geology and geochronology of the suomujarvi complex a new archaean gneiss region in the ne baltic shield finland abstract recent mapping in the eastern central lapland granitoid complex and dating of zircons at the nordsim facility was performed in order to highlight the existence of an 892 km2 area of archaean gneisses not previously identified in southeast lapland finland this archaean enclave is composed of northeast trending km scale bands of orthogneisses with varying mafic content and is named the suomujarvi complex along its southeastern boundary lies the aholanvaara volcano sedimentary package the evolution of the suomujarvi complex began at ca 2810 ma with intrusion of the homogeneous jumisko biotite tonalite granodiorite in the west this lithology becomes increasingly heterogeneous to the east where muscovite and an inherited ca 2870 ma old rock component represented by zircon cores are present these orthogneisses were later intruded by the 2755 18 ma 1 protolith of the miehinkavaara amphibole and biotite bearing diorite tonalite gneisses producing diffuse contacts characterized by gradual increases in mafic minerals all of the orthogneisses were then migmatized metamorphosed after ca 2730 ma the aholanvaara volcano sedimentary package was deposited its main quartzite unit was dominantly derived from an unknown 2731 17 ma 1 source but it also contains a ca 3 4 ga detrital component the absence of 2800 2820 ma detrital material in these sediments indicates they were not derived locally from the suomujarvi complex instead they share affinities with supracrustal packages further south in posio east in the belomorian province and north in the central lapland greenstone belt widespread voluminous intrusion of granites occurred in the area surrounding the suomujarvi gneisses and left a marked metamorphic overprint on the gneisses during the ca 1 8 extensional collapse of the svecofennian and lapland kola orogens multi scale folding produced subhorizontal southwest plunging tight to isoclinal upright folds and the dominant axial planer northeast trending fabric in the area this fabric is cut by a northwest trending fabric in the southwestern part of the area geological characteristics of the suomujarvi complex and ages of the orthogneisses within it indicate that it most likely represents the southwestern margin of the archaean belomorian block

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

atmospheric pumping a mechanism causing vertical transport of contaminated gases through fractured permeable media as the barometer falls gases are drawn upward out of the permeable earth into the atmosphere conversely a rising barometer pushes air downward in a homogeneous permeable medium these cyclical gas motions are piston like and nearly reversible so they contribute only modestly to the net transport of contaminant gases in a fractured permeable medium however the fractures will generally serve as breathing passages for all of the gas filled porosity greatly increasing the amplitude and nonuniformity of vertical motions the resulting transport process may be orders of magnitude more significant than molecular diffusion according to the theoretical analysis presented here analytical solutions are first derived for the sinusoidal pressure response of a medium containing identical vertical fractures equally spaced by slabs of permeable matrix material these solutions are then used to constrain the relationship between fracture aperture and fracture spacing based on field comparisons between surface and subsurface pressure variations the final phase of the analysis addresses the diffusive and advective transport of an inert trace gas which is carried by an oscillatory flow along a fracture having permeable walls a maximum rate of transport is predicted to occur for an intermediate fracture spacing which is typically a few meters

porphyroblast inclusion trail geometries in the grand canyon evidence for non rotation and rotation abstract porphyroblasts in paleoproterozoic supracrustal rocks of the grand canyon grew during a progressive deformation that involved shortening and heterogeneous development of ne striking subvertical foliation s2 that crenulates and transposes an earlier foliation s1 inclusion trails in syn s2 porphyroblast cores show a remarkably consistent nw strike across the regional transect suggesting a lack of appreciable porphyroblast rotation relative to the trace of the ne striking s2 foliation these apparently non rotated porphyroblast cores thus preserve a regional nw striking s1 orientation porphyroblast rims and younger porphyroblasts locally overgrow ne striking s2 indicating that porphyroblast growth and continued non rotation spanned s2 apparent non rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening in contrast single thin sections show domains of these non rotated garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45 this documents local rotation of garnets relative to s2 during a heterogeneous late or post s2 reactivation of the foliation thus these types of studies should continue to look for both rotational and non rotational behaviors at all scales and continue to specify the frame of reference

the three dimensional geometry of simulated porphyroblast inclusion trails inert marker viscous flow models a fluid dynamic model for a rigid spherical porphyroblast growing in a viscous fluid matrix undergoing simple shear deformation predicts an arrangement of captured inert inclusions that is remarkably similar to the spiral pattern observed in synkinematic rolled garnets the model assumes only creeping stokes flow of the matrix and a kinetic model for the growth of the porphyroblast of the form rm km where r is the crystal radius is the amount of shear induced rotation and m and k are kinetic parameters for rotation less than 18d j b thompson and j l rosenfeld s simple ring model provides a surprisingly accurate description of the three dimensional geometry of the central inclusion surface in the simulated porphyroblasts although marker planes originally parallel to the shear plane are deflected around the porphyroblast their intersections with its surface remain approximately circular for much larger amounts of rotation than anticipated by rosenfeld the polar coordinate equation t r of the double spiral formed by the trace of the central inclusion surface in a section through the centre of simulated porphyroblasts is also surprisingly simple as follows rm 6 5k m this relationship was observed over the complete range of rotation angles investigated 0 720d and could form a reasonable basis for estimating the growth rotation history of rolled garnets from the shape of their spiral inclusion trails two dimensional sections through the simulated porphyroblasts parallel to the rotation axis yield clamshell rosenfeld s term inclusion geometries similar to the controversial millipede patterns observed in many natural porphyroblasts

a 3d study of aligned porphyroblast inclusion trails across shear zones and folds inclusion trails si within 741 equant garnet porphyroblasts show a consistent orientation across a 100 m2 outcrop of precambrian metapelite with later d2 folds and shear zones in posio finland variation in si orientations is not systematic with respect to these later folds and shear zones but instead represents microfolding and warping of s1 before the formation of d2 structures the development of the d2 folds and shear zones is investigated by comparing the pattern of si in the outcrop to those predicted for porphyroblasts in tangential longitudinal strain axial slip flexural flow pure shear partitioned flexural flow and partitioned pure shear folds si patterns are also compared with models for non coaxial and coaxial shear zones along with corroborating evidence from structures in the outcrop and matrix the comparisons indicate that d2 folds and shear zones in the outcrop formed in response to nw se axially symmetric shortening during which garnet porphyroblasts did not rotate relative to each other the axial plane of f2 folds and d2 shear zone boundaries

reading history of folding from porphyroblasts abstract the observation that inclusion trails in porphyroblasts commonly exhibit a more or less constant orientation over a large area even though the enclosing rocks have been folded has been considered puzzling by geologists and has generated considerable debate an analysis of vorticity during multilayer folding in response to layer parallel shortening shows that the observation is in fact theoretically expected folding of multilayers in response to layer parallel shortening bulk folding can be considered kinematically as a time variable combination of flexural flow and pure shear components the rotation of a spherical porphyroblast during folding is related to the vorticity history in the vicinity of the porphyroblast by a set of differential equations for likely folding histories in nature the solutions to these equations are in agreement with naturally observed patterns therefore it is unnecessary to invoke other mechanisms such as assuming porphyroblasts to be in islands irrotational with respect to the fold axial plane more importantly the theory allows constraints to be placed from current data sets of porphyroblast inclusion trail geometry on the history of folding and the competence properties of rocks at the time of folding

a new spin on non rotating porphyroblasts implications of cleavage refraction and reference frames abstract it has been claimed that rigid porphyroblasts which grow before or during folding and concurrent cleavage development do not rotate with respect to the geographical reference frame grf even if the straining is non coaxial bell 1985 bell and johnson 1990 the explanation offered is based on strain partitioning it is argued that the initial orientations of early fabrics included as internal foliations s i in the porphyroblasts have been preserved after polyphase deformation and even after successive orogenies according to the strain partitioning model the porphyroblasts are fixed in domains of coaxial straining microlithons and are isolated from the non coaxial straining associated with the enveloping septa s e this hypothesis and also its discussions both pro and contra suffer from insufficient attention to reference frames we therefore attempt to demonstrate a the need for rigorous treatment of reference frames in geological interpretations b that in a folding situation grains that do not rotate with respect to their immediate matrix generally rotate with respect to the grf c that lack of porphyroblast rotation with respect to the grf demands a rare folding mechanism slip fold model and d that the non rotation hypothesis is in conflict with heterogeneous deformation cleavage refraction finally we question the validity of the evidence in a study by fyson 1980 cited in support of non rotation with respect to the grf during folding fyson reported orientations of s i that are constant after folding over a large area this scenario is a product of selective data acquisition in summary our investigation shows that the lack of porphyroblast rotation with respect to a grf during folding while possible is not universal the development of microstructures e g curved s i is only related to the local deformation path the characterisation of which does not rely on the grf

fold mechanisms in the canton schist constraints on the contribution of flexural flow abstract fold mechanisms operating in the canton schist have been resolved using the geometric relationship between folds and spiral inclusion trail geometries of the four end member fold mechanisms tangential longitudinal strain folding and slip folding are unable to produce the observed inclusion trail and fold geometries but a combination of flexural flow and pure shear folding is consistent with the geometric constraints the maximum flexural flow component during each fold event was determined from the geometric data during f 3 and f 4 flexural flow produced 27 and 37 of measured limb rotation respectively which corresponds to a maximum of 24 limb rotation by flexural flow in each fold event to satisfy the geometric constraints the remainder of limb rotation must be a product of pure shear folding the maximum possible component of flexural flow folding during f 3 and f 4 increases with increased variation in vorticity between layers in the rock mass in the flexural flow pure shear model a maximum of 28 of inclusion trail curvature is produced by rotation of porphyroblasts relative to irrotational fold limbs with a minimum 72 curvature due to rotation of fold limbs relative to irrotational porphyroblasts all model solutions produce less than 8 syn folding porphyroblast rotation relative to geographic coordinates

stereological analysis of fractures in the roselend tunnel and permeability determination 1 fracture traces and water fluxes have been determined along the 128 m long roselend tunnel french alps fractures can be classified into two families large fractures which intersect the tunnel and small fractures which partially intersect it three different zones in the tunnel are distinguished with mild low and high water fluxes starting from the entrance a stereological analysis of the trace length probability densities of small fractures provides the fracture diameter probability density distribution which is best approximated by a power law large fractures are assumed monodisperse with a 5 m estimated radius the generated fracture networks obtained by combining large and small fractures do percolate while networks consisting of only small fractures do not percolate computed macroscopic permeabilities of the fracture networks are in agreement with the observed water fluxes when the fracture permeability is a power law of its lateral extent with an exponent equal to 3

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

foliation development and refraction in metamorphic rocks reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation its role has generally been insufficiently emphasized because evidence is best preserved where por phyroblasts which contain inclusion trails are present reactivation occurs when progressive shearing operating in a synthetic anastomosing fashion parallel to the axial planes of folds changes to a combination of coarse and fine scale zones of progressive shearing some of which operate antithetically relative to the bulk shear on a fold limb reactivation of earlier foliations occurs in these latter zones reactivation decrenulates pre existing or just formed crenulations generating shearing along the decrenulated or rotated pre existing foliation planes partitioning of deformation within these foliation planes such that phyl losilicates and or graphite take up progressive shearing strain and other minerals accom modate progressive shortening strain causes dissolution of these other minerals this results in concentration of the phyllosilicates in a simi lar but more penetrative manner to the for mation of a differentiated crenulation cleavage except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds reactivation can generate bedding parallel schistosity in multideformed and meta morphosed terrains without associated folds heterogeneous reactivation of bedding gen erates rootless intrafolial folds with sigmoidal axial planes from formerly through going struc tures reactivation causes rotation or refrac tion of axial plane foliations forming in the same deformation event causing reactivation in those beds or zones in which an earlier foli ation has been reactivated and results in destruction of the originally axial plane foli ation at high strains reactivation also provides a simple explanation for the apparently wrong sense but normally observed rotation of gar net porphyroblasts whereby the external foli ation has undergone rotation due to antithetic shear on the reactivated foliation alter natively the rotation of the external foliation can be due to its reactivation in a subsequent deformation event porphyroblasts with inclusion trails com monly preserve evidence of reactivation of earl ier foliations and therefore can be used to ident ify the presence of a deformation that has not been recognized by normal geometric methods because of penetrative reactivation reac tivation often reverses the asymmetry between pre existing foliations and bedding on one limb of a later fold leading to problems in the geo metric analysis of an area when the location of early fold hinges is essential the stretching lineation in a reactivated foliation can be rad ically reoriented potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events other aspects of reac tivation however can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized for example d grown porphyroblasts may be dis solved against reactivated s and hence appear to have grown syn d

deformation history of the otago schists new zealand from progressively developed porphyroblast matrix microstructures uplift collapse orogenesis and its implications abstract foliation overprinting relationships preserved in porphyroblasts and the surrounding matrix in the otago schists indicate development of up to six penetrative foliations during the late paleozoic mesozoic rangitata orogeny s1 s3 and s5 formed with steep dips and s2 s4 and s6 formed with gentle dips s1 and s2 are preserved only as inclusion trails in porphyroblasts because of matrix foliation reworking during progressive deformation s3 s6 are preserved in the matrix as well as in porphyroblasts and are generally correlatable from one to the other inclusion trail asymmetries define the location of the orogenic uplift core during the rangitata orogeny when the porphyroblasts grew the location of this core is consistent with macroscale geological and structural observations alternating steep and gently dipping foliations are inferred to indicate alternating horizontal compression and extension predictions made by an orogenic model compatible with continually alternating horizontal compression and extension compare favorably with geometries observed in the schists reconstruction of the gross geometry of the otago schists during the rangitata orogeny based on the three dimensional geometry of porphyroblast inclusion trails is similar to reconstructions based on regional geological considerations and plate tectonic calculations

numerical simulations of spiral shaped inclusion trails can 3d geometry distinguish between end member models of spiral formation numerical 3d simulations of the development of spiral inclusion trails in porphyroblasts were conducted in order to test the proposals that a 3d spiral geometry differs between the rotation and nonrotation end member models of spiral formation proposed in the literature and b 3d spiral geometry can be used as a criterion to distinguish between the two end member models in rocks four principal differences are identified between the two sets of simulations smoothness of spiral curvature spacing of foliation planes alignment of individual foliation planes either side of the sphere representing the porphyroblast and spiral asymmetry with respect to matrix shear sense of these differences only spiral asymmetry and possibly the alignment of individual foliation planes are diagnostic criteria for distinguishing between the end member models in the absence of a readily applied test to distinguish the end member models interpretation of spiral inclusion trails is problematic it is necessary to determine complementary evidence to distinguish porphyroblast rotation or nonrotation during spiral formation

metamorphic and structural evolution of the early proterozoic puolankajarvi formation finland i structural and textural relations the amphibolite facies puolankajarvi formation pjf occupies the western margin of the early proterozoic kainuu schist belt ksb of northern finland the lower and middle parts of the pjf consist of turbiditic psammites and pelites and tempestitic semipelites this report concentrates on the pelitic lithologies which include quartz two mica plagioclase schists with variable amounts of garnet staurolite andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations the ksb forms a major north south trending synclinorium between two archaean blocks it contains both autochthonous and allochthonous units and is cut by faults and shear zones the pjf lies on the western side of the ksb and is probably allochthonous the formation has undergone six major deformation phases d1 d2 d3a d3b d4 and d5 during d3a d5 the maximum principal stress 1 changed in a clockwise direction from south west to north east between d2 and d3 the intermediate principal stress 2 changed from horizontal to vertical and the interval between d2 and d3 marks a transition from thrust to strike slip tectonics relict structures in the porphyroblasts indicate the following mineral growth deformation evolution in the pjf 1 throughout the pjf there was a successive crystallization of garnet syn d1 poryphyroblastic biotite inter d3 4 and staurolite inter d3 4 during the pre d4 stage 2 a syn d4 inter d4 5 crystallization of kyanite sillimanite fibrolite porphyroblastic tourmaline magnetite rutile cordierite and muscovite biotite plagioclase pseudomorphs after staurolite was most localized at and near d4 shear zones 3 a syn to post d5 generation of andalusite ilmenohematite and sheet silicates after staurolite and after cordierite occurred near d5 faults the evolution outlined here permits the relative dating of the pjf parageneses which is used in the second part of the study tuisku laajoki 1990 and together with the knowledge of the pressure temperature conditions during various growth events makes it possible to compile pressure temperature deformation paths for the pjf

relationships between foliation development porphyroblast growth and large scale folding in a metaturbidite suite snow lake canada ahstractxomplex relationships exist between cleavage development metamorphism and large scale folding in the well bedded polydeformed staurolite grade metaturbidite of the burntwood suite internal paleoproterozoic trans hudson orogen at snow lake manitoba canada it is demonstrated a that cleavage in anisotropic pelitic rock develops whenever microfolding is possible and that commonly initiation of a cleavage which is pervasive on the scale of a fold predates folding b how a new axial planar fabric can develop on one fold limb of a symmetrical fold and not on the other and c how two cleavages of different generations can be present in adjacent beds it is further shown that porphyroblasts rotate with respect to geographical coordinates during folding finally dissolution of cleavage septa is suggested here as an alternative mechanism for the generation of schistosity the burntwood suite is exposed on the dismembered limb of a macroscopic isoclinal f2 structure and preserves a domainal cleavage sz which locally grades into a schistosity s developed from crenulation of a generally bedding parallel s cleavage that is axial planar to f isoclinal folds formed at 1 84 ga porphyroblast growth coincided with crenulation of st early during f2 folding at 1 815 1 8 ga early stages of development are recorded by inclusion trails sj in the porphyroblasts during fz flexural flow folding variations in magnitude of bedding parallel shear in lithologies of different competency resulted in a strong s refraction and thus heterogeneous strains between beds independent of shear magnitude and resulting j s angle s and s2 remained sub orthogonal everywhere and thus porphyroblasts and the enveloping s rotated by equal amounts with respect to ss as the different magnitudes of porphyroblast rotation in different beds could not be exactly balanced by the counteracting rotation of the fold limbs same magnitude for all beds during fold tightening most porphyroblasts also rotated with respect to geographical coordinates s2 was crenulated prior to f3 large scale folding where favourably oriented f3 crenulations were tightened on the eastern f3 limb and unfolded by sinistral layer parallel shear on the western limb where f2 and f3 layer parallel shears were of opposite and the same sense respectively as a result the initial developmental stages of an ss are developed only on the eastern f3 limb and there only in incompetent layers whereas sz is preserved in the competent layers on the western limb s2 is preserved and appears axial planar to the f3 structure the s domainal fabric was locally transformed into a schistosity by dissolution of the septa during widespread fluid activity which endured until syn or post fa 5c 1998 elsevier science ltd

regional scale foliation reactivation and re use during formation of a macroscopic fold in the robertson river metamorphics north queensland australia abstract the rocks defining a macroscale antiform 25 km2 in the proterozoic robertson river metamorphics have been affected by four deformations of distinctive style with the fourth deformation d4 being responsible for formation of the macroscale geometry of the fold redistribution of progressive shearing strain due to strain partitioning during progressive d4 deformation resulted in the accommodation of d4 shearing strain along s2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations on the antiformal limbs this commonly resulted in reactivation of s2 because shearing during d4 was in a sense that was antithetic relative to that on the bulk scale of the fold continued d4 deformation caused unfolding of d2 crenulations resulting in straightening of sigmoidally folded s1 and its rotation toward the axial plane of the synchronously forming macroscopic d4 fold in zones where the sense of shear during d4 was the same as that operating on the bulk scale of the fold i e synthetic d4 shearing strain was accommodated dominantly by the favourably oriented approximately axial planar s1 fissility in zones where the progressive shearing component of d4 deformation was relatively more intense the s2 cleavage was also rotated into parallelism with s4 and was also re used as opposed to reactivated detailed microstructural analysis particularly of porphyroblast matrix relationships combined with field observations have resolved the processes operating during folding and reveal that despite the intensity of d4 deformation a separate cross cutting s4 cleavage has rarely been produced at either the meso or microscale similar processes have probably operated at all scales in other orogenic belts

the rotation of garnet porphyroblasts around a single fold lukmanier pass central alps abstract the orientation of the straight internal foliation s i within large 5 mm garnet porphyroblasts has been measured relative to the orientation of the external foliation s e around a single antiform of 0 5 m wavelength which folds the dominant regional foliation the internal foliation is not constant in orientation but varies consistently both with position around the fold and with the porphyroblast ellipticity the dip of s i hinge dip taken as zero is consistently less than the dip of s e it increases with increasing dip of s e and with increasing ellipticity of the porphyroblasts s i effectively defines a fold with an opening angle greater than that in the external foliation the opening angle of this fold in s i decreases with increasing porphyroblast ellipticity the observed variation in the orientation of s i can be explained qualitatively by a flattened flexural flow model for fold development as could be expected for folding of a pre existing strongly anisotropic foliation the measurements clearly demonstrate that rotation of porphyroblasts relative to geographical co ordinates did occur during the development of this fold and that a model based on the classical theories of rotation of stiff inclusions in a weaker viscous matrix is most appropriate

porphyroblast non rotation during crustal extension in the variscan lys caillaouas massif pyrenees abstract the dominant foliation s 2 in the variscan lys caillaouas massif central pyrenees france spain formed in a subvertical orientation during crustal thickening d 2 subsequent non coaxial crustal extension produced a subhorizontal crenulation cleavage s 3 whereby s 2 was folded and rotated andalusite staurolite cordierite and biotite porphyroblasts grew very early during this extension d 3 and included s 2 as straight curved and weakly crenulated inclusion trails these inclusion trails exhibit a strong subvertical preferred orientation which is independent of the local magnitude of d 3 strain the dip angle of the external main foliation s 2 or the aspect ratio and shape orientation of porphyroblasts this indicates that porphyroblasts did not rotate in the d 3 deformation field and hence preserve the orientation of s 2 at the time it was trapped in the porphyroblast relative to the bulk d 3 flow plane porphyroblasts could maintain a stable orientation in the d 3 flow due to complete accommodation of the bulk flow vorticity by heterogeneous shear strain the lack of vorticity of local volumes of zero shear strain such as porphyroblasts was balanced by concentration of shear strain in planar zones anastomosing around them it is likely that s 2 maintained a subhorizontal orientation throughout the d 3 crustal extension which would imply that porphyroblasts also remained stationary relative to the earth s surface during d 3 whereas s 2 in the deforming matrix experienced shear induced rotation a late brittle ductile folding event d 4 involving differential rigid block rotations dispersed the orientation of inclusion trails to some degree these late rotations could be corrected for by artificial unfolding of the f 4 folds

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

flexural flow folding does it occur in nature flexural flow produces a continuous deformation equivalent to that of a flexed deck of cards of vanishing card thickness it is assumed to be a common process by which parallel folds are formed in nature in competent anisotropic rock layers the strain pattern produced by flexural flow contrasts strongly with that produced in parallel folds formed by the bending or buckling of isotropic competent layers but it has not been documented in nature two basic questions are thus 1 if anisotropy is sufficiently large will flexural flow develop and 2 if so are competent rocks sufficiently anisotropic for them to respond to deformation by flexural flow a measure of planar anisotropy a is given by the ratio of viscosity in shortening to viscosity in shear finite element modeling indicates that if the value of this ratio is greater than about 50 the strain pattern in a buckle fold in a competent layer will be almost indistinguishable from one of flexural flow such a high anisotropy however is unlikely in natural competent layers the most texturally anisotropic rocks tend to be those rich in phyllosilicates and these usually behave in an incompetent fashion common competent rocks such as those rich in quartz and feldspar are rarely significantly anisotropic a strong preferred crystallographic orientation as a result of crystal plastic flow can produce anisotropic layer behavior but a is unlikely to exceed 10 which is insufficiently great for folding by flexural flow we thus conclude that flexural flow is unlikely in single competent layers the aggregate response to deformation however of composite layers of alternating competent and incompetent rocks may be highly anisotropic and on average correspond to flexural flow

strain compatibility and shear zones is there a problem in analyzing deformation in rocks it is important to ensure that the solutions obtained satisfy strain compatibility this creates a challenge to understanding patterns of strain associated with shear zones in which measured strain may appear incompatible with the strain in the shear zone walls flattening strains are common in natural shear zones with locally straight and parallel boundaries to satisfy compatibility conditions such strains require volume loss across the shear zone or deviations from plane strain with or without discontinuities between the shear zone and the wall rock in the case of shear zones for which there is no evidence of volume loss or discontinuities along the shear zone walls problems of strain compatibility may be resolved if individual shear zones are linked together in an appropriate fashion shear zones commonly occur in anastomosing arrays and simple configurations of such arrays and the strains associated with them are examined it is shown that local transpression with strain compatibility can be accounted for in this way quite complex local strain patterns can develop in simple arrays

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia abstract splays are common features of many rivers and deltas forming as a result of overbank flooding and sediment deposition but they have rarely been described from modern dryland settings this paper investigates splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central australia here splays vary from small lobate or tongue shaped features 1 km long to larger elongate features up to 4 km long and are supplied with rare floodwater and sediment through well defined breaches in the upper parts of the main channel banks in proximal and medial parts splay channels are incised up to 2 m into pre splay fine grained floodplain sediments but remain elevated between 0 5 m and 2 m above the main channels only in the very distal parts are splay channels less deeply incised either grading into the floodplain surface or terminating as sediment lobes that are deposited upon the floodplain aerial photographs and field observations indicate that many splays develop rapidly during large floods and are modified during infrequent subsequent floods four main stages of splay development are proposed 1 aggressive overbank flow results in bank line breaching and channel scour on the floodplain but little sediment deposition 2 coarse bed sediment is diverted from the main channel preventing further deepening at the bank line breach and slowing channel scour 3 increasing coarse sediment transport through the bank line breach and scoured channels leads to the formation of distal sediment lobes 4 subsequent floods divert around these distal lobes sometimes leading to further channel scour and lobe formation on the northern plains splays contribute to downstream decreases in the discharge and size of the main channels and splay deposits form local accumulations of gravelly sand in the predominantly fine grained floodplain successions some splays ultimately may evolve into distributaries or anabranches the dominantly erosional bases of these splay channels contrast with typical descriptions of many humid region splays but they do not display any geomorphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting

porphyroblast rotation eppur si muove in a number of recent papers the theory has been postulated that porphyroblasts as a rule do not rotate with respect to geographical coordinates and can be used to determine the original orientation of older foliations complex inclusion patterns in spiral garnets have even been used to advocate a new model of orogenesis involving several alternating phases of horizontal shortening and extension critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws millipede structures used as proof for flow partitioning can also form by other flow geometries evidence quoted to support irrotational behaviour of porphyroblasts is unsound porphyrobl asts do occur in sets with a preferred orientation of the internal foliation trace but these cannot be shown to represent original orientations microstructures which resemble truncation planes in spiral garnets are used as evidence that these structures developed by several phases of deformation and as proof for periodic extension and horizontal shortening in orogenesis they can however also be explained by intermittent growth of a rotating porphyroblast during a single phase of deformation finally porphyroblast sets in which orientation is a function of aspect ratio indicate that porphyroblast rotation with respect to kinematic axes does occur in at least some situations

metamorphic and structural evolution of the early proterozoic puolankajrvi formation finland ii the pressure temperature deformation composition path the paragenetic relationships between sillimanite andalusite kyanite chlorite cordierite biotite garnet and staurolite in the early proterozoic puolankajarvi formation pjf together with mineral compositions are used to construct a partial petrogenetic grid for metapelites with significant mn content mno 0 1 0 5 by adding a six phase invariant point over the garnet absent invariant point for mn free amf phases the grid and textural relations of the pjf are used to construct part of the p t deformation path for the pjf relatively short deformation pulses and associated flow of oxidizing fluid along shear zones were responsible for the paragenetic and compositional changes during cooling and decompression at 600 500 c and 6 0 2 5 kbar oxidation led to decreased fe2 and further stressed the importance of mn increased mn divalent cations a tectonothermal evolution of the kainuu schist belt is presented which includes crustal thinning and steepening of a previously established thermal gradient this was followed by thrusting and folding of the isotherms into a thermal antiform on the western side of the belt

barometric pumping of a fractured porous medium barometric pumping plays a crucial role in the release of trace gases from fractured porous media to the atmosphere and it requires a rigorous and complete modeling in order to go beyond the approximate schemes available in the literature therefore a coupled set of convection and convection diffusion equations for a slightly compressible fluid in unsteady conditions should be solved the numerical methodology is presented and it is applied to conditions close to the ones of the roselend natural laboratory france the precision of the code is assessed and the mechanism of barometric pumping is explained the usual schematization by simple vertical fractures is shown to be only qualitative finally barometric pumping is shown to be efficient in a narrow range of parameter values its efficiency is a decreasing function of the matrix porosity and of the fracture density

development of snowball structure numerical simulation of inclusion trails during synkinematic porphyroblast growth in metamorphic rocks abstract this paper presents a hydrodynamic analysis that simulates the development of snowball structures in porphyroblasts the porphyroblast is represented by a rigid sphere that grows at a constant volumetric rate while the matrix is represented by a newtonian fluid that deforms by simple shear well away from the sphere the marker particles used to track the motion of matrix and porphyroblast are arranged in a straight line before deformation those in the matrix move along flow lines that deflect around the sphere while those in the sphere passively rotate at a constant angular velocity those in the matrix are progressively captured and incorporated within the growing sphere the resultant trails of the marker particles in the sphere simulate the inclusion trails of synkinematic porphyroblasts three main types of trails are distinguished by the appearance of curvature in the sphere single double and triple rotation types single rotation types show very simple spiral trails formed when the initial angle between the marker line and the shear plane is between 0 and 45 double rotation types show a complicated pattern with the marker line showing a reversal of curvature from the porphyroblast center to its margin this type is formed when the angle between the marker line and the shear plane was initially between 45 and 135 when this angle exceeds 135 then triple rotation types appear which are characterized by two reversals of curvature of the marker line from center to the margin the calculated results are found to be very similar to some natural examples implying that such natural spiral patterns in the porphyroblast are produced in the similar way to what happens in the computer simulation

lack of porphyroblast rotation in noncoaxially deformed schists from petrel cove south australia and its implications abstract inclusion trails representing an s 1 cleavage demonstrate the lack of porphyroblast rotation during subsequent highly non coaxial deformations the pelitic schists of the kanmantoo group at petrel cove south australia contain two generations of porphyroblasts the first one consists of cordierite porphyroblasts that formed early in d2 and contain straight to slightly sigmoidal inclusion trails of s 1 the second generation consists of andalusite porphyroblasts that overgrew crenulated s 2 late during d3 several hundred inclusion trail traces from cordierites measured from oriented specimens taken throughout a strongly folded area show a horizontal great circle distribution when plotted and contoured on a stereographic projection hence s 1 was planar and horizontal prior to d2 s 1 measurements on limbs and hinges of a mesoscale d2 foldpair show that folding had little effect on porphyroblast orientation as the s 1 orientation remained constant and subhorizontal around the fold hence porphyroblasts have not rotated during any of the non coaxial deformations accompanying and following their growth this is interpreted as a result of the partitioning of the deformation around them s 1 in the matrix has been totally destroyed by the formation of s 2 as a fully differentiated crenulation cleavage what has previously been regarded as inconsistent senses of shear recorded by porphyroblasts around folds is resolved by the fact that the matrix foliation rotated rather than the porphyroblasts due to the effects of deformation partitioning the presence of a subhorizontal s 1 foliation suggests horizontal movements e g thrusting or detachment faulting during the earliest phase of the adelaidian orogeny