structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

kinematic analysis of deformed structures in a tectonic melange a key unit for the manifestation of transpression along the zagros suture zone iran kinematic analysis of melange fabrics provides critical information concerning tectonic processes and evaluation of the kinematics of ancient relative plate motion systematic kinematic analysis of deformed structures within a tectonic melange exposed along the zagros suture zone elucidates that this zone is an ancient transpressional boundary the melange is composed of a greywacke and mudstone matrix surrounding various lenses blocks and ribbons of radiolarian chert limestone sandstone pillow lava tuff serpentinite shale and marl the deformation fabrics of the melange suggest that the melange units were tectonically accreted at shallow levels within a subduction complex resulting in layer parallel extension and shearing along a nw se trending suture that juxtaposes the afro arabian continent to the south and the central iranian microcontinents to the north the tectonic melange is characterized by subhorizontal layer parallel extension and subsequent heterogeneous non coaxial shear resulting in alternating asymmetric and layer parallel extensional fabrics such as p y fabrics and boudinaged layers kinematic data suggest that the melange formed during oblique subduction of the neo tethys oceanic lithosphere in late cretaceous time kinematic shear sense indicators reveal that the slip direction n9 e to n14 e during accretion related deformations reflects the relative plate motion between the afro arabian continent and central iranian microcontinents during late cretaceous to miocene times

a back arc origin for the neoarchean megacrystic anorthosite bearing bird river sill and the associated greenstone belt bird river subprovince western superior province manitoba canada this study presents extensive new field petrographic and whole rock major and trace element and sm nd and pb isotope data for the neoarchean megacrystic anorthosite bearing bird river sill spatially associated neoarchean supracrustal rocks of the bird river greenstone belt and the mesoarchean maskwa lake ttg batholith i manitoba canada field observations indicate that the 2743 ma bird river sill was emplaced into the northern lamprey falls formation in an oceanic setting and subsequently intruded by the ca 2725 ma maskwa lake batholith ii the northern lamprey falls formation is in fault contact with the overlying peterson creek formation which in turn is unconformably overlain by the bird river formation the peridotites of the bird river sill were subjected to intense alteration and ductile shearing resulting in the redistribution of many major and trace elements and resetting of their sm nd and u th pb isotopic systems the gabbro leucogabbro and anorthosite units of the sill largely retain their primary igneous textures and minerals ree and hfse compositions and sm nd and pb isotope signatures these geochemical and petrographic data indicate that these units of the bird river sill crystallised from a hydrous ca and al rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma the major and trace element geochemistry and initial end values of the bird river sill 0 53 to 4 72 and the northern lamprey falls formation 1 40 to 2 66 of the bird river greenstone belt crystallised from parental magma batches that were derived by partial melting of variably depleted mantle sources high magnesian andesites from the bird river formation formed by partial melting of subducting oceanic crust and or slab sediments and subsequent interaction with sub arc mantle peridotite whereas the peterson creek formation dacites and maskwa lake ttg batholith i tonalites formed by partial melting of juvenile lower arc crust the major and trace element geochemistry of the bird river sill and greenstone belt and the maskwa lake ttg batholith i and geological relationships and lithological associations point to a transition from continental arc to continental back arc magmatism along the southern active margin of the north caribou terrane at ca 2743 ma resulting in the formation of the maskwa lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent combined field observations and geochemical data indicate that the bird river sill and bird river greenstone belt are a dismembered archean subduction related ophiolite that marks a suture zone between the winnipeg river subprovince and the maskwa lake microcontinent

microstructural and fabric studies from the rocks of the moine nappe eriboll nw scotland abstract microstructures and quartz c axis fabric diagrams from mylonites and psammitic moine schists collected in traverses across the lower levels of the moine nappe in the eriboll area are presented on approaching the moine thrust from the kyle of tongue the following microstructural sequence is encountered interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post tectonic relaxation these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite grade mylonite belt and finally adjacent to the moine thrust into reworked lower chlorite grade mylonites although there is some local variation the overall quartz c axis fabric is an incomplete asymmetric type i girdle the main variation is the development of type ii girdles in the reworked ultrafine grained mylonites the extent of the mylonitization is more extensive than previously reported studies of folds within the mylonite belt have revealed eye structures and small scale folds many are sheath folds they cannot be unequivocally correlated with large scale recumbent folds within the moine nappe the results presented indicate that mylonitization is not limited to a single phase and raises the possibility that there may be earlier caledonian or possibly precambrian structural elements present in the eriboll region moines prior to much of the mylonitization

the reliability of asymmetric c axis fabrics of quartz to determine sense of vorticity abstract asymmetric c axis fabrics of quartz are commonly used to determine sense of vorticity in ductile shear zones this method seems to work if the fabric pattern resembles a model fabric proposed by lister and hobbs 1980 usually however c axis fabrics are rather vague the reliability of such vague fabrics was tested in a major shear zone with known sense of vorticity only 62 of the c axis fabrics predict the correct sense great care should therefore be taken in applying this method to determine sense of vorticity

understanding kinematic data from the moine thrust zone in terms of a kinematics based mathematical model of deforming thrust wedges abstract a salient recess pair along the moine thrust zone exhibits significant lateral kinematic variations specifically the more steeply dipping portion of the thrust surface associated with the recess shows evidence for greater strain magnitudes and more out of the plane motion than the adjoining salient additionally a subset of the grain shape data has grain shape approximations with their long axes oriented perpendicular to the regional transport direction to more fully understand these kinematic features we developed a kinematics based mathematical model of thrust wedge deformation our numerical model predicts an increase in strain magnitude as well as non plane strain flattening in the region of a thrust ramp consistent with the strain patterns observed within the recess along the moine thrust zone furthermore in the absence of a lateral confining boundary condition much of the model thrust wedge experiences transport perpendicular maximum extension except for the region closest to the thrust where simple shear deformation is dominant using our numerical model we predict the amount of lateral confining strain needed to induce plane strain deformation ps as well as the amount of lateral confining strain necessary to make the long axis of the strain ellipsoid parallel with the transport direction parallel we have found that the incremental parallel can be as much as 42 of the incremental strain magnitude thus we consider transport perpendicular lineations like those seen along the moine thrust zone to be the result of lateral confining boundary conditions that are weak enough to allow transport perpendicular maximum extension

a 10 000 yr geomagnetic secular variation record from three australian maars summary detailed within lake magnetic stratigraphies enable composite declination and inclination versus depth curves to be derived for sets of 6m cores from each of three volcanic crater lakes in western victoria comprehensive radiocarbon chronologies for each sequence enable reliable timescales to be assigned to the variations in direction of the remanence the resultant time scaled curves are in satisfactory agreement and are therefore thought to provide an estimate of the geomagnetic secular variation in se australia during the last 10 000 yr amplitude attenuation occurs as an increasing function of age and probably results from long time scale remagnetization processes mean directions coincide with the geocentric axial dipole field directions at the three sites inclination oscillations are stronger and better resolved than those in declination whereas the opposite tends to be true for the european records reduction of records to a common virtual site suggests that the difference is largely of geometrical origin comparisons of vgp paths do not reveal a common dipole wobble component the sense of looping of the magnetic vector favours westward rather than eastward drift of features of the non dipole field relative field strength estimates based on normalization of nrm with respect to measures of the magnetic mineral content do not agree with australian and worldwide archaeointensity results

significance of folds and mylonites at the grenville front in ontario the grenville front is defined as the northwestern limit of the grenville structural province of the canadian shield stockwell 1964 wynne edwards 1972 it strikes northeast southwest and is approximately 1 600 km long extending from lake huron to the labrador sea the front separates the grenville structural province from either the nain churchill superior or southern structural province and is commonly associated with mylonitic fabrics see higgins 1971 for terms usage which strike approximately parallel to the front and dip steeply to the southeast

paleomagnetic record from glacial varves in alto rio mayo patagonia argentina abstract paleomagnetic study of a 3 m thick sequence containing 302 glacial varves outcropping in alto rio mayo valley shows strong intensities a declination shifting gradually to the west and an increasing negative inclination virtual geomagnetic pole positions reveal a migration of the geomagnetic pole from ne canada sw greenland to a position around nw siberia the deposit is believed to represent the interval between the recession of the last glacial maximum and a final ice front oscillation with a readvance in the alto rio mayo valley thought to have occurred shortly before 13 500 yr b p

the geology and structural evolution of a caledonian fold and ductile thrust zone kyle of tongue region sutherland northern scotland the middle to upper proterozoic metasedimentary rocks of the northern highland moine in sutherland are disposed in a series of major caledonian c 470 430 ma metamorphic thrust nappes produced by nw wnw directed shortening and reworking of parts of a pre existing precambrian c 1000 ma orogen lewisian inliers occur throughout sutherland and represent tectonically interleaved units of the original basement gneisses upon which the moine cover sequence was deposited in the western moine nappe of the kyle of tongue region the rocks are deformed by two main episodes d1 and d2 which are thought to represent the effects of precambrian and caledonian orogenesis respectively little basement cover interleaving or large scale folding occurred during the d1 event in the moine nappe but the associated metamorphism to ut leust garnet grade produced peak metamorphic mineral assemblages subsequent caledonian reworking d2 produced a foreland propagating system of wnw directed ductile thrusts and related major f2 sheath folds both of which resulted in much of the observed moine lewisian interleaving metamorphism associated with d2 decreases in grade westwards occurring at similar or slightly lower grades than the earlier d1 event this retrogressive overprint is most obvious at the w margin of the moine nappe the d2 folds and ductile thrusts are geometrically and kinematically similar to the caledonian structures recognized within the moine thrust zone to the west the very low 5 ramp angles suggesting that the thrusts must have very shallow dipping trajectories through the crust f2 fold formation is attributed to an early phase of wnw directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued wnw directed thrusting the occurrence of internal extension in the orogenic wedge during active thrust related foreshortening implies that the shear strength of the material close to the basal detatchment s was continuously reduced i e wedge motion became supercritical in this context strain softening processes within the ductile thrust mylonites were probably of major significance

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

kinematic vorticity flow analysis and 40ar 39ar geochronology related to inclined extrusion of the hp lt metamorphic rocks along the zagros accretionary prism iran abstract the zagros accretionary prism in southwestern iran is exposed along the nw se trending of the zagros thrust system and inclined zagros transpression zone this accretionary prism consists of two units the upper sedimentary melange unit on top and the high pressure metamorphic melange unit at the bottom both units show characteristics of a tectonic wedge the upper unit consists of type i ii and iii melanges which display s c and c type shear band cleavages quartz ribbons and rectangular or fish head boudins the lower unit fabrics display and type porphyroclasts and quartz ribbon mylonites these fabrics formed from a combination of 60 5 simple shear and 39 5 pure shear both components were involved in a lateral exhumation of the high pressure low temperature metamorphic rocks in an inclined transpression wedge shaped geometry the estimated kinematic vorticity number wk was calculated from quartz c axis patterns rotation of porphyroclasts and orientation of finite strain with respect to shear zone boundaries using the mean estimated wm value of 0 84 the inclination angle for the thrust wedge on top of the ne subducting neo tethyan oceanic lithosphere is 18 the 40ar 39ar plateau ages of early generations of biotite from the lower metamorphic melange are 119 95 0 88 ma and 112 58 0 66 ma this late aptian age is related to early thrusting and formation of hp lt metamorphic rocks the dating of two amphibole samples from the amphibolite yields a weighted mean age of 91 23 0 89 ma this turonian cenomanian age suggests a later metamorphic event associated with subduction and obduction of the neyriz ophiolite and later lateral extrusion of hp lt metamorphic rocks along the inclined zagros accretionary prism

dating deformation and cooling in the caledonian thrust nappes of north sutherland scotland insights from 40ar 39ar and rb sr chronology 40 ar 39 ar and rb sr mineral ages have been determined from various lithologies exposed in the caledonian foreland and structurally overlying thrust nappes of north sutherland scotland rb sr muscovite ages of c 428 c 421 and c 413 ma obtained from moine thrust zone mylonites are interpreted to date closely regional thrusting during the late silurian to early devonian 40 ar 39 ar muscovite ages within the lower parts of the moine nappe are mostly anomalously old with respect to rb sr analyses of muscovites from the same samples it is likely that this discrepancy results from a component of extraneous or excess argon 40 ar 39 ar hornblende ages and rb sr and 40 ar 39 ar muscovite ages obtained from structurally higher metamorphic units in the caledonian thrust nappes generally range between c 440 ma and c 410 ma these ages are interpreted to date cooling during and following d 2 regional thrusting and folding within internal sectors of the nappe sequence a possible tectonic model involves the silurian collision of baltica with scottish segments of laurentia resulting in the scandian orogeny and broadly coeval moine thrust zone d 2 structures were superimposed on structures and metamorphic fabrics formed during a regional mid ordovician tectonothermal event dated previously at c 470 460 ma syn d 2 temperatures were generally 600 c and sufficient to achieve more or less complete thermal rejuvenation of rb sr and 40 ar 39 ar isotopic systems in muscovite and hornblende even in areas of low d 2 strain

the geometry and kinematics of flow perturbation folds abstract minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow using structures associated with wnw directed caledonian thrusting in n scotland we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction these define a series of larger scale fold culminations reflecting surging flow or depressions reflecting slackening flow that are bisected by transport parallel culmination and depression surfaces the fold patterns suggest a dominance of layer normal differential shearing due to gradients in shear strain normal to transport culmination surfaces are marked by along strike reversals in the polarity of structural facing and vergence of minor folds which contrary to classic fold patterns define reverse asymmetric relationships culmination surfaces separate folding in to clockwise z folds and anticlockwise s folds domains relative to the transport lineation the dip of fold axial planes systematically increases as their strike becomes sub parallel to transport resulting in a 3d statistical fanning arrangement centred about the transport direction thus mean s and z fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved culminations display convergent fold patterns with fold hinges becoming sub parallel to transport towards the culmination surface and underlying detachment whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface thus around culminations and depressions there are ordered scale independent relationships between transport direction shear sense fold facing vergence and hinge axial plane orientations the techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow

a magnetotelluric survey of ophiolites in the neyriz area of southwestern iran a wide band magnetotelluric study of the ophiolitic zone of the zagros orogenic belt was conducted in the neyriz area of southwestern iran the purpose of the study was to image subsurface structures electrically and relocate the main zagros thrust fault in the region the thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence the fault zone with a nw se geological trend is parallel to the zagros orogenic belt and separates the neyriz ophiolite assemblage from the adjacent sanandaj sirjan metamorphic zone magnetotelluric data were collected along a sw ne profile across the geologic strike the study included 18 stations and modeling was performed using a 2 d inversion scheme analysis of both modes of magnetotelluric data te and tm clarifies the signatures of large resistivity variation in the study area due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite radiolarite zone in the north eastern and southwestern parts of the 2 d resistivity models respectively

secular variation as recorded in lake sediments a discussion of north american and european results type curves depicting secular variations in declination and inclination through holocene time have been constructed by stacking data from individual cores of lake sediment from east central north america and the u k there is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 14 c years b p but from 4500 to 8500 14 c years b p the correlation is strongly negative which we interpret in terms of fluctuations in intensity of standing geomagnetic sources a source located under the atlantic ocean appears to have been dominant before ca 4500 14c years b p and then another similar source located either to the east or to the west of both observation areas gained dominance however the inclination type curves show optimum positive correlation for a phase shift of ca 400 years suggesting that the geomagnetic sources that caused them were drifting westward at ca 0 2 degrees a year there is no way of adjusting the timescales so as to obviate this apparent paradox secular variations of the total geomagnetic vector have been investigated by noting the sense clockwise anticlockwise of looping of the v g p paths though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core as demonstrated by specific radial dipole models

sense of nappe emplacement from quartz c axis fabrics an example from the betic cordilleras spain preferred crystallographic orientation patterns fabrics in zones of ductile deformation beneath nappes are a potentially useful indicator of the sense of nappe translation quartz c axis fabrics from a shear zone beneath the pile of higher betic nappes in the betic cordilleras spain have distinctly asymmetric topologies revealed by constructing a fabric skeleton these skeletons closely resemble those produced by computer simulations of quartz fabric development basedon taylor bishop hill theory the fabric asymmetry indicates a northerly sense of transport for the nappes consistent with field evidence from the same area analyses of both c and a axis fabrics based on asymmetric intensity distributions within the fabric give equivocal results and may reflect the presence of earlier preferred orientation patterns predating nappe emplacement

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

correlation of late pleistocene glaciolacustrine and marine deposits by means of geomagnetic secular variation with examples from northern new york and southern ontario abstract the geomagnetic secular variation record retained by glaciolacustrine and marine sediments at nine sites in northern new york and southern ontario provides a means for stratigraphic correlation of glacial deposits for the time period between about 12 600 to 9900 14c yr b p measurement of the depositional remanent magnetism of sediments deposited in glacial lake iroquois and the champlain sea has produced a geomagnetic secular variation curve that represents the time period immediately following deglaciation about 12 600 14 c yr b p the curve varies from about 358 to 344 declination and 51 to 61 inclination over approximately 180 valve years marine sediments of the champlain sea have preserved a record approximately 1500 yr long that varies from about 2 to 29 declination and 47 to 60 inclination these combined glacial paleomagnetic records may also correlate with those from glacial sequences beyond our study area the shape and amplitude of the secular variation record in glaciolacustrine and marine sediments from the western adirondack borderland show agreement with other glacial varve secular variation records and suggest possible correlations with secular variation curves from lake cores

discussion on evidence for non plane strain flattening along the moine thrust loch strath nan aisinnin north west scotland by mathew strine and steven f wojtal

structures and microstructures related to steady state mantle flow in the neyriz ophiolite iran abstract structural analysis in the well exposed and well preserved neyriz ophiolite suggests that it is a relatively undisturbed piece of oceanic lithosphere detailed structural mapping of high t deformation mantle flow revealed the presence of three elliptical shaped diapirs trending nw se these diapirs are characterized by vertical mantle foliations associated with vertical plunging stretching lineations which progressively incline toward parallelism with the gently ne dipping moho the nw se direction of asthenospheric upwelling of diapirs is parallel with the orientations of the vertical sheeted dike complex this suggests that the neyriz ophiolite was created by two nw se palaeo ridge axes these palaeo ridges are classified as fast spreading ridges these ridges are segmented by a dextral palaeo transform fracture zone this is consistent with fast spreading ridges comparison between the neyriz and oman ophiolites reveals that they show similar characteristics most of the oman palaeo ridge systems are oriented nw se and nnw se they also display similar sheeted dike complex orientations and crustal thickness variations these two ophiolites originally were part of the neo tethyan oceanic lithosphere and afterwards were separated by the oman line during continental collision of the iranian micro continent and afro arabian continent

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

a preliminary palaeomagnetic study of the holocene and late wurmian sediments of lac du bouchet haute loire france summary lac du bouchet is a small maar type lake located at 45 n 4 e in central france the bottom sediments have been sampled down to a depth of 6 m with mackereth type corers a chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations throughout the period covered which runs from 22 000 years ago through to the present time declination and inclination variations consist of oscillations of up to about 30 amplitude no abnormally large amplitude excursions in direction are recorded the main contribution of this study is the extension of the european secular variation sv records to the interval from 13000 bp years before present to 22000 bp the sediments deposited through this interval ancient dryas intc wurm originated from the erosion of the internal walls of the volcano spectrum analyses of the declination and inclination curves from 13000 to 22000 bp reveal peaks at periods of 3000 4000 1800 1150 and 700 yr cross spectrum analysis shows strong coherence of the two signals at the longest of these periods the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector possibly suggesting westward drifting sources although the palaeomagnetic sv record through the holocene 0 10300 bp is compressed into 1 7m of highly organic sediments the principal features exhibited by the type curves established for uk can be identified and the resulting magnetic ages are consistent with the palynology the sense of rotation of the magnetic vector is biased equally in both senses the late glacial 10300 bp to 13000 bp is characterized by oscillating climatic conditions recent dryas allerod and bolling which resulted in poorly resolved and probably discontinuous palaeomagnetic sv records which show a strong bias to anti clockwise rotation all ages are given in radiocarbon years

geomagnetic secular variations in greece through the last 6000 years obtained from lake sediment studies summary geomagnetic declination and inclination secular variation curves have been constructed back to 6000 yr bp from palaeomagnetic measurements of sediments cored in lakes volvi trikhonis and begoritis greece they have been dated by palynological studies and by comparison with observatory and archaeomagnetic records radiocarbon ages are all affected by the old carbon factor cores from all three lakes contain a thin ash layer which we associate with the pompeii eruption of vesuvius in ad 79 averaged curves of secular variations of declination and inclination have been computed for the whole study the broader scale pattern of secular variations is basically similar to lake sediment records from the uk poland and switzerland though finer detail in the structure of some of the principal features is resolved

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

basement cover relationships and structure within the moine rocks of central and southeast sutherland inliers of archaean to early proterozoic lewisian basement crop out within the proterozoic moine succession of central and se sutherland both the lewisian basement and the moine cover have been affected by two major tectonic events the earliest event probably occurred during the late proterozoic and involved minor folding and upper greenschist to upper amphibolite facies metamorphism intense deformation largely obliterated the unconformable relationship between the lewisian and moine rocks and imposed a planar banding on the basement lithologies subsequent reworking during the caledonian orogeny 500 400 ma resulted in ductile thrusting and folding and was associated with a widespread low to mid amphibolite facies metamorphic overprint within this context the lewisian inliers of central and se sutherland occur in two distinct structural settings i as allochthonous sheets which rest on major caledonian ductile thrusts ii in the cores of nw verging intensely curvilinear sheath folds of caledonian age

mixing in flow perturbations a model for development of mantled porphyroclasts in mylonites abstract the presence of a rigid object such as a porphyroclast in simple shear flow induces a flow perturbation with a relatively simple geometry domains of closed and open flow lines are bounded by a separatrix that surrounds the porphyroclast the concept of a separatrix to describe heterogeneous flow gives rise to some new insights in the development of porphyroclasts with a mantle of recrystallized material in mylonites the final shape of the recrystallized mantle depends on separatrix shape but also on the degree of intersection of the mantle and the separatrix and therefore on relative mantle width this model can explain the coexistence of several types of mantled porphyroclast in a single mylonite sample the occurrence of porphyroclasts that lack wings at high strain values and the presence of symmetric winged mantled porphyroclasts in ductile shear zones

holocene geomagnetic secular variation records from north eastern australian lake sediments summary secular variation records have been obtained from cores from lakes barrine and eacham two north eastern australian volcanic crater lakes the results from several cores have been stratigraphically correlated and then stacked and smoothed the chronology provided by radiocarbon dating indicates that the lake eacham sequence spans the last 5700 calendar years the time scale for the lake barrine record is less weil constrained but it

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

an illustration of the advantages of a complete texture analysis described by the orientation distribution function odf using quartz pole figure data abstract the preferred orientation of eight crystal directions of quartz was measured with an x ray texture goniometer for selected quartz bearing tectonites with a clear pattern of c axis preferred orientation the advantages of data analysis by means of calculations of the orientation distribution function and other forms of data presentation derived from the odf are demonstrated

plagioclase and quartz preferred orientations in a low grade schist the roles of primary growth and plastic deformation albite in a lower greenschist facies haast schist from new zealand has a maximum of poles to 001 parallel to the elongation direction 010 forms a girdle about grain length plastic deformation with slip dominantly on 010 001 is rejected as the orienting mechanism because 1 the relationship of crystal axes to grain length would require multiple slip systems incompatible with the lack of rotation of albite twin planes 2 euhedral terminations and other textural features indicate primary crystallization 3 albites in quartzose layers are less elongate than those in sheet silicate rich layers the albite fabric is ascribed to growth anisotropy and competition in response to stress driven solution and growth during formation of metamorphic segregation layering this explanation is compatible with the common observation that plagioclase is brittle at low metamorphic grades associated weak quartz c axis fabrics are ascribed to plastic deformation and this deformation enhanced a dimensional preferred orientation already established by primary growth

on the current state of lake sediment palaeomagnetic research summary it is now possible to obtain high quality records of geomagnetic secular variations in the direction though not the magnitude of the ancient field type curves are presented for four regions namely north america west europe argentina and australia constructed by fitting cubic splines to data stacked from numbers of cores of post glacial sediments ways in which the results can be used to investigate the grosser properties of the ancient field are discussed

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

palaeomagnetic data from late glacial glaciolacustrine sediments at rio corintos chubut argentina abstract the characteristics of the pleistocene glaciations in patagonia have been partially solved on the basis of the moraine extent and the varve counting method and by analogy with the weichselian glaciation in scandinavia caldenius 1932 established a pattern of patagonian glaciations which has remained acceptable palaeomagnetic studies of glacial sediments have been started with the aim of testing this correlation a 78 m late glacial glaciolacustrine sequence was studied and sampled at rio corintos chubut province argentina 43 10 s 71 15 w approximately 1089 varves were counted they were thought to have been deposited between 25 000 13 000 years bp the samples showed strong and stable values of remanent magnetization magnetite was identified as the carrier of the remanence magnetic susceptibility and intensity were recorded and the expected variations according to grain size were obtained recorded declination values oscillated from 30 w to 15 e and inclination values from 8 to 56 this shift can be attributed to the secular variation of the geomagnetic field declination and inclination values were smoothed using a stratigraphic window which covered 40 varves assuming that a single varve implies that one year has passed the secular variation curve can be presented against a scale time which spans approximately 1200 years the corresponding vgp path showed 100 clockwise motion in the region of northern canada a correlation with other vgps recorded in lago buenos aires for late glacial varves has been established

lake sediment record of the geomagnetic secular variation in britain during holocene times summary a palaeomagnetic record of the geomagnetic secular variation during the last 10000 years has been obtained from 10 cores of sediment from loch lomond scotland lake windermere north england and llyn geirionydd north wales a time scale is provided by 30 radiocarbon age determinations and pollen analyses on several of the cores the main swings and much fine detail of both declination and inclination records repeat well between cores and between lakes and the overall record is much more detailed than previous european records the new record shows that neither declination nor inclination swings have been periodic over the past 10000 years but that the main swings have become progressively shorter in duration during that time each swing is characterized by fine detail which enables use of the record as a secondary method of dating other european sediments the motion of the geomagnetic vector has been predominantly clockwise throughout the time period spanned but confirms a period of anticlockwise motion from 1100 to 600 bp first discovered by british archaeomagnetic investigations the record agrees with british and czechoslovakian archaeomagnetic records but not with japanese archaeomagnetic or north american lake sediment records this suggests that the secular changes are controlled by local growing and decaying drifting sources rather than by wobbling of the main geomagnetic dipole

paleomagnetic vectors and tilted dikes abstract where tectonic deformation reorients rocks without penetrative strain their paleomagnetic vectors may be restored to their original attitudes by untilting for strata paleomagnetic inclination is readily restored but the tilt axis must be precisely known if paleodeclination is required for dikes without the knowledge of the rotation s neither declination nor inclination of the paleomagnetic vector can be uniquely defined furthermore back rotating dike orientations to an upright attitude assumes primary verticality whereas primary dike dips are bimodal across the spreading axes e g troodos ophiolite cyprus in the cyprus ophiolite the dikes of the limassol forest transform zone are tilted due to uplift of the mantle sequence rocks and deflected against the arakapas fault their paleomagnetic vectors may be restored rotating about the two axes defined by the strike and the vertical or about a net axis that is possibly the actual tectonic rotation axis this net axis is determined from the tectonic regional dispersion of the dike orientations in this test case the results of the restorations differ slightly but underline the difficulty in selecting the best restoration procedure and the greater difficulty of restoring the paleomagnetic data from dikes vis a vis strata for dikes it is recommended that the paleomagnetic vectors are restored using average dike orientations to minimize the inaccuracies due to the large primary variation in dike orientation

review of platinum group element distribution and mineralogy in chromitite ores from southern iran abstract the platinum group element pge potential of four chromite mining districts in southern and south eastern iran was investigated using geochemical and mineralogical methods a total of 20 mines and prospects were sampled in the neyriz ophiolite the abdasht and sikhuran complexes in the esfandagheh district and the faryab district chromitite and dunite were analysed for major trace element and pge geochemistry and the platinum group element mineralogy pgm in all mining districts chromites analysed from mantle tectonite probable transition zone and cumulate zone chromitites are refractory low in tio2 pge concentrations range from 5000 ppb with median values around 200 ppb in all complexes investigated the 187os 188os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial os mantle normalised pt ir ratios are generally low in the sikhuran complex pods of sulphide bearing chromitite 0 2 1 2 sulphides occur in a dunite zone interpreted as an ultramafic cumulate zone chromites in chromitite are characterised by high cr 75 81 and intermediate mg numbers 45 65 in associated dunite chromites cr 74 80 are more fe rich mg 29 50 chromitite and dunite carry up to 5 2 ppm total pge and are characterised by high normalised pt ir up to 5 pt pd large degrees of partial melting in a supra subduction zone geotectonic setting probably in a two stage melting process produced boninitic melts that crystallised refractory cr mg rich and ti poor chromite in some cases sulphide saturation was achieved during and after chromite formation saturation during chromite crystallisation is evidenced by ni cu rich sulphide droplets preserved in some chromitites sulphur saturation after chromite crystallisation governed the formation of ni rich sulphide liquids that exsolved monosulphide solid solution which later crystallised to form pge bearing pentlandite platinum and pd did not partition into pentlandite but crystallised as pt and pd rich pgm associated with the pentlandite

different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the neyriz mantle diapir iran abstract a deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the neyriz mantle diapir in iran were analyzed using electron backscatter diffraction ebsd both samples have the common amphibole crystallographic preferred orientation cpo in which 100 lies perpendicular to foliation and parallel to lineation amphibole grains in the layered gabbro sample have little internal deformation whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries there is a subtle change in cpo as a function of grain size in the mylonitic gabbro coarse grains porphyroclasts have a 100 cpo oriented with the main foliation reference frame whilst fine grains have a 100 cpo oriented with the c shear bands detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems most particularly 110 control intracrystalline deformation schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to c shear bands it is unlikely that the slip systems that control intracrystalline deformation are important in cpo formation we interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the c shear bands by grain boundary sliding this rigid body rotation possibly in combination with easy slip on 100 are considered the main cause of cpo formation amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to cpo

are there ancient soils in the 3 7 ga isua greenstone belt greenland abstract a lens of black schist within 3 7 ga quartzites of greenland may be earth s oldest known alluvial paleosol the suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite but it has a truncated top above dark gray grading down to gray color ptygmatically folded surface cracks filled with silt grains and large sand crystals unusual for sedimentary or metamorphic rocks the paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends and by chemical analysis showing molar weathering ratios and ree distribution like those of soils the schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the isukasia area the protolith can be reconstructed as a saponite clay with a salt rich horizon of kieserite like other acid sulfate paleosols of the early earth models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 510 ppm o2 and 820 201 ppm co2 and humid cool temperate paleoclimate the profile has organic 13cpdb consistently of 24 2 to 27 4 and modest raleigh distillation near the top similar consistent values and trends are produced by decay of organic matter in living soils but biotic carbon isotopic composition of sediments is erratic from bed to bed and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle thus life in this very ancient soil is not precluded by our analyses but ultrastructural and geochemical testing of carbon particles would further test this hypothesis

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

the geology of the kuh e dalneshin area of southern iran and its bearing on the evolution of southern tethys the kuh e dalneshin area lay close to the northern margin of the arabian continent during mesozoic and palaeogene times recent mapping has shown that except perhaps in the late cretaceous it was inactive until it collided with central iran in the miocene the collision represented by a neogene melange zone in the ne resulted in the zagros orogeny and is expressed in the shelf sediments by folding parallel faulting and a possible arcuate sinistral shear in the late cretaceous a localized deepening of the shelf dates from the cenomanian a narrow trough in the w subsided to abyssal depths in the turonian following a period of faulting a more wide spread subsidence affected the shelf in the coniacian and santonian autochthonous cherts and associated sediments accumulated basaltic volcanics were erupted in the ne part of this basin and permian to cenomanian neritic exotic blocks were derived presumably from an uplift at the shelf edge the rocks from the outer edge of this basin slid westwards over their autochthonous counterparts probably in the campanian accompanied by further gliding of masses of earlier mesozoic cherts which had previously been emplaced on the continental margin a sheet of mantle peridotite partly serpentinized and enclosing neritic and metamorphic exotics in turn slid into the basin shelf sedimentation was renewed in the late campanian or maestrichtian but the transgression did not reach the area until the eocene the derivation and emplacement of the allochthonous rocks in the late cretaceous are still imperfectly understood alternative models are discussed one involving gliding from an aborted spreading ridge along the continental margin the other more conventionally based on subduction processes a preference is expressed for the former

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

construction of type curves of geomagnetic secular variation for dating lake sediments from east central north america type curves of palaeomagnetic secular variations of declination and inclination through the last 12 000 radiocarbon years have been constructed by combining data from the minnesota lakes with paral

evidence for non plane strain flattening along the moine thrust loch srath nan aisinnin north west scotland we report quartz c axis patterns grain shape fabrics and microstructures for 11 mylonitic quartzites and quartz phyllosilicate schists from a transect across the moine thrust at loch srath nan aisinnin north west scotland in the footwall samples collected more than 42 m normal distance from the thrust surface quartz c axis textures indicate a general flattening strain i e 0 k 1 samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c axis patterns recrystallized hanging wall fault rocks exhibit random c axis patterns on the scale of a standard thin section relict footwall grains provide the closest approximation of finite strain they have octahedral shear strains s between 1 10 and 1 47 and exhibit general flattening k values 0 0524 0 659 the long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ese in contrast recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction furthermore these samples have grain shape k value ranges from 0 157 to 0 295 recrystallized hanging wall grain shapes exhibit the lowest octahedral shear strains s 0 532 0 733 and largest mean k values 0 351 0 961 of this sample set the long axes of the mean recrystallized hanging wall grain shapes are parallel to transport similar to that of relict footwall grains unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation the mylonitic foliation and penetrative lineation are consistent with a wnw shearing direction however both were folded during later deformation increments recrystallized grains in footwall quartzites suggest a 305 320 azimuth for the shearing direction the best fit axis of the poles to the foliation is 18 145 close to the mean trend plunge of mesoscopic fold hinges at this site 18 151 both suggest a nnw azimuth for the shearing direction thus structural microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from 286 to 331

a structural analysis of the moine thrust zone between loch eriboll and foinaven nw scotland abstract the northern part of the moine thrust zone as exposed around the valley of srath beag sutherland was developed by thrusts propagating in the tectonic transport direction deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time the foreland has been progressively accreted onto the overriding moine thrust sheet by duplex formation a process which has continuously folded the roof thrust and the rocks above its hanging wall fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging wall a complex history of extensional and compressional strains normal to the transport direction folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography therefore there may be no correlation between structures across or along the thrust zone this variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

geomagnetic secular variations 0 14 000 yr bp as recorded by lake sediments from argentina summary palaeomagnetic and sedimentological studies carried out on the bottom sediments from three lakes in western argentina are described at the within lake level five lithological units could be broadly defined and were useful as a general guide in defining detailed correlations based on the magnetic susceptibility k and intensity of remanent magnetization j in one of the three lakes el trebol it was possible to derive a closer lithologically based correlation and this was consistent with that based on the magnetic parameters inter lake correlation was derived from the inclination and declination patterns considered as a pair also taking account of a set of radiocarbon age determinations to remove any ambiguities all the individual measurements were transformed to a time scale prior to stacking to produce type curves the records for laguna el trebol and brazo de campanario extend back to about 6000 radiocarbon years before present while lago morenito provides a record extending back to about 14000 radiocarbon years before present the records exhibit variations with similar overall characteristics to those previously obtained for europe and north america but there appears to be no correlation with the northern hemisphere patterns on a swing to swing basis both clockwise and anticlockwise precession of the geomagnetic vector is evident both from spectral analysis of declination inclination pairs and vgp paths

thrust sequences and the significance of low angle out of sequence faults in the northernmost moine nappe and moine thrust zone nw scotland in common with most thrust belts the moine thrust zone scotland preserves a foreland propagating sequence of thrusting however significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial remapping of the northernmost moine thrust zone has defined a thick belt of mylonites in the centre of which lies the ductile wnw directed moine thrust the base of the mylonite belt however is marked by a regionally significant low angle west directed brittle fault here termed the lochan riabhach thrust that truncates thrust related structures in its footwall comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at durness indicates that the lochan riabhach thrust cuts down section in its footwall and therefore appears to be extensional locally it is unequivocally breached by underlying imbricate thrusts we prefer to interpret the lochan riabhach thrust as an extensional fault that formed during a hiatus in thrust stacking we relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common although possibly unrecognized features in many other thrust belts