst in the southern walvis basin which have been associated with increased regional upwelling this suggests that the high srs reflect high productivity high mar c org attributed to strong wind and upwelling intensity during cool climate periods low ssts in accordance with orbital forcing patterns observed in our spectra

tectonothermal analysis of high temperature mylonitization in the coimbra cordoba shear zone sw iberian massif ouguela tectonic unit portugal evidence of intra continental transcurrent transport during the amalgamation of pangea abstract mylonites in pelitic and quartzofeldsphatic gneisses from the ouguela tectonic unit coimbra cordoba shear zone sw iberian massif have been studied as an example of high temperature ductile deformation associated with transcurrent tectonics detailed microstructural and p t analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650 750 c and 7 5 9 5 kbar quartzofeldsphatic gneisses and 730 790 c and 7 5 9 5 kbar pelitic gneisses to retrograde conditions at 500 575 c and 4 5 5 5 6 5 7 5 kbar quartzofeldsphatic gneisses and 525 600 c and 3 5 4 5 5 5 7 5 kbar pelitic gneisses following the metamorphic peak exhumation was very fast the p t trajectory which does not reach the curve for granite melting is distinct that of isothermal decompression instead the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks the horizontal component of exhumation path calculated for middle and shallower crustal levels sum to ca 57 km to 94 km for the pressure peak assuming this offset acted in the visean during a time interval of ca 9 ma the estimated exhumation horizontal slip rate is in the order of 6 3 to 10 4 mm yr which corresponds to an exhumation oblique slip exhumation rate of 6 6 to 10 7 mm yr for ductile deformation these values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra continental shear zones such as the still active karakoram fault 8 3 mm yr in the himalayas the coimbra cordoba shear zone is therefore a typical intra continental transcurrent zone with ten to one hundred kilometre along strike mass movement of material that aided the exhumation of deep crustal rocks study of this large scale structure in the sw iberian massif is therefore central to models of orogenic deformation during the amalgamation of pangea

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

rb sr evidence for the presence of ordovician gsranites in the deformed basement of the badajoz cordoba belt sw spain a rb sr whole rock investigation of suites of samples from two orthogneiss bodies in the almendralejo area of the oporto portalegre badajoz cordoba belt points to an ordovician age for the granitic magmatism about 470 ma for the almendralejo gneiss peralkaline metagranite and about 425 ma for the ribera del fresno gneiss leucocratic subaluminous metagranite both with high initial87sr 86sr ratios this points to a post ordovician hercynian age for the metamorphism and tectonic deformation that affected the area including the gneisses and rules out the alleged cadomian or even older age

isotopic dating of the mylonitization of the azuaga group in the badajoz cordoba belt sw spain on the basis of rb sr muscovite dating an age of about 355 ma late tournaisian early visean is assigned to the m1 metamorphism and associated mylonitization of the ultramylonites of the las grullas formation in the azuaga group badajoz cordoba belt this age may possibly be related to the accretion process suturing of the ossa morena zone to the central iberian zone thought to be responsible for the metamorphism and mylonitization muscovite from the valencia de las torres formation displays a somewhat lower rb sr age of about 337 ma the corresponding k ar ages indicate that after the termination of the metamorphism it took some 20 ma before the rocks had cooled below the closure temperature of muscovite to k ar whole rock rb sr data indicate that the las grullas ultramylonites were probably derived from ordovician granitoids

bromine counts from xrf scanning as an estimate of the marine organic carbon content of sediment cores 1 xrf sediment core scanning technology is increasingly used to quantify sediment composition the overall good correlation between biophilic halogen bromine br and sedimentary total organic carbon toc potentially allows the fast estimation of down core toc profiles by xrf scanning in order to test this approach we present data from the arabian sea and a mediterranean brine basin comparing xrf core scanning br data with discrete sample toc analyses overall br counts and toc show a clear correlation except when stable carbon isotope and c n data indicate intervals characterized by enhanced input of terrestrial organic matter hence solid phase br is exclusively associated with marine organic matter moc and can be used as a direct estimate of this parameter after a calibration is established high pore water br in the brine core steepens the br toc correlation but after salt correction shows a nearly identical gradient to that of the arabian sea core

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

radiometric discrimination of pre variscan amphibolites in the ediacaran serie negra ossa morena zone sw iberia new results on the geochronology of metabasites hosted by the neoproterozoic late ediacaran serie negra of the ossa morena zone iberian massif are presented these rocks record a protracted subduction at least since the latest cryogenian followed in a continuum by early cambrian rifting the serie negra country rocks are continental crust forearc segments of the thinned continental margin of gondwana the metabasite suite exhibits e and n morb as well as volcanic arc signatures and discloses the existence of a diverse late ediacaran early cambrian magmatism across the ossa morena zone protoliths of the e morb metabasites are clearly ediacaran they exhibit geochemical characteristics typical of younger island arcs notably of their forearc zones affected by extension soon after the initiation of subduction radiometric dating of n morb metabasites shows that these rocks are significatively younger than their country rocks whose age has been stratigraphically and paleontologically constrained as ediacaran they record early cambrian rifting developed in a continuum after a protracted subduction metabasite petrological zonation permits to reconstruct a single ediacaran subduction zone located to the s of the current omz and dipping towards the n in present day geographical coordinates

combined zircon upb and luhf isotopes study of magmatism and high p metamorphism of the basal allochthonous units in the sw iberian massif ossa morena complex upb and luhf isotope ratios measured in zircon crystals from magmatic and metamorphic rocks provide crucial information to understand the origin and tectonothermal evolution of allochthonous terranes and can contribute to paleogeographic reconstructions and to the understanding of orogenic processes we investigate the age of magmatic and tectonothermal events and the provenance of a high pressure unit from the ossa morena complex sw iberian massif zircons for la icp ms analyses were separated form garnet bearing amphibolites retroeclogites and quartzo feldspathic gneisses from the central unit coimbra cordoba shear zone recent geological correlations propose that the high pressure units cropping out in the ossa morena complex of the sw iberian massif are equivalent to the basal allochthonous units of nw iberian massif and one of the objectives of the study is to check the validity of the proposed correlation the new obtained ages presented in this study indicate that the high pressure metamorphism of the central unit developed in the variscan cycle the metamorphic zircons reveal two distinct events a younger one well constrained by an age of 339 4 ma and an older one of 377 19 ma the former coincide with the age of the high grade and post high p metamorphism described in other areas of the ossa morena complex whereas the second one is interpreted as the age of the high p metamorphism the igneous ages of metabasites and metagranitoids c 585 615 ma and c 480 577 ma respectively reveal a diachronism between magmatic events in allochthonous units from the sw and the nw iberian massif suggesting that both domains were located in different parts of the north gondwanan margin the magmatic ages of the metagranitoids resemble the evolution described for the lower paleozoic rifting in the ossa morena complex the relatively juvenile character of the basic and acid cadomian magmatism in the central unit revealed by the new luhf data indicate limited mixing of mantle melts with crustal components which is compatible with a generation in a continental magmatic arc these similarities in the metamorphic and magmatic evolution support the correlation of the basal allochthonous units from the sw and nw domains of the iberian massif

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

correlation between seismic anisotropy and major geological structures in sw iberia a case study on continental lithosphere deformation in this paper we postulate a correlation between the lithospheric structure and seismic anisotropy inferred from geophysical data and the major structures and tectonic history past tectonothermal features of the outcropping basement rocks in sw iberia oblique plate convergence orogenesis and terrane accretion have been documented for pre mesozoic times in this area however the deviations and areal variations observed between the azimuthal anisotropy of seismic waves and the structural trends of the accessible crust indicate that the tectonic model of orogen parallel lithospheric deformation does not appear to succeed in relating hercynian surface geology and the structural geology of the subcontinental lithospheric mantle the model of polyorogenic lithosphere deformation proposed accounts for this divergence and is supported from additional perspectives including the manifestation of thermal effects induced in the crust by lithospheric mantle shear zones the incidence of crust mantle coupling and or decoupling and the scales of subcrustal heterogeneity

the amphibolites from the ossa morena central iberian variscan suture southwestern iberian massif geochemistry and tectonic interpretation abstract basic rocks included in the ossa morena central iberian variscan suture of sw iberia have been studied in order to decipher the pre orogenic evolution along this major boundary these rocks appear as garnet free amphibolites and garnet bearing amphibolites previous geochronological data on these basic rocks suggest the existence of two different protolith ages upper precambrian and lower palaeozoic ordovician upper precambrian amphibolites form small size up to several metres thick lense dike or elongated shaped bodies while lower palaeozoic ones form elongated bodies with thicknesses up to several hectometres the chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas as for the originally basaltic rocks their geochemical characteristics indicate a shallow melting with different proportions of at least three end members namely i a morb like asthenospheric source ii a more enriched plume derived or subcontinental lithosphere mantle source and iii a continental crust component moreover upper precambrian and lower palaeozoic amphibolites have different geochemical affinities in the former group we envisage an advanced intra continental rifting situation or an intra continental back arc setting related to the cadomian orogeny while for the latter we propose an environment of oceanic crust formation predating variscan subduction and collision the recognition of these remains of oceanic affinity basic rocks along the omz ciz boundary allows us to propose that some sort of ocean may well have existed between the two zones during early palaeozoic times and that this ocean was presumably closed at a later stage probably during the devonian by oceanic subduction under the ciz crust

u pb dating of granodiorite and granite units of the los pedroches batholith implications for geodynamic models of the southern central iberian zone iberian massif the first u pb geochronological results on the magmatic alignment of the los pedroches batholith are presented the batholith is composed of a main granodioritic unit several granite plutons and an important acid to basic dyke complex all of them intrusive after the main variscan regional deformation phase d1 along the boundary between the ossa morena and central iberian zones sw iberian massif zircons from samples on both extremes of the granodiorite massif record nearly simultaneous magmatic crystallization at ca 308 ma while the emplacement of granite plutons was diachronic between 314 and 304 ma the u pb results combined with new field and textural observations allow to better constrain the age of variscan deformations d2 and d3 across the region while the age of d1 remains imprecise transcurrent d2 shearing tightening of d1 folds occurred around 314 ma lower westphalian in relation to the emplacement of the first granitic magmas d3 faults and shear bands bearing a strong extensional component developed at ca 308 ma upper westphalian associated to the intrusion of the main granodiorite pluton granodiorite of the batholith together with available geochemical and geophysical information these results point to the variscan reactivation of lithospheric fractures at the origin and subsequent emplacement of hybrid magmas within this sector of the massif

lithospheric transition from the variscan iberian massif to the jurassic oceanic crust of the central atlantic abstract a 1000 km long lithospheric transect running from the variscan iberian massif vim to the oceanic domain of the northwest african margin is investigated the main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic belt the variscan iberian massif to the adjacent jurassic passive margin of sw iberia and across the transpressive and seismically active africa eurasia plate boundary the modelling approach incorporates available seismic data and integrates elevation gravity geoid and heat flow data under the assumptions of thermal steady state and local isostasy the results show that the variscan iberian crust has a roughly constant thickness of 30 km in opposition to previous works that propose a prominent thickening beneath the south portuguese zone spz the three layers forming the variscan crust show noticeable thickness variations along the profile the upper crust thins from central iberia about 20 km thick to the ossa morena zone omz and the ne region of the south portuguese zone where locally the thickness of the upper crust is

a new model for the hercynian orogen of gondwanan france and iberia abstract the hercynian orogen of gondwanan france and iberia is a collage of disparate terranes separated by faults plate reconstructions indicate that the orogen developed during a massive dextral transpression as laurentia slid along the boundary of gondwana and as laurentia baltica and east avalonia rotated clockwise to form pangea it is proposed therefore that the collage is the result of this transpression and that it is an amalgamation of displaced tectonostratigraphic terranes which represent the dismembering of the gondwanan shelf with its stable platform ordovician devonian sedimentary sequence and the neighbouring rheic ocean specifically it is proposed that 1 the slice of the gondwanan shelf that now forms north brittany was displaced dextrally from nw africa around iberia by more than 2000 km 2 the ophiolites oceanic material and volcanic arcs of south armorica and the massif central represent slices of the rheic ocean displaced dextrally along the gondwana margin and shuffled in amongst slices of gondwanan shelf 3 the convergent component of the transpression is represented by thrusts throughout the region the leon terrane of france and ophiolite bearing klippe in nw iberia the ibero armorican arc was formed by wrapping the mobile dextral shear belts about a rigid iberian basement block the dextral porto tomar shear zone represents the more important shearing in iberia and the iberian sinistral shears are interpreted as bookshelf type structures subsidiary to the dextral shearing

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

transcurrent continental tectonics model for the ossa morena zone neoproterozoic paleozoic evolution sw iberian massif portugal the ossa morena zone sw iberian massif was affected by continuous orogen parallel transcurrent continental tectonics from the neoproterozoic to the carboniferous times involving transtension tt and transpression tp processes that co existed together occurred separately in neighbouring regions by the means of strain partitioning or even worked diachronically a first stage of transpression tp1 took place during the late neoproterozoic lower cambrian as a result of cadomian arc continent collisional processes structures generated by transtension tt1 from cambrian to lower devonian were related to strong lithosphere stretching responsible for the development of basins controlled by major detachments tilting rifting and important tectono thermal diachronic processes denudation phenomena and inhibition of sedimentation related with thermal uplift asthenosphere upwelling and consequent subsidence caused by isostatic equilibrium involving generalized transgressions were processes responsible for major unconformities the variscan tp2 tt2 episodes that followed diachronically tp1 tt1 by maintaining the orogen parallel transport direction were concomitant with syntectonic deposition of continental basins in the omz and foreland basins in the spz tt2 local transtension and tectonic exhumation of deep crustal rocks along major shear zones favoured the opening of tectonic troughs filled up by sediments and volcanism tp2 shortening have generated fold axes parallel to the orogen strike and composite dissymmetric flower structures

regional significance of kilometric scale north east vergent recumbent folds associated with east to south east directed shear on the southern border of the central iberian zone hornachos oliva region variscan belt iberian peninsula on the southern border of the central iberian zone there are two sectors with different styles of deformation to the south west in the hornachos sector large scale recumbent folds associated with ductile shearing can be seen this shearing is characterized by a direction of movement parallel to the fold axes and can be correlated for 150 km along strike the k values of the strain ellipsoid range from 0 8 to 2 0 stretching in the x direction parallel to the recumbent fold axes is more than 100 to the north east in the oliva sector first phase folds are upright and the strain intensity is lower than in the hornachos sector metamorphic geometric and kinematic considerations lead us to conclude that the shearing in the hornachos sector is better explained as conjugate to a main shear zone along which the southern border of the central iberian zone is moved onto the ossa morena zone this main thrust is at present obliterated by a left lateral extensional shear zone that affects a high pressure exotic unit located between the central iberian and the ossa morena zones this high pressure unit constitutes a suture of the variscan belt in the iberian peninsula

relative timing of transcurrent displacements in northern gondwana u pb laser ablation icp ms zircon and monazite geochronology of gneisses and sheared granites from the western iberian massif portugal abstract the variscan belt of western and central europe was formed by the oblique subduction of the rheic ocean and the collision of laurussia with gondwana during the late palaeozoic we present field relationships and new u pb la icp ms zircon and monazite ages for variscan gneisses and granites from a key section of the western iberian massif the martinchel section records the interplay of two kilometre scale variscan transcurrent shear zones active in the gondwana basement of pangaea the porto tomar fault zone ptfz and the coimbra cordoba shear zone ccsz different kinematic models have been invoked to explain the formation and evolution of these major variscan structures mainly based on assumptions made in the absence of reliable radiometric ages we show that 1 ductile deformation and metamorphism were active in the ccsz during the visean serpukhovian c 335 318 ma and created conditions for amphibolite facies metamorphism and coeval emplacement of granites and 2 later ductile brittle deformation related to dextral movements along the ptfz overprinted the earlier foliation and folds derived from the ccsz deformation and deformed the previously intruded granites u pb dating of zircon and monazites yield c 335 ma ages for the ductile deformation developed under amphibolite facies metamorphic conditions in the martinchel gneisses of the ccsz the gneisses were intruded by granites at c 335 318 ma and both were later deformed under ductile brittle conditions by dextral motion on the ptfz the geometry of the martinchel gneisses typical of the ccsz changed from one of thrusting to one of normal faulting by refolding of the early foliation stretching lineation and asymmetric structures related to the later ptfz dextral shear episode this pattern of interference is not fully considered in previous models and may lead to incorrect tectonic interpretations according to our data and recently published ages we suggest that the ptfz was active after the serpukhovian kasimovian since the c 318 308 ma granites are deformed by north south 170 dextral shear planes these data are critical to the interpretation of large scale carboniferous transcurrent displacements in northern gondwana iberian massif and bear upon global models of crustal deformation that emphasize the importance of long lived dextral movements during the collision between northern gondwana and laurussia following the closure of the rheic ocean

a new model for the hercynian orogen of gondwanan france and iberia discussion the study by shelley and bossieare 2000 is an important contribution to the discussion concerning the ibero armorican arc iaa generation model this model comes as one of a sequence of previous ideas already published in several papers bard 1971 matte and ribeiro 1975 lefort and ribeiro 1980 ribeiro et al 1980 burg et al 1981 brun and burg 1982 julivert 1987 ribeiro et al 1990 dias and ribeiro 1995 ribeiro et al 1995 silva 1997 a common feature of the interpretations in these papers is the great importance attributed to two major transcurrent faults the dextral porto tomar shear zone ptsz and the sinistral tomar badajoz cordoba shear zone bcsz to explain the extension of the iberian structures into the armorican massif in the form of trace the arcuate shape of the iaa the large scale migration rotation and accretion of microplates controlled by oblique interactions have been a matter of controversy since researchers applied different kinematic models to reconstruct such global tectonic movements for example badham 1982 interpreted the hercynides as the result of dextral interaction between europe and africa and considered the arcuate geometry of the iaa as a secondary structure in contrast matte 1986 compared the present example of the himalayas with the structures of the iaa in terms of an indentation of crustal fragments and proposed the progressive development of the symmetrical outward lateral escape of blocks in france and iberia in this model the indentor migration caused opposing strike slip movements on either side represented by the major transcurrent faults of the armorican massif and the iberian massif this geometry and kinematics were considered to be syngenetic with the formation of the arcuate structure of the iaa however the main geologic lines linking these western european hercynian massifs through major shear zones are still questionable e g lefort 1989 in this comment we believe that the regional approach regarding the transcurrent faults of hercynian iberia bcsz and ptsz cannot be considered to be well established in the following text we will comment on signi cant omissions concerning available work of the last decade and also focus our discussion on eld data and the structural interpretation of hercynian iberia portugal and spain

land ocean linkages over orbital and millennial timescales recorded in late quaternary sediments of the japan sea late quaternary sediments of the japan sea are characterized by centimeter to meter scale alternations of dark and light layers which are synchronous basinwide high resolution analyses of the sediments from ocean drilling program site 797 reveal that deposition of the meter scale alternations reflect variations in paleoceanographic conditions which were closely associated with glacio eustatic sea level changes through the modulation of the volume and character of the influx to the sea through the tsushima strait the centimeter to decimeter scale alternations reflect millennial scale variations which are possibly associated with dansgaard oeschger d o cycles with each dark layer appearing to correspond to an interstadial this variability is attributed to the development of a humid climate in central to eastern asia and the consequent increase in discharge from the huanghe and changjiang rivers during interstadials this caused expansion of the east china sea coastal water ecscw which penetrated more strongly into the japan sea the increased influence of the lower salinity nutrient enriched ecscw reduced deep water ventilation and enhanced the surface productivity leading to the development of anoxic bottom waters and deposition of the dark layers thus the centimeter to decimeter scale alternations of the dark and light layers record wet and dry cycles in central to eastern asia possibly associated with d o cycles

shrimp u pb geochronological constraints on the timing of the intra alcudian cadomian angular unconformity in the central iberian zone iberian massif spain new shrimp u pb ages of detrital zircons from neoproterozoic low grade metasandstones of the schist graywacke complex central iberian zone iberian massif sampled just below and above the intra alcudian unconformity at two selected locations contribute to reconstruct the geodynamic evolution of iberia during the cadomian orogeny in the north gondwana margin the youngest zircons i e maximum depositional age in the lower alcudian are c 580 576 ma while those in the upper alcudian are c 555 552 ma the obtained remarkable time gap of about 21 ma supports the existence of a tectonic event in between this event resulted in moderate folding without related foliation metamorphism that verticalized the lower alcudian previous to the deposition of the upper alcudian additional evidence of late cadomian tectonothermal events elsewhere in iberia also fit in the interval c 560 550 ma combined with other geological data the most probable maximum depositional ages are c 580 560 ma for the lower alcudian previous to the late cadomian folding event and c 550 540 ma for the upper alcudian previous to the deposition of the overlying pusian group and lower cambrian sandstones and limestones a comparison of the new zircon age spectra with possible source areas verifies recent studies that point to the cadomian foreland in the north gondwana continent affected by the pan african orogeny the west african craton and or the saharan metacraton furthermore ongoing cadomian arc related magmatism in iberia c 605 545 ma could have contributed as a local zircon source the end of the cadomian activity is marked by a transient stage ephemeral lower cambrian platform which preceded widespread cambro ordovician rifting of north gondwana

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

plumbotectonic evolution of the ossa morena zone iberian peninsula tracing the influence of mantle crust interaction in ore forming processes the ossa morena zone in the variscan belt of the iberian peninsula is characterized by a complex geotec tonic evolution it hosts a neoproterozoic paleozoic sequence that records the existence of two orogenic events cadomian and variscan separated by rift and stable platform stages middle cambrian to late devonian volcanogenic and sediment hosted massive sulfides skarns magmatic ni cu deposits synmetamorphic and intragranitic and perigranitic veins and replacement bodies are related to different magmatic hydrothermal systems ranging in age from 650 ma to about 250 ma the lead isotope signatures from 26 different mineral deposits show variable ratios of 206pb 204pb 17 49 18 44 207pb 204pb 15 47 15 67 and 208pb 204pb 37 37 38 78 that have low average values 9 7 but high values 38 3 as a whole the lead isotope data define an irregular cluster that plots around the stacey and kramers 1975 growth curve but clearly marks the existence of two main ore forming events during cadomian and variscan times the most radiogenic samples are from some minor zn pb stratiform sedimentary exhalative mineralization they are interpreted as having a dominantly crustal derivation 10 1 in which most of the lead was leached from a pre cadomian continental basement with a long crustal history this basement has strong geochemical affinities with the oldest rocks of the iberian terrane however most of the ore samples investigated plot below the stacey and kramers 1975 reference curve indicating polyphase but variable mixing of lead from upper crustal and mantle lead reservoirs at the terrane scale clear mixing lines between the two end member reser voirs are evident only in the oldest volcanic rock and sedimentary rock hosted massive sulfide deposits related to the cadomian late neoproterozoic early cambrian magmatic arc in the other deposit types lead isotopes have more restricted values the crustal lead is interpreted as having been derived from ancient crust 2 1 2 5 ga and the derived shales montemolin formation that predated the cadomian orogeny the mantle derived lead may have been derived from three different sources neoproterozoic tholeiitic orthoamphibolite syn cadomian calc alkaline andesite or calc alkaline metaluminous plutonic rocks intruded during the variscan orogeny the first two sources probably controlled the lead isotope signatures of the cadomian massive sulfide deposits and the variscan synmetamorphic veins the input of primitive lead is especially significant during variscan times 350 300 ma and is interpreted as having been related to the existence of a large primitive reservoir that controlled the variscan magmatic metallogenic and hydrothermal evolution of the ossa morena zone this primitive lead is recognized in most of the magmatic hydrothermal deposits including the typically crust derived w and pb bearing perigranitic hydrothermal systems likewise associated plutonic rocks have more primitive lead isotope signatures than their counterparts in other iberian terranes the lead isotope evolution of the region is consistent with recent geophysical data that suggest that a deep large mafic sill was intruded below the ossa morena zone during variscan times this sill was a likely key factor in the different pb isotope evolution of the ossa morena zone compared with the nearby iberian terranes and the entire western mediterranean province which are char acterized by lengthy crustal histories with very little input of primitive lead the extensive mantle crust inter action probably explains the wide variety and the unusual metallogenic features of the ossa morena zone

paleoceanographic implications and cyclostratigraphy of variations in well log data from the western slope of the ulleung basin east sea cyclic variations of density and gamma ray logs achieved from the two drill sites of ubgh2 1 1 and ubgh2 1 2 in the western slope of the ulleung basin were investigated to reveal their paleoceanographic implications and establish cyclostratigraphy of the well logs integration of the core log and seismic data from the two sites indicates that the sediments of the drilled intervals mostly consist of hemipelagic muds with a few sporadic mass transport deposits intercalated only in the downslope site of ubgh2 1 1 comparison with variations in sediment composition suggests that ratios of terrigeneous to biogenic materials which were modulated by orbital scale climate changes should be responsible to the well log variations the established cyclostratigraphy of the well logs based on correlation with marine oxygen isotope records indicates such climatic modulation of the sedimentation in the western slope of the ulleung basin has persisted at least since 2 6 ma consistent with the variation reported from the oki ridge