differential stress control on the growth and orientation of flame perthite a palaeostress direction indicator abstract albite flames result from the replacement of k feldspar by albite through alkali exchange in flame perthites from deformed granites within the grenville front tectonic zone replacement is driven by retrograde breakdown of plagioclase and hydration of k feldspar within the strain fabric of the host rock albite flames preferentially form subparallel to the inferred maximum compression direction and also develop at high stress points such as grain to grain contacts flame tips are generally parallel to the normal perthite crystallographic direction i e the murchison plane which is the orientation of the plane of minimum crystallographic lattice misfit between the albite and k feldspar we propose that the morphology of an albite flame is controlled by the coherent propagation of its tip into the k feldspar crystal lattice and that the orientation of the flames with respect to the rock fabric is controlled by the differential stress imposed during metamorphism this type of replacement requires dry conditions in which there would be inefficient dissolution reprecipitation along completely incoherent interfaces normal replacement and increased strength of feldspar under these conditions coherency is maintained to avoid a very large kinetic barrier the differences in lattice misfit along the murchison plane between k feldspar and albite requires either high elastic strain energy if the fit is coherent or lattice dislocation energy if the fit is semi coherent such energy might be expected to inhibit the replacement growth of albite lamellae in a host feldspar however compression parallel to the murchison plane reduces the lattice misfit along the compression direction this results 1 in an increase in the helmholtz free energy of the k feldspar and 2 in a reduction in the helmholtz energy of the albite lamellae or of the semi coherent interface thus albite replacement allows a decrease in net free energy without the destruction of the alumino silicate framework and is therefore favoured under conditions of high differential stress since flame growth is controlled by the imposed differential stress and not by strain in the host rock flame perthite has potential as a palaeostress direction indicator

assessment of petrophysical surface data of the harsit granitoid in trabzon in northeastern turkey the harsit granitoid in northeastern turkey comprises four separate granite units that are apparently unfoliated the harsit granitoid was investigated here by using microstructural petrofabric and anisotropy of magnetic susceptibility ams data the structural data of the granitoid were found to be highly compatible with the zonation recognized from ams measurements the orientation of magnetic fabrics within the granite units indicates that tectonic deformation might have occurred coevally with the magmatic emplacement of the intrusion when we evaluated the manners on the scale of the pluton that the disruption took place in the form of uplifting probably related to a rapid migration of the volcanic front and the documented change from deep sea sedimentation predominant until late cretaceous to shallower environments during the paleocene the possible tectonic control of fault systems on the ascent and emplacement of the granitic magma in the study area however cannot be completely ruled out because the harsit granitoid is situated at or very near the naf systems in northern anatolia in any case the intrusion is clearly discordant to the regional deformational features formed during the collisional events between the eurasia and anatolia plates in northern turkey

emplacement and deformation of granites during transpression magnetic fabrics of the archean sparrow pluton slave province canada abstract the sparrow pluton is part of the prosperous suite of two mica granites that crop out within amphibolite grade meta greywackes of the southern yellowknife domain in the slave province of the canadian shield the magnetic susceptibility k and the anisotropy of magnetic susceptibility ams were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and late archean tectonics in the region paramagnetic fe phyllosilicates biotite chlorite and very fine grained magnetite contribute to k and to the ams the magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional d 2 strain field the horizontal magnetic lineations in the sparrow pluton suggest a horizontal stretching component associated with the regional d 2 event the zonation defined by k values is compatible with a fold pattern trending parallel to the regional f 2 folds and s 2 foliation and to the magnetic fabric trends in the pluton the intensities and symmetries of the ams also define map patterns that are consistent with d 2 deformation microstructural study indicates the pluton recorded d 2 strain as it crystallized and cooled from the solidus demonstrating syn d 2 emplacement the results indicate the pervasive structural patterns in the sparrow pluton are an integral part of the regional strain field and that they are kinematically consistent with a transpressive d 2 strain regime mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

probing seismic anisotropy in north iberia from shear wave splitting the mantle anisotropic features at the northwestern part of the iberian peninsula have been investigated by the shear wave splitting technique from different temporary array deployments during the last 6 years successive seismic transects were instrumented from east to west to sample areas affected by the alpine compressional tectonics western pyrenees and cantabrian mountains as well as hinterland parts of the iberian variscan belt a remarkable consistency is found in the retrieved anisotropic parameters throughout the study area with an average fast velocity direction close to e w delay times up to 1 5 s are observed in most transects but lower values not exceeding 1 s are measured at the westernmost part although the averaged values are compatible among the different stations a significant variation of the splitting parameters is observed in each station with respect to the backazimuth direction this azimuthal dependence denotes a complex distribution at depth of the anisotropic features and cannot be explained by considering single layer anisotropic models either with hexagonal or orthorhombic symmetry systems and or dipping axes of symmetry synthetic models including two distinct anisotropic layers with an orthorhombic symmetry provide a satisfactory fit in all the retained models the two layers exhibit different thicknesses the most prominent contribution to the observed anisotropy comes from a lower layer consistently oriented close to e w whereas the thinner second layer located above modulates the result and its anisotropic parameters may change along the transect to account for the observed differences in splitting delay times the dominant e w layer throughout could not be associated to major compressional events as in the westernmost part the variscan terrains exhibit n s main lineaments it may rather correspond to an anisotropic imprint around the lithosphere asthenosphere transition related to the eastward displacement of the iberian plate due to the mesozoic extensional processes during the opening of the north atlantic and bay of biscay domains the weaker anisotropic layer could be associated either with crustal anisotropic materials or more likely to an additional anisotropic signature within the lithosphere led by major variscan and alpine orogenic processes

formation and deformation of hyperextended rift systems insights from rift domain mapping in the bay of biscay pyrenees the bay of biscay and the pyrenees correspond to a lower cretaceous rift system including both oceanic and hyperextended rift domains the transition from preserved oceanic and rift domains in the west to their complete inversion in the east enables us to study the progressive reactivation of a hyperextended rift system we use seismic interpretation gravity inversion and field mapping to identify and map former rift domains and their subsequent reactivation we propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen this study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation two spatially and temporally distinct rift systems can be distinguished the bay of biscay parentis and the pyrenean basque cantabrian rifts while the offshore bay of biscay represent a former mature oceanic domain the fossil remnants of hyperextended domains preserved onshore in the pyrenean cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain both domains accommodated most of the shortening the final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes the complex 3 d architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system these results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the bay of biscay and pyrenean domains

new models for evolution of magma poor rifted margins based on a review of data and concepts from west iberia and the alps direct observation and extensive sampling in ancient margins exposed in the alps combined with drill hole and geophysical data from the present day iberia margin result in new concepts for the strain evolution and near surface response to lithospheric rupturing at magma poor rifted margins this paper reviews data and tectonic concepts derived from these two margins and proposes that extension leading to thinning and final rupturing of the continental lithosphere is accommodated by three fault systems each of them characterized by a specific temporal and spatial evolution during rifting of the margin by its fault geometry and its surface response the data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting the place of final break up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres during final break up the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes

thermal imprint of rift related processes in orogens as recorded in the pyrenees 19 the extent to which heat recorded in orogens reflects thermal conditions inherited from 20 previous rift related processes is still debated and poorly documented as a case study we 21 examine the mauleon basin in the north western pyrenees that experienced both extreme 22 crustal thinning and tectonic inversion within a period of 30 myrs to constrain the time 23 temperature history of the basin in such a scenario we provide new detrital zircon fission 24 track and u th sm he thermochronology data the role of rift related processes in 25 subsequent collision is captured by inverse modeling of our thermochronological data using 26 relationships between zircon u th sm he ages and uranium content combined with 27 thermo kinematic models of a rift orogen cycle we show that the basin recorded significant 28 heating at about 100 ma characterized by high geothermal gradients 80 c km our 29 thermo kinematic modeling and geological constraints support the view that subcontinental 30 lithospheric mantle was exhumed at that time below the mauleon basin such a high 31 geothermal gradient lasted 30 myr after onset of convergence at 83 ma and was relaxed 32 during the collision phase from 50 ma this study suggests that heat needed for ductile 33 shortening during convergence is primarily inherited from extension rather than being only 34 related to tectonic and or sedimentary burial this should have strong implications on tectonic 35 reconstructions in many collision belts that resulted from inversion of hyper extended rift 36 basins

central gulf of aden conjugate margins yemen somalia tectono sedimentary and magmatism evolution in hybrid type margins abstract magma rich and magma poor passive margins are generally studied separately yet the spatial evolution from one type to another is not well understood central gulf of aden margins are at the transition between these two types of margins based on new seismic data published wells and gravity data we determine seismic stratigraphy and structural patterns we map the distinct crustal domains continental ocean continent transition oct oceanic domains and propose a tectonic evolution of the yemeni and somalian conjugate margins in an oblique rifting context the most striking results are the significant segmentation narrowness and asymmetry of conjugate margins as well as the gradual eastward decrease in the intensity of magmatic activity of these hybrid type margins west of bosaso hami fracture zone bhfz the hami ahl medo segment presents magmatic type margin features in the distal yemeni margin related to the strong influence from the afar plume seaward dipping reflectors develop since the late syn rift stage 21 18 ma near the xiis al mukalla fracture zone conversely east of bhfz the bosaso sayhut margins segment is characterized by magma poor margins for which the exhumation stage is characterized by the unroofing of mantle along multiple detachment faults in the oct and shortly followed by diachronous volcanism 18 ma the central gulf of aden is affected by a major uplift during the final syn rift stage and up to oct formation this event is associated with the formation of erosional surfaces in the east and with gravitational deformation taking place on decoupling surfaces at the base of the syn rift sequence lower ghaydah fm that lead to the sliding of syn rift units on top of the exhumed mantle rocks in the oct

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

permian magmatism and basin dynamics in the southern pyrenees a record of the transition from late variscan transtension to early alpine extension

magnetic fabrics and microstructures of the post collisional aegirine augite syenite triunfo pluton northeast brazil abstract anisotropy of magnetic susceptibility ams and microstructures of the neoproterozoic triunfo alkaline pluton situated in northeast brazil have been investigated in order to characterize fabrics formed during its emplacement bulk magnetic susceptibility in the pluton varies between 0 15 10 3 and 25 10 t 3 si and the mean magnetic anisotropy ratio is around 1 04 aegirine augite and secondary magnetite are the main carriers of the magnetic susceptibility with the former controlling the directions of the anisotropy magnetic foliation and lineation are consistently oriented throughout the pluton foliation typically dips gently to the northeast and lineation plunges gently to the east these structures were mostly acquired at the magmatic and sub magmatic stages the shape of the magnetic ellipsoid is dominantly oblate a feature that may reflect either the uniaxial oblate ams ellipsoid of the aegirine augite crystals and or a vertical compaction of the magma in its final stages of crystallization the foliation pattern suggests the pluton has a tabular shape within the basement rocks magma emplacement is inferred to have occurred by its lateral migration along a flat lying crustal structure after fracture propagation and ascension of the magma from a lower crust mantle source late fine grained vertical dykes with the same mineral composition as the triunfo syenite cross cut the wall rocks of the pluton and close the alkaline magmatism in the area the ne trending alignment of alkaline plutons and the nne trending syenitic dykes indicate that the emplacement of such bodies occurred along a crustal extension event with a minimum compressive stress horizontal trending roughly e w

tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean example of the western pyrenees 1 in this paper we describe the tectonosedimentary evolution and its subsequent inversion of a basin that underwent extreme crustal thinning in a transtensional setting ahead of a propagating ocean in the western pyrenees the labourd mauleon area situated in the western pyrenees at the termination of the v shaped bay of biscay is an ideal natural laboratory to study how such complex basins evolve in time and space because of a mild inversion of the basin during pyrenean compression the rift structures and their relations to basement rocks and sediments are exposed and can be directly studied in the field the basin shows a complex polyphase evolution that starts with left lateral dominated transtension in latest jurassic early aptian time this event is overprinted by a late aptian early albian extension that is related to the counterclockwise rotation of iberia away from europe leading to the opening of the bay of biscay during this stage the late triassic to jurassic carbonate platform was stretched salt migrated and detachment faults exhumed upper and lower crustal and mantle rocks to the seafloor the final structure of the basin resembles a sag basin floored by exhumed rocks overlain by extensional allochthons and compartmentalized by n40 to n60 transfer faults the sedimentary architecture is characterized by late aptian synrift sediments e g urgonian limestones that were deposited in fault bounded basins and are overlain by thick latest aptian to albo cenomanian sediments e g flysch noir that define a sag sequence the complex tectonosedimentary evolution of the basin is associated with salt tectonics and overprinted by a major magmatic thermal event that postdates mantle exhumation

magnetic petrology of the devonian achala batholith argentina titanohaematite as an indicator of highly oxidized magma during crystallization and cooling summary devonian magmatism in the eastern sierras pampeanas cordoba argentina is represented by elliptical porphyritic batholithic late to post orogenic monzogranites emplaced in a metamorphic plutonic basement two groups of devonian granitoids are distinguished 1 metaaluminous to weakly peraluminous hornblende biotite bearing granitoids and 2 peraluminous biotite muscovite bearing granitoids groups 1 and 2 are characterized by accessory phases of sphene magnetite with high susceptibility and ilmenite monazite uraninite with low susceptibility respectively the magnetic assemblage of the group 2 achala batholith predominantly paramagnetic is the subject of this paper forty three sites were sampled in the achala monzogranite overall it is weakly magnetic with a mean magnetic susceptibility of 1 5 10 4 si twenty one sites showed titanohaematite as the magnetic carrier of a stable remanence the titanohaematite is an accessory mineral and contains exsolved disc shaped intergrowths of ilmenite the weak magnetism of achala monzogranite facies contrasts with the magnetic character of other devonian porphyritic batholithic monzogranites in the region the virtual absence of magnetite together with the presence of exsolved titanohaematite and mn ilmenite indicate crystallization and cooling under more oxidizing conditions oxidizing conditions continued into the subsolidus causing late stage reactions of ilmenite host grains to ilmenite rutile haematite and titanohaematite to ilmenite poorer haematite rutile these reactions resulted in low susceptibility values and a strong reduction of natural remanent magnetization nrm intensities thermal demagnetization up to 620 640 c isolated steeply dipping dual polarity remanence directions the coincidence of the achala paleomagnetic pole with the 380 360 myr segment of the gondwana apparent polar wander path and the u pb zircon age implies that the remanence was acquired soon after emplacement

internal structure and emplacement of the papoose flat pluton an integrated structural petrographic and magnetic susceptibility study the papoose flat pluton of eastern california is commonly cited in the geologic literature as a classic example of a forcibly emplaced pluton although the relative importance attributed to magmatic versus tectonic processes in controlling the structural evolution of the pluton has been controversial reexamination of this late cretaceous 83 ma pluton using a combination of new structural petrographic microstructural and anisotropy of magnetic susceptibility ams data on the pluton wall rock system has shown that the pluton is an inclined and internally zoned tabular structure assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12 15 km initial pluton formation involved magma ascent in a vertical west northwest striking feeder dike which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying cambrian metasedimentary rocks leading to formation of a southwest dipping sill subsequent sill inflation accompanied by horizontal infilling from the feeder dike at the base of the sill resulted in deformation and vertical translation of previously emplaced magma pulses and local raising of the sill roof facilitated by thermal weakening as the wall rock temperatures progressively rose during emplacement of successive magma pulses cooling from the roof of the pluton downward resulted in cessation of vertical inflation on the west side of the pluton and promoted lateral expansion toward the northeast and floor depression below the eastern part of the pluton we have been unable to document any regional scale structures e g of equivalence to similar age synplutonic strike slip shear zones in the sierra nevada batholith to the west that may have controlled emplacement of the papoose flat pluton however this fact does not preclude the likelihood that the country rocks were subjected to a regional deviatoric stress field at this time indeed tectonic overpressuring was probably essential for producing sufficiently high magma pressures for laccolith formation at this midcrustal level simple thermal modeling using microstructural and thermobarometric data indicates that the total duration of emplacement of the pluton did not exceed 30 000 yr this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional scale deformation

depositional features and stratigraphic sections in granitic plutons implications for the emplacement and crystallization of granitic magma abstract many granitic plutons contain sheet like masses of dioritic to gabbroic rocks or swarms of mafic to intermediate enclaves which represent the input of higher temperature more mafic magma during crystallization of the granitic plutons small scale structures associated with these bodies e g load cast and compaction features silicic pipes extending from granitic layers into adjacent gabbroic sheets indicate that the sheets and enclave swarms were deposited on a floor of the magma chamber on granitic crystal mush and beneath crystal poor magma while the mafic magma was incompletely crystallized these structures indicate way up typically toward the interior of the intrusions and appear to indicate that packages of mafic sheets and enclave concentrations in these plutons are a record of sequential deposition hence these plutons preserve a stratigraphic history of events involved in the construction filling replenishment and crystallization of the magma chamber the distinctive features of these depositional portions of plutons allow them to be distinguished from sheeted intrusions which usually preserve mutual intrusive contacts and dike sill relations of different magma types the considerable thickness of material that can be interpreted as depositional and the evidence for replenishment suggest that magma chamber volumes at any one time were probably much less than the final size of the pluton thus magma chambers may be constructed much more slowly than presently envisaged the present steep attitudes of these structures in many plutons may have developed gradually as the floor of the chamber along with the underlying solidified granite and country rock sank during continuing episodes of magma chamber replenishment these internal magmatic structures support recent suggestions that the room problem for granites could be largely accommodated by downward movement of country rock beneath the magma chamber

modelling sediment load in a glacial meltwater stream in western norway summary the objective of this work is to provide a tool for modelling and temporal reconstruction of the sediment load in a proglacial valley in norway for this purpose we have developed the sediment load climatological model selclim to predict climate induced sediment load based on seasonal precipitation and temperature inputs the model is tested using observational data for the period 1968 2000 the main development of the nigardsbreen valley system is likely the combined effects of pulsed erosion over multiple nivoglacial cycles of expansion and retreat at interdecadal scale pulsed flushes and runoff erosivity erosion the model used discharge and surrogate predictors to model annual sediment load the study attempted at relating historical and instrumental glacier front movement thermal spring summer conditions snow and glacier ice ablation and summer rainfall with total summer sediment accumulation to assess interannual to centennial scale variability of sediment load during the little ice age glacier expansion and following retreat at nigardbreen

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

structure and u pb dating of the saint arnac pluton and the ansignan charnockite agly massif a cross section from the upper to the middle crust of the variscan eastern pyrenees in the agly massif pyrenees two variscan plutons the saint arnac pluton and the ansignan charnockite intrude different levels of a c 10 km thick crustal section the saint arnac pluton intrudes through upper crustal rocks and the ansignan charnockite cuts mid crustal country rocks a structural study of these plutons based on the anisotropy of magnetic susceptibility technique combined with a kinematic study of the country rocks shows that the structures and emplacement modes of the plutons are compatible with those of the other plutons of the pyrenees emplaced during the d 2 transpressive phase u pb dating on zircons from the saint arnac pluton yields a 308 3 1 2 ma age for a diorite and a 303 6 4 7 ma age for a granodiorite the charnockite was previously dated at 315 ma the emplacement ages of these two intrusions are thus separated by at least 5 ma first numerous sills and laccoliths such as the ansignan laccolith were injected in the middle crust and this induced heating and thickening subsequently the large saint arnac pluton was injected in the upper crust at the beginning of formation of a gneissic dome

hercynian tectonics in the pyrenees a new view based on structural observations around the bassie s granite pluton abstract the structural study of the metasedimentary rocks surrounding the bassie s granite pluton allows definition of the succession and kinematics of the hercynian tectonic events in this part of the pyrenees and supports a model of syntectonic emplacement of the pluton the following sequence of events has been identified i an early phase of n s compression essentially resulting in southwards thrusting ii emplacement of the granite body along a ne sw sinistral strike slip shear zone which could be a late effect of the first phase iii a major dextral and transpressive phase responsible for themain fold structures trending on average n100 and following a sigmoidal pattern around the granite body this succession of events and their kinematics could be extended to the whole hercynian basement of the pyrenees

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

neoproterozoic to early palaeozoic events in the sierra de san luis implications for the famatinian geodynamics in the eastern sierras pampeanas argentina the application of the sensitive high resolution ion microprobe shrimp u pb dating technique to zircon and monazites of different rock types of the sierra de san luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains additional constraints on the famatinian metamorphic episode are provided by pb pb stepwise leaching experiments on one staurolite and two garnet separates the results indicate that the sedimentary precursors of the conlara metamorphic complex have a maximum age of c 590 ma whereas the pringles metamorphic complex metasediments appear to be sourced from the pampean orogen in the early cambrian folded xenoliths within the c 496 ma el penon pluton suggest that the host conlara metamorphic complex underwent a pampean compression from a 208 pb 232 th monazite age of 478 ma for a migmatite from the nogoli metamorphic complex the structural evolution of this basement complex appears to be entirely post pampean onset of the famatinian high grade metamorphism between c 500 ma and c 450 ma follows a period of crustal extension on the western outboard of gondwana and might not be related directly to a mid ordovician accretion of the cuyania terrane

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

the tectono sedimentary evolution of a hyper extended rift basin the example of the arzacq mauleon rift system western pyrenees sw france in this paper we present a sedimentary and structural analysis that together with maps sections and new ar ar data enable to describe the tectono sedimentary evolution of the mauleon hyper extended rift basin exposed in the w pyrenees hyper extension processes that ultimately resulted in exhuming mantle rocks are the result of the subsequent development of two diachronous detachment systems related to two evolutional stages of rifting an initial late aptian early albian crustal thinning phase is first recorded by the development of a crustal necking zone controlled by the north vergent southern mauleon detachment system during a subsequent exhumation phase active faulting migrates to the north with the emplacement of the northern mauleon detachment system that exhumed north section thinned continental crust and mantle rocks this diachronous crustal thinning and exhumation processes are also recorded by the diachronous deposition of syn tectonic sedimentary tracts above the two supra detachment sub basins syn tectonic sedimentary tracts record the progressive exhumation of footwall rocks along detachment systems tectonic migration from the southern to the northern mauleon detachment system is recorded by the coeval deposition of sag deposits above the necking zone basin and of syn tectonic tracts above exhumed rocks north section located on a hanging wall situation related to the mauleon hyper extension structures the arzacq basin also records a major crustal thinning phase as shown by its subsidence evolution so as by deep seismic images the absence of major top basement structures and its overall sag morphology suggest that crustal thinning processes occurred by decoupled extension of lower crustal levels contrasting with the southern mauleon detachment system reconciling observations from the mauleon and arzacq basins we finally propose in this paper that they were the result of one and the same asymmetric crustal thinning and exhumation processes where extension is accommodated into the upper crust in the mauleon basin lower plate basin and relayed in ductile lower crust below the arzacq basin upper plate basin

regional scale cretaceous albitization in the pyrenees evidence from in situ u th pb dating of monazite titanite and zircon abstract albitization is a common metasomatic process active in various geodynamic contexts in the northern pyrenees there are several occurrences of albitites but until now only one occurrence has been dated 117 ma ar ar dating this paper presents new u th pb ages for several albitite occurrences throughout the pyrenees to test whether they are contemporaneous and if so to specify the regional extent of the albitization event ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 8 and 98 2 ma respectively the zircon u th pb isotopic system did not record this cretaceous metasomatic event even when grains were selected in metasomatically zr enriched rocks or in hydrothermal structures millimetre sized veins cross cutting granitoids we argue that the total time span of 20 ma recorded by albitites corresponds to a long lived hydrothermal system that was active during the rotation of iberia around europe along the north pyrenean fault because albitization and talc mineralization have the same spatial and temporal distribution in the pyrenees we argue that these two metasomatic phenomena are two independent records of this single regional scale long lived hydrothermal system

pluton emplacement during transpression in brittle crust new views from analogue experiments analogue experiments were used to investigate pluton emplacement during transpression in brittle crust models consisted of silicone putty placed within slots in a rigid basement and an overburden of dry sand the models were transpressed causing emplacement of silicone intrusions into the overlying sand space was made for intrusions by upward displacements of overburden along oblique reverse shear zones in some cases accompanied by extensional shearing above the intrusions silicone putty was progressively accreted to the bases of uplifted blocks and growing intrusions rode up the shear zones leaving space for accretion of more silicone the resulting intrusion shapes depended largely on the geometries of shear zone systems which in turn depended on overburden thickness the intrusion profiles and the structural relationships between intrusions and host in the experiments are comparable to some natural examples a magma accretion model may thus account for pluton emplacement and batholith assembly within some transpressive orogens

sigmoidal structures featuring dextral shear during emplacement of the hercynian granite complex of cauterets panticosa pyrenees abstract the three plutons of the cauterets panticosa granite complex are studied by the anisotropy of magnetic susceptibility ams technique as expected from the dominantly paramagnetic nature of the rocks the magnetic susceptibility zonation accurately compares with the lithological zonation detailed examination of the linear and planar structural patterns of the plutons allows the difficult problem of their formation mechanisms to be explored the sigmoidal pattern of the linear structures only results from regional tectonics and points unequivocally to a dextral shear component for the responsible tectonic event the concentric planar structures while also affected by regional tectonics retain some imprint of magma emplacement the microstructures observed in the granitoids point to a continuous deformation from the magmatic state to the high temperature solid state in the country rocks structures and kinematic criteria underline the transpressive character of the d2 hercynian event of the pyrenees which resulted in a partitioning between zones where either compression or dextral strike slip were dominant all these data constrain a new model of d2 syn tectonic emplacement for the plutons of the cauterets panticosa complex and remarkably illustrate structural features resulting from a transpressive regime

magnetic susceptibility of the mont louis andorra ilmenite type granite pyrenees a new tool for the petrographic characterization and regional mapping of zoned granite plutons we have observed remarkably consistent patterns of concentric zonation in the values of low field magnetic susceptibility measurements over the variscan age mont louis andorra granite pluton of the eastern pyrenees of andorra spain and france this zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature as defined by chemical analysis and the magnetic susceptibility magnitude of the rocks it reflects the dominantly paramagnetic nature of the granite carried by fe bearing silicates and this is demonstrated by the fact that low field susceptibilities of representative specimens are almost equal to 1 their high field susceptibilities and 2 their calculated susceptibilities using fe contents assuming a null ferromagnetic contribution we conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons

extensional salt tectonics in a contractional orogen a newly identified tectonic event in the spanish pyrenees a new coniacian early santonian extensional salt tectonics event has been identified in the cotiella thrust sheet of the central spanish pyrenees we present a new interpretation of the cotiella thrust system and describe exceptional outcrop examples of postrift gravity driven extensional growth faults the thrust sheet contains three kilometer scale extensional faults with as much as 5 5 km of coniacian lower santonian growth strata in their hanging walls these postrift listric growth faults detached on the prerift triassic evaporites on the uplifted and eroded southern flanks of early cretaceous rift basins early alpine late santonian maastrichtian contractional deformation reactivated and reversed the displacement on both the early cretaceous rift faults and some of the postrift salt detachment faults hanging walls of the listric growth faults deformed into major antiforms but the original extensional geometries are well preserved continued alpine shortening incorporated these reactivated extensional fault systems into the thin skinned thrust sheets of the southern pyrenees these postrift extensional structures account for significant thickness variations in the postrift succession and for along strike changes in the styles of the thrust structures the late cretaceous extensional salt tectonics is interpreted to result from gravity induced northward sliding on the early cretaceous rift margin toward the axis of the rift basin the listric growth faults are similar to those found on the gulf of mexico margin in the southern north sea and the south atlantic margins

control of magnetic rock fabrics by mica preferred orientation a quantitative approach magnetic anisotropy analysis represents a well established and frequently used method in structural geology several attempts were made to relate the anisotropy of magnetic susceptibility ams to rock fabrics and to discover the sources of the ams in gneissic rocks paramagnetic phyllosilicates with lattice dependent magnetic properties are assumed to control the whole rock ams i e it is dominated by mica lattice preferred orientation texture in this study ams was modelled on the basis of the mica texture the rock composition and compared to the experimentally determined ams the orientations of the modelled and experimental tensors agree quite well whereas all the other characteristic parameters shape anisotropy mean susceptibility display large differences it has been concluded that accessory high susceptible phases as well as the diamagnetic rock constituents may have significant influence on ams consequently mica pole figure measurements cannot be replaced by ams measurements without control of the sources of ams it was also concluded that the march model to evaluate strain from mica preferred orientations is not valid for highly strained rocks beside the fact that the obtained strains are too low the deformation is generally inhomogeneous which leads to an overprint of the mica texture since the microstructural fabric elements which originate in different strain regimes prolate or oblate may create similar modifications of the mica orientation patterns it is obvious that the relationship between ams and deformation is rather complex

multistage emplacement of the mount givens pluton central sierra nevada batholith california the ca 90 ma mount givens pluton is one of the largest granodioritic to granitic intrusions in the sierra nevada batholith of california emplacement of the pluton occurred during a critical time in the tectonic evolution of the central sierra nevada magmatic arc marked by a transition from regional contraction to dextral transcurrent shear a model for the emplacement of the intrusion is developed based on detailed mapping of the pluton and its wall rocks and characterization of its internal structure by measurements of the anisotropy of magnetic susceptibility ams at 351 stations one of the key results of the study is documentation of a strong correlation between petrologic and structural fabrics in the pluton and determination that these fabrics reflect internal magma chamber dynamics more than regional tectonic strain the 80 km long 15 30 km wide pluton crystallized from a multiphase three segment magma chamber marked by a bulbous northern lobe and linear central and southern segments the pluton is interpreted to be tabular in shape with a thickness of 5 km most of the space for the pluton was created by piecemeal block downdrop of the magma chamber floor along three principal fracture sets the most important of which were steeply dipping northwest trending fractures formed parallel to the structural grain of the arc and vertical north trending extension fractures formed in response to a component of arc parallel dextral shear some of these fractures acted as magma conduits episodically filling the pluton as source rocks became depleted in melt an initial voluminous intrusive event stage 1 quickly filled the southern chamber with granodiorite magma but only partially filled the northern and central chambers stage 2 magmatism involved underplating of megacrystic granite in the northern chamber and lateral flow of a large batch of this magma from the northern to the central chamber the latter delineated by a 20 km long belt of megacrystic granite containing subhorizontal magnetic lineations that connects the pluton segments floor downdrop eventually ceased to be an effective space making process in the northern lobe and renewed magmatism stage 3 led to expansion and doming of the chamber as the northern lobe cooled a ring fault ruptured within the viscoelastic stage 1 2 carapace allowing ring dike intrusion stage 4 and sinking of a central flap of consolidated material the temporal and spatial variations in emplacement mechanisms demonstrated for the mount givens pluton i e fracture generation floor downdrop underplating inflation ring diking suggest that end member models e g fracture vs diapir are oversimplifications of the pluton assembly process

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

crustal structure and evolution of the pyrenean cantabrian belt a review and new interpretations from recent concepts and data this paper provides a synthesis of current data and interpretations on the crustal structure of the pyrenean cantabrian orogenic belt and presents new tectonic models for representative transects the pyrenean orogeny lasted from santonian 84 ma to early miocene times 20 ma and consisted of a spatial and temporal succession of oceanic crust exhumed mantle subduction rift inversion and continental collision processes at the iberia eurasia plate boundary a good coverage by active source vertical incidence and wide angle reflection and passive source receiver functions seismic studies coupled with surface data have led to a reasonable knowledge of the present day crustal architecture of the pyrenean cantabrian belt although questions remain seismic imaging reveals a persistent structure from the central pyrenees to the central cantabrian mountains consisting of a wedge of eurasian lithosphere indented into the thicker iberian plate whose lower crust is detached and plunges northwards into the mantle for the pyrenees a new scheme of relationships between the southern upper crustal thrust sheets and the axial zone is here proposed for the cantabrian belt the depth reached by the n dipping iberian crust and the structure of the margin are also revised the common occurrence of lherzolite bodies in the northern pyrenees and the seismic velocity and potential field record of the bay of biscay indicate that the precursor of the pyrenees was a hyperextended and strongly segmented rift system where narrow domains of exhumed mantle separated the thinned iberian and eurasian continental margins since the albian cenomanian the exhumed mantle in the pyrenean rift was largely covered by a mesozoic sedimentary lid that had locally glided along detachments in triassic evaporites continental margin collision in the pyrenees was preceded by subduction of the exhumed mantle accompanied by the pop up thrust expulsion of the off scraped sedimentary lid above to the west oceanic subduction of the bay of biscay under the north iberian margin is supported by an upper plate thrust wedge gravity and magnetic anomalies and 3d inclined sub crustal reflections however discrepancies remain for the location of continent ocean transitions in the bay of biscay and for the extent of oceanic subduction the plate kinematic evolution during the mesozoic which involves issues as the timing and total amount of opening as well as the role of strike slip drift is also under debate discrepancies arising from first order interpretations of the adjacent oceanic magnetic anomaly record

in situ la icp ms u pb titanite dating of na ca metasomatism in orogenic belts the north pyrenean example in the pyrenees in association with the rotation of the iberian plate around europe during the mid cretaceous a na ca metasomatism is recognized as a complementary record of the hydrothermal activity that led to na metasomatism albitization and talc chlorite mineralization it affected metasedimentary rocks as well as hercynian granites in situ laser ablation icp ms u pb analyses of titanite grains formed in albitites during metasomatism date the na ca metasomatism between 110 and 92 ma the temperature of the na ca metasomatism is estimated to be approximately 550 c both the time constraints and temperature estimates suggest that the na ca metasomatism is related to the low p high t north pyrenean metamorphism

petrological arguments for high temperature extensional deformation in the pyrenean variscan crust saint barthelemy massif ariege france abstract the saint barthelemy massif is one of the variscan massifs of the north pyrenean zone thermobarometric analysis has been completed across a sequence comprising from bottom to top basal granulitic gneisses the main mylonite band and migmatites pt estimates are correlated with textures which represent progressive stages of gneissification and mylonitization the main foliation and the mylonitic textures were developed during the retrogressive evolution of the lp ht granulitic metamorphism retrogressive conditions are characterized by high heat flow associated with a pressure reduction the main penetrative deformation of the saint barthelemy massif is interpreted as resulting from a continuous extensional event

emplacement related deformation of granitoid magmas southern adamello massif italy the lago della vacca suite in the tertiary adamello massif italian alps is a small 10 km 2 crudely zoned granitoid pluton emplaced at a pressure of 3 5 kbar into folded mesozoic sediments and precursor intrusive rocks field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early passive stoping by the mafic to intermediate composition marginal units to later forceful emplacement of the more silicic core marginal units of the pluton have highly discordant stoped outer contacts against folded triassic carbonates and precursor mafic ultramafic rocks in contrast the core units have steeply inward dipping contacts against these marginal units magmatic foliations defined by the planar alignment of plagioclase amphibole and biotite and by mafic enclaves are well developed in all units they describe steeply inward dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin lineations are rarely present the most intensely foliated rocks occur in the marginal units as anastomosing sinuous bands that wrap around lenses of less intensely foliated granitoid three dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain strain ellipsoids are weakly prolate within the core of the pluton becoming increasingly oblate as strain increases toward the pluton margin the short axis z of the strain ellipsoid is uniformly subnormal to the foliation suggestive of radial flattening textures of the marginal rocks which record high strain coupled with amphibole plagioclase thermometry indicate that deformation occurred in the magmatic or submagmatic state 650 780 c and was accompanied by significant migration of interstitial melt the documented transition from passive to forceful emplacement and final geometry of the suite resulted from temporal changes in the rheology of both the magma and adjacent country rocks pulses of low density hydrous magma were initially emplaced by stoping denser country rocks on cooling and solidification these magmas contracted by 15 20 creating an unpressured volume which allowed subsequent magma pulses to be emplaced into the pluton core these pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas emplacement of the lago della vacca suite was therefore the product of several internal or magma related processes an interaction between stoping radial expansion and volume contraction on solidification of the hydrous magmas the heterogeneous distribution of strain in the marginal units reflects local variations in temperature and most importantly in the distribution of interstitial melt during emplacement

crustal scale en echelon p shear tensional bridges a possible solution to the batholithic room problem the apparent paradox of voluminous granitoid emplacement in an overall compressional magmatic are has been somewhat alleviated by the idea of pluton emplacement in tension cracks and dilational jogs within arc parallel strike slip fault systems as an alternative hypothesis we propose that an en echelon p shear array provides a continuous batholith scale zone of dilation along the trend of active magmatism and that it is favored by transpressional tectonics in the sierra nevada california the late cretaceous granitoids of the cathedral range intrusive epoch are consistent with emplacement into bridges between p shears because 1 they are elongated oblique to the magmatic trend consistent with p shear orientation within a dextral system 2 they were all emplaced within 5 10 m y along a 300 km trend resulting in a displacement rate of 1 cm yr across the arc and 3 syn to late magmatic dextral shear zones bound and crosscut the plutons as in a p shear tensional bridge

ophicalcites from the northern pyrenean belt a field petrographic and stable isotope study brecciated and fractured peridotites with a carbonate matrix referred to as ophicalcites are common features of mantle rocks exhumed in passive margins and mid oceanic ridges ophicalcites have been found in close association with massive peridotites which form the numerous ultramafic bodies scattered along the north pyrenean zone npz on the northern flank of the pyrenean belt we present the first field textural and stable isotopic characterization of these rocks our observations show that pyrenean ophicalcites belong to three main types 1 a wide variety of breccias composed of sorted or unsorted millimeter to meter sized clasts of fresh or oxidized ultramafic material in a fine grained calcitic matrix 2 calcitic veins penetrating into fractured serpentine and fresh peridotite and 3 pervasive substitution of serpentine minerals by calcite stable isotopic analyses o c have been conducted on the carbonate matrix veins and clasts of samples from 12 pyrenean ultramafic bodies we show that the pyrenean ophicalcites are the product of three distinct genetic processes 1 pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity 2 ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity and 3 polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid cretaceous we highlight a major difference between the eastern and western pyrenean ophicalcites belonging respectively to the sedimentary and to the hydrothermal types our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins but a post depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out finally we discuss the significance of such discrepancy in the characteristics of the npz ophicalcites in the frame of the variable exhumation history of the peridotites all along the pyrenean realm

lithospheric anisotropy beneath the pyrenees from shear wave splitting we investigate upper mantle anisotropy beneath the pyrenean range along three n s profiles across the mountain belt the results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere we present the results of two other profiles that ran in 1995 1996 and 1996 1997 in the eastern and western part of the belt respectively and propose an interpretation of the whole results teleseismic shear waves sks skks and pks are used to determine splitting parameters the fast polarization direction and the delay time t teleseismic shear wave splitting in the eastern pyrenees displays homogeneous values trending n100 e and t values in the range 1 1 to 1 5 s a station located in the southern massif central 100 km north of the range is characterized by different splitting parameters n70 e t 0 7 s in the western part of the belt anisotropy parameters are similar across the whole belt n110 e and t 1 3 to 1 5 s most of the measured delay times including those obtained in the central part of the range are above the global average of the sks splitting around 1 s at the belt scale is generally poorly correlated with recent estimations of the absolute plate motion which predicts a fast direction ranging between n50 e and n80 e instead the orientation of n100 e is parallel to the trend of the pyrenean belt but also to hercynian preexisting structures this parallelism supports an anisotropy primarily related to frozen or active lithospheric structures we show that a signature related to the pyrenean orogeny is likely for the stations located in the internal domains of the belt by contrast the anisotropy measured at the stations located on the external parts of the belt could reflect a pre pyrenean hercynian deformation we suggest that a late hercynian strike slip deformation is responsible for this frozen upper mantle anisotropy and that the pyrenean tectonic fabric developped parallel to this preexisting fabric finally no particularly strong splitting is related to the north pyrenean fault commonly believed to represent the plate boundary between iberia and eurasia

thermal control on the modes of crustal thinning leading to mantle exhumation insights from the cretaceous pyrenean hot paleomargins the pre rift mesozoic sequences of the cretaceous passive margins fossilized in the north pyrenean zone npz are characterized by high temperature deformation in relation with thinning of the continental basement our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the pre rift sedimentary cover and the loci of extreme crustal stretching geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic pre rift sediments indicate an early attenuation of the rifted continental crust this leads us to propose a mechanism of rifting involving boudinage of the continental crust the lateral extraction of the paleozoic basement occurred under the pre rift cover that is decoupled on the triassic clays and evaporates the thermal conditions allowing coeval ductile deformation of the crust and of the pre rift sediments leaded to the widening of basins devoid of large faulted blocks we discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high temperature metamorphism of the pre rift sediments in the npz albian cenomanian flysch sequences were deposited synchronously with the syn metamorphic ductile deformation of the pre rift sequences since the base of the flysch deposits also recorded locally the high temperature tectonic event we propose an original mechanism for the evolution of the basins involving continuous deformation of the pre rift metamorphic sediments at the scale of the pyrenean domain our results suggest a strong lateral variability in the tectonic style of passive margins in direct link with their thermic pattern

preorogenic exhumation of the north pyrenean agly massif eastern pyrenees france 1 the pyrenees north of the north pyrenean fault display a complex structure involving a succession of peridotite massifs basement massifs and mid cretaceous to late cretaceous basins located in a narrow domain which was affected by a mid cretaceous preorogenic high temperature low pressure metamorphism the late cretaceous basins were interpreted either as pull apart basins formed during transcurrent motion of iberia relative to eurasia or as remnants of a larger extensional basin recent models support that peridotite massifs result from the exhumation of the mantle during this preorogenic event the northern boundary of the agly basement massif shows evidence of ductile deformation of the basal formations of the agly sedimentary cover macroscopic and microscopic kinematics indicators consistent with asymmetry of crystallographic fabrics suggest normal sense of shear and thus suggest detachment at least partial of the mesozoic cover from its basement triassic to early cretaceous limestones are mylonitic and consistently shows a foliation a ns to ne trending lineation shear criteria suggesting top to the north shearing and locally boudinage at the microscopic scale mylonites are characterized by a very fine grain size frequently 10 m they contain larger partially recrystallized calcite parent grains and undeformed quartz grains with calcite fringes crystallized in pressure shadows in these mylonites calcite systematically shows a weak but well defined crystallographic preferred orientations suggesting ht dislocation creep combined to diffusion creep and possibly grain boundary sliding in the finest fraction of the mylonites paleotemperatures determined using raman spectrometry suggest synkinematic temperature conditions in the range 340 390 c in good agreement with observed microstructures and calcite cpo the mylonitic fabric in mesozoic limestones is folded by ew trending pyrenean folds north of the agly basement massif attesting that this fabric formed before the pyrenean orogeny these data consistently support preorogenic extension under medium temperature conditions of the northern agly massif and likely of the massif itself since simultaneously 100 ma a mid cretaceous basin opened south of the basement massif we suggest a model of preorogenic exhumation of the agly massif in response to the regional extension associated to the rotation of iberia this model may explain the exhumation of the north pyrenean massifs during a single preorogenic event that allowed the opening of extensional basins and the exhumation of the lithospheric mantle all these structures being subsequently reworked during the pyrenean orogeny

evidence for shear zones in the lower crust offshore britain lower crustal reflections seen on british institutions reflection profiling syndicate birps deep seismic reflection data recorded offshore the united kingdom are commonly short convex up and crosscutting consequently the lower crust should not be described as layered as much of the reflecting structure is not fully resolved and may be quite complex moreover the lower crustal reflective zone lcrz is not a uniformly reflective layer but contains reflective bands that wrap around nonreflective transparent zones because transparent zones probably correspond to a lack of a reflecting fabric understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust although transparent zones may have a variety of explanations e g large shear pods or frozen magma bodies the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip line field theory and by drawing analogies with structures developed in highly tectonised high grade metamorphic terrains this interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms in particular extension may be accommodated by the relative movement of low strain lozenges along intervening zones of simple shear the lower crust may be stretched during extension but it must also transfer deformation between the upper crust and the mantle the observation on birps data of mantle reflections interpreted as mantle shear zones implies that deformation in the mantle is localised however the manue reflections are never colinear with faults in the upper crust requiring at least locally a component of sub horizontal bulk strain within the lower crust which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the lcrz

bursting the bubble of ballooning plutons a return to nested diapirs emplaced by multiple processes a popular model for the emplacement of roughly spherical plutons is that of ballooning or in situ inflation of a magma chamber in a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent while the hotter tail of magma continues to rise and expand the already crystallized outer margin this expansion forms a concentric gneissic to mylonitic foliation and flattening type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures our reexamination of three supposedly ballooned plutons ardara ireland cannibal creek australia papoose flat california and evaluation of published descriptions of many others indicate that this model is largely incorrect deflections of country rock structures strains and porphyroblast matrix relationships indicate that only minor to moderate expansion usually 30 or less occurred during emplacement that other emplacement mechanisms must have occurred and that regional deformation continued during and after emplacement internal structures indicate that when magma chamber expansion did occur it did so by flow of magma that magmatic foliations and lineations formed late in the magma chamber evolution that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion and that only minor internal subsolidus deformation results from emplacement this study indicates that many plutons previously interpreted as post tectonic ballooning plutons are better viewed as syntectonic nested diapirs emplaced by a variety of country rock material transfer processes this nested diapir model implies that magma ascent may occur by rise of large magma batches instead of transport in dikes followed by ballooning that magma chamber dynamics differ from that in the ballooning model and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt

mylonite dominated footwall geometry in a shear zone central pyrenees a major ductile shear zone in the gneissic core of the saint barthelemy massif central pyrenees is characterized by an asymmetric shear strain profile defined as mylonite dominated footwall geometry the shear zone is part of a low angle fault system in the massif which caused thinning of a sequence of lithologic units and isograds the shear zone is interpreted as a low angle normal fault zone of probably cretaceous age predating alpine crustal shortening in the central pyrenees

mechanisms for folding of high grade rocks in extensional tectonic settings abstract this review of structures developed in extensional high grade terrains combined with results of centrifuge analogue modelling illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust thickened orogen several extensional fold mechanisms such as folding within detachment shear zones are similar to those in contractional settings the metamorphic p t t path and not fold style or mode of formation is therefore required to determine the tectonic setting in which some folds developed other mechanisms such as rollover above and folding between listric normal shear zones and folding due to isostatic adjustments during crustal thinning are unique to extensional tectonic settings several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation it is shown that isoclinal recumbent folds refolded by open upright folds may develop during regional extension in the deep crust folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment or from enhanced back rotation of layers between normal shear zones during back rotation folding layers rotated into the shortening field undergo further buckle folding and all may rotate towards orthogonality to the maximum shortening direction this mechanism explains the presence of many transposed folds folds with axial planar pegmatites and folds with opposite vergence in extensional terrains examples of folds in high grade rocks interpreted as forming during regional extension included in this paper are from the grenville province of canada norwegian caledonides albany mobile belt and leeuwin complex of western australia ruby mountains in the basin and range province of nevada the at sund area of greenland the napier complex of enderby land in east antarctica and the kigluaik mountains in western alaska

formation of the trimouns talc chlorite deposit pyrenees from persistent hydrothermal activity between 112 and 97 ma for many economically important deposits precise duration and age of hydrothermal ore formation are poorly known we tackle these questions for the large trimouns talc chlorite deposit by u pb analyses of xenotime and monazite which crystallized within cm size geodes in the hanging wall dolostones formation of geodes and ree minerals occurred in relation to 300 350 c 0 2 0 3 gpa metasomatism along a host shear zone sixteen xenotime fragments broken off from four single euhedral grains define an age range from 112 to 97 myr and a monazite age of 99 myr some ages of individual fragments are grain specific whereas others show the same range of dates given the analytical uncertainties on individual fragments about 1 myr the absence of inherited components and the fact that no thermal event has affected these crystals after their formation these ages are interpreted to record continuous crystallization between 112 and 97 ma this means that the trimouns deposit was produced by hydrothermal activity lasting for at least 16 million years occurring along a shear zone through which fluids continuously or episodically circulated to allow ion transport from the country gneisses to the deposit locality persistent deformation and high heat flow related to the transtensional regime that opened the bay of biscay seems to be the most likely tectonic scenario to explain this activity

deformation history inferred from magnetic fabric in the southwestern okcheon metamorphic belt korea abstract magnetic fabric and rock magnetic studies have been carried out for the non fossiliferous low to medium grade metasedimentary rocks in the southwestern part of the okcheon metamorphic belt omb two major metamorphic events in the study area were previously recognized regional metamorphism m 1 in the late carboniferous to early permian and contact metamorphism m 2 due to the intrusion of granite in the middle jurassic the metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones magnetic foliation is the dominant magnetic fabric in the biotite zone while magnetic lineation prevails in both garnet and staurolite zones we interpret the metamorphism related deformation history of the study area based on magnetic fabrics magnetic mineralogy and previously reported metamorphic evolution as follows penetrative nw dipping cleavage represented by magnetic foliation was formed in the study area by prevailing nw se shortening event during the m 1 regional metamorphism in the late carboniferous early permian this shortening event is interpreted to be associated with the collisional event between the north and south china blocks cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of jurassic granite in the northwestern area

cretaceous albitization and dequartzification of hercynian peraluminous granite in the salvezines massif french pyrenees abstract the salvezines massif in northern french pyrenees has undergone extensive albitization the massif consists of gneisses and a leucogranite hosted in paleozoic sediments schists and carbonates the leucogranite was emplaced at the end of the hercynian orogeny the granite underwent monazite fractionation during magmatic stages which induced lowering of ree and th contents in the most evolved rocks hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the ree patterns of the granite and through the fractionation of ratios like zr hf out of the charac charge radius controlled range the granite underwent partial to complete sub solidus albitization progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz episyenitization together with new albite formation hydrothermal muscovite with a sheaf like structure sometimes crystallised in albitites inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic the initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one a cnk 1 albitization induced gains in na and al and losses in si and k leaching of ree and u is also identified during albitization as well as the non charac behaviour of some elements 18o values of quartz and muscovite in albitized granites are in the magmatic range about 12 and 9 5 respectively feldspars acquired high 18o values during albitization up to 15 muscovite grains from albitites have higher 18o values than inherited ones and tend towards isotopic equilibrium with albite the albitizing fluids have interacted with high 18o rocks probably local carbonates prior to alter granite muscovites inherited from magmatic stages display very characteristic disturbed 39ar 40ar age spectra with saddle shapes when a muscovite associated with albitization provided a plateau age at 117 5 ma for this alteration event albitization in the salvezines massif just preceded the regional talc chlorite hydrothermal mineralization both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the iberian plate around europe the north pyrenean metamorphism is identified by 39ar 40ar analyses at ca or younger than 100 ma

modification of the regional stress field by magma intrusion and formation of tabular granitic plutons granitic intrusions typically have large aspect ratios with an horizontal major axis about 4 8 times the vertical one combined structural and geophysical gravity data allows distinction between two main types of plutons flat floored plutons the most represented are rather thin 3 4 km and extend in every horizontal direction with a gently dipping floor toward several root zones these contrast with the thick 10 km wedge shaped plutons more elongated in one direction with a few root zones we interpret the shape of flat floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma magma is preferentially emplaced into the plane 1 2 perpendicular to the least principal stress component 3 this plane is initially vertical except for compressional conditions this dilation causes a local re organization of the stress field by increasing the minor and intermediate principal stress components when they overcome the lithostatic load a drastic change in the orientation of the opening plane results switching from vertical to horizontal this constitutes a change from vertically oriented dike shaped intrusions to sub horizontal laccoliths crustal anisotropies vertical faults or horizontal zones of different rheology also contribute to modify the shape of the intrusions the continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings rather we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement

geophysical and structural signatures of syntectonic batholith construction the south mountain batholith meguma terrane nova scotia summary the late devonian south mountain batholith is a very large 7000 km2 composite peraluminous granitoid complex situated within the meguma terrane of the northern appalachians it is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380 370 ma during the acadian orogeny i e during the collision of gondwana with the eastern margin of north america a significant geophysical and geological database makes the south mountain batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen gravity models reveal the plutons have flat or gently dipping floors at approximately 7 0 km depth and aspect ratios 651 they are underlain by deeper 10 km elongate northeast southwest trending roots that may indicate magma feeder zones dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale the very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement space was mostly created by vertical displacements of country rocks the data are consistent with a laccolithic model for syntectonic batholith assembly the laccolithic plutons may have been emplaced at the base of the meguma supergroup metasedimentary rocks suggesting a maximum thickness of approximately 7 0 km for the supracrustal rocks in the meguma terrane

structural interplay between plutons during the construction of a batholith cerro aspero batholith sierras de cordoba argentina magnetic fabric patterns and microstructures of granitic rocks provide evidence of structural modifications as a result of the coalescence of two plutons alpa corral and talita during the construction of a large granitic batholith cerro aspero in the sierras pampeanas of cordoba argentina the alpa corral granite is a small and nearly circular pluton while the larger talita pluton displays a marked n s elongation the nw quadrangle of the alpa corral pluton underwent a deformation resulting in foliation trajectories that outline a crescent moon pattern whose inner arc is concordant with the alpa corral talita contact the highest values for the magnetic susceptibility and its anisotropy are concentrated in the coalescence zone between both plutons the magmatic and sub magmatic nature of the microstructures demonstrates that such a deformation was caused by the southward overriding of the largest talita pluton while the alpa corral pluton was not yet fully crystallized rb sr geochronology indicates that this process took place 369g9 ma ago long after the famatinian ordovician to early devonian deformation of the country rocks which confirms a synmagmatic rather than a regional deformational event q 2006 elsevier ltd all rights reserved

styles of extension offshore mid norway and implications for mechanisms of crustal thinning at passive margins 1 eocene magmatic breakup along the mid norway rifted margin was preceded by extreme jurassic cretaceous crustal thinning in a magma poor environment along the se borders of the rift top basement detachment faults with heaves on the order of 15 40 km evolved in at least two stages to become the boundaries between moderately thinned 20 30 km thick crust and a 100 200 km wide highly extended area with crustal thicknesses generally between 2 and 12 km under the present day more and voring basins in the footwalls of the basin flank detachments lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults under the most highly thinned areas a more distal set of deep seated basin floor detachments incised and extended remnant crust and probably the upper mantle leaving as little as 5 km of continental crust to be preserved under thick synrift and postrift deposits we suggest that basin flank detachments such as the ones described above hold the potential to reduce the crustal thickness down to the critical value required for embrittlement of continental crust thus they prepare the ground for incision by the basin floor detachments which may become responsible for exhumation of deep crust or continental mantle if extension is allowed to proceed

deep structure of the west african continental margin congo zaire angola between 5 s and 8 s from reflection refraction seismics and gravity data summary the deep structure of the west african continental margin between 5 s and 8 s was investigated using vertical reflection and wide angle reflection refraction techniques during the zaiango project a joint programme conducted in 2000 april by ifremer and totalfinaelf to penetrate below the salt layer a non conventional low frequency seismic source was used in the single bubble mode together with ocean bottom instruments hydrophones and seismometers and a 4 5 km long streamer that recorded multichannel seismic reflection mcs the data show that the continental crust thins abruptly over a lateral distance of less than 50 km from 30 km thick below the continental platform based on gravity data to less than 4 km thick below the lower congo basin that formed prior to the aptian salt deposition this subsalt sedimentary basin 180 km wide 4 km thick with velocities varying from 4 7 km s 1 to 5 8 km s 1 at the bottom is located between the foot of the continental slope and the oceanic domain it is underlain by crust of an intermediary or transitional type between continental crust and what can be recognized as oceanic crust in the transitional zone a crustal upper layer is present below the pre salt sedimentary basin 3 to 7 km thick with velocities increasing from 5 8 km s 1 at the top to 6 8 km s 1 at the bottom of the layer this layer appears to thin regularly from 6 7 km thick below the depocentre of the pre salt sedimentary basin to 3 4 km thick below the western termination of the basin below this upper crustal layer an anomalous velocity layer 7 2 to 7 8 km s 1 is documented below the eastern side of the basin where the crustal thinning is at a maximum the origin of this layer is unknown several arguments like rifting duration between 15 ma and 30 ma or the absence of seaward dipping reflectors precludes the hypothesis of underplated mantle material but other hypotheses such as serpentinized material or high grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks are plausible near the ocean termination of the basin the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles 7 11 and 14 having a dense ocean bottom seismometer hydrophone obs obh spacing on these profiles an anomalous velocity layer is present in the westernmost part of the transitional zone below the basement ridge and in the oceanic domain this layer absent on profile 3 may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process

3d architecture of a complex transcurrent rift system the example of the bay of biscay western pyrenees abstract the parentis and arzacq mauleon basins located in front of the v shaped oceanic propagator in the bay of biscay present evidence for extreme crustal thinning in this paper we investigate the 3d structure of these rift basins based on field observations and the interpretation of seismic data we compare these results with those obtained from two different and independent inversion methods first a 3d gravity inversion and second the standard euler deconvolution for the mauleon basin our results show that the positive gravimetric anomaly identified above its southern margin is the consequence of two shallower high density bodies that are separated by the pamplona fault and a deeper high density body the high density bodies can be explained by the presence of mid crustal and mantle rocks that were exhumed or uplifted at shallower depth during early cretaceous rifting before they were reworked and integrated to the pyrenean chain during compression phase also during this reactivation phase some slices of the exhumed mid crustal and mantle rocks were sheared off and were integrated in the present day thrust belt in the mauleon basin for the parentis basin we can demonstrate based on seismic data and gravimetric inversion methods a decrease in extension from west to east which is compatible with the v shape geometry of the overall basin along strike a change in the fault geometry from downward concave top basement detachment faults to upward concave high angle faults can be observed eastwards i e towards the termination of the basin a key structure controlling the evolution of the parentis basin is the east west trending ibis fault we interpret this fault to have initially formed as a strike slip fault before it was reactivated during later crustal thinning at present it forms the limit between an upper plate sag basin to the north and a lower plate sag basin floored at least locally by a top basement detachment faults to the south the strong asymmetry of the basin is supported by the shape of the basin and the results of standard euler deconvolution

rock and magnetic fabric of the renca batholith sierra de san luis argentina constraints on emplacement abstract the renca batholith belongs to a group of post collisional plutons that intruded the early paleozoic basement of the sierra de san luis during late devonian to early carboniferous combined microstructural petrofabric and anisotropy of magnetic susceptibility ams studies indicate that interactions between magmatism and emplacement related fabrics played the main role in the strain patterns with a very subordinate far field control by regional deformation this concentrically foliated zoned elliptical granitoid is made up by porphyric granodiorites and equigranular monzogranites that are associated with syenites and monzonites two major magmatic associations are present i a mg k monzonitic association represented by syenitic monzonitic and monzodioritic enclaves and synplutonic dykes and a porphyroid monzodioritic to melagranodioritic facies and ii a high k calc alkaline association that constitutes the major part of the concentrically zoned batholith with an external porphyroid granodiorite to monzogranite unit 1 and a central equigranular monzogranite unit 2 bulk susceptibility k shows a bimodal distribution that corresponds respectively to unit 1 ferromagnetic and unit 2 paramagnetic similar distribution is observed in the intensity of natural remanent magnetization nrm and ams degree p the steep to vertical inward dipping foliation that is recognized in the porphyroid granitoids is parallel to the x y flattening plane of the enclaves the synplutonic dykes attitude and the contact between the two major units of the batholith the ams defines a predominantly foliated fabric with a distribution of magnetic foliation planes generally subvertical and parallel to the boundaries of the ring like unit 1 these internal features are concordant with the axial planes of the meso and microscale folding of the regional nne s2 s3 in the ductile aureole that developed in the medium grade metamorphic ordovician country rocks foliation results from a combination of both magmatic and high temperature solid state flows the latter as the result of the emplacement of the central monzogranite which caused a radial compression k 0 as measured using strain indicators suggests flattening unit 2 shows a planar magnetic fabric shallowly dipping outwards and dominant subhorizontal lineations the magnetic fabrics of this body agree with other fabric parameters and are consistent with a magmatic fabric there is a strong correlation between rock and magnetic fabrics these fabrics reflect internal magma dynamics whereas regional tectonic exerted a far field control on the expansion of the pluton

emplacement of the ardara pluton ireland new constraints from magnetic fabrics rock fabrics and age dating the ardara pluton as part of the donegal batholith was intruded into neoproterozoic metasediments and metadolerites at mid crustal levels the emplacement mechanism of the ardara granite is very controversial and mechanisms ranging from diapirism ballooning and stoping followed by nested diapirism have been proposed magnetic fabrics rock fabrics and k ar dating of micas are used here to constrain the emplacement history the compositional zoning of the ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite whereas the intervening tonalite is of intermediate nature the magnetic carriers are characterized by the anisotropy of the magnetic susceptibility ams thermomagnetic measurements and through high field analyses hfa the separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the ams in the quartz monzodiorite both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite biotite is responsible mainly for the ams in the tonalite and granodiorite the magnetic foliation can be directly related to the macroscopic foliation and also to the d4 structures in the country rocks the foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip towards the central granodiorite the magnetic foliation dies out although plagioclase texture measurements indicate a weak magmatic shape fabric with the exception of the tail the kmax axes magnetic lineation vary from steeply to gently plunging the so called lineation factor is approximately 1 01 and therefore points to a less significant axial symmetry these observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of k 0 flattening strain texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high temperature solid state conditions the age of the intrusion is still under discussion but a new cooling age was determined by k ar dating of biotite at 403 7 8 ma corresponding to a temperature range between 450 and 300 c for a mylonite along the southern contact between the ardara pluton and the country rock a k ar muscovite age of 378 8 7 ma indicates a minimum age for the shear zone when the ardara pluton must have already been cooled down below 350 50 c

sediment routing system and sink preservation during the post orogenic evolution of a retro foreland basin the case example of the north pyrenean aquitaine bay of biscay basins abstract we investigated here the evolution of the sediment routing system i e the sediment transport and deposition evolution along successive depositional topographies and environments and the sink i e deposited sediments preservation in a foreland basin from the period of mountain belt shortening to its post orogenic stage the studied system is the north pyrenean retro foreland basin from 50 ma to today which is composed of a subsiding platform the aquitaine basin fed by the erosion of the pyrenees passing laterally to a slope and a deep sea plain the bay of biscay deep basin the ultimate area of deposition this study is based on a double seismic stratigraphic and structural analysis of an extensive seismic dataset and on an age model of the sediments combining biostratigraphy orbitostratigraphy and sequence stratigraphy with a time resolution of 0 1 ma four major periods of deformation corresponding to a single or a set of stratigraphic sequence boundaries were characterized the pyrenean shortening decrease may be recorded by a basin scale uplift at 49 8 ma late ypresian the paroxysm of the piggy back shortening and related uplift is dated at 35 8 ma priabonian the end of the pyrenees shortening transition to post orogenic conditions is well dated between 27 1 and 25 2 ma chattian a major west european scale deformation of possible mantle origin uplifted the basin from 16 4 to 10 6 ma late burdigalian to early tortonian these major periods of deformations controlled the sink preservation at the first order through the ratio between the accommodation space created by subsidence on the aquitaine platform asub and the siliciclastic sediment supply coming from the erosion of the pyrenees ssc a general model is proposed at the time of the foreland basin when asub ssc most of the sediments are preserved on the platform as a progradational aggradational wedge up to 25 2 ma here at the time of the post foreland evolution when asub the continental sediment routing system is mainly provided along nearly flat alluvial plains with extensive lakes and or humid zones i e the local base levels of the alluvial fans the aquitaine retro foreland was never overfilled

depositional environments provenance and paleoclimatic implications of ordovician siliciclastic rocks of the thango formation spiti valley tethys himalaya northern india abstract recently published findings indicate that the ordovician period has been much more dynamic than previously anticipated thus making this period significant in geological time the ordovician of india can best be studied in the spiti region because the spiti basin records the complete uninterrupted history of excellent marine sedimentary rocks starting from cambrian to paleogene which were deposited along the northern margin of india due to these reasons the geochemical data on the ordovician rocks from the spiti region is uncommon the present geochemical study on the ordovician thango formation sanugba group is mainly aimed to understand the provenance and the paleoclimatic conditions the sandstones are the dominant lithology of the thango formation with intercalations of minor amount of shales detailed petrographic and sedimentological analysis of these rocks suggest that three major depositional environments viz fluvial transitional and marine prevailed in the basin representing transgressive and regressive phases the major and trace element ratios such as sio2 al2o3 k2o na2o and la th sc discrimination diagram suggest that these rocks were deposited in passive margin tectonic settings various geochemical discriminants and elemental ratios such as k2o na2o al2o3 tio2 la sc th sc cr th zr sc gd yb n and pronounced negative eu anomalies indicate the rocks to be the product of weathering of post archean granites the striking similarities of the multi elemental spider diagrams of the studied sediments and the himalayan granitoids indicate that sediments are sourced from the proterozoic orogenic belts of the himalayan region chemical index of alteration cia values of the studied sediments 55 72 suggest that the source rocks underwent low to moderate degree of chemical weathering the span of the cia values 55 72 recorded in the sediments from the spiti region may have resulted from varying degrees of weathering conditions in the source area this in turn reflects variable climatic conditions prevailing during the ordovician period which is consistent with the global studies the high cia values observed by numerous studies on siliciclastic sediments may imply warm paleoclimate attributed to an increase in atmospheric carbon dioxide and or methane levels inducing an acidic weathering environment and enhanced silicate weathering while the low cia values indicates prevalence of arid glacial conditions where negligible chemical alteration is possible this study documents the noteworthy events like fluctuating rise and fall atmospheric co2 and sea levels during the ordovician period a conclusion which is consistent with the other such studies on the phanerozoic glaciation or mass extinction events on gondwana supercontinent

magnetic fabric and microstructures of late paleozoic granitoids from the north patagonian massif evidence of a collision between patagonia and gondwana abstract widespread late paleozoic magmatism in northern patagonia is a target to test hypotheses on the long standing question over the origin of patagonia in recent years a dispute over whether it is an accreted crustal block that collided with gondwana in paleozoic times or an autochthonous part of south america has taken place as part of a multidisciplinary study an integrated microstructural and magnetic fabric study was carried out on the late carboniferous yaminue complex and the early permian navarrete plutonic complex both exposed in the northeastern corner of the north patagonian massif 40 5 s 67 0 w other investigated units are the late carboniferous tardugno granodiorite the newly defined cabeza de vaca granite and the late permian san martin pluton over 300 oriented cores from 60 sites were collected for anisotropy of magnetic susceptibility ams measurements a systematic analysis of around 100 petrographic thin sections was performed to characterize the microstructures of the different magmatic units microstructures in the yaminue complex are indicative of a transition from magmatic to solid state deformation microstructures of the orthogneiss of tonalitic composition suggest an early stage in the emplacement history of this complex the cabeza de vaca granite intrusive in yaminue complex is the most evolved unit and records less intense high temperature solid state deformation which suggests that the stress field that controlled the emplacement of the yaminue complex outlasted it according to petrologic and structural considerations the navarrete plutonic complex has been subdivided into three facies i e robaina guanacos and aranda respectively microstructures of the navarrete plutonic complex are mostly magmatic to submagmatic versus the solid state fabric that characterizes the robaina facies at the contact with the yaminue complex combined analyses of ams and microstructural data lead us to suggest that the yaminue complex cabeza de vaca granite and possibly tardugno granodiorite were intruded during a major compressional event associated with top to the ssw thrusting this event is most likely related to a frontal collision of the north patagonian massif and the southwestern gondwana margin at around 300 ma the navarrete plutonic complex and san martin pluton were emplaced after that tectonic event which must have ended by 281 ma previous magmatic geochronological and paleomagnetic data that suggest close connection of the north patagonian massif with the south american gondwana blocks during the paleozoic can be reconciled with a late paleozoic collision by a model of a para autochthonous north patagonian massif that rifted away from gondwana after the ordovician and collided again in the late carboniferous early permian

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

mantle exhumation crustal denudation and gravity tectonics during cretaceous rifting in the pyrenean realm sw europe insights from the geological setting of the lherzolite bodies 1 the pyrenean peridotites lherzolites form numerous small bodies of subcontinental mantle a few meters to 3 km across exposed within the narrow north pyrenean zone npz of mesozoic sediments paralleling the north pyrenean fault recent studies have shown that mantle exhumation occurred along the future npz during the formation of the albian cenomanian pyrenean basins in relation with detachment tectonics this paper reviews the geological setting of the pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the bearn region only two types of geological settings have to be distinguished among the pyrenean ultramafic bodies in the first type sedimented type or s type the lherzolites occur as clasts of various sizes ranging from millimetric grains to hectometric olistoliths within monogenic or polymictic debris flow deposits of cretaceous age reworking mesozoic sediments in dominant proportions as observed around the lherz body in the second type tectonic type or t type the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed triassic rocks and have fault contacts with units of deformed jurassic and lower cretaceous sediments belonging to the cover of the npz these deformed mesozoic formations are not older that the aptian early albian they are unconformably overlain by the albian cenomanian flysch formations and have experienced high temperature low pressure mid cretaceous metamorphism at variable grades such a tectonic setting characterizes most of the lherzolite bodies exposed in the western pyrenees these geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the mesozoic cover in the preorogenic evolution of the pyrenean realm in the light of such evidence a simple model of basin development can be inferred involving extreme thinning of the crust and mantle uprising along a major detachment fault we demonstrate coeval development of a crust mantle detachment fault and generalized gravitational sliding of the mesozoic cover along low angle faults involving triassic salt deposits as a tectonic sole this model accounts for the basic characteristics of the precollisional rift evolution in the pyrenean realm

syn acadian emplacement model for the south mountain batholith meguma terrane nova scotia magnetic fabric and structural analyses the anisotropy of magnetic susceptibility and the structural geology of the late devonian south mountain batholith meguma terrane nova scotia are used to characterize its emplacement and structural evolution the south mountain batholith is the largest peraluminous granitoid complex within the appalachian orogen it is a composite batholith made up of stage 1 granodiorites and monzogranites and stage 2 leucomonzogranites and leucogranites the anisotropy of magnetic susceptibility was used to map the biotite petrofabric at the scale of the batholith the biotite fabric pattern is similar to the pattern of folding and shearing of the feldspar megacryst foliation documented at the outcrop scale in the stage 1 plutons the magnetic foliation is deformed into a girdle about the horizontal northeast southwest trending magnetic lineation which is parallel to regional fold axes and extension lineations in the country rocks in the stage 2 plutons a horizontal northeast southwest to east west trending magnetic lineation is also present but the horizontal orientation of the magnetic foliation is widely preserved the folding and shearing of the foliations and the horizontal northeast southwest trending lineation that is pervasive throughout the south mountain batholith are interpreted to record acadian tectonic deformation of the plutons as they crystallized acadian deformation continued during the latest stages of crystallization resulting in the localization of mineral deposits and greisens along shear zones faults and joints a model is presented for the syntectonic emplacement of the south mountain batholith as a laccolithic complex

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

early paleozoic orogenic belt of the andes in southwestern south america result of laurentia gondwana collision the late precambrian to early paleozoic age rock units of the pampean ranges the puna and the north patagonian massif of southwestern south america constitute the famatinian orogenic belt they are interpreted as an ordovician collisional belt between the occidentalia terrane and the gondwana craton they include mafic and ultramafic belts of neoproterozoic to early paleozoic age an intense tectonothermal event resulted from the collision syntectonic granitoids represent crustal melting in that collision syntectonic to late tectonic foreland basins developed the recently proposed juxtaposition of laurentia and east antarctica australia in the neoproterozoic raises the possibility that laurentia and western south america were close together in the early paleozoic and therefore that the famatinian belt resulted from laurentia gondwana collision occidentalia which is bordered by a cambrian carbonate platform similar to that of eastern north america may be a sliver detached from laurentia during late ordovician time

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

submagmatic microfractures in granites microstructures in hercynian granite plutons of the pyrenees indicate fracturing of plagioclase crystals in the presence of melt this is represented mainly by quartz and or feldspar that fills fractures electron microprobe analyses demonstrate that in compositional agreement with the rims of the zoned plagioclase the feldspar infillings are more sodic with decreasing melt fraction fracturing resulted from concentration of stress at contacts between grains while the granite was submagmatic i e when the melt was below the critical fraction for magmatic flow material compositionally far from eutectic is petrologically the best candictate to develop submagmatic fractures because during cooling the volume fraction of melt remains low for a large temperature range a strong tendency for microfractures to be oblique to flow foliation may constitute a new criterion for shear sense in magmas

petrogenesis of the postcollisional middle devonian monzonitic to granitic magmatism of the sierra de san luis argentina abstract middle devonian granitoids intruded the eastern sierras pampeanas basement ca 600 km east of the inferred proto pacific margin of gondwana along which a ca 390 ma collisional event developed in the sierra de san luis voluminous middle devonian 393 382 ma batholiths are composed of i to a type hybrid monzonite and granite suites shoshonite and subordinated high k series stocks synplutonic dikes and enclaves make up the monzonite suite rocks are metaluminous alkali calcic magnesian porphyritic or equigranular monzonite quartz monzonite monzodiorite and scarce monzogabbro high k and subordinated shoshonite series metaluminous to mildly peraluminous magnesian alkali calcic to calc alkalic porphyritic or equigranular quartz monzonite granodiorite monzogranite and equigranular leucomonzogranites make up the granite suite plutons and batholiths only a small group of highly evolved granites are ferroan sio 2 46 62 cr ni v sc lile lree th zr and variable sr y la yb n and tb yb n smooth eu eu moderate na 2 o ca 3 5 and troughs at nb and ta for monzonite suite rocks suggest an subduction related enriched lithospheric mantle source sm nd data t dm 0 98 1 08 ga end 380 ma 0 66 1 47 and 87 sr 86 sr i 0 703520 0 704203 are compatible with an enriched mantle source the metaluminous porphyritic quartz monzonite monzogranite and the mildly peraluminous equigranular biotite monzogranites of the granite suite are characterized by relatively moderate al 2 o 3 cao and 87 sr 86 sr i high lile cr variable sr y la yb n and eu eu and low rb sr dm 1 20 1 28 ga end 380ma 3 02 to 3 3 87 sr 86 sr i 0 706578 0 707027 and the biotite monzogranites t dm 1 31 ga end 380ma 3 3 87 sr 86 sr i 0 707782 would share a common source the equigranular alkali calcic leucomonzogranites are characterized by rb sr 1 5 asi 1 05 1 18 and ga al 2 6 3 9 end 380 ma 3 74 to 3 95 and 87 sr 86 sr i 0 710743 0 712955 which would point to metasedimentary or felsic igneous crustal sources nevertheless their t dm 1 36 1 38 ga is considerably younger than the mean 1 8 1 6 ga eastern sierras pampeanas crustal residence age and less radiogenic middle devonian magmatism would record an episode of crustal growth by enriched mantle derived magma input and variable degrees of partial melting of a lower crustal source at the waning stages of the achalian orogeny

the las chacras potrerillos batholith pampean ranges argentina structural evidences emplacement and timing of the intrusion within the southern part of the sierra pampeanas the sierra de san luis argentina a series of extensive intrusive bodies are regarded to post date the famatinian cycle but were emplaced during the achalian a period of heterogeneous deformation along crustal scale fault zones the largest of those is the las chacras potrerillos batholith that is situated at the northern end of the transpressive sinistral guzman shear zone this composite pluton exhibits three sub domains that comprise two granitoid sub units each the southern potrerillos stock muscovite bearing red granite and biotite bearing red granite and the central biotite porphyritic granite and giant porphyritic granite and northern domain equigranular granite and porphyritic granite of the las chacras stock the crystallisation ages of the biotite porphyritic granite is around 381 ma u pb on zircons and pb pb on sphene while the host rock was already cooled below 350 c at 420 ma thermal modelling approaches favour a pulsed intrusion with a duration of 1 5 ma the emplacement was followed by rapid cooling below the muscovite cooling temperature biotite cooling ages in different sub units reflect either a long lasting cooling history of approximately 30 ma which is supported by the modelling or a reheating effect at around 350 ma devonian age determinations on the fault rocks and granitoids point to a syn tectonic emplacement of the batholith the pluton is interpreted to be positioned at the crossover of sinistral shear zones the origin of this nne directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along a secondary set of nnw trending sinistral faults the final emplacement is due to a subsequent ballooning of the batholith following the direction of space creation this model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field as well as magnetic fabric data our results indicate that the emplacement is syn kinematic with respect to the achalian deformation event

retrograde metamorphic reactions in deforming granites and the origin of flame perthite microprobe analyses of feldspars in granite mylonites containing flame perthite give compositions that invariably plot as three distinct clusters on a ternary feldspar diagram orthoclase or92 97 albite and oligoclase andesine the albite occurs as grains in the matrix as flame shaped lamellae in orthoclase and in patches within plagioclase grains we present a metamorphic model for albite flame growth in the k feldspar in these rocks that is related to reactions in plagioclase rather than alkali feldspar exsolution flame growth is attributed to replacement and results from a combination of two retrograde reactions and one exchange reaction under greenschist facies conditions reaction 1 is a continuous or discontinuous across the peristerite solvus reaction in plagioclase in which the an component forms epidote or zoisite most of the albite component liberated by reaction 1 stays to form albite in the host plagioclase but some na migrates to form the flames within the k feldspar reaction 2 is the exchange of k for na in k feldspar reaction 3 is the retrograde formation of muscovite as sericite and has all of the chemical components of a hydration reaction of k feldspar the si and al made available in the plagioclase from reaction 1 are combined with the k liberated from the k feldspar to produce muscovite in reaction 3 the muscovite forms in the plagioclase rather than the k feldspar as a result of the greater mobility of k relative to al the composition of the albite flames is controlled by both the peristerite and the alkali feldspar miscibility gaps and depends on the position of these solvi at the pressure and temperature that existed during the reaction using an initial plagioclase composition of an20 the total reaction can be summarized as 20 oligoclase 1 k feldspar 2 h2o 2 zoisite muscovite 2 quartz 15 albiteplagioclase 1 albiteflame this model does not require that any additional feldspar framework be accreted at replacement sites na and k are the only components that must migrate a significant distance e g from one grain to the next allowing al to remain within the altering plagioclase grain the resulting saussuritization is isovolumetric the temperature and extent of replacement depends on when and how much water infiltrates the rock the fugacity of the water and therefore the pressure of the fluid may have been significantly lower than lithostatic during flame growth

magnetic evidence for slow seafloor spreading during the formation of the newfoundland and iberian margins there is considerable debate concerning the nature and origin of the thin crust within the ocean continent transition oct zones of many passive non volcanic continental margins located between thinned continental and true oceanic crust this crust is usually found to be underlain by upper mantle material of 7 2 7 4 km s velocity at shallow depths 1 2 km it has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading one of the examples of occurrence of such a crust are the conjugate margins of newfoundland and iberia here we present an interpretation of magnetic data from these regions to show that their oct zones are underlain by crustal material formed by slow seafloor spreading 6 7 mm yr soon after iberia separated from the grand banks of newfoundland in the late jurassic similarities in the magnetic anomalies and velocity distributions from these regions with those from the sohm abyssal plain a region lying immediately south of the newfoundland basin and formed by seafloor spreading at a similar rate of spreading give further support to such an interpretation the idea that these regions were formed by unroofing of upper mantle during rifting of iberia from newfoundland may be likely but the presence of weak magnetic anomalies in these regions which bear all the characteristics of seafloor spreading anomalies makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading the similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the north atlantic and that of the labrador sea suggest that this oct velocity structure may be the norm rather than the exception across those passive non volcanic margins where the initial seafloor spreading was slow furthermore the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges fl 2000 elsevier science b v all rights reserved

basement cover decoupling and progressive exhumation of metamorphic sediments at hot rifted margin insights from the northeastern pyrenean analog we compile field data collected along the eastern part of the north pyrenean zone npz to point to a tectonic evolution under peculiar thermal conditions applying to the basin sediments in relation with the opening of the cretaceous pyrenean rift based on this compilation we show that when thinning of the continental crust increased isotherms moved closer to the surface with the result that the brittle ductile transition propagated upward and reached sediments deposited at the early stage of the basin opening during the continental breakup the pre rift mesozoic cover was efficiently decoupled from the paleozoic basement along the triassic evaporite level and underwent drastic ductile thinning and boudinage we suggest that the upper albian and upper creta ceous flysches acted as a blanket allowing temperature increase in the mobile pre rift cover finally we show that continuous spreading of the basin floor triggered the exhumation of the metamorphic ductily sheared pre rift cover thus contributing to the progressive thinning of the sedimentary pile in a second step we investigate the detailed geological records of such a hot regime evolution along a reference section of the eastern npz we propose a balanced restoration from the mouthoumet basement massif north to the boucheville albian basin south this section shows a north to south increase in the ht pyrenean imprint from almost no metamorphic recrystallization to more than 600 c in the pre and syn rift sediments from this reconstruction we propose a scenario of tectonic thinning involving the exhumation of the pre rift cover by the activation of various detachment surfaces at different levels in the sedimentary pile in a third step examination of the architecture of current distal passive margin domains provides confident comparison between the pyrenean case and modern analogs finally we propose a general evolutionary model for the pre rift sequence of the northeastern pyrenean rifted margin

crustal structure and orogenic material budget in the west central pyrenees surface and subsurface data are combined to construct a crustal scale cross section of the western central pyrenees france and spain at the boundary between the european and iberian plates the position of moho reflections in the ecors arzacq reflection profile suggests a tectonic wedge of european crust and upper mantle had indented the iberian plate at lower crustal levels the european wedge is overlain by an upper iberian wedge thus constituting a double stacked wedge geometry the upper wedge was delaminated and deformed giving rise to the pyrenean orogenic prism manifested as a bivergent fan in upper crustal levels the underthrust lower iberian plate has been imaged to depths of 55 60 km but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km total orogenic contraction calculated from surface structures is about 75 80 km which was accomplished between the latest cretaceous and the early miocene at an averaged rate of 1 2 mm yr consistent with these moderate values exhumation of the orogen is much less than in the more shortened eastern parts of the range paleozoic basement and preorogenic mesozoic rocks are little eroded and much of the exhumation involved cannibalization of early foreland basins which once covered the entire poorly emergent orogen at this transect this resulted in a continuous process of sediment recycling and coupled with a considerable lateral arrival of material a bulk negative erosion sedimentation budget

pyrenean orogeny and plate kinematics 1 the evolution of the pyrenees a mountain range between iberia and eurasia has remained the subject of many debates between geologists and geophysicists for a long time by combining the identification of seafloor spreading anomalies a33o to m0 in the bay of biscay with those in the north atlantic we have derived a position of a mean pole of rotation for the entire opening of the bay of biscay four hundred kilometers of shortening took place between the iberian and eurasian plates in the pyrenean domain during the opening of the bay of biscay from chrons m0 to a33o time 118 to 80 ma the deep seismic etude continentale et oceanique par reflexion et refraction sismique ecors profile shot across the pyrenees and teleseismic data show the presence of two distinct slabs which dip to the north the southern slab is linked to the subduction of the neo tethys ocean which was created from late jurassic to early aptian simultaneously elongated back arc basins formed along the future pyrenean domain this slab was active from at least 118 ma early aptian to 100 ma late albian the northern slab active since 85 ma is linked to the subduction of the lower continental crust located south of the pyrenean domain in the upper crust normal faults as well as the north pyrenean fault became reverse faults and former back arc basins were inverted giving rise to the uplift of the pyrenees as a double asymmetrical wedge