post collisional oblique convergence along the thelon tectonic zone north of the bathurst fault nwt canada abstract the thelon tectonic zone ttz separates the slave from the churchill rae structural provinces of the northwest canadian shield along the ttz in the tinney hills overby lake area collision was initiated with northwestward overthrusting of granulite facies gneiss onto the slave province accompanied by thinskinned shortening of the proterozoic cover in the succeeding transpressive stage shortening structures assumed a se rather than nw vergence a deformation front migrated westward into the slave craton edge causing thick skinned folding of the cover probably including the detachment and transpression was partitioned in craton edge and allochthon into zones of shortening shortening strike slip and strike slip ductile deformation along the trace of the late nw striking brittle bathurst fault occurred at this time this zone links dextral nne trending transpressive zones in which the shortening structures have opposite vergence se north of the fault and nw to the south the last episode of convergence tectonic escape may have been accomplished during brittle deformation on the bathurst fault in the slave foreland and the reworked slave craton edge the role of heterogeneous basement deformation is very important the basement had a complex archean structure which was overprinted by systems of proterozoic shear zones the bulk strains accomplished by these are expressed as upright folds in the stratiform cover causing characteristic synclines

microstructures and c axis fabrics indicative of quartz dissolution in sheared quartzites and phyllonites abstract quartzites sheared in low metamorphic grade shear zones that extend along the contact with migmatitic gneissic complexes in the quadrilatere ferifero region southeastern brazil show quartz microstructures indicating deformation by dissolution dissolution occurred adjacent to the foliation planes preferentially in quartz porphyroclasts oriented with their 0001 planes parallel to the foliation dissolution worked as an efficient mechanism to remove porphyroclast edges smoothing foliations and enabling strain accommodation both dissolution and crystal plasticity were competitive crystallographically controlled processes operating in different domains of the evolving microstructure dissolution occurred along the foliation planes while isovolume crystal plasticity and accompanying recrystallization operated essentially in the quartzose domains between folia phyllonites formed in the most strained domains of the shear zones fine grained quartz grains in phyllonites show uncommon c axis fabrics c axis oriented at low angles to the stretching lineation which were interpreted to form either by direct reprecipitation or by residual concentration of grains unfavourably orientated to undergo dissolution domainal c axis fabrics reflect a deformation partitioning path with increasing participation of solution transfer strengthening c axis maxima at low angles with the stretching lineation at the expense of crystal plastic processes during progressive phyllonitization

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

heterogeneous brittle ductile deformation at shallow crustal levels under high thermal conditions the case of a synkinematic contact aureole in the inner northern apennines southeastern elba island italy abstract we present an example of interaction between magmatism and tectonics at shallow crustal levels in the late miocene the metamorphic units of the eastern elba island northern apennines were intruded at very shallow crustal levels by a large pluton 60 km 2 with the development of an hectometre sized contact aureole defined by growth of low pressure high temperature mineral assemblages p max max 650 c structural data show that the contact aureole is associated with a km sized antiform of the foliation and by several metre to decametre thick high strain domains consisting of strongly foliated rocks containing synkinematic ht lp mineral assemblages and ductile shear zones of variable thickness these shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration associated with the thermal peak of contact metamorphism to subgrain rotation and bulging recrystallization the latter mostly associated with the cataclastic overprint these transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation deformation accompanied the development and cooling of the contact aureole which recorded the switch from high temperature ductile to low temperature brittle conditions the geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks

interpreting stable isotope records from freshwater snail shell carbonate a holocene case study from lake golhisar turkey the oxygen and carbon stable isotope ratios from fossil snail shells within a small intramontane lake in southwest turkey are used to highlight the potential and problems of using freshwater snail carbonate as a palaeoenvironmental proxy two species gyraulus piscinarum and valvata cristata yielded different isotope ratios at the same sampling intervals probably due to differences in water isotope composition between different microhabitats isotope ratios from a number of individual shells from the same sampling intervals representing 7 25 years show large ranges up to 8 for 18o for each species only by analysis of a significant number of species specific shells 5 from each sampling interval can a true understanding of environmental change be obtained averages of the data provide an insight into longer term climatic variation while the ranges provide an estimate of short term decadal environmental variability

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

determining vorticity axes from grain scale dispersion of crystallographic orientations kinematic shear sense indicators are critical tools for tectonic interpretation of shear zones the appropriate plane in which to interpret shear sense indicators is the vorticity normal surface which is often inferred using fabric foliation lineation orientation strain modeling however suggests that such fabrics can be unreliable for determining a kinematic framework we demonstrate the application of a new quantitative method crystallographic vorticity axis cva analysis that utilizes rotation statistics to calculate dispersion axes from crystallographic orientations at the grain scale we apply cva analysis to samples from three shear zones that exhibit distinct kinematics and deformation geometries in all cases the aggregate of calculated grain scale dispersion axes yields a preferred axis of crystallographic vorticity at the specimen scale that is coincident with the independently determined bulk vorticity axis this new method allows the position of vorticity axes to be evaluated independently of foliation and lineation information and without assumptions about the kinematics of deformation

an audio magnetotelluric investigation of the otjiwarongo and katima mulilo regions namibia abstractas an additional opportunistic component to the southern african magnetotelluric experiment natural source audio magnetotelluric amt data were acquired during phase iv to investigate the local scale electric conductivity subsurface structure in the otjiwarongo and katima mulilo regions namibia as an aid in locating the installation points for high voltage direct current earth electrodes the study showed that the shallow subsurface of areas containing one measurement site in the otjiwarongo region and three sites in the katima mulilo region have appropriate high conductivities for the optimal placement of the earth electrodes both of the amt surveys are situated close to the edge of the orogenic neo proterozoic damara mobile belt dmb previous studies all suggest the existence of a highly conductive midcrustal zone which correlates well with the spatial location of the dmb two dimensional inverse modeling of the otjiwarongo amt data confirms the existence of the high conductive zone at m

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

deep seismic profiling in the nosop basin botswana cratons mobile belts and sedimentary basins abstract deep seismic reflection data from the kalahari of western botswana has been interpreted in terms of the tectonics of southern africa the 600 km of 12 fold data exhibit good quality reflections in the range 5 15 s twt the data outline the nosop basin a depositional centre with greater than 10 km of palaeozoic and younger sediments five prominent reflecting horizons are traced in the basement underlying the sediments on the basis of the seismic data the nosop is now interpreted as a single basin the previous multi basin interpretation of the aeromagnetic data esults from features within the magnetic basement the uppermost two basement units are interpreted as proterozoic sediments that rest on archean crust the seismic data show that the crust appears to thicken to the southeast as the kaapvaal craton is approached at the same time the sedimentary cover progressively thins suggesting that the western edge of the kaapvaal is an extended margin interpreted deep crustal shear zones indicate a major terrane boundary separating the proterozoic from the archean depths to basement inferred from aeromagnetic data are consistent with the depth to the seismic boundary associated with the top of the archean some of the aeromagnetic expression may be due to karoo volcanic intrusives in the basement a thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby ghanzi chobe belt and allows the mid crustal layering to be dated as late proterozoic

crustal lineaments and shear zones in africa their relationship to plate movements abstract many of the major lineaments in southern africa are major ductile shear zones with large displacement occurring within though often bounding orogenic belts an example is the boundary to the limpopo belt in botswana and zimbabwe however some of these shear zones only record slight displacement when considered on a crustal scale they are merely planes recording differential movement on much larger flat to gently dipping shear zones where the boundary to the orogenic belt is a low angle thrust zone these different types of shear zones are clearly shown in the pan african belt of zambia where large ene trending lineaments have been recorded recent work has shown the northern group of shears to be large lateral ramps for example the rocks of the copper belt are part of an ene verging thrust package the southern boundary of which is a major oblique to lateral ramp in southern zambia shears are more analogous to major transform faults they form as tear faults separating zones of different thrust vergence a possible plate tectonic model is given for this part of africa showing the different relative plate movement vectors estimated from the geometry of the pan african shear zones

the okahandja lineament and its significance for damaran tectonics in namibia the okahandja lineament is considered to represent the southern margin of a magmatic arc produced by the northerly subduction of the damran ocean between 750 and 520 ma ago sediments of the orogen and granitoid intrusives produced by subduction and crustal melting underwent shear deformation between 675 and 575 ma in a low angle zone of sinistral sense shear

the generation of ductile and brittle shear bands in a low angle mylonite zone in the saint barthelemy massif french pyrenees a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies the last major structures formed in the zone are isolated shear bands divided into three types anastomosing inhomogenous ultramylonitic shear bands type i are subparallel to the mylonitic foliation in the gneiss sg most of these bands developed by ductile deformation processes only planar homogeneous ultramylonite bands type ii are usually oblique to sg they generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation type iii shear bands are planar and oblique to sg they consist of pseudotachylyte weakly affected by ductile deformation type i ii and iii bands seem to represent progressively younger structures on a local scale linked to falling p t conditions the systematic variation in orientation of the different shear bands with respect to sg is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy

the eigenvalues of steady flow in mohr space abstract the eigenvalues of the velocity gradients tensor and of the position gradients tensor describing instantaneous flow or incremental deformations are easily determined in mohr space as the points of intersection between a mohr circle and the horizontal mohr axis intersections occur only if the tensor has real eigenvalues eigenvectors associated with the eigenvalues are directions of no rotation a line of material particles under an eigenvector does not rotate during steady flow although the line is instantaneously stretching at a rate equal to its corresponding eigenvalue for non spinning steady sub simple shearing there will always be two eigenvectors the angle between the eigenvectors is dependent on the ratio of pure to simple shearing rates this angle decreases as the pure shearing component decreases relative to the simple shearing component superimposed increments of pure shear and simple shear produce an equivalent set of non rotated material lines but the deformation histories of these lines are rotational the influence of eigendirections on geological structures is constrained by the rheology of a rock during deformation in homogenous steady flow no influence may be obvious spatially periodic material softening along eigenvectors during bulk hardening may result in a complex fabric of intersecting shear zones the geometry of the fabric elements provides important information on the bulk deformation of the rock particularly on the bulk shear sense

recrystallization of quartz after low temperature plasticity the record of stress relaxation below the seismogenic zone abstract quartz microfabrics in rocks from the silvretta basal thrust and the defereggen antholz vals dav shear zone in the eastern alps are analysed by polarized light and electron microscopy the microfabrics from both shear zones record a switch from low temperature plasticity at transient high stress to recrystallization at relaxing stresses at greenschist facies conditions the development of new grains is dominantly by subgrain rotation and subsequent strain induced grain boundary migration in areas of localized high strain developed during initial low temperature plasticity the findings suggest that new grains develop at almost random crystallographic orientations at fast rates of stress relaxation i e at low stress as indicated by recrystallized quartz zones in the silvretta fault rocks in contrast at slow rates of stress relaxation new grains develop at moderately high stresses with crystallographic preferred orientation characterized by high schmid factor for basal glide as indicated by vein quartz samples from the dav shear zone both recorded histories with transient peak stresses and different rates of stress relaxation are interpreted to be related to seismic activity of the fault systems this study demonstrates that characteristic microfabrics provide important information about the deformation history of natural shear zones developed in different tectonic regimes

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

palaeomagnetism of neoproterozoic glacial rocks of the huabei shield the north china block in gondwana abstract a palaeomagnetic study is reported of reddened facies fengtai formation of neoproterozoic glacial rocks which underlie early cambrian rocks with disconformity in the huabei north china sino korean block the diamictite preferred age 620 600 ma carries a dual polarity remanence residing in hematite of the red matrix the mean direction derived from 62 samples is d i 205 9 32 4 95 3 9 yielding a pole position at 233 e 62 n tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction although clasts near the base of the formation are largely overprinted overlying early cambrian sediments of the houjiashan and yutaishan formations have a similar remanence also of dual polarity d i 205 5 32 1 95 3 9 32 samples the magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and or the overlying cambrian succession red shales from the liulaobei formation 890 840 ma near the base of the neoproterozoic succession in the huabei block yield a contrasting remanence of d i 59 4 75 3 95 7 1 19 samples equivalent to a pole position at 150 e 43 n pole positions from this study accord with lower middle cambrian poles from the australian and south china blocks with north china sited adjacent to northeastern australia in accordance with recent biogeographic and palaeogeographic models although the palaeolatitude derived from the fengtai diamictite 17 is cambrian in age and probably later than the glaciation the correlation with australia implies that glaciation in north china took place in low palaeolatitudes 20 and reinforces the view that late neoproterozoic glaciation at least at this perimeter of gondwana occurred at low latitudes neoproterozoic cambrian poles from north china south china and australia fail to conform to the popular rodinia reconstruction with the latter blocks adjacent to western north america and also fail to support the view that gondwana formed by the fusion of widely separated eastern and western segments during pan african orogeny instead they accord to poles from africa and south america in a precursor of gondwana protogondwana the subsequent palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike slip motion during pan african deformation following extinction of the afro arabian arc

dynamic recrystallization of quartz under greenschist conditions abstract the sem electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions the degree of recrystallization has been determined at 10 20 50 and 100 respectively we demonstrate that dynamic recrystallization is a sequential process involving subgrain grain rotation and grainboundary migration it initiates via subgrain polygonization on a single crystal slip system and continues via subgrain and neoblast rotation on several slip systems in the early stages of recrystallization

oroclines and pseudo oroclines abstract in the last few decades a variety of mechanisms have been proposed for the genesis of oroclines and pseudo oroclines poinconnement p tapponnier for the alpine himalayan chain plate bending on a vertical axis h p laubscher in the mediterranean sea change of the thrust direction l b harris in corsica axial plunge of a nonplane thrust surface r sacchi in mozambique whirl tectonics m gidon for the western alps large scale ductile shear m p coward and m c daly for the lufilian arc of zambia tear faulting j goguel for the western alps high strain at boundaries of polygonal miniplates m b katz for precambrian africa differential displacement connected with different thermal regimes m p coward for the damara orogenic system mesozoic basin pattern inheritance e v artyushkov and m a baer in the alpine carpathyan region and large scale shearing of weakened continental margins w m van der linden in the mediterranean area have all been proposed several combinations of these models have also been suggested after a brief review the criteria of recognition of some models are discussed the importance of the distribution pattern of the minor structures is stressed by drawing inspiration from some of the ideas of hans laubscher especially the emphasis laid on the uncoupling of the surficial peels from the rest of the crust as an example two case histories are reported from the western alps and from southern africa

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

an sem study of porosity and grain boundary microstructure in quartz mylonites simplon fault zone central alps a backscattered and secondary electron sem study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde simplon fault zone established three characteristic components 1 fine isolated pores 1 m diameter are scattered across two grain interfaces preferentially concentrated on surfaces in extension pores are uncommon on three grain junctions and there is no evidence for fluid interconnectivity along three and four grain junctions the fine porosity may develop by accumulation of original mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding dynamic cavitation implies that the ductile mylonitic deformation is at least locally dilatant and therefore pressure sensitive 2 large vug like pores up to mm scale extend along multi grain boundaries observed in all samples they are most common in the higher initial temperature coarse grained samples with a microstructure dominated by grain boundary migration recrystallization grains bordering this connected porosity develop perfect crystal faces undecorated by fine pores or pits the irregular lobate optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces the coarse porosity is probably due to accumulation during dynamic recrystallization of co2 rich fluid with a high wetting angle against quartz 3 in one sample interconnected sinuous ridges 0 2 m high are observed to follow three and four grain junctions and disjoint into more isolated worms and spheroidal globules on two grain interfaces these are transitional to more branching vein like or convoluted brain like forms the brain like and globular forms have been observed with varying frequency through the range of samples with the globules attaining sizes of up to 60 m vein structures have also been observed on intragranular fractures these topologies do not match across adjoining surfaces and must have developed into free space the ridge vein brain spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established they could represent unstable meniscus necking of a thin grain boundary phase of low viscosity developed due to quasi adiabatic shear and or local stress induced dilatancy during microcracking

fabric anisotropy controls faulting in the continental crust the influence of pre existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust e g nanga parbat massif nw himalayas where earlier brittle structures have been annealed the kinematics of young faults formed in a single continuing tectonic regime nnw compression are distinctly different depending upon the orientation of the early ductile foliations around them faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping thus structural anisotropy does control faulting in the continental crust a result with important implications for seismogenesis fluid flow and basin evolution

development of crystallographic preferred orientations by nucleation and growth of new grains in experimentally deformed quartz single crystals abstract deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation cpo development cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol of added water and 20 mg of mn2o3 powder in a griggs solid medium deformation apparatus in different crystallographic orientations with compression direction i parallel to ii at 45 to and 45 to and iii parallel to the experiments were performed at a temperature of 800 c a confining pressure of 1 2 gpa a strain rate of 10 6 s 1 to bulk finite strains of 14 36 the deformed samples were analysed in detail using optical microscopy electron backscatter diffraction ebsd and transmission electron microscopy tem two different microstructural domains were distinguished in the deformed samples i domains with undulatory extinction and deformation lamellae and ii domains with new recrystallised grains within the domains of undulatory extinction crystal plastic deformation caused gradual rotations of the crystal lattice up to 30 away from the host orientation new recrystallised grains show a strong cpo with c axis maxima at 45 to the compression direction this is the case in all experiments irrespective of the initial crystallographic orientation the results show that c axes are not continuously rotated towards the new maxima the new grains thus developed through a mechanism different from subgrain rotation recrystallisation new grains have a subeuhedral shape and numerous microcavities voids fluid channels and fluid inclusions at their grain boundaries no host control is recorded in misorientation axes across their large angle grain boundaries new grains might have been created by nucleation from solution in the m scale voids and microfractures the cpo most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions

a km scale triaxial experiment reveals the extreme mechanical weakness and anisotropy of mica schists grandes rousses massif france abstract the development of andersonian faults is predicted according to theory and experiments for brittle frictional deformation occurring in a homogeneous medium in contrast in an anisotropic medium it is possible to observe fault nucleation and propagation that is non andersonian in geometry and kinematics here we consider post metamorphic brittle frictional deformation in the mechanically anisotropic mylonitic mica schists of the grandes rousse massif france the role of the mylonitic foliation and of any other source of mechanical anisotropy in brittle frictional deformation is a function of orientation and friction angle according to the relative orientation of principal stress axes and foliation a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle frictional fractures and faults if the foliation is not utilized the rock behaves as if it was isotropic and andersonian geometry and kinematics can be observed if the foliation is utilized the deviatoric stress magnitude is buffered and andersonian faults fractures cannot develop in a narrow transition regime both andersonian and non andersonian structures can be observed we apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the grandes rousses schists defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated this approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction thus the study area can be seen as a km scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica schists in this way we infer a coefficient of friction w e a k 0 14 for brittle frictional failure of the foliation or 20 of the isotropic coefficient of internal friction

reprint of extensional shear band development on the outer margin of the alpine mylonite zone tatare stream southern alps new zealand abstract schistose mylonitic rocks in the central part of the alpine fault af at tatare stream new zealand are cut by pervasive extensional c shear bands in a well understood and young natural ductile shear zone the c shears cross cut the pre existing mesozoic aged foliation displacing it ductilely synthetic to late cenozoic motion on the af using a transect approach we evaluated changes in geometrical properties of the mm cm spaced c shear bands across a conspicuous finite strain gradient that intensifies towards the af precise c attitudes c foliation dihedral angles and c s intersections were calculated from multiple sectional observations at both outcrop and thin section scales based on these data the direction of ductile shearing in the alpine mylonite zone during shear band activity is inferred to have trended 20 clockwise down dip of the coeval pacific australia plate motion indicating some partitioning of oblique slip motion to yield an excess of dip slip relative to plate motion azimuth or some up dip ductile extrusion of the shear zone as a result of transpression or both constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary szb across all examined parts of the shear zone the mean dihedral angle between the c shears and the mylonitic foliation s remains a constant 30 1 1 the aggregated slip accommodated on the c shear bands contributed only a small bulk shear strain across the shear zone 0 6 0 8 uniformity of per shear slip on c shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain hardening rheology such that they locked up at a finite shear strain inside c bands of 12 15 shear band boudins and foliation boudins both record extension parallel to the szb as do the occurrence of extensional shear band sets that have conjugate senses of slip we infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with w k

a magnetotelluric study of the damara belt in namibia 1 regional scale conductivity anomalies abstract the namibian margin is dominated by the late proterozoic to early cambrian fold belts of the damara orogen which wrap around and separate the congo and kalahari cratons this mosaic of relatively soft fold belts and hard cratons apparently controlled the path for the opening of the south atlantic in the early cretaceous the continents split along the coast parallel fold belts of the damara orogen while the inland fold belt damara belt was effected by extension and widespread igneous intrusion but never developed to the rift stage this paper is concerned with the interpretation of magnetotelluric mt data along a 200 km nw se profile across the damara belt in nw namibia the regional two dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones i a generally very resistive upper crust which is typical for the granites and metasediments of the damara belt ii two subvertical conductors in upper to mid crustal levels which correlate with major tectonic zone boundaries and iii a highly conductive middle to lower crust in the southern part of the profile the geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust we interpret the high electrical conductivity in terms of graphite or other forms of mineralization enrichment along the shear planes this zone of crustal weakness may have originated in pre damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the cretaceous rifting and magmatism associated with the opening of the south atlantic

quartz c axes parallel to stretching directions in very low grade metamorphic rocks abstract quartz preferred orientations with c axes parallel to stretching directions are widespread in slates phyllites and very low grade schists of the south island new zealand in general these rocks lack evidence for plastic deformation of quartz and often preserve the original clastic quartz which naturally tends to be elongate parallel to the c axis the c axis preferred orientation can be explained therefore by mechanical rotation of clastic grains during deformation so that long axes became parallel to the stretching direction plastic deformation does however play a role in some very quartz rich layers which display a preferred orientation due to basal slip with c axes at high angles to the stretching direction solution transfer is important in enhancing the contrasts between pelitic and quartz rich lithologies but its role in producing c axis preferred orientations by competitive anisotropic growth or solution seems important only in slickenfibres and in fibrous quartz veins of shear zones where quartz displays very strong preferred orientations with c axes parallel to the local stretching direction in contrast the non fibrous quartz pressure shadows in some muscovite rich layers display a c axis preferred orientation similar to adjacent quartz rich sand layers and it probably results from nucleation of pressure shadow quartz on pre existing mechanically oriented quartz clasts

brittle shear zones in experimentally deformed quartz single crystals abstract intracrystalline shear zones developed in a quartz single crystal that was experimentally deformed to 26 finite strain under conditions favourable for macroscopically ductile deformation the crystal was uniaxially compressed parallel to c in the presence of 1 vol of added water at a temperature of 800 c a confining pressure of 1200 mpa and a strain rate of 10 6 s 1 we investigated how the shear zones initiated and how their structure evolved with ongoing deformation detailed crystallographic orientation analyses were carried out with the help of the electron backscatter diffraction ebsd technique and complemented by transmission electron microscopy tem the shear zones appeared to develop along planar microcracks oriented parallel to crystallographic rhomb planes further microcracking resulted in cataclastic shear zone development new grains developed by rotation of fracture fragments the rotation of the fragments in the shear direction was probably facilitated by the high water pressure and the relatively high porosity in the shear zone shear zones finally exhibit a strong crystallographic preferred orientation cpo our results suggest that cataclastic deformation processes may lead to shear zone development the cpo developed within the shear zones through dynamic recrystallisation i e new grains with c oriented approximately perpendicular to the shear zone boundary grew at the cost of other new grains

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

the zuccale fault elba island italy a new perspective from fault architecture the zuccale fault central eastern elba island has been regarded since the 1990s as a low angle normal fault that records neogene crustal extension in the inner tyrrhenian side portion of the northern apennines the flat lying attitude of the fault zone and the strong excision of thick nappes were the main reasons for this interpretation previous structural and petrographic studies have focused primarily on the fault rocks themselves without map scale investigation of the structural setting and deformation structures in the hanging wall and footwall blocks furthermore despite the complex history proposed for the zuccale fault the timing of deformation has not yet been constrained by radiometric age data we present the findings of recent geological studies on eastern elba island that provide significant new insight on the nature and tectonic significance of the zuccale fault we document in detail the architecture of breccias and cataclasites that comprise the zuccale fault our new observations are consistent with a purely brittle deformation zone that crosscuts older early middle and late miocene regional and local tectonic structures the activity on the fault postdates emplacement of the late miocene porto azzurro pluton and it displaces a previously formed nappe stack 6 km eastward without any footwall exhumation or hanging wall block rotation these new data raise questions about the development of misoriented faults in the upper crust

localisation of plastic flow in the mid crust along a crustal scale fault insight from the hatagawa fault zone ne japan abstract this study examines plastic flow in fault rocks exposed along the hatagawa fault zone hfz of ne japan the fault zone developed in 110 ma granitoids ceased activity by 98 1 2 5 ma three different fault rocks mylonites with microstructures a and b and cataclasite are exposed along the fault microstructure a formed at the brittle plastic transition the temperature conditions for microstructure b were higher than those for microstructure a those for the cataclasite were lowest microstructure a is exposed in limited areas maximum length extent of approximately 6 km along the hfz the distribution of microstructure a is considered to represent the latest stage localised zones of plastic flow associated with strain weakening accompanied by dynamic recrystallisation of feldspar suggesting the restriction of plastic displacement to certain intervals at depth ranges with p t conditions of the brittle plastic transition many zones of localised deformation containing crush zones are observed in rocks with microstructure a suggesting that numerous fractures nucleated due to the ductile fracturing of highly deformed fine grained feldspar in the outcrop extent of microstructure a the nucleation of large earthquakes was possibly promoted by interaction between fractures that nucleated by ductile fracture and stress concentrations associated with the restricted development of plastic displacement

misoriented faults in exhumed metamorphic complexes rule or exception abstract low angle normal faults and other weak faults are common in the metamorphic core of collisional orogens they frequently show a phyllosilicate rich mylonitic foliation that was reactivated under brittle conditions experimental and theoretical works indicate that mechanical anisotropies exert a substantial influence on shear failure and frictional sliding eventually inhibiting the nucleation and propagation of new andersonian shear fractures and favoring the localization of brittle failure along the pre existing foliations metamorphic phyllosilicate rich rocks may show a friction coefficient varying from 0 6 at high angles to the foliation to 0 2 0 4 for shear along the inherited foliation to test the influence of mechanical anisotropies on the development of non andersonian faults we have applied a modified slip tendency analysis to three misoriented phyllosilicate rich faults of the european alps the analysis accounts for anisotropy in friction coefficients and has been named anisotropic slip tendency analysis here we show that brittle deformation along misoriented phyllosilicate rich foliations is more probable than the development of new andersonian faults the presence of a well developed network of weak phyllosilicate rich faults may influence the overall structural style and mechanical properties of the brittle lithosphere in collisional orogens

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

comparison of geoelectric and seismic reflection models of the zambezi valley basins northern zimbabwe the mana pools and lower zambezi karoo sedimentary basins lie within the zambezi mobile belt in northern zimbabwe the subsurface apparent resistivities measured at both locations are extremely low magnetotelluric mt data along a profile across part of the lower zambezi basin have been inverted using rapid relaxation inversion smith booker 1991 to find the minimum structure needed to fit the data and compare with an earlier forward model the resistivity models of both the mana pools and the lower zambezi basins are then compared with their structure revealed from seismic reflection data the resistivity structure of the mana pools basin is well modelled as a series of different resistivity layers whose boundaries are defined by the seismic data however the resistivity structure of the lower zambezi basin cannot be matched easily to the seismic structure additional structure with no seismic expression is required there is a conductive feature in the two basins in the upper karoo sandstone layer that extends below the seismic basement beneath the lower zambezi basin this indicates that the conductors may represent different types of features in the two basins consistent with their proposed different tectonic origins a resistive unit is present within the sediments in the lower zambezi basin that may represent intercalated basalt dykes giving an anisotropic mt response it has been suggested that there might be similar thin basalt layers within the sediments of the mana pools basin but these could not be resolved by mt methods the low resistivity of the basement particularly beneath the lower zambezi basin is remarkable and may result from a high degree of either chemical or tectonic alteration to the underlying rocks due to metamorphic processes and tectonic disruption during rift formation the presence of the lower zambezi basin conductor at depths greater than the seismic basement is consistent with observations to the west in the adjacent damara mobile belt where similar conductors have been seen in regions with no sedimentary cover

crystallographic preferred orientation development by dissolution precipitation creep abstract crystallographic preferred orientations cpos in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms where deformation is achieved by the movement of dislocations in this paper we investigate the possibility of cpo development by dissolution precipitation creep or pressure solution a numerical model is presented which simulates the development of a grain aggregate that deforms by reaction controlled dissolution precipitation creep grains are simulated as rectangular boxes that change their shape by growth or dissolution of their surfaces depending on the normal stresses acting on the individual surfaces grains can also rotate due to an applied vorticity for non coaxial deformation and if they have a non equidimensional shape for each strain increment stress that is applied to the grains is the same for all grains while individual grains deform and rotate by different amounts a variety of cpos develop at moderate strains depending on the reaction rates of the different crystal surfaces and type of deformation uni axial shortening plane strain pure shear and simple shear the modelling results confirm that dissolution precipitation creep may play a role in cpo development in rocks

evolution of shear zones through the brittle ductile transition the calamita schists elba island italy abstract a network of shear zones that evolved through the brittle ductile transition is exposed in the calamita schists elba island italy the shear zones formed during late miocene contractional deformation coeval with high grade contact metamorphism 650 c related to the emplacement of plutonic rocks at shallow crustal levels 7 10 km an early stage high metamorphic grade foliation was overprinted by mylonitic deformation that progressively localized on low metamorphic grade shear bands producing s c mylonites during cooling of contact aureole localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning between hard high grade relics and soft shear bands softening of shear bands occurred likely due to fluid influx and retrograde growth of fine grained phyllosilicates the interconnection of anastomosing shear bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing mylonites with a composite mylonitic foliation and c shear bands an estimate of the vorticity number wk of the flow of 0 3 0 5 was obtained from the orientation of c shear bands measured at the meso and thin section scale close to the brittle ductile transition the growth of soft phyllosilicates allowed c shear bands to act as precursory structures to brittle deformation localized into an array of low angle faults and shear fractures

grain coarsening in contact metamorphic carbonates effects of second phase particles fluid flow and thermal perturbations under contact metamorphic conditions carbonate rocks in the direct vicinity of the adamello pluton reflect a temperature induced grain coarsening despite this large scale trend a considerable grain size scatter occurs on the outcrop scale indicating local influence of second order effects such as thermal perturbations fluid flow and second phase particles second phase particles whose sizes range from nano to the micron scale induce the most pronounced data scatter resulting in grain sizes too small by up to a factor of 10 compared with theoretical grain growth in a pure system such values are restricted to relatively impure samples consisting of up to 10 vol micron scale second phase particles or to samples containing a large number of nano scale particles the obtained data set suggests that the second phases induce a temperature controlled reduction on calcite grain growth the mean calcite grain size can therefore be expressed in the form d 1 4 c2 eq rt dp fp m where c2 is a constant q is an activation energy t the temperature and m the exponent of the ratio dp fp i e of the average size of the second phases divided by their volume fraction however more data are needed to obtain reliable values for c2 and q besides variations in the average grain size the presence of second phase particles generates crystal size distribution csd shapes characterized by lognormal distributions which differ from the gaussian type distributions of the pure samples in contrast fluid enhanced grain growth does not change the shape of the csds but due to enhanced transport properties the average grain sizes increase by a factor of 2 and the variance of the distribution increases stable d18o and d13c isotope ratios in fluid affected zones only deviate slightly from the host rock values suggesting low fluid rock ratios grain growth modelling indicates that the fluid induced grain size variations can develop within several ka as inferred from a combination of thermal and grain growth modelling dykes with widths of up to 1 m have only a restricted influence on grain size deviations smaller than a factor of 1 1 to summarize considerable grain size variations of up to one order of magnitude can locally result from second order effects such effects require special attention when comparing experimentally derived grain growth kinetics with field studies

pan african rifting and orogenesis in southern to equatorial africa and eastern brazil abstract the upper proterozoic pan african belts of africa and brasiliano belts of south america are assumed to have evolved from an elaborate system of continental rifts which formed on the west gondwana continent 1100 1000 ma ago reviews of the damara belt kaoko belt gariep belt saldania belt west congolian belt lufilian arc and zambezi belt of southern and equatorial africa and the ribeira mantiqueira belt of brazil show the applicability of the continental rifting model to all these belts strict application of the rifting model describing the formation and further evolution of rift structures in the continental crust leads to the conclusion that the west congolian belt as traditionally described is incomplete and has to be supplemented by the mayumbian belt and a rift structure situated farther to the west the lufilian arc is assumed to have evolved from two rift structures separated from each other by the later domes region the golfe du katanga which branches off northeastwards from the lufilian arc is interpreted as an aulacogen shaba aulacogen the pan african brasiliano rift system is comparable to the mesozoic arctic north atlantic rift system with regard to size and distribution of rifts ocean floor spreading and opening of a proto south atlantic ocean most probably occurred along a main line of rifting situated between the west coast of africa and eastern brazil the wedge like ocean may have terminated at the sao francisco congo cratonic bridge between northeastern brazil and gabon cameroon there is no evidence of ocean floor spreading in the zambezi belt the lufilian arc and the traditional west congolian belt which are arranged along another major line of rifting closure of the pan african brasiliano rift system took place during two successive orogenic episodes the zambezi belt lufilian arc and west congolian belt underwent their main orogenic deformation with prevailing ene and wsw oriented tectonic transportation directions during the katangan episode at 900 750 ma the transcontinental sinistral mwembeshi shear zone adjusted opposite movement directions in the zambezi belt and lufilian arc and at its western prolongation concurrently supported opening of the khomas trough in the southern damara belt the proto south atlantic ocean still opening during the katangan episode was gradually closed during the damaran episode at 750 500 ma closure proceeded from north to south and was accompanied by northwest directed subduction underneath the brazilian plate and southeast directed tectonic transportation in the kaoko damara and gariep belts in the previously formed west congolian belt lufilian arc and zambezi belt a second deformation phase characterized by strike slip faulting and local emplacement of nappes occurred during this episode

weakening and strain localization produced by syn deformational reaction of plagioclase there are many observations in naturally deformed rocks on the effects of mineral reactions on deformation but few experimental data in order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase h2o more albitic plagioclase zoisite kyanite quartz we utilized fine grained 2 6 m plagioclase aggregates of two compositions an54 and an60 both dried and with 0 1 0 4 wt h2o present in shear deformation experiments at two sets of conditions 900 c 1 0 gpa in the plagioclase stability field and 750 c 1 5 gpa in the zoisite stability field dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep but samples with 0 1 to 0 4 wt water deformed in the zoisite stability field showed extreme strain localization into very narrow 1 3 m shear bands after low shear strain in these samples the microstructures of reaction products in the matrix differ from those in the shear bands in the matrix large up to 400 m zoisite crystals grew in the direction of finite extension and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic in the shear bands the reaction products albitic plagioclase zoisite white mica and traces of kyanite form polyphase aggregates of very fine grained 0 1 m dislocation free grains most of the sample strain after 2 has occurred within the shear bands within which the dominant deformation mechanism is inferred to be diffusion accommodated grain boundary sliding dagbs the switch from dislocation creep in dry samples deformed without reaction to dagbs in reacted samples is associated with a decrease in flow stress from 800 to 200 mpa these experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine grained polyphase assemblage leading to a switch in deformation mechanism and significant weakening thus localization of deformation in polyphase rocks may occur on any pressure p temperature t path where the equilibrium composition of the constituent minerals changes

contrasting deformation of s and i type granitoids in the lachlan fold belt eastern australia abstract several examples of contrasting deformation under greenschist facies conditions of closely associated silurian devonian s type peraluminous and i type mainly metaluminous granitoid plutons of very similar age occur in the lachlan fold belt of eastern new south wales australia they occur in and or near the wologorong batholith the wyangala batholith and the kosciusko batholith the s type granitoids generally are strongly foliated and locally mylonitic whereas most of the i type granitoids typically show alteration to hydrous minerals chlorite epidote and actinolite but are unfoliated or only locally weakly foliated commonly fractured and have spaced ductile shear zones the more obvious and penetrative deformation in the s type granitoids probably is due mainly to their larger contents of relatively weak and or ductile minerals especially quartz and biotite and to the common occurrence of quartz in relatively large patches these features providing more continuous strained volumes and enabling quartz and mica rich folia to develop between resistant grains of feldspar in most of the i type granitoids the smaller amounts of quartz and biotite together with the common occurrence of quartz in smaller more scattered patches produce more localized strained volumes and may give rise to local stress concentrations that lead to fracturing the latter provides access to fluids responsible for hydrothermal alteration however some i type granitoids have high enough proportions of less ductile minerals to develop foliations although typically this occurs only adjacent to major faults some variation in the deformation of s types also is to be expected thus the main factor controlling the development of foliations appears to be the ratio of strong to weak or brittle to ductile minerals rather than the s i subdivision with which it generally correlates although other possible factors need further investigation such contrasting styles of deformation could lead to the erroneous interpretation that the massive i types are post deformation plutons and that the s types which commonly are slightly older are foliated by syn emplacement deformation whereas in fact both are pre deformation granitoids

timing and pt evolution of whiteschist metamorphism in the lufilian arc zambezi belt orogen zambia implications for the assembly of gondwana abstract one of the world s most extensive occurrences of whiteschists talc kyanite schists is located in south central africa with several exposures along a ca 700 km northwest southeast striking zone the metamorphic evolution and age relations of whiteschists and associated rocks from four localities three in the lufilian arc and one in the zambezi belt were investigated in the lufilian arc whiteschists associated garnet amphibolites and biotite kyanite garnet gneisses record peak metamorphic conditions of about documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes

coupled micro faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the sambagawa metamorphic rocks southwest japan abstract in the sambagawa schist southwest japan while ductile deformation pervasively occurred at d 1 phase during exhumation low angle normal faulting was locally intensive at d 2 phase under the conditions of frictional viscous transition of quartz c 300 c during further exhumation into the upper crustal level accordingly the formation of d 2 shear bands was overprinted on type i crossed girdle quartz c axis fabrics and microstructures formed by intracrystalline plasticity at d 1 phase in some quartz schists the quartz c axis fabrics became weak and finally random with increasing shear accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica we interpreted these facts as caused by increasing activity of pressure solution the quartz grains were dissolved mostly at platy quartz mica interface and precipitated with random orientation and pinned by mica thus having led to the obliteration of existing quartz c axis fabrics in the sheared quartz schist the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands but also to enhanced dissolution at the interphase boundaries

late cretaceous reactivation of major crustal shear zones in northern namibia constraints from apatite fission track analysis abstract namibia s passive continental margin records a long history of tectonic activity since the proterozoic the orogenic belt produced during the collision of the congo and kalahari cratons in the early proterozoic led to a zone of crustal weakness which became the preferred location for tectonism during the phanerozoic the pan african damara mobile belt forms this intraplate boundary in namibia and its tectonostratigraphic zones are defined by ductile shear zones where the most prominent is described as the omaruru lineament waterberg thrust oml wt the prominance of the continental margin escarpment is diminished in the area of the central and northern zone of the damara belt where the shear zones are located this area has been targeted with a set of 66 outcrop samples over a 550 km long 60 km broad coast parallel transect from the top of the escarpment in the south across the damara sector to the kamanjab inlier in the north apatite fission track age and length data from all samples reveal a regionally consistent cooling event thermal histories derived by forward modelling bracket this phase of accelerated cooling in the late cretaceous maximum palaeotemperatures immediately prior to the onset of cooling range from ca 120 to ca 60 c with the maximum occurring directly south of the omaruru lineament because different palaeotemperatures indicate different burial depth at a given time the amount of denudation can be estimated and used to constrain vertical displacements of the continental crust we interpret this cooling pattern as the geomorphic response to reactivation of basement structures caused by a change in spreading geometry in the south atlantic and south west indian oceans

evolution of quartz microstructure and c axis crystallographic preferred orientation within ductilely deformed granitoids arolla unit western alps abstract we have studied quartz microstructures and the c axis crystallographic preferred orientations cpos in four granitoid samples representative of increasing ductile shear deformation from a weakly deformed granitoid stage 1 to a mylonitic granitoid stage 4 the quartz c axis cpo measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area all the samples belong to the austroalpine arolla unit western alps and were deformed at greenschist facies conditions the quartz c axis cpo was analyzed using a u stage and the optical orientation imaging technique the magmatic plagioclase forming more than 50 of the volume of the granitoid is extensively replaced by a mica rich aggregate even in weakly deformed samples of stage 1 these aggregates flow to form an interconnected weak matrix with increasing deformation wrapping relatively less strained quartz grains that undergo dominantly coaxial strain recrystallization of quartz ranges from less than 1 in the weakly deformed granitoid to up to 85 in the mylonitic granitoid with average grain strain of 41 and 64 respectively with increasing strain and recrystallization quartz grains in the granitoids show a sequence of transient microstructures and cpos crystal plastic deformation is initially accomplished by dislocation glide with limited recovery and at 50 grain strain it results in a cpo consistent with dominantly basal a slip at 60 grain strain recrystallization is preferentially localized along shear bands which appear to develop along former intragranular cracks and the recrystallized grains develop a strong c axis cpo with maxima orthogonal to the shear band boundaries and independent of the host grain orientation within the granitoid mylonite at an average quartz grain strain of 64 recrystallization is extensive and the c axis cpo of new grains displays maxima overlapping the host c axis orientation and therefore unrelated to the bulk sense of shear the host controlled cpo is inferred to reflect pervasive recrystallization by progressive subgrain rotation the switch from shear band control to host control on c axis cpo occurred between 40 and 70 of recrystallization in the quartz mylonite the quartz c axis cpo develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions this study indicates that the cpo evolution of quartz may significantly differ in cases of polymineralic vs monomineralic rocks under the same deformation conditions if quartz in the polymineralic rock behaves as a strong phase

c axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid rich greenschist conditions the quartz v axis orientations and microstructures have been analyzed in an originally single crystal buckled quartz vein of greenschist metamorphic grade from the sambagawa metamorphic belt southwest japan the basal 0001 plane was inclined by ca 10 20 to the quartz vein surface before deformation and the flexure essentially kink which produced the fold was accomplished by basal 0001 slip alone in the hinges i e flexural slip folding in addition to crystal plasticity dissolution microstructures are ubiquitous in the quartz vein dynamic recrystallization resulted from both subgrain rotation and grain boundary migration rotation recrystallization associated with basal 0001 slip resulted in host controlled v axis orientation distribution along a great circle whereas migration recrystallization perhaps assisted by intergranular fluid resulted in a penetrative development of a weak x elongation maximum v axis fabric nowhere rotated by the folding the migration recrystallization is evidenced by large v axis misorientations across the grain boundaries as well as irregular grain shape based on the weak v axis fabric development and lack of grain shape fabric solution precipitation creep may have dominated in the fluid assisted grain boundary migration regime and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein

simulating coseismic deformation of quartz in the middle crust and fabric evolution during postseismic stress relaxation an experimental study abstract non steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake driven episodic deformation in the middle crust three types of experiments were carried out using a modified griggs type solid medium deformation apparatus all three start with high stress deformation at a temperature of 400 c and a constant strain rate of 10 4 s 1 type a some are followed by annealing in the stability field of quartz for 14 15 h at zero nominal differential stress and temperatures of 800 1000 c type a b or by annealing for 15 h at 900 c and at a residual stress type a c the quartz samples reveal a very high strength 2 gpa at a few percent of permanent strain the microstructures after short term high stress deformation type a record localized brittle and plastic deformation statisc annealing type a b results in recrystallisation restricted to the highly damaged zones the new grains aligned in strings and without crystallographic preferred orientation indicate nucleation and growth annealing at non hydrostatic conditions type a c results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation in this case however the recrystallised zone is several grain diameters wide the grains are elongate and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation quartz microstructures identical to those produced in type a b experiments are observed in cores recovered from long valley exploratory well in the quaternary long valley caldera california with considerable seismic activity the experiments demonstrate the behaviour of quartz at coseismic loading type a and subsequent static annealing type a b or creep at decaying stress type a c in the middle crust the experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

new constraints on the evolution of the inner northern apennines by k ar dating of late miocene early pliocene compression on the island of elba italy

crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins abstract quartz veins within rieserferner pluton underwent deformation during post magmatic cooling at temperature around 450 c different crystallographic orientations of cm sized quartz vein crystals conditioned the evolution of microstructures and crystallographic preferred orientations cpo during vein parallel simple shear up to high shear strains 10 for glide in ribbons with c axis sub parallel to the shear zone vorticity y axis ribbons with c axis orthogonal to y xz type ribbons were stronger and hardened more quickly they show lower aspect ratios and fine grain size 10 20 m recrystallization along sets of microshear zones szs exploiting crystallographic planes distortion of xz type ribbons and recrystallization preferentially exploited the slip systems with misorientation axis close to y new grains of szs initiated by subgrain rotation recrystallization sgr and thereupon achieved high angle misorientations by a concurrent process of heterogeneous rigid grain rotation around y associated with the confined shear within the sz dauphine twinning occurred pervasively but did not play a dominant role on sz nucleation recrystallization became widespread at 2 and pervasive at 10 ultramylonitic quartz veins are fine grained 10 m similar to new grains of sz and show a cpo banding resulting in a bulk c axis cpo with a y maximum as part of a single girdle about orthogonal to the foliation and orientations at the pole figure periphery at moderate to high angle to the foliation this bulk cpo derives from steady state sgr associated with preferential activity in the different cpo bands of slip systems generating subgrain boundaries with misorientation axes close to y the cpo of individual recrystallized bands is largely inherited from the original crystallographic orientation of the ribbons and therefore vein crystals from which they derived high strain and pervasive recrystallization were not enough to reset the initial crystallographic heterogeneity and this cpo memory is explained by the dominance of sgr this contrast with experimental observation of a rapid erasure of a pristine cpo by cannibalism from grains with the most favourably oriented slip system under dominant grain boundary migration recrystallization

a magnetotelluric model of the mana pools basin northern zimbabwe the mana pools sedimentary basin lies within the zambezi mobile belt in northern zimbabwe new and preexisting magnetotelluric data and the available seismic reflection data are used to constrain the basin structure and the depth to the electrical basement long period magnetotelluric lmt data were collected at five stations along a 60 km north south profile across the mana pools basin and onto the southern escarpment these data augment an existing audiofrequency amt data set from 11 sites in the same area the subsurface apparent resistivities measured at periods sampling the basin are very low a few m after processing both data sets the estimated impedance tensor is decomposed showing that the resistivity structure of the mana pools basin can be modeled two dimensionally the algorithm is used to show that there is no systematic offset in magnitude between the amt and lmt data sets before they are combined minimum structure resistivity models of the mana pools basin compare well with the information from reflection seismic data and support its previous description as a half graben basin of 7 km depth the excellent conductor in the mana pools basin is quite different to those seen elsewhere in the orogenic belt in that it is a feature of the sedimentary fill rather than the basement the resistivity of the basement is low but no localized good conductor is observed these low resistivities may result from a high degree of either chemical or tectonic alteration to the underlying rocks due to metamorphic processes and tectonic disruption during rift formation

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

experimental investigation of incipient shear failure in foliated rock abstract it has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout earth s crust however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation here we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50 mpa intact samples of pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165 with respect to the imposed shear direction foliation orientations of 0 30 produce initial failure at the lowest shear stress while samples oriented at higher angles are less favorable resulting in higher peak failure strength in all samples post peak development of through going deformation zones in the r1 riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material as shear strain increases all samples approach a residual apparent friction defined by the ratio of shear strength to normal stress of 0 4 however the final deformation microstructure depends strongly on the orientation of the pre existing foliation when fabric is oriented at low angles to the shear direction

texture development of recrystallised quartz polycrystals unravelled by orientation and misorientation characteristics abstract the development of microstructures and textures i e crystallographic preferred orientations during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope in the investigated sample series of quartz rich rocks originating from different deformation regimes the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains criteria could be established which characterise the dominant recrystallisation process and its influence on texture development it is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of soft and hard orientations the analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation recrystallisation and grain growth the influence of twinning after the dauphine law was also investigated observations of a progressive reduction in the population of dauphine twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation a mechanical origin for twinning and possible influence on texture development was therefore discarded

dynamic analysis and two types of kink bands in quartz veins deformed under subgreenschist conditions abstract in order to clarify deformation mechanisms and behaviours of quartz in a low temperature regime in the earth s crust microstructural analyses particularly on kink bands have been carried out for quartz veins moderately deformed under subgreenschist conditions both the dominance of subbasal deformation lamellae and geometry of kink bands suggest that basal 0001 slip was the sole active slip system in the deformed quartz on a morphological basis kink bands in the quartz were classified into two types type i is characterized by conjugate and narrow bands with angular hinge zones and type ii by a wide monoclinal band dynamic analyses using deformation lamellae and kink bands have revealed that type i kink bands were formed in grains with basal plane sub parallel to the compression axis whereas type ii kink bands were formed in grains with basal planes inclined to it using a numerical model of kinking of elastic multilayers modified after honea and johnson tectonophysics 30 197 239 1976 changes of the level of yielding stress for kinking and the width of kink bands as a function of the angle between the slip plane and the compression axis have been examined the theory predicts that type i kink bands were formed at a higher stress level than type ii kink bands and hence occurrence of type i kink bands suggests that a significant strain hardening occurred in the deformed quartz veins the theory also well explains the fact that the width of type i kink bands 0 to 10 is narrower by an order of magnitude than type ii kink bands 10 to 80

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

quartz pressure solution influence of crystallographic orientation abstract the present contribution examines the relationship between pressure solution and the crystallo graphic orientation of detrital quartz in cleaved sandstones which exhibit a high degree of pressure solution the majority of the detrital grains examined have undergone pressure solution and as a result they have acquired smooth truncated margins along certain cleavage planes the amount of pressure solution varies significantly among the detrital grains this analysis indicates that quartz offers variable resistance to pressure solution along different crystallographic directions quartz grains with a small angle between c axis and the z axis of shortening exhibit the least amount of pressure solution whereas grains with c axes oriented about 50 to z manifest the highest degree of the pressure solution

the effect of dauphine twinning on plastic strain in quartz we present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies t 300 400 c granitoid protomylonite from the arolla unit of the nw alps mechanical dauphine twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane z almost orthogonal to the compression direction z twin orientation twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb r nearly parallel to the compression direction r twin orientation in contrast we report the lack of twinning in two porphyroclasts already oriented with one of the r planes orthogonal to the compression direction one twinned porphyroclast has been investigated with more detail it shows the localization of much of the plastic strain into discrete r twins as a consequence of the higher amount of elastic strain energy stored by r twins in comparison to z twins the presence of dauphine twins induced a switch in the dominant active slip systems during plastic deformation from basal regions without twinning to and pervasively twinned regions dynamic recrystallization is localized along an r twin and occurred dominantly by progressive subgrain rotation with a local component of bulging recrystallization part of the recrystallized grains underwent rigid body rotation approximately about the bulk vorticity axis which accounts for the development of large misorientation angles the recrystallized grain size piezometer for quartz yields differential stress of 100 mpa the comparison of this palaeostress estimate with literature data suggests that mechanical dauphine twinning could have a potential use as palaeopiezometer in quartz bearing rocks

structural analysis of mylonitic rocks in the cougar creek complex oregon idaho using the porphyroclast hyperbolic distribution method and potential use of sc type extensional shear bands as quantitative vorticity indicators abstract mylonitic rocks of the cougar creek complex of northeastern oregon and west central idaho provide an opportunity to document the deformational structures produced during general non coaxial shear within quartz feldspar mylonites and to explore the potential role of sc type extensional shear bands in vorticity analysis well developed feldspar porphyroclasts within six mylonite zones were utilized to estimate bulk kinematic vorticity w k using the porphyroclast hyperbolic distribution phd method w k values for the cougar creek mylonites range from w k 0 26 to w k 0 37 synthetic and antithetic shear band inclinations were measured relative to observed shear zone boundaries within five mylonite zones with estimated w k values and compared to the non coaxial flow field geometries and eigenvector orientations in each mylonite zone synthetic sc type shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the acute bisector ab of the eigenvectors similarly antithetic shear band populations show a range of inclination near the obtuse bisector ob of the eigenvectors we infer that sc type extensional shear bands form initially parallel to ab and ob and rotate towards the flow plane with progressive deformation decreasing their inclination relative to the shear zone boundary ab and ob have significance in the strain field in that they represent orientations of maximum angular strain rate thus planes perpendicular to ab and ob are mechanically favorable for small zones of localized simple shear shear bands within the heterogeneous bulk strain of the mylonite orientation analysis of populations of sc type shear bands may provide a direct quantitative means of estimating w k

reconnaissance geochronology tectonothermal evolution and regional significance of the middle proterozoic choma kalomo block southern zambia abstract new u pb zircon 40 ar 39 ar rb sr and sm nd data provide the first numerical chronology for the tectonothermal evolution of the choma kalomo block an extensive middle proterozoic terrane in southern zambia to the south of the pan african zambezi belt the choma kalomo batholith a large plutonic body in the central part of the block is shown to be a composite intrusion emplaced from c 1345 to 1200 ma based on u pb zircon ages and detailed mapping of intrusive relations early phases of the batholith were intruded as syntectonic to late syntectonic plutons during development of a penetrative s 1 fabric in the metasedimentary country rocks of the choma kalomo block d 2 folding produced the regional ne sw structural trends in the block and possibly occurred at c 1230 ma based on an rb sr isochron from an early phase of the batholith containing a penetrative s 2 foliation an u pb zircon age of c 1200 ma from post tectonic granite provides a younger limit to the timing of d 2 40 ar 39 ar mineral ages from the posttectonic granite and sm nd and 40 ar 39 ar mineral ages from amphibolites in country rock paragneisses range from c 1200 to 500 ma recording post d 2 cooling followed by subsequent low grade pan african overprinting along the southern margin of the zambezi belt d 1 in the choma kalomo block at c 1345 ma is substantially younger than early proterozoic orogenesis in adjacent terranes to the east in zimbabwe indicating that a major precambrian discontinuity beneath the mid zambezi valley separates these regions d 1 in the block is similar in age to plutonic and deformational events dated at c 1350 1400 ma in the irumide belt of central and northern zambia suggesting that these two terranes on each side of the younger cross cutting zambezi belt are parts of a single originally continuous middle proterozoic mobile belt continuity between the choma kalomo block and broadly coeval terranes to the southwest in namibia and south africa also appears likely and all of these areas are considered to form parts of a middle proterozoic orogenic province lying west of the kaapvaal and zimbabwe cratons

v pull apart microstructures a new shear sense indicator abstract granitoids deformed in a low grade shear zone show conspicuous development of asymmetric pull apart microstructures in feldspars characterized by v shaped gaps between the separated fragments v pull aparts they occur beside active shear planes and result from rigid body rotation of one fragment relative to the other the shear sense in the adjacent flow surface can be easily determined through the asymmetry normally shown by v pull aparts where the fracture wall at a lower angle to the shear plane indicates which fragment has rotated synthetically to the adjacent shearing another diagnostic feature is the foliation deflection that occurs adjacent to the rotated fragment the v shaped gaps are infilled by recrystallized quartz mica aggregates showing quartz c axis patterns indicative of strong participation of crystal plastic processes in these domains evidence of crack seal by precipitation c axes parallel to the direction of separation have also been found the rigid body rotation indicated by these microstructures worked as a mechanism of removal of protruding porphyroclast edges enabling the smoothing of the shear planes rigid body rotation should operate in circumstances where other mechanisms such as dissolution or crystal plasticity did not succeed in eliminating undulations along the shear planes

evidence for dominant grain boundary sliding deformation in greenschist and amphibolite grade polymineralic ultramylonites from the redbank deformed zone central australia abstract microstructural and textural investigations by scanning sem and transmission electron microscopy tem techniques have been performed on samples taken across two quartzo feldspathic mylonite zones from the redbank deformed zone central australia one has been deformed at greenschist facies gs the second at amphibolite facies am conditions with increasing strain the rock type changes from protomylonite to mylonite to ultramylonite the protomylonites and mylonites consist of alternating quartz and polymineralic quartz feldspar bands at the highest strains a homogeneous fine grained polymineralic ultramylonite occurs shear zone geometry and microscale structures indicate that these ultramylonites experienced higher strains and were weaker than the encapsulating protomylonites and mylonites tem and sem studies of the ultramylonites reveal a rectangular to square grain shape a continuous alignment of grain and interphase boundaries across several grain diameters a grain size gs 0 5 m am 5 11 m less than the equilibrium subgrain size and open and void containing grain and interphase boundaries analysis of local textures by electron back scatter diffraction ebsd in the sem showed a very weak crystallographic preferred orientation cpo for the quartz the grain misorientation relationships are not consistent with dislocation creep being the dominant deformation mechanism all structures are of the type expected if grain boundary sliding processes had contributed significantly to the deformation consequently the deformation of such quartzo feldspathic rocks and by implication the rheology of the redbank deformed zone must have been controlled by the mechanical properties of these fine grained polymineralic ultramylonites deforming by grain boundary sliding processes this is in contrast to the pure quartz bands which deformed by dislocation creep mechanisms and were less important in the rheology of the redbank deformed zone

geochemistry of marbles and calc silicate rocks in the pan african zambezi belt zambia abstract amphibolite facies marbles and calc silicate rocks make up an important part of the supracrustal sequence in the pan african zambezi belt of zambia major and trace element geochemistry shows that dolomite marbles in the sequence were derived from nearly pure dolostones calcite marbles were derived from impure limestones calc silicate schists were derived from calcareous shales and massive calc silicate rocks were derived from calcareous sandstones a pronounced compositional banding in abundant calc silicate gneisses now folded and transposed reflects original interbedding of carbonate rich and pelite rich sediments trends in major and trace elements in the calcite marbles and calc silicate rocks are controlled largely by original variations in the amount of silicate phases admixed with caco3 during deposition and these compositional trends have been preserved into the amphibolite facies dolomite was present in significant amounts only in the protoliths to the dolomite marbles and played little or no role in the evolution of the calcite marbles and calc silicate rocks correlation of most major and trace elements with al2o3 indicates that an alumina bearing phase made up a major fraction of the material admixed with caco3 the use of the niggli parameter al alk reveals that the al2o3 was carried largely by clays and detrital micas other components contributed by the phyllosilicates include sio2 tio2 feo na2o k2o ba rb cr ni and zn sr was added with the carbonate fraction during deposition and detrital zircon may have been an important carrier of zr and y some of the massive calc silicate rocks are geochemically distinct from the other calc silicate rocks suggesting that admixture of other detrital phases such as quartz and feldspar played a more important role in governing element contents in these rocks evidence of significant alkali metasomatism is shown only by some of the massive calc silicate rocks in which na2o replaced k2o

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

tectonic evolution of the cuvette centrale zaire the cuvette centrale central basin of zaire is an infra cambrian to recent basin with up to 9 km of sedimentary section basin subsidence can be largely explained by a postulated late proterozoic failed rift and subsequent thermal relaxation seismic reflection profiles and well data reveal a palaeozoic development of the basin punctuated by two regional deformational episodes one in the early palaeozoic late cambrian and a second in the late palaeozoic late permian triassic both episodes result from ne sw contraction with the late palaeozoic episode being largely a reactivation of the early palaeozoic structures the timing of the two deformations correlates with collisional events at continental margins some distance from the congo basin in the early palaeozoic the deformation appears to be related to the late stages of the west congolian fold and thrust belt and coincides regionally with the pan african event sensu stricto the late palaeozoic deformation coincides with collisional tectonics along the southern margin of gondwana and the generation of the cape and sierra ventana foldbelts it is postulated that in both the early and late palaeozoic africa and gondwana experienced widespread intracontinental deformation generated by distant collisional processes much as central asia is experiencing today

structural control on the dikulushi cu ag deposit katanga democratic republic of congo the dikulushi cu ag deposit is located in the kundelungu plateau region in the southeast katanga province of the democratic republic of congo the kundelungu plateau is a triangular shaped area to the north of the lufilian arc which contains part of the central african copperbelt the largest known sediment hosted copper province in the world the lufilian arc and the kundelungu plateau comprise neoproterozoic sedimentary rocks of the katanga supergroup that attain an overall thickness of more than 7 000 m the lufilian orogeny imposed a northeast directed shortening on the kundelungu plateau region forming several gentle northwest oriented anticlines above subparallel detachment surfaces at depth a north northwest oriented thrust is exposed in the western part of the dikulushi mine marked by ductile deformed shale that was transported upward and is in faulted contact with an overlying dolomite sandstone sequence this upward emplacement of the shale caused brecciation and the local development of kink folds in the dolomite sandstone sequence the brecciated layers in the west and nonbrecciated sandstone layers in the central part of the mine are crosscut by several east west and northeast oriented faults this complex set of faults produced a damage zone that was a favorable site for the migration of mineralizing fluids the faults and brecciated layers contain cu pb zn fe mineralization consisting of chalcopyrite bornite and chalcocite with minor pyrite arsenopyrite sphalerite and galena with dolomite calcite and quartz as gangue minerals the northeast oriented fault direction was reactivated several times with dextral strike and oblique slip fault movement these fault zones were sites of remobilization and enrichment of the previously deposited cu pb zn fe mineralization the remobilized sulfides consist of ag rich chalcocite with barite calcite and quartz as gangue minerals the study has shown that the deposit of dikulushi is structurally controlled by a crosscutting set of east west and northeast oriented faults similar structural settings in the mine and in the dikulushi region represent important future exploration targets

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz

new polygonal grains formed by dissolution redeposition in quartz mylonite abstract arrays of polygonal grains are common along original grain margins subgrain boundaries and intragranular fractures of deformed quartz porphyroclasts we investigated these internal polygonal grains within quartz porphyroclasts of a partially recrystallized quartz mylonite deformed in the presence of fluid in a greenschist facies shear zone from quadrilatero ferrifero southeastern brazil optical observation showed profuse fluid inclusions and muscovite flakes inside the deformed porphyroclasts observation by sem revealed the presence of crystal faces on the internal polygonal grains domainal c axis analysis showed that the internal polygonal grains have a preferred orientation similar to the host porphyroclasts c axes at low angles to the stretching lineation but different from the fabric of recrystallized matrix grains type 1 cross girdle based on these observations we suggest a mechanism of in situ solution reprecipitation where the new grains are nucleated in the intergranular fluid filled pores present along the internal discontinuities of the porphyroclasts the grains grow with their boundaries largely in contact with fluid allowing the development of crystal faces at least during the early growth stages after the new grain reaches the opposite pore wall further grain growth should occur through ordinary grain boundary migration mechanisms driven by the strain energy difference between the new grain and the host porphyroclast we conclude that in these rocks in situ solution reprecipitation operated in conjunction with crystal plastic processes accounting for the nucleation of new grains during deformation recrystallization in the presence of fluid under low grade metamorphic conditions