tectonothermal evolution of the badajoz cordoba shear zone sw iberia characteristics and 40ar 39ar mineral age constraints field characteristics indicate that the badajoz cordoba shear zone has had a long polyphase tectonothermal evolution 40ar 39ar cooling ages recorded within central ductile sectors of the badajoz cordoba shear zone range from ca 370 to 360 ma amphibole and 340 to 330 ma muscovite these are interpreted to date post metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant variscan tectonothermal overprint these results combined with field relationships indicate that the badajoz cordoba shear zone experienced at least 15 km of uplift relative to adjacent areas during late paleozoic sinistral transpression the data also indicate that individual structural units within the shear zone experienced contrasting uplift histories both in total amount and rate the 40ar 39ar cooling ages indicate complete variscan rejuvenation of all older intracrystalline argon systems within central sectors of the badajoz cordoba shear zone

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

paleothermal structure of the point san luis slab of central california effects of late cretaceous underplating out of sequence thrusting and late cenozoic dextral offset late cretaceous shale and sandstone turbidites of the point san luis slab are isoclinally folded locally sheared and faulted but their severity of stratal disruption is relatively mild when compared to adjacent polymictic melange of the franciscan complex we tested the interpretation of a trench slope basin origin for these strata by documenting their paleothermal structure including contacts between turbidites and melange values of mean random vitrinite reflectance rm from turbidites are 0 9 1 7 estimates of maximum paleotemperature are 135 200 c melange matrix samples yield rm values of 1 1 2 5 with an average of 1 5 and peak temperatures between 160 and 240 c the turbidite over melange contact is locally cooler over warmer and was folded after peak heating the relatively high paleotemperatures cast doubt on a shallow slope basin model i e 1 2 km burial depth we suggest instead that thermal maturation of the point san luis slab occurred much deeper in an accretionary prism 10 15 km where offscraped trench wedge deposits were faulted against underplated melange the paleothermal structure was offset and tilted after peak heating by two out of sequence faults late oligocene to pliocene strata rest unconformably above the franciscan and there is a significant gap in thermal maturity across this unconformity with no evidence to show that franciscan rocks were reset thermally following the main episode of uplift and erosion three dimensional orientations of isoreflectance surfaces on opposite sides of the san gregorio san simeon hosgri fault system also can be used to test interpretations of strike slip neotectonics the failure to match these geometries among suspected piercing points at point san luis cambria and point sur favors a suggestion that differential post thermal peak dextral offset of franciscan basement has not exceeded 10 15 km

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

the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia correlation of three high p and low to intermediate t belts from nw and sw iberia allows recognition of a single terrane affected by late devonian continental subduction during the variscan orogeny this terrane is located in the lower part of the allochthonous complexes of nw iberia and in a lower position in most of the ossa morena zone that therefore emerges as another allochthonous complex of iberia the ossa morena complex together with the allochthonous complexes of nw iberia form a huge rootless nappe that was transported towards the variscan foreland during final assembly of pangea

tectonic evolution of the iberian margin of gondwana and of correlative regions a celebration of the career of cecilio quesada

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

age and characteristics of the loma del aire unit sw iberia implications for the regional correlation of the ossa morena zone abstract finding of ca 620 ma old zircons in a volcanic rock of the loma del aire unit one of many structural divisions in the ossa morena zone some years ago attracted much attention to this unit which would contain if proven the oldest rocks so far dated not only in ossa morena but also in the entire iberian massif in this paper new field petrographic whole rock geochemical and sm nd isotope data as well as new tims u pb zircon ages are presented which collectively allow a much better characterization and dating as cambrian of the rock sequence in the loma del aire unit the previously found ca 620 ma old zircon population is now interpreted as xenocrysts incorporated in the cambrian magma our data together with recently published provenance studies bear important implications concerning the nature and evolution of the underlying lithospheric basement of the ossa morena zone which shares many characteristics with the neoproterozoic peri west african craton arc systems currently exposed in the precambrian inliers of the moroccan anti atlas the north armorican zone of brittany normandy and the channel islands cadomian arc and the saxo thuringian and tepla barrandian zones of the bohemian massif a correlation of the ossa morena zone with these arc systems in the neoproterozoic and early paleozoic is thus proposed

petroleum generation simulation over six years of hydrocarbon formation from torbanite and brown coal in a subsiding basin abstract dry samples of torbanite and soft brown coal from which mineral matter and material soluble in chloroform have been removed have been artificially matured in an attempt to simulate conditions in a subsiding sedimentary basin with a constant geothermal gradient sealed tubes were increased in temperature by 1 c per week beginning at 100 c a tube containing each source material was opened and analysed at yearly intervals for six years olefins and carbon monoxide were not significant products at any stage at less than 300 c torbanite generated a paraffinic crude oil in 35 yield while the brown coal gave a wet co2 rich natural gas in most respects these experiments which are possibly as slow as can be realistically planned within a human time scale appear to have duplicated geochemical hydrocarbon generation

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

the georges bank basin a quantitative basin analysis study using a one dimensional 1d fluid flow compaction model applied to wells of the georges bank basin east coast usa has enabled a simulation to be made of the history of the sedimentation basement subsidence tectonic activity geoheat flux evolution maturation of organic matter hydrocarbon generation and migration tendency based on the analysis of the basement tectonic activity the basin s development is related to the formation and the evolution of the east edge of the north american plate through the simulation of geoheat flux variation maturation of organic material and hydrocarbon generation at different positions in the basin at different time periods are analysed additionally from the modelling of excess fluid pressure porosity and permeability variations in the strata the potential hydrocarbon migration tendency and stratigraphic traps in the basin are discussed

determination of paleoheat flux from vitrinite reflectance data a method is presented for determining paleoheat flux from the reflectance of vitrinite collected from wells the maturation of vitrinite is modeled using a time temperature integral that assumes pseudo first order kinetics in testing the numerical algorithm it was necessary to introduce a critical temperature of 295 k below which vitrinite does not mature this result is consistent with the latest findings of independent researchers the degree of consistency obtained from a number of wells is not perfect much of the inconsistency can be attributed to large uncertainties in the vitrinite reflectance readings to test the method heat flow history was determined for the north sea basin and the national petroleum reserve of alaska quantitative results were obtained detailing the increase and or decrease of heat flux with time and were consistent with the qualitative behavior inferred for paleoheat flow from current geologic models

u pb geochronology and petrology of the late paleozoic gil marquez pluton magmatism in the variscan suture zone southern iberia during continental collision and the amalgamation of pangea the origin of plutonic complexes that stitch suture zones developed during collision is not well understood in southern iberia the pulo du lobo suture zone pdlz is intruded by the syn to postcollisional gil marquez pluton gmp thought to be part of the sierra norte batholith u pb la icpms zircon data on various phases of the gmp yield from oldest to youngest 1 a 354 4 7 6 ma unfoliated gabbro 2 a 345 6 2 5 ma foliated intermediate phase 3 a 346 5 5 4 ma unfoliated porphyritic granite 4 a 335 1 2 8 ma unfoliated biotite granite this sequence is consistent with cross cutting relationships observed in the field the range in ages is consistent with interpretations that the gmp is part of the composite ca 350 308 ma snb inherited ages preserved in the gmp intermediate and felsic phases indicate that its magmas traversed through south portuguese zone and pdlz crust during emplacement the ca 345 ma emplacement of the late kinematic foliated intermediate phase constrains the age of late stage strike slip deformation within the pdlz and the lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement nor during its subsequent history the unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision while the ca 335 ma biotite granite intrudes the santa ira flysch thereby providing a tight constraint for the latest stage of deformation in the pdlz

development of local orthorhombic fabrics within a simple shear dominated sinistral transpression zone the arronches sheared gneisses iberian massif portugal

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

polyphase late paleozoic tectonothermal evolution of the southwestern meguma terrane nova scotia evidence from 40ar 39ar mineral ages 40ar 39ar incremental release ages of hornblende muscovite and biotite from a variety of granitic stocks and host metamorphic rocks suggest a complex late paleozoic tectonothermal evolution for the southwestern meguma terrane regional d1 folding with cleavage formation under greenschist lower amphibolite facies m1 metamorphic conditions occurred at ca 400 410 ma and was followed by emplacement of a series of granitic stocks ranging in age between ca 375 and 315 ma these were emplaced at relatively shallow crustal levels and developed contact metamorphic aureoles of variable grade these are locally superposed on m1 regional metamorphic assemblages and result in a complex isograd pattern 40ar 39ar mineral ages suggest episodes of contact metamorphism occurred at 1 360 375 ma possibly related to emplacement of the south mountain batholith or temporal equivalents 2 350 356 ma around the port mouton pluton and northeastern shelburne pluton 3 ca 315 325 ma near the wedgeport pluton and in

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

crystallographic preferred orientations and microstructure of a variscan marble mylonite in the ossa morena zone sw iberia the mylonitic marbles of the cherneca shear zone a variscan sinistral strike slip structure developed in the ossa morena zone sw iberia related to the emplacement of the santa olalla igneous complex were studied in detail different mylonites from protomylonites to ultramylonites were analyzed by electron back scattering diffraction ebsd in order to determine the crystallographic preferred orientation cpo of calcite the cpos show very good agreement with experimental torsion work giving suitable interpretations of the operative slip systems and strain quantifications thus the obtained natural cpos validate the experimental approaches a wide range of observed textures and microstructures are mainly controlled by the amount of shear strain the presence of secondary phases and the development of antithetic subsidiary shears the effect of secondary phases and the nucleation of antithetic shear bands on texture development are discussed

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

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

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

effect of a dolerite intrusion on triterpane stereochemistry and kerogen in rundle oil shale australia abstract stereochemical changes of triterpanes present in extracts from an immature oil shale sequence intruded by a 3 m dolerite sill have been studied by gas chromatography mass spectrometry gc ms the steric configuration of the hopanes was observed to change from one dominated by the thermally less stable 17 h 21 h configuration at some distance from the intrusion to one dominated by the thermally more stable 17 h 21 h and 17 h 21 h configurating in the immediate vicinity of the intrusion in addition severe alteration of the kerogen appeared to have taken place as a result of the contact metamorphism and high concentrations of extractable organic matter were observed below the intrusion characterization of the kerogens by curie point pyrolysis has enabled the effects of the intrusion on the shales to be monitored

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

thermal effects of magmatic sills on coal seam metamorphism and gas occurrence igneous intrusions in coal seams are found in 80 of coal mines in the huaibei coalfield china and coal and gas outburst accidents have occurred 11 times under a 120 m thick sill in the haizi mining field the magma s heat had a significant controlling effect on coal seam gas occurrence based on theoretical analysis experimental tests and site validation we analyzed the temperature distribution following magma intrusion into coal measure strata and the variations in multiple physical parameters and adsorption desorption characteristics between the underlying coal seams beneath the sill in the haizi mining field and coal seams uninfluenced by magma intrusion in the adjacent linhuan mining field the research results show that the main factors controlling the temperature distribution of the magma and surrounding rocks in the cooling process include the cooling time and the thickness and initial temperature of the magmatic rock as the distance from sill increases the critical effective temperature and the duration of sustained high temperatures decrease the sill in the haizi mining field significantly promoted coal seam secondary hydrocarbon generation in the thermally affected area which generated approximately 340 m3 t of hydrocarbon in the magma affected area the metamorphic grade micropore volume amount of gas adsorption initial speed of gas desorption and amount of desorption all increase fluid entrapment by sills usually causes the gas pressure and gas content of the underlying coal seams to increase as a result the outburst risks from coal seams increases as well

a study of the effect of igneous intrusions on the structure of an australian high voltage bituminous coal abstract samples of heat affected coal taken near two igneous intrusions dykes in the victoria tunnel seam of the newcastle coalfield have been investigated by chemical analysis petrography fourier transform infrared spectroscopy and solid state 13 c nmr spectroscopy visual inspection of the seam near the intrusion showed four distinct zones these have been called in order of increasing distance from the dyke the massive cinder the banded cinder heat affected coal and apparently unaffected coal the samples show an increase in structural change as the distance from the dyke decreases with the samples taken from the massive cinder nearest the dyke being classified as semicokes ftir and 13 c nmr results together with the results of dipolar dephasing nmr experiments suggest that the predominant structural change in the heat affected zone is replacement of aromatic ethers and or phenols with hydrogen leading to an increase in proton aromaticity closer to the intrusion bond rupture leads to a marked loss of aliphatic groups while in the most affected region adjacent to the dyke aromatic crosslinking has also occurred to form coke like material

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

outline of upper precambrian and lower paleozoic evolution of the iberian peninsula according to u pb dating of zircons in the northwestern part of the iberian peninsula recent geological studies have defined the following stratigraphic sequence from bottom to top basal orthogneiss of miranda do douro is overlain by a complex of schist and greywacke which have been assigned to the cambrian or late precambrian these are followed by the ollo de sapo gneisses of volcanic sedimentary origin which in turn are overlain by the fossiliferous layers of the lower ordovician the schist and greywacke complex and the lower part of the ollo de sapo formation are transected by granitic dikes viana del bollo orthogneiss all these rocks were folded faulted metamorphosed and intruded by granite plutons during variscan times the emplacement of the miranda do douro and viana del bollo orthogneisses have been respectively dated at 618 9 ma and 465 10 ma by the u pb method on zircons these data yield lower and upper limits for the deposition of the ollo de sapo formation and demonstrate the occurrence of a precambrian basement pan african or cadomian orogeny in the variscan belt in the northwestern part of the iberian peninsula the emplacement of viana del bollo orthogneiss could be genetically related to the main phase of crustal thinning 470 520 ma characteristic of the prevariscan evolution of western europe typology and u pb data of the ollo de sapo zircons indicate the occurrence of inherited old precambrian zircons and yield an age of 325 3 ma for the variscan main thermal event in the area considered four possible evolutionary models for the u pb system of the ollo de sapo zircons are discussed

the effect of igneous intrusive bodies on sedimentary thermal maturity abstract the high temperatures of igneous intrusives cause localized thermal maturity of sediments far in excess of the regional variation previous studies have shown that igneous bodies cool in less than about a million years for all but the most exceptional geological conditions two case studies are provided which show how the increase in thermal maturity around an igneous body can be used to assess the temperature of the intrusive at emplacement and also the time of intrusion corollorative implications for localized hydrocarbons generation migration and accumulation are also discussed briefly

ediacaran palaeozoic tectonic evolution of the ossa morena and central iberian zones sw iberia as revealed by sm nd isotope systematics abstract sm nd isotopic analyses of palaeozoic sedimentary and igneous rocks in the southwest iberian massif western end of the european variscan belt are presented in order to unravel its complex poly orogenic evolution during the closure of the rheic ocean and the amalgamation of pangea the gondwanan margin in southwest iberia sw iberia is subdivided into the ossa morena and central iberian zones separated by the badajoz cordoba shear zone which represents a cryptic suture zone between these terranes the relationships between these terranes and between units preserved within the suture zone e g the sierra albarrana group during the palaeozoic and neoproterozoic are controversial sm nd isotopic studies of representative sedimentary sequences covering the entire pre variscan record of the ossa morena and central iberian zones show very similar characteristics from the uppermost ediacaran onwards these data indicate that their accretion to one another must have been completed by the late neoproterozoic ediacarin that time an event assigned to cadomian orogeny and that they never separated substantially from each other since that time the sm nd isotopic composition of the sierra albarrana group metasedimentary rocks is similar to that of the pre cadomian sequences of the ossa morena zone serie negra suggesting derivation from a common source the common provenance of the palaeozoic sequences in the two zones is identical to that of the pre cadomian serie negra of the ossa morena zone which in accordance with the data presented herein and published u pb zircon data indicates a west african affinity

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

new seismic gap hypothesis five years after we use earthquake data from 1989 1994 to test a forecast by nishenko based on the seismic gap theory we refer to this forecast as the new seismic gap hypothesis because it is the first global forecast based on the seismic gap hypothesis that considers the recurrence time and characteristic earthquake magnitude specific to each plate boundary segment nishenko s forecasts gave probabilities that each of about 100 zones would be filled by characteristic earthquakes during periods of 5 10 and 20 years beginning on the first day of 1989 only the first of these can be tested now we used three tests based on 1 the total number of zones filled by characteristic earthquakes 2 the likelihood that the observed list of filled zones would result from a process with the probabilities specified in nishenko s hypothesis and 3 the likelihood ratio to that of a poissonian null hypothesis the null hypothesis uses a smoothed version of seismicity since 1977 and assumes a gutenberg richter magnitude distribution we used both the harvard centroid moment tensor and the national oceanic and atmospheric administration preliminary determination of epicenters catalogs in our test we also used several different magnitude cutoffs in our tests because nishenko s forecast did not specify a clear relationship between the characteristic earthquake magnitude and the threshold magnitude for a successful prediction using a strict interpretation that only earthquakes equal to or larger than the characteristic magnitude should be counted both catalogs show only two qualifying earthquakes in the entire area covered by the forecast the predicted number is 9 2 and the discrepancy is too large to result from chance at the 99 confidence level the new seismic gap hypothesis predicts too many characteristic earthquakes for three reasons first forecasts were made for some zones specifically because they had two or more earthquakes in the previous centuries biasing the estimated earthquake rate second open intervals before the first event and after the last event are excluded in calculation of recurrence rate third the forecast assumes that all slip in each zone is released in characteristic earthquakes of the same size while in fact considerable slip is released by both smaller and larger earthquakes the observed size distribution of earthquakes is inconsistent with the characteristic hypothesis instead of a deficit of earthquakes above the characteristic limit earthquake numbers are distributed according to the standard gutenberg richter relation by lowering the magnitude threshold for qualifying earthquakes it is possible to reduce the discrepancy between the observed and predicted number of earthquakes to an acceptable level however for every magnitude threshold we tried the new seismic gap model failed the test on the number of filled zones or the likelihood ratio test or both at least at the 95 confidence level

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

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

probing the composition of unexposed basement south portuguese zone southern iberia implications for the connections between the appalachian and variscan orogens geochemistry and sm nd and u pb magmatic zircon isotope data from a postcollisional batholith that crosscuts the allochthonous south portuguese zone spz of southern iberia suggest that the base

cadomian vs variscan evolution of the ossa morena zone sw iberia field and 40ar 39ar mineral age constraints abstract six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the ossa morena zone of the iberian massif these include 1 the obejo valsequillo domain north of the badajoz cordoba shear zone 2 the sierra albarrana structural unit immediately south of the badajoz cordoba shear zone and 3 the thermal some exposed within the olivenza monesterio antiform zafra monesterio domain hornblende from foliated amphibolite in the obejo valsequillo domain siera negra unit displays internally discordant 40ar 39ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation 36ar 40ar vs 39ar 40ar isotope correlations are generally well defined and yield ages of c 550 560 ma these are interpreted to date cooling following late precambrian early paleozoic cadomian tectonothermal activity this was followed by slight variscan rejuvenation at c 375 400 ma muscovite from this unit displays internally discordant age spectra which reflect more extensive variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c 560 ma hornblende and muscovite from the sierra albarrana domain sierra albarrana group record 40ar 39ar plateau and isotope correlation ages which range between c 392 ma and 351 ma these results indicate that complete variscan rejuvenation was followed by relatively rapid post metamorphic cooling hornblende separated from various lithologie elements exposed in the olivenza monesterio antiform yield variable results hornblende within a mafic xenolith in the monesterio granodiorite records an isotope correlation age of c 553 ma which is interpreted to date the last cooling through appropriate argon retention temperatures hornblende within amphibolite of the montemolin series lower serie negra records internally discordant age spectra which reflect extensive variscan c 390 400 ma rejuvenation of intracrystalline argon systems which had initially recorded ages c 500 ma muscovite was concentrated from several lithologie elements in this area including 1 mylonitic monesterio granodiorite 2 metasedimentary rocks of the montemolin succession displaying high grade sillimanite potassium feldspar bearing assemblages and 3 migmatitic schist of the siere negra group these muscovite concentrates display similarly discordant 40ar 39ar age spectra which suggest significant but incomplete variscan rejuvenation occurred at c 400 ma this appears to have affected intracrystalline argon systems which had initially cooled through appropriate closure temperatures sometime prior to c 450 ma following an initial high grade cadomian metamorphism the 40ar 39ar results clearly reflect a variable complex and polymetamorphic evolution for the ossa morena zone internal consistency of data within each regional tectonic unit compared with marked contrast between the units provides evidence of the extremely heterogeneous variscan tectonothermal overprint the results demonstrate that the present tectonic architecture of the ossa morena zone was not solely the result of late variscan wrench tectonics but instead a consequence of processes active since at least the lower middle devonian juxtaposition of regional structural units with contrasting tectonothermal histories and recording variable rates and extents of uplift are consistent with maintainence of an overall transpressional variscan tectonic regime

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 revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

constraints on the crustal structure of the internal variscan belt in sw europe a magnetotelluric transect along the eastern part of central iberian zone iberian massif 1 the iberian massif is the best exposed segment of the european variscan belt it includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the late paleozoic assembly of the pangaea supercontinent in sw iberia these units are the central iberian zone ciz ossa morena zone omz and the south portuguese zone spz a 210 km long ne sw magnetotelluric profile was carried out through the ciz from the omz ciz boundary toward the north reaching the tagus cenozoic basin data dimensionality analysis resulted in a suitable 2 d electrical resistivity structure allowing a 2 d inversion of the data set complementary available geophysical data deep seismic gravity and aeromagnetic and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2 d electrical resistivity structure of the ciz crust the results show for the upper crust the existence of diverse conductive resistive bodies that correlate well with known geological features sedimentary basins faults granitic plutons mineralized systems a mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement the upper section of this band connects with several elongated shallow conductors providing further evidence for the existence in the central iberian zone of a complex decollement system where the major faults are rooted such a crustal architecture is viewed as the northward continuation of the variscan large scale structures previously recognized in the southern sectors omz and spz

genesis and evolution of a syn orogenic basin in transpression insights from petrography geochemistry and sm nd systematics in the variscan pedroches basin mississippian sw iberia abstract the pedroches basin is an outstanding syn orogenic variscan mississippian depocenter located in the vicinity of and overstepping onto the neoproterozoic cadomian suture between the ossa morena and central iberian zones of the sw iberian massif its formation appears to be related to transtensional processes at a major releasing bend in the reactivated suture during sinistral escape of the ossa morena zone from the zone of frontal variscan collision between northern gondwana and laurussia subsequent basin inversion resulted in compartmentalization and internal deformation and was probably related to transpression at a restraining bend along the reactivated suture badajoz cordoba shear zone basin fill consists of both igneous and metasedimentary rocks generally deposited in a shallow marine storm dominated platform environment by using new petrographic geochemical and sm nd isotopic data we attempt to characterize 1 the sources and geological evolution of the pedroches basin fill and 2 the processes involved in basin development and sediment supply and dispersal sampling was carried out in both sedimentary and igneous rocks belonging to three major structural units within the central part of the basin guadiato unit guadalbarbo unit and south pedroches valley unit the combined nd isotopic and geochemical data from the pedroches mississippian metasedimentary rocks indicate an upper continental crust provenance and a high degree of sedimentary recycling of these materials throughout the basin i e they are derived from old recycled upper crust without any significant juvenile component despite the presence of interbedded basaltic rocks the igneous rocks in turn correspond to submarine basalts and trachy andesitic basalts plus some hybrid rocks including crust derived components concerning the mafic rocks major element compositions reveal a tholeiitic and transitional tholeiitic character for the guadalbarbo unit rocks morb type and an alkaline affinity for those of the guadiato unit trace element analyses along with sm nd isotopic ratios suggest two kinds of provenance sources for the pedroches basin syn sedimentary igneous rocks 1 an important crust contaminated and or old enriched mantle source for the guadiato unit and the pedroches batholith and 2 a time integrated depleted mantle source for the guadalbarbo rocks in which no significant interaction with continental crust components is discernible these latter rocks suggest that a strong attenuation or even almost complete tectonic and or thermal erosion of the continental lithosphere was reached at least locally in this region

multistage boron metasomatism in the alamo complex central iberian zone spain evidence from field relations petrography and 40ar 39ar tourmaline dating the alamo complex consists of structural metamorphic domes surrounded by low grade metasedimentary rocks of upper proterozoic to lower cambrian age that form part of the schist graywacke complex central iberian zone spain tourmaline is ubiquitous throughout the domes in which it occurs in tourmalinites psammo pelitic schists quartzites gneisses migmatites leucogranites aplo pegmatites and quartz veins overall tourmaline compositions can be described within the four component system schorl dravite foitite magnesiofoitite with k2o 0 23 low ca contents mg mg fe 0 25 0 71 and x x na 0 18 0 43 where x vacancies in the x site field relations and petrographic observations combined with tourmaline 40ar 39ar data provide evidence of intense boron metasomatism affecting this region tourmaline 40ar 39ar step heating spectra for cambrian ordovician orthogneisses are complex yielding a pseudo plateau age of 370 ma that is interpreted to reflect variscan rejuvenation of older tourmaline tourmaline 40ar 39ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo plateau ages of 355 342 ma that are unsuitable for precise age determination these ages however are consistent with published ages 340 350 ma for the second variscan deformation d2 tourmaline from fine layered tourmalinite and metasedimentary rocks yield well defined plateau ages of 317 and 315 ma respectively recording an additional metasomatic event concomitant with anatexis and evolution of b bearing granites pegmatites and hydrothermal fluids the different tourmaline forming stages reflect significant boron cycling within the continental crust of the central iberian zone driven by deformation metamorphism and magmatism during the variscan orogeny boron rich aqueous fluids related to cambro ordovician magmatism are considered to be the primary source of boron

zn pb cu massive sulfide deposits brine pool types occur in collisional orogens black smoker types occur in backarc and or arc basins volcanic hosted massive sulfide deposits of zn pb cu type were derived either from seawater dominated buoyant fluids that built mounds on the seafloor e g the ores of the hokuroku basin japan or from saline fluids that reversed buoyancy on mixing with seawater and filled basins on the seafloor e g several ores of the iberian pyrite belt and the mount read province in tasmania the hokuroku ores formed above subduction zones during protracted periods of regional extension but both the iberian pyrite belt and the mount read province formed under extensional stress during continent continent iberia or arc continent mount read collision and orogenesis in the iberian pyrite belt oblique convergence resulted in transcurrent faulting and this may also be the case for the mount read province transcurrent faulting may have allowed easy vertical access for the ore related magmas some of which were sourced in the asthenosphere as well as the blueschist and eclogitic facies emplaced in adjacent terrains and possibly also saline fluids exsolved from the magmas

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

3d gravity modelling of the aguablanca stock tectonic control and emplacement of a variscan gabbronorite bearing a ni cu pge ore sw iberia the aguablanca stock is a variscan mafic pluton located in the ossa morena zone southern iberian massif hosting an unusual ni cu pge mineralization associated with magmatic breccia pipes which intruded its northern part the emplacement of the aguablanca stock and the mineralized breccia pipes are related to the activity of the cherneca ductile shear zone a variscan sinistral shear zone that favoured magma ascent through the upper crust a detailed gravity study has been carried out in order to investigate the 3d geometry of the aguablanca intrusion and to get insights about the emplacement mechanism and tectonic controls of the mineralization the three dimensional gravity modelling shows that the stock has an inverted drop geometry with a feeder zone in contact with the cherneca ductile shear zone the inferred orientation of the feeder zone suggests that the emplacement probably took place along an open tensional crack formed within the strain field of the adjacent cherneca ductile shear zone the modelling of the breccia pipes hosting the ni cu pge ore shows that they are included inside the feeder zone thus their emplacement is probably controlled by successive opening events of this tensional crack

combined thrusting and wrenching in the ibero armorican arc a corner effect during continental collision this paper is an attempt to reconcile strain pattern and deformation history in an overall arcuate structure with continental collision processes geological data are first presented for the early history 400 ma of the variscan orogenic belt in france principal stretch trajectories are on a large scale more or less parallel with the trend of the belt they became strongly oblique to this trend near the basal surfaces of the nappes a kinematic model combining two orthogonal shear components is proposed to explain such a strain pattern it is concluded that the observed strain trajectories result from a combination of a southward thrusting and a sinistral wrenching this conclusion is in good agreement with previous conclusions gained by several local microtectonic studies the geodynamic significance of combined thrusting and wrenching at the scale of the ibero armorican arc is discussed and a progressive deformation model is proposed

deformation of garnets in a low grade shear zone elongated shapes of garnets in high grade metamorphic rocks have been explained as a result of plastic crystal flow or anisotropic growth in the case of low to medium grade metamorphic rocks a satisfactory explanation has not been proposed yet for elongated garnets we have studied elongated garnets grown under low grade metamorphic conditions in a shear zone with a composite planar linear fabric garnet shapes in three dimensions define oblate ellipsoids drawing on evidence from compositional x ray maps it can be deduced that growth zoning is truncated along the long borders of grains whereas subcircular garnets show non truncated concentric growth zoning this fact shows that selective dissolution along planes parallel to the foliation and the c surfaces can be claimed to be the main mechanism responsible for the deformation of the garnets under examination here more specifically dislocation enhanced dissolution which occurs at low temperatures and low dislocation mobility is arguably the mechanism responsible for partially dissolving the garnet grains selective dissolution is expected to yield plane strain oblate ellipsoids i e with a volume loss as measured in the elongated garnets however the rock as a whole has been deformed by plastic flow in a simple shear regime as shown by the existence of numerous shear criteria and the strain calculations performed 0 1997 elsevier science ltd

thermal effects of igneous intrusions on maturity of organic matter a possible mechanism of intrusion abstract this paper assesses thermal maturation of organic matter near a well dated intrusion in the dsdp 41 368 hole near cape verde rise eastern atlantic and near some other intrusions from the u k u s a and east greenland the traditional numerical model with instantaneous intrusion satisfactorily describes the thermal evolution and thermal maturity aureoles of igneous bodies only if the initial temperature of intrusion is close to or even lower than the solidus temperature calculations in this model with more real values of the intrusion temperature and with consideration of latent heat effects lead to considerable over estimation of thermal influence of the intrusion models which consider formation of the intrusive body over time as distinct from instantaneous intrusion and especially models of emplacement of an intrusive body in the shell of relatively cool magmatic rocks show better agreement between the computed and observed vitrinite reflectance profiles consideration of convective heat flow due to hydrothermal activity arising in the vicinity of the intrusion during its formation and cooling helps to explain the difference between maturation profiles above and below some sills

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

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

comparative evolution of the iapetus and rheic oceans a north america perspective abstract differences in the origin and evolution of the iapetan and rheic oceans profoundly affected the development of the ouachitan appalachian variscan orogen the iapetus ocean was initiated in the late neoproterozoic early cambrian by the separation of large continental landmasses laurentia baltica gondwana whereas the rheic ocean originated in the late cambrian early ordovician as the result of the separation of several ribbon continents collectively termed peri gondwanan terranes from the northern margin of gondwana compared to the classic passive margin developed along the laurentian margin of iapetus passive margin successions to the rheic ocean are more limited in extent during convergence both margins of the iapetus ocean developed western pacific type arc back arc systems whereas subduction of the rheic ocean was primarily directed beneath laurentia and was andean in style closure of iapetus involved a number of arc back arc accretionary events and had occurred by the middle silurian as a result of laurentia baltica collision to form laurussia and the accretion of the peri gondwanan terranes rheic ocean closure occurred in the carboniferous by way of continent continent collision between laurussia and gondwana with gondwana on the lower plate within iapetus the formation of ophiolites occurred early in the orogenic cycle and was followed soon after by obduction hp lt metamorphism and foreland directed thrusting that is intimately related to accretionary events these features dominate the northern appalachian orogen where ht lp metamorphism is attributed to post accretionary slab break off events in contrast hp lt metamorphism within the rheic ocean reflects a type subduction during laurussia gondwana collision and is rapidly succeeded by ht lp metamorphism attributed to orogenic collapse ophiolites are rare and were formed either very early early ordovician or very late devonian carboniferous in the evolution of the ocean the early ordovician ophiolites were not obducted until the carboniferous at least 100 million years after their protoliths were formed rheic ocean closure resulted in the formation of large scale bivergent structures within the orogen with symmetry changing on either side of the rheic suture comparison of the evolution of the iapetus and rheic oceans suggests that differences in rift configuration passive margin development and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the ouachita appalachian variscan orogen