geometry and regional significance of neoproterozoic cadomian structures of the narcea antiform nw spain the narcea antiform is located at the hinterland to foreland transition of the variscan belt in the nw of the iberian massif it consists of an antiformal stack of fault propagation folds that developed in the hanging wall of allochthonous thrust sheets during the first variscan deformation event in the palaeozoic rocks the neoproterozoic narcea slates formation cropping out in the core of the narcea antiform records a cadomian deformation event that formed large asymmetric nw vergent folds with related axial planar cleavage developed in the lower greenschist facies these folds are truncated by the lower cambrian angular unconformity lower cambrian conglomerates contain pebbles with predepositional microstructures that are widespread in the underlying neoproterozoic rocks the narcea antiform provides valuable information concerning major tectonic cadomian events the large scale near recumbent cadomian folds developed between 560 and 540 ma top to the nw kinematics can be deduced for the cadomian plate convergence transverse to the variscan convergence in present coordinates it was followed shortly afterwards by large displacements along the northern gondwana margin that resulted in its accretion alongside the essentially undeformed neoproterozoic rocks of the central iberian zone before early cambrian times

paleomagnetic determination of emplacement temperatures of pyroclastic deposits an under utilized tool paleomagnetic data from lithic clasts collected from mt st helens usa volcan lascar chile volcan de colima mexico and vesuvius italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures at mt st helens the temperature of the deposits tdep at three sites from the june 12 1980 eruption was found to be 532 c 509 c and 510 570 c respectively one site emplaced on july 22 1980 was emplaced at 577 c these new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates lithic clasts from pyroclastic deposits from the 1993 eruption of lascar were fully remagnetized above the respective curie temperatures which yielded a minimum tdep of 397 c samples were also collected from deposits thought to be pyroclastics from the 1913 2004 and 2005 eruptions of colima at colima the sampled clasts were emplaced cold this is consistent with the sampled clasts being from lahar deposits which are common in the area and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit tdep of the lower section of the lithic rich pyroclastic flow lrpf from the 472 a d deposits of vesuvius was 280 340 c this is in agreement with other recently published paleomagnetic measurements in contrast the upper section of the lrpf was emplaced at higher temperatures with tdep 520 c this temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections with the upper section containing a greater proportion of vent derived material that was initially hot our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation

syntectonic anatexis and magmatism in a mid crustal transpressional shear zone an example from the hercynian rocks of the eastern pyrenees abstract hercynian metamorphism and magmatism in the cap de creus peninsula eastern pyrenees occurred contemporaneously with non coaxial deformation in a transpressive regime an example of this has been taken from a migmatite complex in the northern coast the studied area is formed by partially melted sillimanite schists together with two different suites of igneous rocks a calc alkaline magmatic sequence consisting of small granitoid bodies and later peraluminous leucogranites and pegmatites all these rocks occur within a ne sw to e w trending sub vertical high strain zone where a first schistosity is tightly folded and transposed the relations between folds and crenulation cleavage in the metasediments and deformational fabrics in the granitoids and migmatites provide good evidence for the penecontemporaneity of deformational processes magmatism and migmatization furthermore the analysis of cross cutting relationships between different granitoids as well as the observation of their different degrees of deformation verifies that magmatism in this area took place as synkinematic consecutive intrusions from the more basic magmas to the peraluminous acid dykes

the maladeta granite polydiapir spanish pyrenees a detailed magnetostructural study abstract the maladeta granitic complex spanish central pyrenees has been subjected to a detailed magnetostructural study through the systematic measurement of magnetic susceptibility and anisotropy of magnetic susceptibility at 253 regularly spaced sampling sites due to the dominant paramagnetic property of the granites a first order correlation can be derived between the susceptibility magnitude and the bulk rock iron content hence the petrographic nature of the rock the maladeta complex is shown to be composed of four principal juxtaposed plutons each one being normally zoned i e becoming more basic towards the periphery the largest pluton namely the colomers unit is itself made of a number of sub plutons due to distinct but more or less coeval batches of magmas the various magnetic data complemented by field and microstructural observations allow us to propose that the maladeta polydiapir probably exploited ne and se trending openings of the brittle crust for its emplacement that accompanied or just preceded a sw verging compressive event a later n s compression marked by inverse dextral and e w trending shears is preferably attributed to late variscan rather than to alpine tectonics

comment on identification of an underfilled foreland basin system in the upper devonian of the central pyrenees implications for the hercynian orogeny by souquet et al international journal of earth sciences 92 316 337 2003

emplacement temperature determinations of proximal pyroclastic deposits on santorini greece and their implications this paper presents the results of a study undertaken to determine emplacement temperatures of volcanic deposits on the volcano of santorini greece using palaeomagnetic methods two types of breccia have been identified on santorini lithic lag breccias and ground breccias they have been interpreted as deposits from pyroclastic flows however where the breccia deposits are not intimately associated with ignimbrite their origin is ambiguous emplacement temperature determinations were carried out on lithic clasts within these deposits to assess the origin and to place constraints on their emplacement mechanisms the results of thermal demagnetization show that all the clasts display at present earth s field component of magnetization the earth s field component indicates that these clasts have cooled from elevated temperatures within the deposits therefore these breccia deposits were emplaced hot verifying their origin as primary products of pyroclastic eruption many of the deposits display a wide range of emplacement temperatures and the lowest temperature at any one location probably represents the equilibrium temperature of the deposit an emplacement temperature determination records the highest temperature experienced by the clast before cooling in situ these clasts may have been heated before during or after the eruption in estimating the emplacement temperature it is important to understand at what stage the clasts were heated thermal modelling was carried out to assess the effects of heating of the clasts a during the eruption and subsequent transport by hot gases and juvenile pyroclasts within pyroclastic flows and b by an overlying hot deposit such as a pyroclastic deposit or lava flow results from this study indicate that many of the clasts were hot before incorporation took place this indicates that clasts were either derived from depth within the volcano or that there were abundant rocks at elevated temperature near or at the surface

recent lahars at volcan de colima mexico drainage variation and spectral classification abstract volcan de colima is the most active volcano in mexico and represents a high risk for more than 500 000 people in 1998 the volcano renewed its activity with the extrusion of a lava dome and subsequent lava and block and ash flows during the recent period of activity pyroclastic products did not directly affect villages around the volcano however several lahars did we used lidar topographic coverage aster and landsat images for the recognition of morphological changes in the drainage system and lahar detection for lahar delineation we applied principal components analysis and canonical classification tasseled cap in order to perform a supervised image classification using the maximum likelihood rule algorithm laharz objective delineation of distal debris flow hazard zones has been used and tested using two topographic datasets with different resolutions which provided evidence of the importance of high resolution topographic coverage in hazard assessment finally a hazard map for lahars is presented showing that several villages and ranchos can be affected in contrast due to morphological changes produced by products of the intense explosive activity other populated areas such as la yerbabuena are outside of the high risk zone they could be affected by lahars only in case of cataclysmic eruptions such as the 1913 plinian event

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

late orogenic extension in mountain belts insights from the basin and range and the late paleozoic variscan belt from two examples of orogenic domains some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized the basin and range province and the variscan belt in the french massif central have both suffered important compressional orogenic crustal thickening and the results of late orogenic processes can be observed in the field both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed at the scale of the orogenic domains the most prominent tectonic features are the metamorphic core complexes mcc which expose deformed rocks from the middle crust generally affected by high temperature low to medium pressure metamorphism partial melting and widespread granite emplacement in these mcc large scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations they show a complete evolution from early deep seated ductile deformation generally achieved under high temperature low to medium pressure metamorphism to a late shallow brittle stage characterized by cataclastic deformation the late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits two main geometries of mcc are defined that are characterized by differing geometry and kinematics of low angle shear zones in the first case two low angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical mcc often one system is dominant over the other the second geometry characterizes asymmetrical mcc bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain strain distribution within the lower crust is less well constrained by field observation however analogies between cocorp and ecors deep seismic reflection profiles give important constraints on crustal structure wide zones of highly subhorizontally layered lower crust and a flat high amplitude reflection moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension a kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains a weak hot upper mantle allows large scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation heterogeneous strain is accommodated by low angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust combined pure and simple shear occurs along localized shear zones whereas at the scale of the whole lithosphere deformation nearly corresponds to a vertical pure shear such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics thus as much in the andean type west american cordilleran belt as in the collision type variscan belt late orogenic processes produced similar extensional features

deposition temperature of the ad 472 pollena pyroclastic density current deposits somma vesuvius italy the deposition temperature of the pyroclastic density current pdc deposits emplaced during the ad 472 pollena eruption somma vesuvius italy has been investigated using the thermal analysis of the magnetic remanence carried by lithic clasts embedded within the deposits a total of 310 lithic clasts were collected from all the pdc units in the pollena stratigraphic succession at different distances from the inferred vent and at different stratigraphic levels the temperature reached by each individual clast during residence in the pdc was estimated through stepwise thermal demagnetization with the values from all clasts collected at each site being used to infer the deposition temperature tdep although the sedimentological features of these pdc deposits show some variation the deposition temperature typically falls in the range 300 to 320 c with a maximum range of 260 to 360 c the fairly uniform temperature observed in both the dune bedded and massive deposits points to homogeneity in attainment of tdep for the different deposits and suggests similarity in the depositional regime of the different pdcs and or in heat transfer to lithic fragments similarity in depositional regime was also favoured by the limited control exerted by topography on the distribution of these pdcs with the northern wall of the somma caldera that did not act as a morphological barrier as a result the currents were capable of moving away from the vent without topographic disturbances and thus significant variations in the cooling regime because the pollena eruption is considered similar to the maximum expected event at somma vesuvius the characteristics of its deposits best simulate the likely maximum hazard for the vesuvius region in this regard pollena produced hot dilute pdcs which were able to travel up to 12 km from the vent maintaining high temperatures across this distance

strain partitioning in banded and or anisotropic rocks implications for inferring tectonic regimes abstract among the many concepts that have contributed to the development of structural geology in the last half century two are particularly important these are i the link between the symmetry of the structure the vorticity of the associated deformation process and the tectonic regime and ii the widespread partitioning of strain however a clear understanding of these concepts and the links between them are not always apparent in structural analyses carried out to determine the prevailing tectonic regime of an area through the use of structural symmetries and kinematic indicators this contribution based on field studies from cap de creus experimental data and theoretical concepts highlights some of the problems encountered when attempting to deduce the type of strain and associated regional tectonic regime from field structures the relation between the symmetry of a structure and its associated kinematics is not unique for example it is demonstrated that in mechanically anisotropic materials symmetry variations do not necessarily reflect marked vorticity variations but rather the angular variations between the kinematic framework and the anisotropy planes because of widespread strain partitioning local and general kinematics do not generally match this mismatch is especially accentuated in rheologically heterogeneous rocks nevertheless correct interpretations are possible by performing continuous multi scale structural analyses in which the impact of strain partitioning and of the presence of material anisotropies and heterogeneities are considered

tectonic levels in the palaeozoic basement of the pyrenees a review and a new interpretation reply abstract two different structural domains developed during the main hercynian deformation events were recognized by de sitter zwart in the hercynian basement of the central pyrenees a deep seated level infrastructure made of medium to high grade metasediments and orthogneisses with dome shaped gently dipping foliations contrasts with a shallow level suprastructure of low to very low grade metasediments with rather steep foliations since the recognition of this structural zonation different interpretations have been proposed disagreements concern timing of deformation and tectonic regime considered to be prevalent of the structural development of each tectonic level the proposed interpretation regards the progressive development and differentiation of both structural domains and considers the following factors to be significant for the structural arrangement of the pyrenean segment of the hercynian belt an increase with depth of the horizontal shear component of deformation related to tangential and transcurrent tectonics a marked inhomogeneity of deformation and penetrability of related foliations and the gradual evolution from an earlier compressive crustal shortening event to a late transpressive event responsible for crustal shortening and trend parallel stretching

forecasting catastrophic stratovolcano collapse a model based on mount taranaki new zealand regular large scale edifice collapse and regrowth is a common pattern during the long lifespans of andesitic stratovolcanoes worldwide the 130 k y history of mount taranaki new zealand is punctuated by at least 14 catastrophic collapses producing debris avalanche deposits of 1 to 7 5 km 3 the largest of these sudden events removed as much as one third of the present day equivalent cone the resulting deposits show similar sedimentary and geomorphic features suggesting similar proto edifice characteristics failure trigger mechanisms and runout path conditions each collapse was followed by sustained renewed volcanism and cone regrowth although there are no matching stepwise geochemical changes in the magma erupted instead a stable slowly evolving magmatic system has prevailed last glacial climatic variations are also uncorrelated with the timing or magnitudes of edifice collapse we demonstrate here that if the magmatic composition erupted from stratovolcanoes is constant and basement geology conditions are stable large scale edifice collapse and the generation of catastrophic debris avalanches will be governed by the magma supply rate using a mass balance approach a volume frequency model can be applied to forecasting both the probable timing and volume of future edifice failure of such stratovolcanoes in the mount taranaki case the maximum potential size of a present collapse is estimated to be 7 9 km 3 while the maximum interval before the next collapse is 3

the heating of substrates beneath basaltic lava flows as populations around volcanoes grow the potential for society to be impacted by lava flows is increasing while lava flows are known to ignite bulldoze and or bury structures little is known about potential impacts to buried infrastructure we measure temperature profiles below molten rock to constrain a heat transfer model thermomagnetic and palaeomagnetic measurements on soil samples from beneath a 2014 hawaiian lava flow are then used to verify the model finally we illustrate the model s utility in lava flow hazard assessments by modelling a hypothetical lava flow active for 4 weeks in auckland new zealand the modelling predicts the upper 1 7 m of dry soil would exceed 100 c after 1 week and the upper 3 8 m of soil would exceed 100 c after 4 weeks determining the depth profile of substrate heating has important implications for planning and preparedness e g siting buried infrastructure volcanic impact and risk assessments and decision making before and during lava flow crises e g mitigation measures to be employed

directed blasts and blast generated pyroclastic density currents a comparison of the bezymianny 1956 mount st helens 1980 and soufriere hills montserrat 1997 eruptions and deposits we compare eruptive dynamics effects and deposits of the bezymianny 1956 bz mount st helens 1980 msh and soufriere hills volcano montserrat 1997 shv eruptions the key events of which included powerful directed blasts each blast subsequently generated a high energy stratified pyroclastic density current pdc with a high speed at onset the blasts were triggered by rapid unloading of an extruding or intruding shallow magma body lava dome and or cryptodome of andesitic or dacitic composition the unloading was caused by sector failures of the volcanic edifices with respective volumes for bz msh and shv c 0 5 2 5 and 0 05 km3 the blasts devastated approximately elliptical areas axial directions of which coincided with the directions of sector failures we separate the transient directed blast phenomenon into three main parts the burst phase the collapse phase and the pdc phase in the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere with much of the expansion having an initially lateral component the erupted material fails to mix with sufficient air to form a buoyant column but in the collapse phase falls beyond the source as an inclined fountain and thereafter generates a pdc moving parallel to the ground surface it is possible for the burst phase to comprise an overpressured jet which requires injection of momentum from an orifice however some exploding sources may have different geometry and a jet is not necessarily formed a major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet or alternatively to dynamic pressures and shocks within the energetic collapse and pdc phases internal shock structures related to unsteady flow and compressibility effects can occur in each phase we withhold judgment about published shock models as a primary explanation for the damage sustained at msh until modern 3d numerical modeling is accomplished but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified pdc this view is reinforced by recent modeling cited for shv in distal and peripheral regions solids concentration maximum particle size current speed and dynamic pressure are diminished resulting in lesser damage and enhanced influence by local topography on the pdc despite the different scales of the blasts devastated areas were respectively 500 600 and 10 km2 for bz msh and shv and some complexity involving retrogressive slide blocks and clusters of explosions their pyroclastic deposits demonstrate strong similarity juvenile material composes 50 of the deposits implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred the character of the magma fragmented by explosions highly viscous phenocryst rich variable microlite content determined the bimodal distributions of juvenile clast density and vesicularity thickness of the deposits fluctuates in proximal areas but in general decreases with distance from the crater and laterally from the axial region the proximal stratigraphy of the blast deposits comprises four layers named a b c d from bottom to top layer a is represented by very poorly sorted debris with admixtures of vegetation and soil with a strongly erosive ground contact its appearance varies at different sites due to different ground conditions at the time of the blasts the layer reflects intense turbulent boundary shear between the basal part of the energetic head of the pdc and the substrate layer b exhibits relatively well sorted fines depleted debris with some charred plant fragments its deposition occurred by rapid suspension sedimentation in rapidly waning high concentration conditions layer c is mainly a poorly sorted massive layer enriched by fines with its uppermost part laminated created by rapid sedimentation under moderate concentration weakly tractive conditions with the uppermost laminated part reflecting a dilute depositional regime with grain by grain traction deposition by analogy to laboratory experiments mixing at the flow head of the pdc created a turbulent dilute wake above the body of a gravity current with layer b deposited by the flow body and layer c by the wake the uppermost layer d of fines and accretionary lapilli is an ash fallout deposit of the finest particles from the high rising buoyant thermal plume derived from the sediment depleted pyroclastic density current the strong similarity among these eruptions and their deposits suggests that these cases represent similar source transport and depositional phenomena

the geological significance of the western termination of the merens fault at port vell central pyrenees abstract a detailed local structural map of the merens fault zone at port vell vall ferrera reveals that the main fault related mylonite band splits westwards into a network of mylonite zones these consist of bands of highly strained rocks exhibiting tightened folds transposed foliations and extensive obliteration of pre existing structures evidence of continuity of marble layers across the port vell mylonite zone and structural and metamorphic similarities on both sides of the port vell mylonite band indicate that there is no significant throw across it the mylonite foliation forms parallel to the axial planes of tight folds in or near the mylonite zone which can be related to more open folds in adjacent areas the port vell mylonite band corresponds to a pinched w e axial trending late fold in order to estimate the importance of these results a review of the controversial geological significance of mylonite belts in the pyrenees with special emphasis on the merens fault introduces the paper

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

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

kinematics of orogenic collapse in the variscan pyrenees deduced from microstructures in porphyroblastic rocks from the lys caillaouas massif in the pyrenees a phase of bulk vertical shortening d3 is indicated by a horizontal crenulation cleavage s3 which is developed in a pre existing schistosity s2 porphyroblasts of the lys caillaouas massif regionally preserve 1 1 the initial pre d3 orientation of s2 in the form of a vertical and e w trending preferred orientation of their inclusion trails 2 2 a vertical l2 stretching lineation defined by the elongation of the inclusions these data are consistent with n s compression and crustal thickening during d2 the angle between the inclusion trails s2 i and the external schistosity s2 e records the subsequent rotation of s2 from vertical to gently dipping orientations at higher crustal levels s2 remained steep suprastructure and little d3 deformation is evidenced with depth d3 deformation progressively intensifies and zones can be distinguished with opposite dip directions of s2 opposite apparent porphyroblast rotation senses and opposite f3 crenulation asymmetries these micro structures on a n s section through the lys caillaouas and the garonne dome massifs record a regional partitioning of bulk coaxial vertical shortening this deformation partitioning was probably controlled by a not exposed gneiss dome which behaved relatively rigid as compared to its metasedimentary cover in zones of non coaxial d3 deformation s2 was extended reactivated and forms the dominant schistosity where d3 was locally more coaxial s2 was shortened isoclinally crenulated and s3 forms the macroscopic cleavage the fact that d3 extension in the lys caillaouas massif immediately followed d2 compression and that similar extension is recorded in large parts of the variscan foldbelt suggests gravitational collapse of a thickened orogen this is consistent with the flattening type of d3 strain recognized in a number of pyrenean variscan massifs peak high t low p metamorphic conditions coincide with the onset of orogenic extension when very little crustal thinning could have been accomplished yet this suggests that heat induced strain softening within the orogen may have triggered its collapse

pattern and kinematic polarity of late mesozoic extension in continental ne asia perspectives from metamorphic core complexes 1 late mesozoic extension in ne asia resulted in the development of a large extensional province metamorphic core complexes mccs are the major features in this province and have 40ar 39ar ages of 130 110 ma for the mylonites and u pb zircon ages of 150 110 ma for the integral granitic intrusions based on this and previous studies this paper summarizes major characteristics of these mccs and recognizes a regional kinematic shear sense most mccs in the transbaikalia region sino mongolia border tract and the northwest central portion of the north china craton ncc show a top to the southeast se shear whereas those in the eastern and southern ncc locally underwent top to the northwest nw shear the three largest basins songliao huabei and ordos in north china are located in the transitional zone between domains of opposing shear sense we interpret the extension in the transbaikalia sino mongolia tract and northwestern part of the ncc to reflect late orogenic collapse of thickened crust following middle late jurassic collision along the okhotsk suture the southeastward extension is probably controlled by crustal scale top to the se tangential shear the transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage ecc i e c in sub horizontal ductile shear zones late in orogenic crustal thickening the combined effect of gravitational loading and thermal uplifting is considered to be the origin of the late or post orogenic collapse the top to the nw extension in the ne of the ncc might reflect antithetic sub extensional zones or mesozoic back arc extension as a far field effect of cretaceous pacific plate subduction

on folding and shear zone development a mesoscale structural study on the transition between two different tectonic styles abstract an example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis folds and shear zones were formed under retrogressive conditions after the peak of the regional hercynian metamorphism and both affect a previous regional schistosity a gradual change in structural style on a regional scale is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones geometrical analysis reveals that the bulk structural pattern is compatible with a non coaxial strain history structural variation reflects a gradual change in mechanical behaviour of schists which show a marked variation in crystallinity at the time of deformation

permian clockwise rotations of the ebro and corso sardinian blocks during iberian armorican oroclinal bending preliminary paleomagnetic data from the catalan coastal range ne spain abstract paleomagnetic investigations of early permian and triassic magmatic rocks from catalan coastal ranges provide three magnetic directions which are interpreted as primary magnetizations and partly as overprints the succession of magnetic directions is interpreted in terms of a succession of large scale clockwise rotations which bring the whole assembly of pyrenees catalonia corsica sardinia and maures esterel massifs from the southeast of the massif central to the west during permian this major movement is associated with clockwise rotation of northern limb of the iberian orocline during latest carboniferous and early permian 305 290 ma subsequently permian large scale dextral transtensional extensional shearing operated along aquitanian shear belt between the massif central in the northeast and the iberian variscan massifs in the southwest the latest phase of the rotation in the late permian early triassic is associated with alkaline magmatism probably linked to neo tethys opening activity in western europe

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

thick and thin skinned tectonics in the pyrenees abstract a comparison is made between the gavarnie thrust and the merens fault in the axial zone of the pyrenees the former has a gentle dip and quite a large displacement at least 12 km but does not cut through either hercynian or alpine isograds the latter has a smaller displacement 5 km but dips steeply and cuts through both hercynian and alpine isograds at a high angle on this basis and on the basis of shear zone geometries immediately north of it it is proposed that the merens fault nucleated as a steeply 65 80 dipping structure while the gavarnie thrust nucleated with a shallow attitude the merens fault is not a backward rotated thrust fault nor is it the root zone for any major nappe structure similar steep ductile structures occur within the gavarnie nappe and may reflect considerable internal strain in basement lithologies the relationship between steep and shallow structures is not yet clear the shear zones may pre date the thrusting in which case they may be thick skinned structures affecting the whole lithosphere or they may be contemporary with thrusting reflecting only local thickening above a decollement rheological models can be used to test proposed geometrical and kinematic models for the lithosphere scale evolution of the pyrenees suggested models are dominated by a cool rigid high level mantle wedge beneath the north pyrenean zone which probably controlled the location of north dipping thrust faults thick skinned shortening is possible in thick crust in the axial zone but is very unlikely in the north pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere

rain triggered lahars following the 2010 eruption of merapi volcano indonesia a major risk abstract the 2010 vei 4 eruption of merapi volcano deposited roughly ten times the volume of pyroclastic materials of the 1994 and 2006 eruptions and is recognized as one of the most intense eruption since 1872 however as the eruptive phase is now over another threat endangers local communities rain triggered lahars previous papers on lahars at merapi presented lahar related risk following small scale dome collapse pdcs thus the aim of this study is to provide new insights on lahar related risk following a large scale vei 4 eruption the paper highlights the high number of events 240 during the 2010 2011 rainy season october 2010 may 2011 the frequency of the 2010 2011 lahars is also the most important ever recorded at merapi lahars occurred in almost all drainages located under the active cone with runout distances exceeding 15 km the geomorphic impacts of lahars on the distal slope of the volcano are then explained as they directly threaten houses and infrastructures creation of large corridors avulsions riverbank erosion and riverbed downcutting are detailed through local scale examples related damage is also studied 860 houses damaged 14 sabo dams and 21 bridges destroyed sedimentological characteristics of volcaniclastic sediments in lahar corridors are presented with emphasis on the resource in building material that they represent for local communities risk studies should not forget that thousands of people are exposing themselves to lahar hazard when they quarry volcaniclastic sediment on lahar corridors finally the efficient community based crisis management is explained and shows how local people organize themselves to manage the risk 3 fatalities were reported although lahars reached densely populated areas to summarize this study provides an update of lahar risk issues at merapi with emphasis on the distal slope of the volcano where lahars had not occurred for 40 years and where lahar corridors were rapidly formed

voluminous ice rich and water rich lahars generated during the 2009 eruption of redoubt volcano alaska abstract redoubt volcano in south central alaska began erupting on march 15 2009 and by april 4 2009 had produced at least 20 explosive events that generated multiple plumes of ash and numerous lahars the 3108 m high snow and ice clad stratovolcano has an ice filled summit crater that is breached to the north the volcano supports about 4 km 3 of ice and snow and about 1 km 3 of this makes up the drift glacier on the north side of the volcano explosive eruptions between march 23 and april 4 which included the destruction of at least two lava domes triggered significant lahars in the drift river valley on march 23 and april 4 and several smaller lahars between march 24 and march 31 mud line high water marks character of deposits areas of inundation and estimates of flow velocity revealed that the lahars on march 23 and april 4 were the largest of the eruption in the 2 km wide upper drift river valley average flow depths were at least 2 5 m average peak flow velocities were likely between 10 and 15 ms 1 and peak discharges were on the order of 10 4 10 5 m 3 s 1 the area inundated by lahars on march 23 was at least 100 km 2 and on april 4 about 125 km 2 two substantial lahars emplaced on march 23 and one on april 4 had volumes on the order of 10 7 10 8 m 3 and were similar in size to the largest lahar of the 1989 90 eruption the two principal march 23 lahars were primarily flowing slurries of snow and ice derived from drift glacier and the drift river valley where seasonal snow and tabular blocks of river ice were entrained and incorporated into the lahars despite morphologic evidence of two lahars only a single deposit up to 5 m thick was found in most places and it contained about 80 95 of poorly sorted massive to imbricate assemblages of snow and ice clasts the deposit was frozen soon after it was emplaced and later eroded and buried by the april 4 lahar the lahar of april 4 in contrast was primarily a hyperconcentrated flow as interpreted from 1 to 6 m thick deposits of massive to horizontally stratified sand to fine gravel rock material in the april 4 lahar deposit is predominantly juvenile andesite whereas rock material in the march 23 deposit is rare and not obviously juvenile we infer that the lahars generated on march 23 were initiated by a rapid succession of vent clearing explosions that blasted through about 50 100 m of crater filling glacier ice and snow producing a voluminous release of meltwater from drift glacier the resulting surge of water entrained snow fragments of glacier and river ice and river water along its flow path small volume pyroclastic flows possibly associated with destruction of a small dome or minor eruption column collapses may have contributed additional meltwater to the march 23 lahars meltwater generated by subglacial hydrothermal activity and stored beneath drift glacier may have been ejected or released rapidly as well the april 4 lahar was initiated when hot dome collapse pyroclastic flows entrained and swiftly melted snow and ice on drift glacier the resulting meltwater incorporated pyroclastic debris and rock material from drift glacier to form the largest lahar of the 2009 eruption the peak discharge of the april 4 lahar was in the range of 60 000 160 000 m 3 s 1 for comparison the largest lahar of the 1989 90 eruption had a peak discharge of about 80 000 m 3 s 1 lahars generated by the 2009 eruption led to significant channel aggradation in the lower drift river valley and caused extensive inundation at an oil storage and transfer facility located there the april 4 2009 lahar was 6 30 times larger than the largest meteorological floods known or estimated in the drift river drainage

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

rapid extensional unroofing of a granite migmatite dome with relics of high pressure rocks the podolsko complex bohemian massif the podolsko complex bohemian massif is a high grade dome that is exposed along the suprastructure infrastructure boundary of the variscan orogen and records snapshots of its protracted evolution the dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high to ultrahigh pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards our new u pb zircon ages indicate that the leucocratic migmatites were derived from early ordovician c 480 ma felsic igneous crust the same age is inferred for melting the proto source of the metapelitic migmatites the relics of high to ultrahigh pressure rocks suggest that at least some portions of the complex witnessed an early variscan subduction to mantle depths followed by high temperature anatexis and syntectonic growth of the podolsko dome in the middle crust at c 340 339 ma subsequently the dome exhumation was accommodated by crustal scale extensional detachments similar c 340 ma ages have also been reported from other segments of the variscan belt yet the significance of this tectonothermal event remains uncertain here we conclude that the 340 ma age post dates the high pressure metamorphism the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and finally the extensional unroofing of the complex requires a minimum throw of about 8 10 km we use this as an argument for significant early carboniferous palaeotopography in the interior of the variscan orogen

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

insights into the october november 2010 gunung merapi eruption central java indonesia from the stratigraphy volume and characteristics of its pyroclastic deposits abstract the 2010 eruption of merapi was the second most deadly in the historic record of this volcano claiming over 380 lives by relating the observations of this eruption with detailed examination of deposit distribution stratigraphy and sedimentology a reconstruction of the properties of the pyroclastic density currents pdcs is presented including the valley controlled block and ash flows bafs and widespread energetic pyroclastic surges the distribution volume and mobility characteristics of all types of pdc during the eruption sequence show evidence for levels of intensity unseen since the large scale 1872 and 1930 eruption phases especially during the climactic events of october 26 and november 5 many tephra falls interbedded with pdc units show that most dome collapse events occurred along with and between explosive vulcanian eruptions the 2010 eruption produced very long runout bafs reaching 16 1 km in the kali gendol on november 5 this runout could be explained by its large volume 20 million m 3 around 10 times that of previous merapi bafs during the last 130 years major avulsion of these dense bafs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive pdc deposits spreading avulsed bafs were a particular hazard downstream of 10 km where the landscape is less dissected less clear however is why pyroclastic surges extended up to 10 km from the vent on november 5 and 6 4 km on october 26 these expanded much farther from baf margins 2 km than ever seen before at merapi in one location they were decoupled from valley centered bafs with high momentum traveling initially laterally across steep valley systems before draining downslope at this site on the western side of the upper gendol at around 3 km from source surge decoupling was apparently exacerbated by upstream collision and deflection of high flux hot and gas rich bafs against the cliffs of gunung kendil the 1 4 km long cliff face was impacted directly for the first time in 2010 events and may have been responsible for the formation of larger than normal turbulent ash rich clouds above bafs these results imply that future eruption events under the present summit and upper flow path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard especially now that dense forest has been destroyed on the upper southern slopes of the volcano

structural inheritance in the central pyrenees the variscan to alpine tectonometamorphic evolution of the axial zone estimating structural inheritance in orogens is critical to understanding the manner in which plate convergence is accommodated the pyrenean belt which developed in late cretaceous to paleogene times was affected by cretaceous rifting and variscan orogeny here we combine a structural and petrological study of the axial zone in the central pyrenees to discuss structural inheritance low grade paleozoic metasedimentary rocks were affected by a variscan transpressional event that produced successively 1 regional scale folds 2 isoclinal folding steep pervasive cleavage and vertical stretching synchronous with peak metamorphism 3 strain localization into ductile reverse shear zones the persistence of a relatively flat envelope for the paleozoic sedimentary pile and variscan isograds and the absence of alpine crustal scale faults in the core of the axial zone suggests that the axial zone constitutes a large variscan structural unit preserved during pyrenean orogeny this configuration seems to be inherited from cretaceous rifting which led to the individualization of a large continental block future axial zone against a hyper extended domain along the north pyrenean fault zone this study places the currently prevailing model of pyrenean belt deformation in a new perspective and has important implications for crustal evolution and inheritance in mountain belts more generally supplementary materials raman spectroscopy of carbonaceous materials data and a figure illustrating peak fitting of the raman spectrum of carbonaceous material and raman spectra from the various samples of the pallaresa cross section are available at https doi org 10 6084 m9 figshare c 3906247

lahars at merapi volcano central java an overview abstract merapi volcano in central java is one of the most active volcanoes in the world at least 23 of the 61 reported eruptions since the mid 1500s have produced source deposits for lahars the combined lahar deposits cover about 286 km 2 on the flanks and the surrounding piedmonts of the volcano at merapi lahars are commonly rain triggered by rainfalls having an average intensity of about 40 mm in 2 h most occur during the rainy season from november to april and have average velocities of 5 7 m s at 1000 m in elevation a wide range of facies may be generated from a single flow which may transform downvalley from debris flow to hyperconcentrated streamflow because of the high frequency and magnitude of the lahar events lahar related hazards are high below about 450 600 m elevation in each of the 13 rivers which drain the volcano hazard zone maps for lahar were produced by pardyanto et al volcanic hazard map merapi volcano central java 1 100 000 geol surv of indonesia bandung ii 4 1978 and the japanese indonesian cooperation agency master plan for land conservation and volcanic debris control in the area of mt merapi jakarta 1980 but these maps are of a very small scale to meet modern zoning requirements more recently a few large scale maps 1 10 000 and 1 2000 scale and risk assessments have been completed for a few critical river systems

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

instantaneous record of the geomagnetic field direction of various facies from pyroclastic flow deposits tests for consistency in paleomagnetic directions abstract a comparative study of the paleomagnetism of various lithofacies of pyroclastic flow deposits has been undertaken to examine the accuracy of their remanent magnetization direction at the time of deposition the aso 2 pyroclastic flow deposit which erupted at 141 ka from aso caldera japan was chosen as the site of our investigation because the deposit formed over a short time period and the paleomagnetic directions of the deposit are expected to record substantial contemporaneous volcanism paleomagnetic samples of densely welded lava like rheomorphic and non welded pyroclastic deposits were collected at nine sites from the aso 2 pyroclastic flow deposit remanence directions from densely welded pyroclastic deposits display good within and between site consistency and are considered to accurately record the ambient geomagnetic field direction at the time of emplacement rheomorphic pyroclastic deposits had directions that show good within site consistency and it is considered that this material obtains its remanence direction parallel to the ambient field corresponding to the time of its emplacement remanence directions from non welded pyroclastic deposits show large confidence limits or deviations from their expected directions such deposits would likely be prone to modification of remanence direction introduced from random rotations of remanence carrying material during syn or post depositional stages in conclusion we suggest that among various lithofacies of pyroclastic flow deposits remanence directions observed from non welded pyroclastic deposits may need to be interpreted cautiously

distribution of linear mesostructures in oblique folded surfaces unravelling superposed ordovician and variscan folds in the pyrenees abstract detailed mapping and structural analysis yielded new data on the geometry of ordovician folds in the pyrenees these folds caused the variscan mainphase folds to develop over non horizontal bedding surfaces in the pre upper ordovician rocks as a result the variscan mainphase fold axes and intersection lineations in these rocks present a wide range of orientations bedding surfaces exhibit a complex pattern in map view making it necessary to reconstruct the structure in 3d computer aided reconstruction tools such as foldplot proved useful for visualizing and reconstructing the geometry of such refolded structures in 3d in the absence of map scale key horizons evidence of superposed folds is provided by detailed mapping of mesostructures this requires a good and continuous outcrop over an extended area since one single fold system can account for a complex distribution of the subsequent intersection lineation as a result a cartographic study and a structural analysis are warranted because the direction of the ordovician folds cannot be established only by the orientation of the l2 mesostructures

decollements in slate belts examples from the european variscides and the qin ling belt of central china decollements are conspicuous features of collision belts as a result of shortening and shearing of rocks of different competencies in response to intracontinental subduction and crustal stacking when these decollements occur at upper levels the classical thin skin tectonics of the foredeeps without important internal strain results at mid crustal levels slaty cleavage and foliation development occurs in the more internal parts of the chains in the lower crust near the crust mantle boundary the more severe conditions granulitic metamorphism result in ductile deformation in the deepest parts of the mountain belts examples of middle and lower crust are well documented in central china and the variscan belt of europe from field evidence and deep seismic profiling from these examples it appears more and more likely that most of the mid crustal slate belts showing vertical slaty cleavage at the erosion surface overly flat decollements at greater depth large decollements must occur as well in the lower continental crust and at crust mantle boundary typical features of the deep seismic vertical profiles such as the strongly layered lower crust may represent the image of such deep decollements

the mylonite zones in the pyrenees and their place in the alpine tectonic evolution abstract in the pyrenees the development of mylonites zones is one of the most striking structural features two sets of mylonites of regional extent have been recognized large longitudinal e w to n110 e trending zones e g merens fault and north pyrenean fault and oblique nw se trending zones cross cutting both the hercynian and the post hercynian terrains the longitudinal zones limit the major structural zones of the pyrenees and are associated with nw se en echelons folds in the mesozoic terrains and rotations of rootless plutonic or gneissic massifs acting as competent inclusions in a more ductile matrix in the hercynian basement the oblique mylonite zones limit map scale fold bands and appear as the sheared limbs of these folds the age of the oblique zones and of the major movements along the longitudinal zones is clearly alpine and the en echelons folds seem to have controlled the sedimentation during the upper albian and possibly during the upper cretaceous early movements along the longitudinal zones may have been hercynian the analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole mesozoic period this strike slip shearing was probably associated with an extension perpendicular to it from the permian to the upper cretaceous and then to a shortening component also perpendicular to it from the late cretaceous to the eocene the development of the mylonite zones appears to have predated the major alpine thrusting but to have been reactivated during this thrusting acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones

thermal evolution of a hyperextended rift basin mauleon basin western pyrenees onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma poor rifted margins however critical questions remain concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation in the western pyrenees the mauleon basin preserves the structural and stratigraphic record of cretaceous extension exhumation and sedimentation of the proximal to distal margin development pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating making the mauleon basin an ideal locality to study the temporal and thermal evolution of magma poor hyperextended rift systems through coupling bedrock and detrital zircon u th he thermochronometric data from transects characterizing different structural rifting domains these new data indicate the basin was heated during early rifting to 180 c with geothermal gradients of 80 100 c km the proximal margin recorded rift related exhumation cooling at ca 98 ma whereas the distal margin remained 180 c until the onset of paleocene pyrenean shortening lithospheric scale numerical modeling shows high geothermal gradients 80 c km and synrift sediments 180 c can be reached early in rift evolution via heat advection by lithospheric depth dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments mauleon basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma poor continental rifted margins

a dynamic model of the structural evolution of the hercynian pyrenees abstract the structural history of the hercynian pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes the migmatization and the intrusions of plutonic and basic and ultrabasic rocks the metamorphism can be shown to have progressed upwards with time attaining the presently exposed levels just before during and a short time after the major penetrative regional deformation the intrusion of major plutonic anatectic and gneissic massifs which has determined the metamorphic domes in country rocks was roughly synchronous with the progression of the metamorphism but seems to have continued later in the uppermost levels the basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation thus showing that their intrusion predated this deformation in the lowermost levels these results together with the presently available sedimentological data lead us to propose a dynamic model of the tectonic evolution of the hercynian pyrenees the first step was a crustal extension and thinning attaining its maximum during frasnian time marked in the upper levels by a horst and graben controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low pressure metamorphism and anatexis characteristic of the hercynian pyrenees the second step was a crustal shortening marked by large scale crustal thrusting followed by widespread regional penetrative deformation the flysch troughs formed in front of the progressing and emerging thrusts in lower levels the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels

polyphase magmatic pulses along the northern gondwana margin u pb zircon geochronology from gneiss domes of the pyrenees abstract alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the aston and hospitalet domes of the axial zone of the pyrenees are subjects of a laser ablation icp ms u pb zircon geochronology study the age spectra recorded by detrital magmatic xenocrystic and inherited zircons reveal a more complex nearly continuous paleozoic magmatic history of the variscan basement of the pyrenees than previously known inherited and detrital zircons of mesoarchean paleoproterozoic to ediacaran ages attest to the peri gondwana location of the cambrian sediments that later form the metamorphic core of the variscan pyrenees the youngest magmatic zircon ages fall into the late carboniferous and earliest permian ranging from ca 306 297 ma and represent the emplacement ages of the dikes and small granite intrusions the age spectra of magmatic xenocrystic zircons contain several maxima middle 475 465 ma and late ordovician 455 445 ma early 415 402 ma and late devonian 385 383 ma early 356 351 ma and middle carboniferous ca 328 ma middle ordovician and middle carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded the aston and hospitalet orthogneisses and the soulcem granite the presence of early mid carboniferous magmatic zircons in several samples lends further support to a wide spread early variscan magmatic activity in the central pyrenees the other age peaks do not have equivalent igneous or metaigneous rocks in the central axial zone but are thought to be present in the pyrenean crust not exposed and yet to be identified the diversity of ordovician devonian and carboniferous up to permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre variscan and variscan orogenies the source of the heat for the devonian to early mid carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone lithospheric scale shearing or a mantle plume tuzo

variscan extension in the pyrenees evidence for variscan extension in the pyrenees is summarized and used with other geological and geochronological data to reconstruct the variscan extensional geometry and kinematics at a crustal scale a regionally consistent pattern of ese wnw trending synmetamorphic stretching directions indicates variscan extension roughly parallel to the present alpine trend of the belt the spatial distribution of variscan low pressure high temperature metamorphic domes is interpreted as being possibly due to the activity of few moderately dipping crustal scale shear zones allowing heterogeneous ductile extension of the variscan middle and lower crust extension occurred synchronous with uplift and emergence of large parts of the crust and deposition during the upper carboniferous and permian of up to 2500 m of continental sediments and intercalated basaltic andesitic volcanics in fault bounded extensional halfgrabens the extensional structures and related metamorphism postdate a namurian to early westphalian stage of crustal thickening characterized by thrusting and steep folding it is suggested that variscan extension in the pyrenees was driven by the gravitational potential energy of an initially thickened lithosphere enhanced by detachment of a cold and gravitationally unstable lithospheric root which caused progressive uplift tectonic denudation and a highly anomalous thermal structure of the variscan crust leading to extensive lower crustal melting and granodioritic intrusion during the early permian

palaeomagnetic determination of emplacement temperature of vesuvius ad 79 pyroclastic deposits the city of herculaneum was hurried under 20 m of pyroclastic deposits during the ad 79 eruption of vesuvius whose crater is only 7 km to the east these deposits have been interpreted as the deposits of mudflows1 or hot pyroclastic flows2 5 maury s studies6 of incinerated wood in herculaneum demonstrate heating to at least 400 c we have studied the variation of remanent magnetism with temperature for specimens taken from the deposits including specimens of the matrix material and of embedded lithic fragments we conclude that the temperature of the deposit at emplacement is unlikely to have been greater than 400 c which further supports the interpretation of the pyroclastic deposits at herculaneum as largely ignimbrites hot pyroclastic flow deposits

landscape and sedimentary response to catastrophic debris avalanches western taranaki new zealand abstract catastrophic volcanic debris avalanches reshape volcanic edifices with up to half of pre collapse cone volumes being removed deposition from this debris avalanche deposit often fills and inundates the surrounding landscape and may permanently change the distribution of drainage networks on the weakly incised mt taranaki ring plain volcanic debris avalanche deposits typically form a large wedge shape in plan view over all flat lying fans following volcanic debris avalanches a period of intense re sedimentation commonly begins on ring plain areas particularly in wet or temperate climates this is exacerbated by large areas of denuded landscape ongoing instability in the scarp source region damming of river stream systems and in some cases inherent instability of the volcanic debris avalanche deposits in addition on mt taranaki the collapse of a segment of the cone by volcanic debris avalanche often generates long periods of renewed volcanism generating large volumes of juvenile tephra onto unstable and unvegetated slopes or construction of new domes with associated rock falls and block and ash flows the distal ring plain impact from these post debris avalanche conditions and processes is primarily accumulation of long run out debris flow and hyperconcentrated flow deposits with a variety of lithologies and sedimentary character common to these post debris avalanche units is evidence for high water content flows that are typically non cohesive hence sedimentary variations in these units are high in lateral and longitudinal exposure in relation to local topography the post collapse deposits flank large scale fans and hence similar lithological and chronological sequences can form on widely disparate sectors of the ring plain these deposits on mt taranaki provide a record of landscape response and ring plain evolution in three stages that divide the currently identified warea formation 1 the deposition of broad fans of material adjacent to the debris avalanche unit 2 channel formation and erosion of stage 1 deposits primarily at the contact between debris avalanche deposits and the stage 1 deposits and the refilling of these channels and 3 the development of broad tabular sheet flows on top of the debris avalanche leaving sediments between debris avalanche mounds after a volcanic debris avalanche these processes represent an ever changing and evolving hazard scape with hazard maps needing to be regularly updated to take account of which stage the sedimentary system is in

the afragola settlement near vesuvius italy the destruction and abandonment of a bronze age village revealed by archaeology volcanology and rock magnetism abstract public works in progress in the campanian plain north of somma vesuvius recently encountered the remains of a prehistoric settlement close to the town of afragola rescue excavations brought to light a bronze age village partially destroyed and buried by pyroclastic density currents pdcs of the vesuvian pomici di avellino eruption 3 8 14c ka bp and subsequently sealed by alluvial deposits volcanological and rock magnetic investigations supplemented the excavations careful comparison between volcanological and archaeological stratigraphies led to an understanding of the timing of the damage the buildings suffered when they were struck by a series of pdcs the first engulfed the village located some 14 km to the north of the inferred vent and penetrated into the dwellings without causing major damage the buildings were able to withstand the weak dynamic pressure of the currents and deviate their path as shown by the magnetic fabric analyses some later collapsed under the load of the deposits piled up by successive currents stepwise demagnetization of the thermal remanent magnetization trm carried by potsherds embedded in the deposits yields deposition temperatures in the order of 260 320 c fully consistent with those derived from pottery and lithic fragments from other distal and proximal sites the fairly uniform temperature of the deposits is here ascribed to the lack of pervasive air entrainment into the currents this in turn resulted from the lack of major topographical obstacles along the flat plain the coupling of structural damage and sedimentological analyses indicates that the currents were not destructive in the afragola area but trm data indicate they were still hot enough to cause death or severe injury to humans and animals the successful escape of the entire population is apparent from the lack of human remains and from thousands of human footprints on the surface of the deposits left by the first pdcs people were thus able to walk barefoot across the already emplaced deposits and escape the subsequent pdcs the rapid cooling of the deposits was probably due to both their thinness and heat dissipation due to condensation of water vapour released in the mixture by magma water interaction

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

a rifted tectonic setting for hercynian high thermal gradient metamorphism in the pyrenees abstract hercynian regional metamorphic terrains in the pyrenees contain evidence of very high temperature gradients within the crust during metamorphism with temperatures as high as 700 c attained at 10 12 km below the surface stable isotope studies demonstrate that the crust was simultaneously flushed by marine fluids to at least this depth the absence of any evidence for crustal collision and the upper palaeozoic stratigraphic record for the area suggest that the tectonic setting for the metamorphism was a zone of continental rifting associated with strike slip movement in this zone anatexis occurred at two distinct levels cambro ordovician pelites at the base of the palaeozoic sedimentary pile melted to produce per aluminous magmas while in the lower hercynian crust very large scale melting generated voluminous granodioritic magmas which then invaded high structural levels the thermal structure of the hercynian crust was profoundly influenced by both convective and advective heat transfer due to movement of surface derived aqueous fluids and intrusion of magmas

development of high thermal gradients by coeval transpression and magmatism during the variscan orogeny insights from the cap de creus eastern pyrenees abstract the cap de creus massif in the eastern pyrenees is a fragment of the variscan mid crust geological mapping and meso and micro scale analysis of the relationships between fold structures metamorphic mineral assemblages and fabrics affecting igneous bodies suggest the interaction of three processes deformation metamorphism and magmatism low pressure prograde metamorphism grades laterally from greenschist to amphibolite facies over a distance of 5 km zones of intense metamorphism coincide with transpressional deformation and with the intrusion of small sheets of diorite and granitoid sillimanite schists and small migmatite pods are distributed around the small intrusions of basic intermediate composition the inferred maximum thermal gradient is 80 c km this thermal pattern is similar to patterns observed in other variscan massifs in the pyrenees where high metamorphic gradients are interpreted in different ways in the ne cap de creus the type and distribution of prograde metamorphism was caused or at least enhanced by the intrusion of diorites and granitoids alternatively deformation developed at prograde conditions and synchronously with magmatism displays a highly heterogeneous distribution producing a sub vertical zone of high strain that coincides with domains of highest grade metamorphism and igneous activity this zone of high strain developed in a broad transpressive regime could have favoured the injection of magmatic sheets from deep to mid crustal levels progressive folding from prograde to retrograde metamorphic conditions might account for the narrowing of metamorphic zones

thermal history of volcanic debris flow deposits on the eastern flanks of mt taranaki new zealand implications for future hazards abstract we use palaeomagnetic methods to decipher the thermal histories of a succession of massive to weakly stratified debris flow deposits ngatoro and te popo formations of late holocene age located on the eastern lower flanks of mt taranaki egmont volcano western north island new zealand results from two sites vickers quarry and surrey road quarry both c 9 6 km from the present day summit enable us to distinguish between clast incorporation temperatures of about 400 c and emplacement temperatures between 150 and 200 c consistent with observation of superficial charring and desiccation of outer podocarp hardwood tree trunks at vickers quarry analysis of palaeomagnetic directions and lithofacies architecture suggest that these deposits were likely initiated as a closely spaced succession of block and ash flows bafs that rapidly cooled as they descended the volcano flanks radiocarbon chronology and the widespread occurrence of a palaeosol between the products of the preceding inglewood eruptive phase c 3 4 cal ka b p and the overlying ngatoro formation suggest that these two events are temporally unrelated certainly there is no field evidence of contemporaneous explosive volcanic activity that might be related to the emplacement of ngatoro formation however we suggest that these low temperature deposits might either relate to collapse of a small emergent lava dome or a cooling dome remnant possibly emplaced in the aftermath of the inglewood eruption how collapse was initiated remains uncertain the remnant dome may have been rendered unstable by volcano tectonic or tectonic seismic events and or by adverse meteorological events nevertheless this study demonstrates that edifice collapse events generating potentially hazardous debris flows can occur independent of specific eruptive activity

a new high resolution record of holocene geomagnetic secular variation from new zealand we present the first full holocene palaeomagnetic secular variation record from new zealand the 11 500 year long record from the sediments of mavora lakes comprises composite declination inclination and relative palaeointensity logs compiled from two six metre long cores and the uppermost 1 5 m of another an age model has been developed from 28 ams radiocarbon age determinations on fragments of southern beech lophozonia menziesii and fuscospora cliffortioides leaves the excellent between core correlation in all three components of the field results in a high resolution palaeosecular variation record with precise and accurate age control the variations change in character from high amplitude in phase declination and inclination swings in the earliest part of the record to low amplitude variations in the middle part and declination and inclination swings that are 90 out of phase leading to broad looping of the vector in the upper part of the record that is consistent with westward drifting sources in the outer core the present day field at the site dec 24 2 e inc 70 7 f 59 t represents a rare steep and easterly extreme direction but close to average intensity the palaeointensity is inferred to have varied between about 40 and 90 t with variations that to some extent mirror variations in the virtual axial geomagnetic dipole moment seen from global data but also show some notable differences particularly in the past few thousand years

polymetamorphism and ductile deformation of staurolite cordierite schist of the bossost dome indication for variscan extension in the axial zone of the central pyrenees the bossost dome is an e w trending elongated structural and metamorphic dome developed in cambro ordovician metasedimentary rocks in the variscan axial zone of the central pyrenees a steep fault separates a northern half dome cored by massif granite from an e w trending doubly plunging antiform with granitic sills and dykes in the core to the south the main foliation is a flat lying s 1 2 schistosity that grades into a steeper dipping slaty cleavage at the dome margins three major deformational and two metamorphic phases can be differentiated s 1 2 schistosity is an axial planar cleavage to w vergent recumbent folding that probably occurred in mid westphalian time peak regional metamorphism m 1 is characterized by static growth of staurolite and garnet following thermal relaxation of the previously thickened crust strong non coaxial deformation recording uniform top to the se extension during d 2 a is preserved in staurolite garnet schists in a 1 5 km thick shallowly se dipping zone in the southeastern dome a 500 m thick contact aureole m 2 was imprinted on the regionally metamorphosed rocks following the intrusion the bossost granite during d 2 b more coaxial deformation prevailed during synkinematic growth of m 2 phases in the inner part of the contact aureole around the northern part of the dome where it obliterated d 2 a fabrics progressive non coaxial deformation continued in the southeastern antiform and is recorded by late synkinematic growth of cordierite successive overprinting of the m 1 staurolite garnet assemblage by andalusite and cordierite of m 2 is preserved in the southern part the assemblage muscovite cordierite staurolite biotite is considered metastable given the low mn and zn contents of staurolite and cordierite and interpreted as the result of prograde metamorphism during decompression p t conditions during m 2 were approximately 3 kbar and 600 c pervasive crenulations and mesoscopic to regional southerly verging folds are the result of d 3 nne ssw compression post dating ductile deformation and contact metamorphism polymetamorphic assemblages of the bossost dome preserve a regionally confined zone of ese directed extensional shearing within an overall n s compressional setting exact timing of extensional shearing is not known but can be constrained by recumbent folding during the mid westphalian and granitic intrusions which confine it to late carboniferous time c 305 ma crustal scale flat lying extensional shear zones with similar orientation and time frame are observed in the hospitalet massif of the eastern axial zone this suggests that crustal extension though probably restricted by regional strain partitioning over orthogneiss or intruding granitic bodies within an overall compressive setting was not uncommon in late carboniferous time in the axial zone of the pyrenees

synthesizing large scale pyroclastic flows experimental design scaling and first results from pele pyroclastic flow eruption large scale experiment pele is a large scale facility for experimental studies of pyroclastic density currents pdcs it is used to generate high energy currents involving 500 6500 m3 natural volcanic material and air that achieve velocities of 7 30 m s 1 flow thicknesses of 2 4 5 m and runouts of 35 m the experimental pdcs are synthesized by a controlled eruption column collapse of ash lapilli suspensions onto an instrumented channel the first set of experiments are documented here and used to elucidate the main flow regimes that influence pdc dynamic structure four phases are identified 1 mixture acceleration during eruption column collapse 2 column slope impact 3 pdc generation and 4 ash cloud diffusion the currents produced are fully turbulent flows and scale well to natural pdcs including small to large scales of turbulent transport pele is capable of generating short pulsed and sustained currents over periods of several tens of seconds and dilute surge like pdcs through to highly concentrated pyroclastic flow like currents the surge like variants develop a basal 0 05 m thick regime of saltating rolling particles and shifting sand waves capped by a 2 5 4 5 m thick turbulent suspension that grades upward to lower particle concentrations resulting deposits include stratified dunes wavy and planar laminated beds and thin ash cloud fall layers concentrated currents segregate into a dense basal underflow of 0 6 m thickness that remains aerated this is capped by an upper ash cloud surge 1 5 3 m thick with 100 to 10 4 vol particles their deposits include stratified massive normally and reversely graded beds lobate fronts and laterally extensive veneer facies beyond channel margins

the detection and significance of early deformation in the southern variscan pyrenees spain implications for regional paleozoic structural evolution detailed structural analysis of part of the variscan southcentral pyrenees revealed the occurrence of several deformation generations of which the most important one called the mainphase folding and striking wnw ese seems to be the oldest directional analysis of structural elements related to mainphase folding sedimentary bedding mainphase cleavage small scale foldaxes and intersection lineations shows however that sedimentary bedding must have been non planar before mainphase deformation took place this observation suggests that premainphase folding occurred as well and indeed the areal distribution of intersection lineations in the studied area demonstrates the existence of two early variscan fold systems they are characterized by very open nnw sse and wsw ene folds and have subvertical axial planes and subhorizontal foldaxes in strong contrast to mainphase folds penetrative axial plane foliations did not develop during deformation

using thermal remanent magnetisation trm to distinguish block and ash flow and debris flow deposits and to estimate their emplacement temperature 1991 1995 lava dome eruption at mt unzen volcano japan abstract the 1991 1995 mt unzen eruption kyushu japan produced 13 lava domes approximately 9400 block and ash pyroclastic flows baf resulting from lava dome collapse events and syn and post dome collapse debris flow df events in the field it can be very difficult to distinguish from field facies characteristics which deposits are primary hot baf cold baf or rock avalanche or secondary df deposits in this study we use a combination of field observations and thermal remanent magnetisation trm analysis of juvenile lava dome derived clasts from seven deposits of the 1991 1995 mt unzen eruption in order to distinguish between primary baf deposits and secondary df deposits and to determine their emplacement temperature four major trm patterns were identified 1 type i clasts with a single magnetic component oriented parallel to the earth s magnetic field at time and site of emplacement this indicates that these deposits were deposited at very high temperature between the curie temperature of magnetite 540 c and the glass transition temperature of the lava dome 745 c these clasts are found in high temperature baf deposits 2 type ii clasts with two magnetic components of magnetisation the lower temperature magnetic components are parallel to the earth s magnetic field at time of the unzen eruption temperature estimations for these deposits can range from 80 to 540 c we found this paleomagnetic behaviour in moderate temperature baf or warm df deposits 3 type iii clasts with three magnetic components with a lower temperature component oriented parallel to the earth s magnetic field at unzen the individual clast temperatures estimated for this kind of deposit are usually less than 300 c we interpret this paleomagnetic behaviour as the effect of different thermal events during their emplacement history there are several interpretations for this paleomagnetic behaviour including remobilisation of moderate temperature baf warm df or syn eruptive granular flows 4 type iv clasts with single or two magnetic components that are randomly oriented suggesting an emplacement at ambient temperature these clasts are found in df and syn eruptive rock avalanche deposits containing high proportion of well rounded clasts and non carbonised vegetation we demonstrate that trm analysis associated with field observations can be a viable method for determining the emplacement temperature of hot baf and cold df deposits characterised by similar componentry and sedimentological facies this has implications for hazard assessment of older dormant volcanic systems to reconstruct the eruptive history

the role of strike slip fault systems at plate boundaries a new analysis shows that most 59 plate boundaries have a relative velocity vector that is markedly oblique greater than 22 to the boundary normal a significant proportion 14 have vectors that are nearly 22 parallel to the boundary accommodation of the oblique motion usually involves strike slip faulting but the kinematic role of these faults differs at divergent and convergent boundaries four main types of plate boundary related strike slip faults are distinguished ridge transforms boundary transforms trench linked strike slip faults and indent linked strike slip faults discrimination of the four types should be possible in ancient orogenic belts but is complicated by the common reactivation of the strike slip zones in other roles plate boundary related strike slip faults form major lineaments at the present day ridge transforms have a low preservation potential in continents boundary transforms and indent linked faults often re use old lineaments but trench linked strike slip faulting is an effective method of forming new lineaments in continental crust strike slip faulting in general is less commonly recognized in ancient orogenic belts than its abundance in present plate boundary orogens requires this under recognition results both from poor understanding of strike slip kinematics and from deeper prejudices about the way in which orogenic belts form

considerations about hercynian thrusting in the marimanya massif central pyrenees abstract we can distinguish four main units within the marimanya massif the cambro ordovician detrital series a carbonate unit mainly made up of massive limestones dated as silurian devonian dark slates and limestones dated as middle upper devonian and the late hercynian marimanya intrusive mainly composed of granites and granodiorites mapping has shown that the carbonate units lie on different levels of the cambro ordovician series and even on the dark slates and limestones this abnormal contact is demonstrated to be a thrust the thrusting surface is affected by hercynian folds with vertical axial planes a fold axis trending e w and an associated cleavage moreover it is cut by the late hercynian intrusive consequently considering paleontological and structural data as well as the regional context the most reasonable interpretation is that this structure represents a hercynian thrust probably with southward displacement

hercynian crustal scale extensional shear zone in the pyrenees detailed structural analysis of metasedimentary rocks and gneisses in the hospitalet massif pyrenees has revealed that upright structures in low grade rocks are in a structurally higher position and older than gently inclined to flat lying structures in higher grade rocks the steep structures were sub vertically flattened during formation of the flat lying structures indicating that the higher grade rocks formed as part of a shear zone system a similar conclusion can be drawn from the relation between deformation and metamorphism in the higher grade rocks we suggest that the shear zone may have accommodated crustal extension

circa 546 ma plume related dykes in the 1 ga novillo gneiss east central mexico evidence for the initial separation of avalonia abstract the 1 ga novillo gneiss exposed in an inlier in the front ranges of the laramide belt in east central mexico is cut by an unmetamorphosed ne trending mafic dyke swarm the gneiss represents the northernmost inlier of the oaxaquia terrane that underlies the backbone of mexico titanite and biotite from metasedimentary rocks yielded ages of 928 2 ma concordant u pb age and duplicate 40 ar 39 ar plateau ages of 697 7 and 697 10 ma which are inferred to represent cooling through 660 and 300 350 c respectively on the other hand hornblende from cross cutting ne trending mafic dykes yielded a 40 ar 39 ar plateau age of 546 5 ma which is inferred to closely post date the time of intrusion combined with published ages u pb zircon sm nd garnet k ar hornblende the data suggest that following granulite facies metamorphism at 982 6 ma 8 9 9 7 kbar and 730 775 c the novillo gneiss cooled at a rate of 1 45 c my if this latter rate is extrapolated it would place the rocks at the surface 30 c by 497 ma about 50 my after dyke emplacement geochemistry of the dykes indicates that they are fe rich tholeiites with i low mgo but high tio 2 and other hfse ii smooth mantle normalized incompatible trace element patterns peaking at nb and la and iii intraplate tectonic affinities these features are typical of enriched tholeiites associated with plumes current precambrian paleogeographic reconstructions place avalonia against oaxaquia and subsequently separate avalonia by way of ridge trench collision at 550 ma giving rise to the unique avalonian cambrian fauna and a rapid phase of early cambrian subsidence these events are contemporaneous with emplacement of the novillo mafic dykes suggesting that the plume related magmatism and separation of avalonia from oaxaquia are linked