magnetostratigraphy of the lebo and tongue river members of the fort union formation paleocene in the northeastern powder river basin montana we analyzed paleomagnetic samples and documented the stratigraphy from two sections near miles city montana to determine the geomagnetic polarity stratigraphy and to constrain the age and duration of the lebo and tongue river members of the fort union formation in the northeastern powder river basin the resulting polarity sequence can be correlated to subchrons c29n c26r of the geomag netic polarity time scale by interpolating measured sediment accumulation rates from the base of c28r to the top of c27n and then extrapolating to the top of the tongue river member and the bottom of the lebo member we developed two age models to estimate the durations of the lebo and tongue river members based on the first model which uses different sedimentation rates for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 30 and 1 74 million years and of the tongue river to be between 1 42 and 1 61 million years using the second model which uses the same sedimentation rate for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 33 and 1 76 million years and of the tongue river to be between 1 00 and 1 25 million years our results indicate a decrease in sediment accumulation rates in c27r which is likely the result of a 0 26 to 0 62 million year long depositional hiatus in the middle of c27r represented by the lebo tongue river contact this unconfor mity occurs 2 million years earlier than previously suggested and is likely contempo raneous with unconformities in the williston basin and in southwestern alberta suggesting that it may be regionally significant

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

wuchiapingian early lopingian permian global brachiopod palaeobiogeography a quantitative approach abstract a global presence absence database of 212 wuchiapingian early lopingian permian brachiopod genera from 30 stations is analysed by cluster analysis nonmetric multidimensional scaling and minimum spanning tree to document the global palaeobiogeographical patterns five core groups are revealed by the quantitative analysis and interpreted as representing five marine biotic provinces they are the cathaysian tropical western tethyan tropical himalayan warm temperate austrazean cold temperate and greenland svalbard provinces cold temperate the cathaysian province is composed of many isolated or semi isolated islands situated in the palaeotethys whereas the other four provinces occurred mainly on the continental shelves of pangea the western tethyan province along the western coast of the palaeotethys the himalayan province on the northern margin of gondwanaland the austrazean province along the southeastern margin of gondwanaland and the greenland svalbard province on the northern margin of pangea in addition nonmetric multidimensional scaling helped to identify key biogeographic determinants latitude related thermal gradient appears to have accounted for most of the variance in the data geographic distance and ocean circulation may have also played a major but subordinate role in the delineation and or enhancement of some of the provinces comparison with early and middle permian global marine provincialism indicates that marine biotic provinces had significantly reduced during the lopingian late permian in the lead up to the end permian mass extinction

direction changes in thrusting of the schistes lustres in alpine corsica abstract the direction of thrusting contemporaneous with high pressure low temperature hp lt metamorphism of the ophiolite schistes lustres nappes in cap corse alpine corsica has changed from being towards the northwest to towards the southwest during upper cretaceous obduction similar anticlockwise changes in thrusting have been observed in other regions of alpine corsica calabria and southern betic cordilleras a model is proposed for the alpine evolution of this part of the western alps involving a sinistral component of transcurrent movement added to the northwest thrusting these events have been followed by eocene backthrusting of nappe of southern alpine origin in northwest cap corse towards the southeast with associated backfolding of the underlying schistes lustres

ordovician magmatism in the levezou massif french massif central tectonic and geodynamic implications new u pb dating on zircon yielded ca 470 ma ages for the granitoids from the levezou massif in the southern french massif central these new ages do not support the previous interpretation of these granitoids as syn tectonic intrusions emplaced during the late devonian early carboniferous thrusting the geochemical and isotopic nature of this magmatism is linked to a major magmatic ordovician event recorded throughout the european variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension the comparable isotopic signatures of these granitoids on each side of the eclogite bearing leptyno amphibolitic complex in the levezou massif together with the fact that they were emplaced at the same time strongly suggest that these granitoids were originally part of a single unit tectonically duplicated by either isoclinal folding or thrusting during the variscan tectonics

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

staurolite producing reactions and geothermobarometry of a high pressure thermal aureole in the french massif central biotite garnet staurolite and kyanite isograds in pelitic metasedimentary rocks are developed as a result of thermal metamorphism around syntectonic granitoids in eastern rouergue france temperature estimates range between 400 c and 650 c at about 6 5 kbar geothermobarometry shows a steep isobaric t gradient which is consistent with the interpretation that the metamorphic highs are thermal aureoles high grade rocks show evidence of two staurolite forming reactions in the presence of plagioclase and the absence of chlorite that have not been described previously in the literature the reaction that occurs in the middle staurolite zone alm rich ga ca rich pla na rich mu gro rich ga na rich pla st na poor mu is considered to be prograde whereas the reaction that occurs in the kyanite zone alm rich ga ca rich pla w st ca rich ga na rich pla qz is retrograde the topology of these reactions is illustrated in terms of end member compositions for the systems knafash and kcafash respectively

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

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

strain fabric evolution within and near deformed igneous sheets the sudbury igneous complex canada abstract deformation structures differing in style and orientation within a given terrane are often attributed to distinct tectono metamorphic events however mechanical heterogeneities may locally cause strain perturbations that can have a profound effect on the geometry of such structures here we document highly variable orientations of planar structures metre to kilometre scale folds and shortening directions inferred from brittle fault analysis within the synclinal and layered sudbury igneous complex sic and its huronian host rocks nw se shortening during palaeoproterozoic deformation led to the formation of planar structures in the host rocks that are parallel to the ne sw striking contact of the southern sic during deformation local contact parallel shortening became more important than regional nw se shortening and generated contact orthogonal planar mineral fabrics and folds local contact parallel shortening is attributed to the shape change of the southern sic from a convex outward to a concave inward curvi planar geometry contact parallel shortening accounts for the formation of a previously unidentified kilometre scale buckle fold in the sic and respective axial planar mineral fabrics in huronian host rocks this buckle fold shares similar structural characteristics with known higher order buckle folds of the eastern sic we suggest that non cylindrical buckling and associated mineral fabric development accommodated the shape change of the sic this resulted in mutually perpendicular fabric orientations which are compatible with overall nw se shortening the original shape of the sic seems to have had a profound influence particularly on the geometry of metamorphic foliations our structural analysis supports earlier structural studies advocating that highly discordant planar strain fabrics can be generated by local strain perturbations near igneous sheets under uniform regional shortening

structural characteristics of granitic plutons emplaced during weak regional deformation examples from late carboniferous plutons morocco abstract strain patterns have been investigated around shallow crustal granitic plutons emplaced during weak regional deformation in the western moroccan hercynian belt the plutons are distributed along reactivated deep faults and display elliptical shapes which are consistently oriented with respect to the regional strain field pluton age orientation and ellipticity can be correlated with strain history ductile deformation and contact metamorphism are mainly located in the thermally softened aureoles finite strain gradients towards plutons are accompanied by the development of superimposed microstructures finite strain trajectories and variations of strain ellipsoid shapes exhibit perturbations of the regional strain field around plutons structures internal to the plutons are continuous with regional structures foliation development in granitoids records a decrease in temperature during syntectonic pluton cooling geometry and kinematics of deformation reflect the influence of both regional deformation and pluton intrusion

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

exhumation of the ultrahigh pressure tso morari unit in eastern ladakh nw himalaya a case study exhumation processes of the ultra high pressure uhp tso morari dome nw himalaya are investigated using structural petrological and geochronological data the uhp tso morari unit is bounded by the low grade metamorphic indus suture zone to the ne and mata unit to the sw three deformation phases d1 d2 and d3 are observed only d3 is common to the uhp unit and the surrounding units in the uhp unit the first deformation phase d1 produced upright folds under eclogitic conditions 20 kbar 580 60 c d1 is overprinted by d2 structures related to a nw se trending open anticline this phase is characterized by blueschist mineral associations and corresponds to the quasi isothermal decompression from a depth of 90 km eclogitic conditions up to 30 40 km the final exhumation phase of the tso morari unit is dominated by tectonic denudation and erosion d3 associated with a slight temperature increase radiochronological analyses indicate that the uhp exhumation process began during the eocene exhumation was fast during d1 d2 and slowed down through d3 in oligocene time the change in the deformation style from d1 d2 to d3 in the tso morari unit coincides with changes in the exhumation rates and in the metamorphic conditions these changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge to the vertical uplift of the tso morari unit across the upper crust

geometry classification and kinematics of s c and s c fabrics in the mushandike area zimbabwe planar fabrics in small dextral shear zones in the mushandike area of the zimbabwe archaean craton consist of a sigmoidal primary foliation s displaced by discrete shears c or c these planar fabrics can be divided into porphyroclastic megacrystic and banded types depending mainly on the values of the angles between s and c or c surfaces the structures that define s surfaces and the spacing of c or c surfaces the values of the angles between the foliations are proposed as the basis for a general classification of planar fabrics in shear zones the values of the angles depend on microstructures and strain microstructural evidence shows that slip did not occur on s surfaces but only on c or c surfaces a model for the development of the fabrics proposes that s surfaces lie parallel to the xy plane of the local finite strain ellipsoid in domains of simple shear between c or c surfaces c surfaces may form in the orientation of a coulomb failure surface the field evidence suggests that an increase in bulk strain is at least partly accommodated by an increase in local strain in domains between c or c surfaces as the s surfaces rotate with the local finite strain ellipsoid

strain localisation in an orthogneiss laccolith the pinet massif aveyron southern france abstract sheared granitoids of the pinet laccolith southern france display strain gradients from undeformed protolith to s c mylonites and ultramylonites that formed at amphibolite facies of metamorphism we study shear strain distribution at a regional scale and use image processing techniques to measure strain across continuous shear gradients local perturbations of strain intensity seem to represent early stages of shear strain localisation parallel to the main mylonitic foliation our study suggests that localisation of shear strain shows a periodicity at regional and outcrop scales

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

the himalayan main central thrust pile and its quartz rich tectonites in central nepal as a consequence of the india asia collision a deeply rooted crustal slice has been moved southwards over the northern boundary of the indian plate at the present level of observation the main central thrust pile a rock pile of several kilometers thickness is affected by shear deformation which imprinted a flat foliation and a constant n s trending lineation these tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field the m c t shear zone is studied from the view point of the quartz rich rocks over 100 samples helped to define a microstructural zonation roughly parallel to the thermal one 35 samples particularly rich in quartz have been used for the preferred orientation study of their c axes and a few of them for their a axes strong lattice orientations when undisturbed by mainly secondary recrystallization attest to large plastic strains in quartz the c axes are mostly distributed along unequally populated crossed girdles which may evolve into an inclined single girdle the a axes concentrate perpendicular to the more populated girdle concerning quartz the following is suggested 1 1 the deformation has a dominant shear component responsible for the dominant stretching lineation it could be simple shear in case of inclined single girdles 2 2 one set of a axes line up parallel to the shear direction slip on various planes in the a direction may be an important deformation mechanism 3 3 asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the himalayan thrusting in more than 80 of cases

microtectonic evidence for eastward ductile shear in the jurassic orogen of sw japan abstract two jurassic deformation phases are responsible for the thrust and nappe structures in the infrastructure of the outer belt of the mesozoic chain of sw japan the first phase was synmetamorphic and took place under hp lt conditions microtectonic analysis shows that the penetrative deformation corresponded to a ductile shear directed from w to e parallel to the stretching and mineral lineation at the regional scale the first phase corresponded to the obduction of the greenschist nappes upon the kurosegawa continent during the second phase the greenschist nappes were sliced leading to an apparent reverse metamorphic zoning the first phase structures were then locally reworked by folds giving rise to an apparent westwards sense of shear

regional shear variation in relation to diapirism and folding abstract regional structural trends in the levezou area southern massif central france delineate an antiformal structure deformed granitoids display changes along strike in s c plane relationships and therefore in sense of shear these variations are interpreted to be strain accommodation features that developed during the diapiric emplacement and folding of the granitoid sheets and so cannot be used as kinematic indicators on a regional scale