paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

two contrasted p t time paths of coronitic metanorites of the french massif central are reaction textures reliable guides to metamorphic histories metanorites from two eclogitized metagabbros of the hercynian french massif central preserve coronitic textures of hornblende garnet quartz and or kyanite produced at the expense of the primary magmatic assemblage orthopyroxene and plagioclase using a petrogenetic grid in the cfmash system two possible p t evolutions for the origin of the coronas are evaluated the sequence of reactions involving the formation of hbl ky grt and qtz coronitic assemblages is consistent with an isobaric cooling at high pressure c 1 2 gpa under hydrated conditions however this p t path inferred by using only petrographical observations is inconsistent with the geochronological constraints emplacement of the gabbro at 490 ma and high pressure metamorphism at 410 ma in order to reconcile petrographical observations with geochronological constraints we propose a discontinuous two stage evolution involving a change in water activity with time 1 emplacement and cooling of the norite at low pressure under anhydrous conditions at 490 ma 2 during the hercynian orogeny the norite experienced an increase in pressure and temperature under fluid present conditions adding water to the system implies a dramatic change in the petrogenetic grid topology restricting the orthopyroxene plagioclase assemblage only to high temperatures therefore the breakdown of the unstable magmatic assemblage through apparent retrograde reactions occurred along the prograde p t path which never crossed the equilibrium boundaries of these reactions

paleomagnetism of late paleozoic igneous rocks from southern portugal the upper devonian beja gabbro yields a stable characteristic magnetization with dual polarities inclinations are moderate to steep while declination vary systematically along the arcuate trend of the massif suggesting pretectonic magnetization and oroclinal bending the inclination differs from those of other devonian results from europe suggesting a separation between southern and northern iberia lower carboniferous volcanics from the pyrite belt were remagnetized in middle carboniferous but a few sites were found with a high temperature component which might represent the primary magnetization the paleolatitudinal movement computed from these magnetizations is in excellent agreement with that of africa from late devonian to middle carboniferous we suggest therefore that southermost parts of spain and portugal were part of africa until the final collision events of the hercynian orogeny

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

special section on azimuthal dependence of p wave seismic signatures introduction this special issue is based on papers presented at the postconvention seg workshop on azimuthal dependence of p wave signatures held in dallas in 1997 the main motivation for analyzing the azimuthal variation of seismic traveltimes amplitudes attenuation etc is to obtain reliable information about azimuthal anisotropy in the subsurface another potential application of multiazimuth techniques is in finding andmappingdesirablelateralheterogeneitiesthatcould masquerade as azimuthal anisotropy the last topic has not yet been fully discussed and is not addressed in the special issue butseveralexplorationanddevelopmentscenarioscontainoriented lateral heterogeneities sand channels etc that at the right scale length could be highly visible in properly processed wide azimuth 3 d data themostcommonphysicalreasonsforazimuthalanisotropy are systems of vertical or steeply dipping fractures or tilted transversely isotropic layers both types of azimuthally anisotropic models are discussed here with an emphasis on issues related to fracture detection and characterization in tight formations the extraction offluids is often impossible without exploiting the increased drainage provided by fracture networks therefore geophysicalcharacterizationoffracturedreservoirs is an extremely important problem from an economic standpoint sweet spots of high fracture intensity are not always correlated with faults and other structural features and are difficult to detect using conventional exploration techniques effective exploitation of fractured reservoirs also requires determination of the orientation and connectivity of fractures as well as their spatial relationships to other reservoir heterogeneities most existing seismic methods for fracture characterization are based on analysis of shear wave splitting e g crampin 1985 thomsen 1988 winterstein and meadows 1991 mueller 1992 since split s waves travel along essentially the same raypath the difference between their velocities andreflectionamplitudesdependsprimarilyontheanisotropic properties of the medium rather than on lateral heterogeneity theshear wavetechnology however hasknownshortcomings

palaeomagnetism of the ordovician dolerites of the crozon peninsula france summary in order to obtain a lower palaeozoic pole for the armorican massif and to test the origin of the ibero armorican arc the ordovician dolerites of the crozon peninsula have been palaeomagnetically studied the samples show a multicomponent magnetization which has been revealed by af and thermal demagnetization and thoroughly investigated with rock magnetic experiments polished section examinations and k ar dating four groups of directions have been recognized often superimposed on each other in an individual sample one component d has always the lowest blocking temperatures and coercivities and is considered to be of viscous origin acquired recently in situ or in the laboratory during storage two components a and b are interpreted to be of secondary origin and to correspond to the observed k ar age distribution between 300 and 190myr these ages represent the time interval between two regional thermo tectonic events associated with the hercynian orogeny and the intrusion of dykes related to the early opening of the central atlantic ocean and the bay of biscay a fourth component c could be of ordovician or younger palaeozoic age it is not clear whether the age of the magnetization is pre or post folding but a prefolding age would yield a direction of magnetization similar to ordovician results from the iberian peninsula the latter interpretation suggests a fairly high palaeolatitude which is in agreement with a glacio marine postulated for sediments overlying the dolerite sills

revision of the age of magnetization of the montmartin red beds normandy france summary a new roadcut has enabled us to sample the south dipping limb of the montmartin syncline for a palaeomagnetic reevaluation of an earlier result published by jones van der voo bonhommet in combination with the results previously published in 1979 for the north dipping beds of the syncline a conclusively negative fold test is obtained the resulting magnetization declination inclination 206 3 95 12 palaeopole at 38 s 325 e is interpreted to be of late carboniferous age not late devonian as thought earlier simultaneously we have re evaluated the age of the rocks previously thought to be late devonian on the basis of acritarchs chitinozoans and spores it has not been possible to reconfirm these fossils not even in the same samples as studied originally in contrast the regional presence of early palaeozoic fossils suggests to us an age similar to that of other red beds in the arrnorican massif which have been dated as early ordovician the geodynamic implications of our finding that the montmartin rocks are completely remagnetized however are of no great consequence for the geodynamics of the hercynian belt pre folding magnetization obtained from silurian and devonian rocks in spain and germany argue for the same conclusion as reached erroneously in our earlier study namely that the armorican massif and adjacent parts of hercynian europe were adjointed to north america great britain the baltic shield and the russian platform since at least late devonian time if a medio european ocean existed during the palaeozoic it was virtually closed before the mid devonian and of insignificant width during culm deposition in early carboniferoirs time

paleomagnetic evidence for tectonic rotations in the variscan mountain belt a critical review of devonian and carboniferous paleomagnetic results from the hercynian basement of western and central europe reveals large rotations their geographic distribution suggests a tectonic origin for large scale curved structures such as the ibero armorican arc or the variscan v together with a coherence of the hercynian domain with time geodynamic models which can have produced the observed rotation pattern are discussed it appears that lateral compression due to the accommodation of continental collision as in the india asia convergence is a plausible mechanism although variscan and himalayan mountain belts have different characteristics

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

paleomagnetism in the northeastern central massif france evidence for carboniferous rotations of the hercynian orogenic belt abstract a paleomagnetic study has been carried out on visean volcanics of northeastern central massif standard paleomagnetic techniques have been applied to a hundred of samples alternating field and thermal demagnetizations yield an unusual direction d 78 i 2 95 12 for the 9 best sites as the polarity is unknown two motions can explain this direction either a clockwise rotation of about 45 or an anticlockwise of 135 relative to the armorican massif in both cases the paleomagnetic data reveal important deformations of the hercynian orogenic belt probably due to strike slips in middle carboniferous times

quantitative determination of hydraulic properties of fractured rock using seismic techniques there have been significant advances over the last ten years in the use of the seismic anisotropy concept to characterize subsurface fracture systems measurements of seismic anisotropy are now used to deduce quantitative information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon various equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume scale frequency independence and hence cannot distinguish between micro cracks and macrofractures the latter however control the fluid flow in many oil gas reservoirs as the fracture size and spacing hence fracture storability are essential parameters for reservoir engineers recently a new equivalent medium theory for modelling of wave propagation in media with multi scale fractures has been presented the model predicts velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale micro cracks and equant matrix porosity and formation scale fractures application of this model to field data shows that fracture density and fracture size can be inverted successfully from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data this paper presents the results of the latest development in the seismic characterization of natural fractures with an emphasis on the quantitative determination of fracture sizes

origin of biotite hornblende garnet coronas between oxides and plagioclase in olivine metagabbros adirondack region new york complex multivariant reactions involving fe ti oxide minerals plagioclase and olivine have produced coronas of biotite hornblende and garnet between ilmenite and plagioclase in adirondack olivine metagabbros both the biotite 6 10 tio2 and the hornblende 3 6 tio2 are exceptionally titanium rich the garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks

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

the ancenis terrane an exotic duplex in the hercynian belt of armorica western france abstract the ancenis terrane bordering the loire france consists of precambrian brioverian basement and three paleozoic sedimentary sequences marine ordovician to devonian shelf sediments lower carboniferous synorogenic intermontane sediments the culm and coal bearing namurian deposits the terrane is completely bound by faults and its geology is in marked contrast with that of its neighbours new data and observations along the terrane s southern boundary show that the ordovician devonian sequence occupies a transpressional shear zone and can be described as a tectonic melange the culm outcrop previously interpreted as a sinistral pull apart structure is reinterpreted as a lower carboniferous dextral pull apart and it is shown that carboniferous transpression and thrusting remodelled the pull apart basin into its present mica fish shape with its remarkable concentric structure and within a plate tectonic setting of mega dextral displacements along the northern boundary of gondwana the ancenis terrane can be interpreted as an exotic duplex

calculation of partial melting equilibria in the system na2o cao k2o feo mgo al2o3 sio2 h2o nckfmash a thermodynamic model for haplogranitic melts in the system na2o cao k2o al2o3 sio2 h2o nckash is extended by the addition of feo and mgo with the data for the additional end members of the liquid incorporated in the holland powell 1998 internally consistent thermodynamic dataset the resulting dataset with the software thermocalc is then used to calculate melting relationships for metapelitic rock compositions the main forms for this are p t and t x pseudosections calculated for particular rock compositions and composition ranges the relationships in these full system pseudosections are controlled by the low variance equilibria in subsystems of nckfmash in particular the solidus relationships are controlled by the solidus relationships in nkash and the ferromagnesian mineral relationships are controlled by those in kfmash however calculations in nckfmash allow the relationships between the common metapelitic minerals and silicate melt to be determined in particular the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks particularly relating to open system behaviour of rocks under high grade conditions

a devonian palaeomagnetic pole for armorica summary the palaeomagnetic study of siluro devonian redbeds of the san pedro formation at cabrillanes asturias spain reveals a high temperature and pre tectonic magnetization determined by intersection of great circles as well as conventional averaging fisher this magnetization is carried by the haematitic pigment and is therefore of chemical and postdepositional origin the formation mean direction d 113 i 34 ag5 10 is interpreted as representing a record of a devonian palaeofield the corresponding pole after correction for the opening of the bay of biscay and the tectonic closure of the ibero armorican arc is in good agreement with contemporaneous european poles confirming that the arinorica plate had already joined the laurentia baltica assembly at that time

contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the grenville province of western labrador contrasting coronas around olivines pyroxenes and ilmenites occur in metagabbros in two adjacent terranes in the grenville province of western labrador in the parautochthonous gagnon terrane coronas around olivines display evidence of evolution from double coronas of orthopyroxene and fluorine rich pargasitic amphibole through triple coronas of orthopyroxene garnet clinopyroxene pargasitic amphibole to double coronas of orthopyroxene garnet clinopyroxene parentheses enclose phase occurring as inclusions in previous phase in the allochthonous lac joseph terrane coronas around olivines consist of orthopyroxene clinopyroxene spinel or orthopyroxene pargasitic amphibole spinel brackets enclose phases occurring in symplectic intergrowths possible reactions that explain the development of these assemblages are investigated and it is concluded that the system was closed at the level of the corona the silica required for the olivine plagioclase reaction in gagnon terrane was gene

dependence of reaction kinetics on h2o activity as inferred from rates of intergranular diffusion of aluminium quantitative constraints on the accelerative effects of h2o on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of al in natural systems that are fluid saturated hydrous but fluid undersaturated and nearly anhydrous widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the makhavinekh lake pluton northern labrador combined with time temperature histories from conductive thermal models yield intergranular diffusivities for al from 700 900 c under nearly anhydrous conditions those rates when extrapolated down temperature are approximately three orders of magnitude slower than rates derived from re analysis of garnet resorption coronas formed under hydrous but fluid undersaturated conditions near 575 c in rocks of the llano uplift of central texas which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid saturated conditions in rocks from the picuris range of north central new mexico thus even at constant temperature rates of intergranular diffusion of al and corresponding length scales and timescales of metamorphic reaction and equilibration may vary by as much as seven orders of magnitude across the range of h2o activities found in nature

induced microearthquake patterns and oil producing fracture systems in the austin chalk abstract microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in austin chalk oil reservoirs at two sites in the giddings field texas we deployed three component downhole geophone tools in production wells at depths of 2100 m and greater one near cook s point and two on the matcek lease near caldwell at cook s point we collected 482 microseismic events during a 4000 m 3 25 000 bbl hydraulic stimulation in an offset well we collected 770 events during a similar operation on the matcek lease many seismograms contained reflected phases that constrained location depths to the production zone at the base of the austin chalk by restricting all microearthquake locations to production depths we located 20 of the cook s point events and over 60 of the matcek events at both sites we observed only the fracture wing closest to the observation stations locations formed elongated patterns extending up to 1 km from the stimulation well and trending n60 e parallel to the known regional fracture trend the cook s point seismic zone measured over 100 m in width while long stretches of the matcek seismic zone narrowed to 30 m or less we believe that the width of the seismic zone reflected the density of conductive fractures and thus the volume of the reservoir accessed by the stimulation indeed production rates in the first year following stimulation were much higher at cook s point where we observed the wider of the two seismic zones

palaeomagnetism of the upper carboniferous strzegom and karkonosze granites and the kudowa granitoid from the sudet mountains poland abstract carboniferous granites and granitoids varying in age from 305 to 281 ma from the sudet mountains latitude 51n longitude 16e poland carry dual polarity magnetic components consisting of an inferred primary high temperature a component found in the strzegom granite the karkonosze granite with the bordering karpacz hornfels and the kudowa granitoid and scattered low stability components of mesozoic tertiary age present in all rocks except the strzegom granite the group a component defines a palaeomagnetic pole position d 202 i 12 95 13 0 n 4 calculated palaeomagnetic pole position 42 s 346 e which is in good agreement with palaeomagnetic results from rocks of similar age in western europe thus implying that no major tectonic movement has taken place in the sudets relative to stable europe since upper carboniferous times

modelling frequency dependent seismic anisotropy in fluid saturated rock with aligned fractures implication of fracture size estimation from anisotropic measurements measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures the latter however control the fluid flow in many subsurface reservoirs therefore the fracture size is essential information for reservoir engineers in this study we apply a new equivalent medium theory that models frequency dependent anisotropy and is sensitive to the length scale of fractures the model considers velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale microcracks and equant matrix porosity and formation scale fractures the theory is first tested and calibrated against published laboratory data then we present the analysis and modelling of frequency dependent shear wave splitting in multicomponent vsp data from a tight gas reservoir we invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data

the variscan belt of northern france southern belgium geodynamic implications of new palaeomagnetic data abstract palaeomagnetic investigations were carried out in devonian early carboniferous rocks of the variscan foreland chain of northern france southern belgium in order to reveal the origin of its arcuate shape the brabant parautochthon was sampled in the boulonnais near calais and near tournai while the ardenne allochthon was sampled near maubeuge and in the givet area all the sampled localities yielded characteristic remanent magnetization as a result of stepwise demagnetization and component analysis fold or tilt tests were possible for three localities with negative results indicating pervasive remagnetization the tectonic position was sub horizontal at two localities while the tilt was monoclinal for the rest therefore the acquisition time of the magnetic signals was estimated by comparing the palaeolatitude computed from each magnetic component to the palaeolatitudes of variscan europe calculated after van der voo 1993 three components showing a a southern b a near equatorial and c a northern palaeolatitude are recognized from our data since a pre variscan age of component a observed only in boulonnais at 10 sites is not supported by data it is assigned to an early phase of deformation component b 16 sites was acquired during the peak of the variscan tectonics late westphalian while component c five sites originated during permian times regardless of the palaeolatitudes declinations fall between 190 and 210 thus being conformable with the declinations expected for variscan europe the declinations show no correlation with the arcuate shape of the belt neither are they different in the paraauthochthon and in the allochthon nor in the different components arc formation by moulding of the allochthon on the brabant parautochthon is therefore not supported by these data since this mechanism requires substantial opposed rotations on both wings of the arc the available palaeomagnetic data are conformable with a pre formed arc simply docking to the brabant obstacle of similar shape variable offsets along a main thrust make possible a third model which slightly unfolds the former passive arc

a comparison between phase diagram modelling of metamafic rocks and experimental and independent thermobarometric data abstract the use of equilibrium phase diagrams such as pseudosections has become widespread in recent years but few studies tested the phase relations predicted by these models this work aims at comparing predicted phase relations with mineral assemblages observed in experimental products and in natural samples in this latter case using as reference metamorphic conditions pressures and temperatures determined for metapelites metamorphosed under the same conditions as the mafic rocks pseudosections were calculated with thermocalc for six mafic bulk rock compositions in the range 425 700 c and 2 8 kbar the resulting diagrams can be divided into two groups one in which a classic sequence of mineral assemblages occur and another where quartz is unstable and diopside is stable throughout the results compare well to the independent data in three of the six studied cases the main inconsistencies found in the pseudosections are underestimation of the stability field of garnet and overestimation of the stability fields of diopside and hornblende which is at odds not only with data specific to the rocks studied here but with the literature on metamafic rocks in general in the case of garnet the most likely case of discrepancy is the absence of mn in the activity composition models available on the other hand the issue with diopside and hornblende is much harder to evaluate as it may be related to any of the activity composition models or to relationships between two or more of them nonetheless we can affirm that the overestimation of the diopside stability field is related to sio2 content although there are other factors that influence it

mapping of crustal scale tectonic boundaries in the ossa morena zone using reprocessed iberseis reflection seismic data the iberseis deep seismic reflection profile imaged crustal scale structures in the sw iberian variscan belt crossing the south portuguese zone the ossa morena zone and the central iberian zone in spain two subsets of the profile corresponding to the south portuguese zone ossa morena zone and the ossa morena zone central iberian zone tectonic contacts have been reprocessed with the aim of investigating the influence of cross dip and to better image steeply dipping features alternative strategies for binning midpoints into common depth point cdp bins using different azimuths were examined for synthetic data we show that the choice of the cdp processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile multi azimuth binning normal moveout dip moveout and migration velocity analysis on synthetic and real data show the presence of clear sub vertical upper crustal structures near the south portuguese zone ossa morena zone suture the aroche fault this sub vertical reflectivity that was not imaged earlier projects into a location in the lower crust with low reflectivity

faulting fracturing and in situ stress prediction in the ahnet basin algeria a finite element approach abstract many low efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures accurate and low cost information on basic fault and fracture properties orientation in particular is critical in reducing well costs and increasing well recoveries this paper describes how we used an advanced numerical modelling technique the finite element method fem to compute site specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties structural position and tectonic stress presented are the numerical results of two dimensional plane strain end member fem models of a hydrocarbon bearing fault propagation fold structure interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling however it still allows comparisons with the little available geological and geophysical data in all models the weak mechanical strength and flow properties of a thick shale layer the main seal leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement and results in flexural slip across the shale layer all models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression the stress regime for the formation of new fractures changes from compressional to shear with depth if pre existing fractures exist only sub horizontal fractures are predicted to open thus defining the principal orientation of effective reservoir permeability in models that do not include a blind thrust fault in the basement flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks the folding induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading models incorporating a blind thrust fault correctly predict the formation of secondary syn and anti thetic mesoscale faults in the basement and sediments of the hanging wall some of these faults cut reservoir and or seal layers and thus may influence effective reservoir permeability and affect seal integrity the predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure and proximity to failure these numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs

geoelectromagnetic measurements across the moine thrust and the great glen in northern scotland summary results are presented for a magnetotelluric and magnetovariational study in northern scotland for nine of the 20 sites at which observations were made the results of the analysis are presented fully together with one dimensional 1 d monte carlo inversions for representative sites from the lewisian foreland the north west highlands the great glen region and the grampian highlands for the remaining sites the results are summarized the 1 d models of all sites situated in a traverse extending from the lewisian foreland to the neighbourhood of the highland boundary fault have been used as the basis for preliminary 2 d modelling maps of magnetic field induction vectors and transfer functions for an inducing field perpendicular to the average geological strike are presented for periods of 23 819s the 2 d conductivity models were constrained by both the magnetotelluric and magnetovariational responses the period range of observation in this study was too limited to resolve structural differences in the upper crust but adequate to reveal a moderately good conductor at upper mantle lower crustal depths in the caledonian metamorphic zone in contrast the whole of the crust in the lewisian foreland is highly resistive and there is no evidence of an intermediate conducting layer between the crust and mantle in this region

fracture characterization from near offset vsp inversion a global optimization method incorporating a ray tracing scheme is used to invert observations of shear wave splitting from two near offset vsps recorded at the conoco borehole test facility kay county oklahoma inversion results suggest that the seismic anisotropy is due to a non vertical fracture system this interpretation is constrained by the vsp acquisition geometry for which two sources are employed along near diametrically opposite azimuths about the well heads a correlation is noted between the time delay variations between the fast and slow split shear waves and the sandstone formations

late hercynian remagnetization of tournaisian series from the laval syncline armorican massif france abstract paleomagnetic investigations have been conducted on tournaisian volcanics and sediments from the laval syncline in order to evaluate the consequences of the late variscan tectonic and thermic phases on the southern flank of the syncline anisotropy measurements have yielded maximum susceptibility in the schistosity planes thermal demagnetization exhibits only remagnetizations characterized by a large range of blocking temperatures from 350 to 670 c two phases of remagnetization seem to follow each other the first may have occurred during the tournaisian major orogenic phase the second has taken place during the latest stephanian earliest permian in relation with the latest variscan thermal event all results are in favour of a clockwise rotation by 15 40 of the laval syncline after the latest overprints according to results from other regions of brittany this clockwise rotation involved the whole armorican massif this motion may be related to the latest compression phase which has tightened the ibero armorican arc well before the permo triassic opening of the bay of biscay

palaeomagnetism and 40ar 39ar dating of the tregor dolerites armorican massif france abstract the tregor doleritic dike swarm intrudes the cadomian north tregor batholith armorican massif france alternating field a f and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin section examinations three magnetization components were recognized in addition to a low temperature component with a viscous origin the most frequently observed component named b d 214 i 10 was isolated at 12 sites independent upon their magnetic mineralogy and appears to be a remagnetization negative baked contact test a carboniferous age of acquisition is inferred from comparison of the b component palaeopole 29 s 318 e with an armorican apwp at some sites after demagnetization of the b component higher temperature and coercivity components were isolated their directions fall into two groups named a and c positive baked contact tests were obtained for both the c component d 174 i 50 whose magnetic carrier is magnetite yields a palaeopole 11 s 2 e which is similar to some ordovician palaeopoles from the british isles south of the lapetus suture or eocambrian palaeopoles from the tregor area the a component d 191 i 33 whose magnetic carrier is magnetite partly transformed to leucoxene yields a palaeopole 58 s 336 e and a palaeolatitude 17 9 n not previously observed for the armorican massif in palaeozoic time comparison between this palaeolatitude and those for northern spain from ordovician to carboniferous time suggests a late silurian remanence age this interpretation supports the model of apparent polar wander for gondwana between ordovician and devonian time proposed by van der voo and bachtadse and briden 40 ar 39 ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the a and c components

are the oroclines of the variscan belt related to late variscan strike slip tectonics terra nova 23 241 247 2011 abstract early variscan structures and magnetic anomalies delineate four oroclines along the variscan belt the arcs bend the lithostratigraphic zones of the autochthon and the allochthonous terranes which include ophiolites of the rheic suture and units of gondwanan derivation subducted on both sides of the rheic ocean a transcurrent model dominated by right lateral motion explains the genesis and evolution of the oroclines and a simple plate tectonics scenario links late variscan dextral strike slip movements in the variscides to a broad intracontinental shear zone kinematically equivalent to a transform fault terra nova 00 1 7 2011

palaeomagnetism of the ordovician moulin de chateaupanne formation vendee western france summary a palaeomagnetic study of the moulin de chateaupanne formation reveals two characteristic components of magnetization the first one isolated by thermal treatment at intermediate temperatures is strongly resistant to chemical leaching its direction d 217 i 25 ag5 5 5 and the corresponding pole position lat 21 s long 320 e are typical of the hercynian overprints in the armorican massif the second component carried by fine grained haematite presents a steep downward direction d 228 i 81 ag5 6 pole position lat 34 n long 343 e already recognized in other ordovician rock units from the same area it is resistant to high temperature demagnetization treatment and is believed to be of chemical origin as assessed by a conglomerate test a mutual consistency test with the nearby thouars massif suggests that the tilt of the formation has a sedimentary origin and that the thouars massif has not suffered significant tilt during the hercynian orogenic events

impact of interlayer slip on fracture prediction from geomechanical models of fault related folds understanding and interpreting the timing location orientation and intensity of natural fractures within a geologic structure are commonly important to both exploration and production planning activities here we explore the application of finite element based geomechanical models to fracture prediction our approach is based on the idea that natural fractures can be interpreted or inferred from the geomechanical model derived permanent strains for this analysis we model an extensional fault tip monocline developed in a mechanically stratified limestone and shale sequence because field data exist that can be directly compared with model results the approach and our conclusions however are independent of the specific structural geometry the presence or absence of interlayer slip is shown to strongly control the distribution and evolution of strain and this control has important implications for interpreting fractures from geomechanical models

palaeomagnetism of late precambrian cambrian volcanics and intrusives from the armorican massif france samples of the keratophyre tuffs of treguier 640 12 myr of the upper brioverian series and the microgranite dykes of loguivy 548 24 myr have been collected from the north coast of brittany france the samples were progressively demagnetized with af and predominantly thermal methods blocking temperatures coercivities and acquisition of isothermal remanent magnetization irm indicate that the keratophyre remanence is variably carried in magnetite hematite or occasionally in both after removal of a soft viscous component two stable palaeomagnetic directions were found one in magnetite the other in hematite the characteristic magnetite direction tb 300 580 c is a pre folding primary thermoremanent magnetization trm which after structural correction has a mean declination inclination of 217 9 30 4 the younger hematite component has a mean in situ direction of 320 4 20 3 the microgranite natural remanent magnetization nrm intensity distribution is distinctly bimodal forming high and low 1 6 a m 1 intensity groups upon demagnetization a single stable remanence in magnetite was removed between 300 and 580 c or 20 and 100 mt two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level low intensity samples have a mean direction of 201 9 2 3 and probably resulted from a late devonian early carboniferous local magnetization high intensity microgranites have a mean direction of 200 50 8 and this remanence is shown to be a primary trm palaeopoles obtained from the two keratophyre magnetizations magnetite 44 7 n 116 4 e hematite 39 5 n 233 6 e as well as the high intensity microgranite remanence 5 8 s 339 9 e show good agreement with similar age poles from the apparent polar wander path for the armorican massif comparison of these data to recent palaeomagnetic data from southern england and wales gives additional evidence to link that region with the armorica plate as proposed by van der voo and hagstrum et al

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

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

identifying fracture orientation in a mature carbonate platform reservoir the permian guadalupian san andres reservoir at keystone field winkler county texas is divided into three major stratigraphic units and twelve flow units on the basis of an analysis of multiple shoaling upward cycles of shallow water marine to tidal flat carbonate facies these rocks are now thoroughly dolomitized and cemented with anhydrite and gypsum the distribution of original oil in place was mapped both laterally and vertically most of the resource is in the upper five flow units and the original oil in place map of these upper flow units indicates that the highest concentration of hydrocarbons is in the center of the study area porosities in this reservoir are nearly 10 and permeabilities are generally less than 1 md despite these low matrix permeabilities recently drilled wells initially produced at rates as high as 120 bbl of oil per day although these rates declined an average of 75 in the first 6 months primary recovery from this reservoir is only 8 of original oil in place subvertical fractures in this reservoir are visible in cores and on a microimage log early floodwater breakthrough occurred without increased oil production in a pilot waterflood these production characteristics combined with direct observations of fractures indicate that productivity is dependent on fracture permeability horizontal boreholes perpendicular to the strike of effective fractures and within the part of the reservoir that contains highest remaining oil will maximize primary recovery borehole breakouts and regional stress measurements suggest that the direction of principal horizontal compressive stress is northeast southwest and natural fractures that strike in this direction are most likely to be open and capable of transmitting fluids

paleomagnetic evolution of the central massif france during the carboniferous the present paper aims to synthesize results of a systematic paleomagnetic investigation performed on metamorphic plutonic and volcanic series from the central massif detailed thermal and alternating field demagnetizations yield a large set of paleomagnetic directions several groups of directions corresponding to different age intervals are identified the group d mean direction d 288 i 57 37 s 110 e characterizes late devonian early carboniferous metamorphic and plutonic rocks from limousin the group c directions d 301 i 24 30 s 79 e represent late visean namurian magnetizations present in the major investigated areas the group b directions d 249 i 7 12 n 111 e exist not only in the whole central massif but also in other paleozoic outcrops of the variscan belt they were acquired during the namurian westphalian the group a a directions are the only typically european magnetic directions they have taken place in stephanian autunian times mainly during the kiaman reversed interval interpretation of these directions in terms of geodynamics leads to a probable large s n drift of the massif during the latest devonian early carboniferous followed by two important rotation phases first in the middle carboniferous then at the end of the westphalian these rotations have also affected other massifs of the variscan belt

palaeomagnetism of the ibero armorican arc and the hercynian orogeny in western europe van der voo1 has pointed out that further advances in the understanding of the hercynian orogeny require palaeomagnetic data from well studied orogenic areas we report here palaeomagnetic results of palaeozoic rocks taken in brittany portugal and spain in the ibero armorican arc iaa a large geological feature considered to be a key area for the hercynian orogeny in western europe they show evidence that the iaa formed in two stages extending results obtained in spain by ries et al 2 giving precise constraints for the former shape of the arc we can use pre hercynian palaeomagnetic directions found in spain and portugal to follow its evolution during palaeozoic times this megastructure linking brittany to iberia squeezed between gondwana and the northern continents during the carboniferous did break away from the former block during ordovician times

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

a field investigation of fracture compliance a field measurement of fracture compliance is described the aim was to determine how compliance scales with fracture size and specifically how laboratory measurements of fracture compliance compared with field estimates from sonic and seismic data a test site was constructed consisting of three 40 m vertical boreholes drilled in the floor of a carboniferous limestone quarry detailed knowledge of the rocks in the test area was obtained from core analysis wireline logging and local area fracture mapping seismic cross hole surveys were performed using a sparker source with a dominant frequency of 2000 hz and hydrophone receivers the rocks had a compressional wave velocity anisotropy of 10 which was attributed to the presence of predominantly horizontal partially open fractures estimates of normal fracture compliance within a range from 2 5 10 13 m pa to 3 5 10 12 m pa were obtained from both the cross hole data and the sonic log data this is an order of magnitude greater than values obtained from laboratory experiments which are reported elsewhere

diffusion controlled corona reaction and overstepping of equilibrium in a garnet granulite yenisey ridge siberia diffusion modelling is applied to layered garnet pyroxene quartz coronas formed by a pressure induced reaction between plagioclase and primary pyroxene in a metabasic granulite the reconstructed reaction involves some change in composition of reactant minerals the distribution of minerals between layers is satisfactorily explained by diffusion controlled reaction with local equilibrium in which the diffusion coefficient for al was smaller than those for fe mg and ca by a factor of approximately four diffusion of mg towards plagioclase implies a chemical potential gradient for mgo component in a direction opposite to the changing mg content of garnet this is explained by the influence of al2o3 on the chemical potential of the pyrope end member grain boundary diffusion is suggested to have operated possibly with composition gradients different from those in the bulk minerals chemical potential differences across the corona are estimated from the variation in garnet composition enabling affinity the free energy change driving the reaction to be estimated as 6 9 1 8 kj per 24 oxygen mole of garnet produced this implies that the pressure for equilibrium among the minerals was overstepped by 1 4 0 4 kbar the probable p t conditions of reaction were in the range 650 790 c 8 10 kbar assuming a timescale of reaction between 106 and 108 years estimated diffusion coefficients for fe mg and ca are in the range 9 10 23 to 5 10 20 m2 s 1 these are consistent with experimental values in the literature for solid state diffusion including grain boundary diffusion

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

palaeo stress fields in the variscan foreland during the carboniferous from microstructural analysis in the british isles abstract north of the variscan front the carboniferous series of the british isles is affected by mainly n s trending folds which are almost perpendicular to this e w oriented front flat and unconformable overlying permian strata demonstrate a variscan age for the deformation a microtectonic survey made in dinantian and to a lesser extent namurian calcareous layers has revealed two successive events according to chronological criteria the first event maximum stress e w trending led to the formation of n s folds with local reorientation in the vicinity of major pre existing faults this compression may have been active since the namurian it pre dates a second compressional event orientated n s which is related to the final emplacement of the variscan nappes

new kinematic constraints on the cantabrian orocline a paleomagnetic study from the penalba and truchas synclines nw spain abstract the cantabrian orocline is a large structure that bends the variscan orogen of western europe in nw iberia the extensively studied kinematics of its core the foreland of the orogen indicates that the structure is secondary i e acquired after the formation of the orogenic edifice however the extent of the cantabrian orocline away from its core is under debate in this paper we study the kinematics of the cantabrian orocline beyond the foreland we collected and analyzed samples from the northern and central parts of the truchas syncline which provides new data within the hinterland of the orogen in nw iberia the analysis of 320 samples shows a late carboniferous remagnetization with an e to ne declination and shallow downward inclinations these results suggest a counter clockwise rotation of 60 and peri equatorial but still southern hemisphere latitude for iberia during the uppermost carboniferous early permian this rotation fits with the expected kinematic evolution of the truchas syncline if it indeed was part of the cantabrian orocline

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

paleomagnetism and cross folding in a key area of the asturian arc spain in the northwest corner of spain paleozoic and older rocks outcrop in an arcuate pattern asturian arc concave toward the east the arc may be primary before sedimentation or secondary after sedimentation supporting a secondary origin are many structural features including two sets of folds one radial and one tangential to the arc the villasecino area was selected for a pilot study of the relationships between paleomagnetism and folding detailed structural mapping has shown that radial folds postdate tangential folds interference of the two sets has produced the doubly plunging villasecino anticline forty two oriented paleomagnetic cores were drilled at six sites distributed around the anticline all within red limestones griotte of the alba formation lower carboniferous the typical sample contains 95 by volume of fine grained calcite each grain being rimmed by very fine grained goethite and other iron bearing minerals the natural remanent magnetism nrm is of moderate intensity about 3 10 3 a m 1 readily measured and very homogeneous for each site within site scatter of vectors is low but the site mean varies significantly from site to site the angle between bedding and magnetization vector is nearly constant these data suggest that magnetization predated folding thirteen samples from six sites were subjected to alternating field demagnetization up to 50 mt and this showed the nrm to be very stable thermal demagnetization indicated a unique stable component with blocking temperature in the range 300 600 c chemical demagnetization was attempted using sodium sulphite results suggest that the carriers of magnetization are not iron hydroxides but probably hematite even after demagnetization a simple bedding correction does not yield a good grouping of magnetization vectors we have therefore tested five other more complex models of unfolding based on structural observations the statistically best grouping is produced by a model involving two successive fold phases where first folds have horizontal e w axes and second folds have vertical axes this is compatible with structural evidence after unfolding we obtain d 101 8 i 13 1 95 1 5 for the orientation of the earth s magnetic field in the area if we compare with the direction for the carboniferous of stable europe we get a difference in declination of 50 to 65 according to the model used for closing the bay of biscay possibly in association with tectonic evolution of the asturian arc

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

a magnetotectonic study of the hercynian montagne noire france a paleomagnetic study of eleven sites of devonian limestones sampled along the flanks and hinge of a polyphase hercynian recumbent fold in the montagne noire reveals well defined directions of characteristic remanent magnetization chrm the directions have good within site precision but a large between site scatter which fails to be reduced by a classical tilt correction this is readily understood if widespread remagnetization occurred between the two major hercynian phases of folding a and b which are known to have shaped the montagne noire this remagnetization appears to be an instance of a continent wide chemical event that wiped out much of the magnetic memory of devonian and lower carboniferous rocks in laurasia baltica and northern gondwana possibly due to groundwater circulation following the major hercynian tectonic phases comparison of observed and predicted hercynian magnetic directions allows one to determine through a fairly simple geometric construction the tectonic elements of the later phase b it is found that different rotations affected the flanks and hinge zone of the folds generated by phase a thus demonstrating how a pre existing structure can control the development of deformation in a later tectonic phase the effects of the non cylindrical phase b can next be removed to reveal the broad tectonic features of phase a deformations the reversed kiaman polarity of the chrm allows a determination of the age of phase b which is found to be about 300 310 ma and of the age of the remagnetization itself at about 310 320 ma the search for the age of remagnetization included paleomagnetic measurements at two lower permian sites from the nearby lodeve basin these provide new data for the permian pole of europe and point out the limitations of some earlier studies due to insufficient magnetic cleaning finally the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30 between phase a and phase b this feature already observed in the collision of india against eurasia may be related to the progressive suturing of former subduction zones during the hercynian collision of gondwana against laurasia this study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems we propose the term magnetotectonics for this rapidly expanding field of studies in which the methods of paleomagnetism and structural analysis are closely associated

remagnetizations and postfolding oroclinal rotations in the cantabrian asturian arc northern spain devonian carbonates in the cantabrian arc reveal characteristic magnetizations with coherent shallow upward inclinations the magnetizations appear to be carried by magnetite within site directions are very well grouped but site mean declinations range from easterly to south southwesterly in in situ as well as tilt corrected coordinates as has also been observed in previous studies of other formations in the arc the widely varying declinations of all studies roughly correlate with the overall structural trends of the arc and suggest that the sites underwent rotations in a process that involved folding about vertical axes and tightening of arc upon tilt correction the inclinations of our study on the other hand become scattered and it is concluded that the magnetizations were acquired after late carboniferous folding about horizontal axes the oroclinal rotations therefore also must have occurred after the late carboniferous folding phase the previous paleomagnetic results had been interpreted mostly as primary magnetizations residing in hematite however inclination only fold tilt tests applied to these results suggest that many if not all of the directions were acquired during the earlier stages of the late carboniferous folding thus all paleomagnetic results from the cantabrian arc appear to be remagnetizations but the ages of the remagnetizations vary from pre to synfolding for the mostly hematitic formations to postfolding for the devonian carbonates the reversed polarity inclinations of the hematite bearing formations have mean values ranging from 25 to 5 whereas the carbonates have a mean inclination of 8 on the basis of inclinations predicted for the area from results from stable europe the pyrenees and the iberian meseta the ages of these remagnetizations can be inferred to range from about 320 ma to 260 ma because all the remagnetizations reveal rotated declination patterns the oroclinal rotations occurred well after the main phase of hercynian deformation 320 280 ma while the timing of the rotations is unconstrained at the younger end they must have occurred during or after the permian best estimate is less than 260 ma which is much later than anticipated from other geological considerations

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

mise en evidence par le paleomagnetisme d une importante rotation antihoraire des vosges meridionales entre le viseen terminal et le westphalien superieur the volcano sedimentary formations from the southern vosges are subdivided in two main series a lower visean series characterized by a volcanism of spilite keratophyre type and an upper visean series which includes a normal volcanic association of shoshonitic tendency paleomagnetic study of 50 sites sampled in both series but mostly in the upper one yields three types of directions of characteristic magnetizations the first type corresponds to tertiary and quaternary remagnetizations with low apparent blocking temperatures 350 500 c titano maghemites the second group is formed by remagnetizations which have taken place during late carboniferous early permian times and which show high blocking temperatures of magnetite and mostly titano haematites the mean direction is d 16 i 7 95 9 for 13 sites 43 n 165 e the last group is represented by primary magnetizations of latest visean age and post sudetic remagnetizations with blocking temperatures of magnetite and haematite the mean direction d 323 i 17 95 9 for 18 sites 25 n 228 e deviates from about 60 from the theoretical direction calculated with the early carboniferous european pole position this deviation is interpreted as resulting from a counterclockwise rotation of the southern vosges between late visean and westphalian times one consequence may be the formation of the variscan v due to the anticlockwise rotation of the eastern branch of the chain the northwesterly directions show a variation of the inclinations which may indicate that the rotation was preceded by a relatively significant drift of the vosges to the north

evolution of polycyclic basement complexes in the aracuai orogen based on u pb shrimp data implications for brazil africa links in paleoproterozoic time paleoproterozoic basement of the aracuaorogen is composed of amphibolite and granulite facies orthogneiss units known as mantiqueira and juiz de fora complexes six u pb shrimp crystallization ages for the mantiqueira complex range from 2137 19 to 2041 7 ma studied samples are characterized by the abundance of inherited archean zircon grains tdm model ages ranging from 2 9 to 3 2 ga and strongly negative end t 9t o 13 gneiss protholits were mainly generated by partial melting of older continental material and the mantiqueira complex is related to active margin and syn collisional magmatism high grade gneiss samples of the juiz de fora complex yields crystallization ages of 2119 16 and 2084 13 and inherited zircon grains are absent this plutonic unit much probably evolved within an oceanic magmatic arc setting or on a very stretched continental crust the mantiqueira and juiz de fora complexes together with the kimezian basement of the west congo belt were parts of a paleoproterozoic orogenic system disrupted and deeply reworked during the evolution of the aracuacongo orogen neoproterozoic metamorphic overprint is dated at ca 590 574 ma 2007 elsevier b v all rights reserved

palaeomagnetism of palaeozoic rocks from the cabo de penas asturia spain summary ordovician volcanic samples of the cabo de penas area yielded a primary remanent magnetization d 202 i 78 95 6 which is partially or entirely overprinted by a late palaeozoic remagnetization late devonian early carboniferous sedimentary samples reveal a pre or syntectonic carboniferous remanent magnetization similar to the secondary one of the volcanic samples a soft post tectonic component appears also in few samples for which a late carboniferous age is inferred comparison of the carboniferous remanent magnetizations and structural parameters suggests that in a late stage of the deformation the sites have undergone minor rotations around a vertical axis a strike correction is therefore proposed to take such an effect into account the average of the corrected mean site directions d 178 i 19 95 9 is considered to be a good approximation of the early or middle carboniferous palaeomagnetic direction in asturia

palaeomagnetic investigations in the hercynian mountain belt of central europe abstract the palaeomagnetism of sediments and volcanics of middle devonian to early carboniferous age has been studied in the hercynian realm of the vosges harz and franconian forest france and germany after appropriate cleaning characteristic directions of natural remanent magnetization nrm have been determined which are considered to be of pretectonic origin according to rock magnetic and structural geological evidence a mean palaeopole for the devonian rocks in the areas investigated was determined at 29 7 n and 9 0 e the site latitude derived from the devonian rocks agrees very well with that observed in great britain and britanny the significant difference in longitude between our devonian poles and the other european data may be due to an anticlockwise rotation of central europe with respect to the northern hercynian foreland the carboniferous rocks yield a mean palaeopole at 41 7 n and 340 4 e matching the carboniferous part of the european polar wander path a fold test within carboniferous greywackes from the harz mountains proves a pre folding origin of the nrm of these rocks however this stable magnetization direction indicates local or regional tectonic displacement our results do not support any major latitudinal motions of continental segments in central europe during the hercynian orogeny the palaeozoic palaeomagnetic data from the vosges franconian forest harz britanny and central massif can be used to interpret the hercynides in terms of a large scale shear belt

petrogenesis of the orthogneisses of the mantiqueira complex central ribeira belt se brazil an archaean to palaeoproterozoic basement unit reworked during the pan african orogeny abstract the occidental terrane of the central segment of the brasiliano pan african ribeira belt comprises two crustal scale thrust sheets andrel ndia and juiz de fora domains taken as reworked neoproterozoic products of the sao francisco cratonic margins pre 1 8 ga orthogneisses and associated rocks of the mantiqueira complex comprise the basement for rocks of the andrel ndia depositional cycle within the andrel ndia tectonic domain geochemical data indicate that the mantiqueira complex comprises rocks that can be grouped as follows intermediate to acid calc alkaline rocks and a transitional basaltic series on the basis of quantitative analysis of the lithogeochemical data these lithotypes cannot be related statistical and or petrological criteria made it possible to define suites and or groups within each one of those units and to constrain petrogenetic models based mostly on their ree data simple least square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves the results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the mantiqueira complex the partial melting processes might have taken place under oxidising conditions typical of tectonic settings associated with the generation of calc alkaline rocks

tectonic implications of precambrian sm nd dates from the southern sao francisco craton and adjacent aracuai and ribeira belts brazil abstract the archean and paleoproterozoic sao francisco craton of eastern brazil is surrounded on all sides by brasiliano pan african orogens the n ne trending orogen that separates the eastern edge of the southern sao francisco craton from the atlantic coast can be divided into the largely greenschist and amphibolite facies aracuai belt on the west and the largely granulite facies ribeira belt on the east a pronounced linear gravity and magnetic anomaly the abre campo discontinuity defines the boundary between these two belts we obtained sm nd mineral ages and whole rock sm nd model ages for garnet bearing metamorphic rocks along an e w transect across the southern sao francisco craton the southern aracuai belt and the ribeira belt at about latitude 20 s a recrystallization age of 2 1 ga from metasediments recrystallized during the development of the classic dome and keel province of the southern sao francisco craton the quadrilatero ferrifero indicates that dome emplacement occurred during the waning stages extensional collapse of the transamazonian collisional orogeny seven mineral ages from the southern aracuai and ribeira belts date the thermal peak of metamorphism at between 538 and 589 ma confirming that these belts were pervasively remobilized during the brasiliano event samples from the aracuai belt yield either archean 2 6 ga or transamazonian 2 1 2 3 ga tdm model ages indicating that the protoliths were either fragments of the sao francisco craton crust or were sediments derived from that craton which presumably had been stretched to form a thinned continental margin during meso and neo proterozoic rifting events notably a mixed meta pelite and metabasite sequence in the southern aracuai belt the dom silverio group has transamazonian ancestry and thus may represent oceanic sediments deposited on or east of this stretched margin and then thrust back onto the continent to mark a collisional suture between the sao francisco block and an archean crustal sliver to the east the rocks of the ribeira belt i e the region east of the abre campo discontinuity have younger model ages tdm 1 6 2 0 ga indicating that this belt was not originally part of the sao francisco craton it may represent an accreted transamazonian terrane therefore the abre campo discontinuity marks an important crustal boundary possibly a suture brasiliano shear zones appear to steepen and root into the abre campo discontinuity suggesting that it formed in brasiliano time

using shear waves to predict lateral variability in vertical fracture intensity during the last five years the use of shear wave splitting in geophysical exploration has made the transition from research project to very promising field technique the measurement of the azimuth of the fast shear wave and analysis of the s wave splitting or birefringence are now commonplace furthermore a very timely practical application is suggested by the recent proposal that reflectivity on post rotation s1 and s2 data sets should indicate lateral variation of anisotropy this could be of great value because the determination of lateral variation in fracture intensity in a vertically fractured sedimentary layer is necessary to insure high efficiency in a horizontal drilling program designed to drain the fractures

a kinematic model for the collision and complete suturing between gondwanaland and laurussia in the carboniferous maps of middle to late carboniferous strike slip faults in western europe northern africa and eastern north america have been compiled from geological gravity and magnetic onshore and offshore data a large majority of these faults can be interpreted in terms of a logical syn to post collisional pattern superimposed on the initial suture between the african i e gondwana and north american european laurussian continental margins the pre collision positions of the major continents are derived from paleomagnetic data and when combined with the general distribution of the strike slip faults lead to a kinematic model of the complex middle late carboniferous collision in this model the west african precambrian craton acted as a rigid promontory or die with respect to the partially rigid plastic zones between it and the large north american craton these partially rigid plastic zones comprise the external appalachian and canadian maritime margins the mauritanide and hercynian margins of north

reorientation of remanent magnetism during tectonic fabric development an example from the waynesboro formation pennsylvania u s a abstract a paleomagnetic study of the late early early middle cambrian waynesboro formation in south central pennsylvania reveals a pre folding magnetization which is close to the expected cambrian field at three sites while other site mean directions are rotated by as much as 82 from the expected cambrian field these results are based on detailed thermal demagnetization analysis of 84 red siltstone and sandstone samples from nine sites collected over a 163 km 2 area local faulting fault block rotation and remanence acquisition during rapid apparent polar wander do not appear to be responsible for the observed remanence deviations total strain at the sites as indicated semiquantitatively by the anisotropy of magnetic susceptibility ams of 28 samples has been studied to test the hypothesis that deformation has rotated the remanence directions ams data vary considerably both between sites and within sites and show a path of progressive deformation on a flinn diagram there is a strong positive correlation between the deviation of a sample s remanence from the expected cambrian field and its position along the deformation path suggesting that finite strain fabric development has rotated the magnetizations the study area is structurally contained in the south mountain anticlinorium whose southern margin forms the displacement transfer zone to the southern appalachian alleghenian thrust sheets elliott 1976 if thrusting had continued and incorporated the waynesboro rocks into these thrust sheets the rotated magnetizations could have been incorrectly interpreted to result from rigid thrust sheet rotation

the effect of whole rock mno content on the stability of garnet in pelitic schists during metamorphism garnet bearing mineral assemblages are commonly observed in pelitic schists regionally metamorphosed to upper greenschist and amphibolite facies conditions modelling of thermodynamic data for minerals in the system na2o k2o feo mgo al2o3 sio2 h2o however predicts that garnet should be observed only in rocks of a narrow range of very high fe mg bulk compositions traditionally the nearly ubiquitous presence of garnet in medium to high grade pelitic schists is attributed qualitatively to the stabilizing effect of mno based on the observed strong partitioning of mno into garnet relative to other minerals in order to quantify the dependence of garnet stability on whole rock mno content we have calculated mineral stabilities for pelitic rocks in the system mno na2o k2o feo mgo al2o3 sio2 h2o for a moderate range of mno contents from a set of non linear equations that specify mass balance and chemical equilibrium among minerals and fluid the model pelitic system includes quartz muscovite albite pyrophyllite chlorite chloritoid biotite garnet staurolite cordierite andalusite kyanite sillimanite k feldspar and h2o fluid in the mno free system garnet is restricted to high fe mg bulk compositions and commonly observed mineral assemblages such as garnet chlorite and garnet kyanite are not predicted at any pressure and temperature in bulk compositions with xmn mn fe mg mn 0 01 however the predicted garnet bearing mineral assemblages are the same as the sequence of prograde mineral assemblages typically observed in regional metamorphic terranes temperatures predicted for the first appearance of garnet in model pelitic schist are also strongly dependent on whole rock mno content the small mno contents of normal pelitic schists xmn 0 01 0 04 are both sufficient and necessary to account for the observed stability of garnet

paleomagnetism of the late ordovician thouars massif vendee province france a paleomagnetic study of the late ordovician thouars massif reveals very steep components of magnetization directed upward and downward and with thermally discrete blocking temperatures of 550 to 580 c this characteristic magnetization yields a mean direction with declination inclination 340 83 with k 27 and 95 7 based on 15 sites the south paleopole is located at 34 n 5 e a partial magnetic overprint of hercynian age is observed in sites close to the south armorican shear zone the 15 sites are spread over some 20 by 30 km and the internal consistency of the site mean directions indicates that differential tilting of parts of the massif has not occurred since it appears to be unlikely that the entire massif was tilted we infer that the characteristic direction is primary and that the paleopole adequately represents the local magnetic field in the late ordovician the paleopole for the thouars massif supports previously obtained results from other ordovician rocks in hercynian europe but it is the first result to pass modern reliability criteria the high ordovician paleolatitudes of the armorican massif other areas in hercynian europe and north western africa indicate that gondwana and armorica were adjacent until late ordovician time low paleolatitudes for late devonian time suggest that armorica underwent rapid northward motion at a rate of about 10 cm yr during the silurian and earliest devonian armorica is thought to have collided with north america and the baltic shield russian platform before or during the early devonian forming the old red continent the collisions produced the caledonian and acadian orogenic belts

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

details of the gabbro to eclogite transition determined from microtextures and calculated chemical potential relationships permian aged metagabbros from the eclogite type locality in the eastern european alps were partially to completely transformed to eclogite during eoalpine intracontinental subduction microtextures developed along a preserved fluid infiltration and reaction front in the gabbros record the incipient gabbro to eclogite transition allowing the details of the eclogitization process to be investigated original anorthite rich igneous plagioclase is pervasively replaced by fine grained intergrowths of clinozoisite kyanite and na rich plagioclase where plagioclase was in contact with igneous orthopyroxene 100 200 m thick bimineralic coronae of symplectic kyanite and diopsidic clinopyroxene form along the edges of the grains the rims of igneous orthopyroxene develop a complementary bimineralic corona of diopsidic clinopyroxene and garnet igneous clinopyroxene does not show any breakdown textures however jadeite content gradually increases towards the rims in addition exsolution lamellae inherited from the igneous clinopyroxene become progressively more jadeitic as eclogitization proceeds given that the igneous plagioclase is pervasively replaced by clinozoisite kyanite and na rich plagioclase whereas kyanite diopside symplectites are confined to narrow rim zones we suggest that the development of these textures was controlled by the im mobility of different elements on different length scales the presence of hydrous minerals in the core of anhydrous plagioclase indicates that h2o diffusivity occurred on a mm scale by contrast the size of the anhydrous diopside kyanite and diopside garnet symplectites indicate that fe mg ca na diffusivity was limited to a 10s of m scale chemical potential relations calculated in the idealized ncash chemical system show that the clinozoisite kyanite albite intergrowths formed due to an increase of h2o to plagioclase whereas all other elements remained effectively immobile on the scale of this texture fluid conditions indicated by this texture span from virtually dry conditions ah2o 0 15 to h2o saturation and therefore does not imply that the rocks were ever fluid saturated calculations in the cmas and ncfmas systems show that the gabbro to eclogite transition is characterized by the growth of garnet diopsidic clinopyroxene and kyanite due to diffusion of ca na and mg fe along a cao na2o mgo feo chemical potential gradient developed between orthopyroxene and plagioclase compositional domains the anhydrous nature of the textures indicate that the gabbro to eclogite transition is not driven by hydration however increased h2o acts as a catalyst that increases diffusivity of all elements and rates of dissolution precipitation allowing the overstepped metamorphic reactions to occur our results show that crustal eclogite formation requires low h2o content confirming that true eclogites are dry rocks