calcite twin widths and intensities as metamorphic indicators in natural low temperature deformation of limestone abstract twins in naturally deformed coarse grained limestones of the northern subalpine chain france show a direct relationship of twin width and intensity number of twins per mm to the peak metamorphism under diagenetic and anchimetamorphic conditions the transition from dominant thin twinning to dominant thick twinning occurs between a mean vitrinite reflectance ro of 1 60 and 2 60 or temperatures of between 150 and 200 c below the transition temperature increasing strain is apparently accommodated by adding new twins and above the transition temperature increasing strain is accommodated by twin widening similar transitions from thin to thick twinning with increasing metamorphism are also present in the helvetic alps the prealps and the central appalachian valley and ridge province these data for naturally deformed limestones support the interpretation from experimental studies of a temperature effect on twin width and intensity and indicate that the twinning in the northern subalpine chain occurred at or near peak metamorphism

pure shear and simple shear calcite textures comparison of experimental theoretical and natural data abstract the pattern of lattice preferred orientation texture in deformed rocks is an expression of the strain path and the acting deformation mechanisms a first indication about the strain path is given by the symmetry of pole figures coaxial deformation produces orthorhombic pole figures while non coaxial deformation yields monoclinic or triclinic pole figures more quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models for this comparison a representation in the sensitive three dimensional orientation distribution space is extremely important and efforts are made to explain this concept we have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the taylor model we were able to simulate the observed changes with strain history coaxial vs non coaxial and the profound texture transition which occurs with increasing temperature two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results a marble from the santa rosa mylonite zone in southern california displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear a limestone from the tanque verde detachment fault in arizona has a monoclinic fabric from which we can interpret that 60 of the deformation occurred by simple shear

the coalification pattern in the northern apennines and its palaeogeothermic and tectonic significance the rocks of the northern apennines predominantly consist of non metamorphic terrigeneous deposits flysches and molasses some of which are preorogenic some synorogenic and others postorogenic with respect to the nappe tectonics miocene as plant fragments frequently occur in these sediments a study of coal rank based on reflectance measurements on vitrinites rm mean value of the random reflectance in non polarized light contributes to the clarification of the relation between the orogenic and the palaeogeothermal development the determination of the rm values of more than 180 samples from outcrops and three deep drillings revealed some important features within the pile of liguride and tuscanide nappes the coal rank increases from the uppermost nappe to the lower nappes until lowgrade metamorphism is reached in the lower tuscanides in the single nappes the rank decreases from the tyrrhenian coast internal zone towards the po plain external zone this regional trend is disturbed only locally by young post coalification tectonics in the uppermost liguride nappe m antola unit a pre oligocene i e pre apenninic thermal event was detected postorogenic heating is connected with the magmatic activity of late miocene to pleistocene age in tuscany except for these preorogenic and postorogenic thermal events the main coalification is generally younger than the emplacement of the nappes in the nappe pile during the apenninic orogeny in the miocene but it is older than the last thrust movements and the final tensional tectonics in the internal zones of the chain for these reasons the main regional thermal event has to be considered as synorogenic or more precisely as late synorogenic

dating of deformation phases using k ar and 40ar 39ar techniques results from the northern apennines abstract paleozoic to oligocene metasedimentary rocks present in the alpi apuane region of the northern apennines italy have been sequentially deformed during a tertiary progressive deformation in an attempt to date the individual deformation episodes over 50 conventional k ar and 1140 ar 39ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known mineralogy x ray diffractometry and thin section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3 4 kbars and 300 400 c during all deformational phases pre existing micas were variably crenulated during each subsequent deformational phase both k ar and 40ar 39ar analyses were carried out on 0 6 2 m 2 6 m and 6 20 m size separates of the phengitic white mica although the k ar apparent ages range from 11 to 27 ma and are consistent with available stratigraphic constraints the 40ar 39ar age spectra display variable internal discordancy these isotopic data indicate that 1 both the k ar and 40ar 39ar total gas ages decrease as the degree of crenulation increases 2 the k ar and 40ar 39ar total gas ages decrease as grain size decreases 3 for each sample characteristics of the 40ar 39ar age spectra depend upon grain size with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and 4 phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures the isotopic results are interpreted to indicate that the major deformation phase d1 occurred at approximately 27 ma with subsequent pulses ending by c 10 ma these results may be combined with finite strain data to suggest that the region was deformed at strain rates between 10 15 and 10 14 s 1 a 27 ma age indicates mid oligocene initiation of plate tectonic activity in the western mediterranean and concomitant deformation in the northern apennines

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

natural calcite c axis fabrics an alternate interpretation microstructures and crystallographic fabrics in natural calcite mylonites from newfoundland canada are discussed and compared with other natural and experimental examples in the newfoundland rocks porphyroclasts deformed mainly by twinning and intracrystalline slip and the recrystallized grains deformed by dislocation creep assisted by dynamic recrystallization by grain boundary migration t 350 c porphyroclast c axis fabrics consist of a broad point maximum oriented normal to the shear plane recrystallized grains in the ultramylonites have strong c axis fabrics consisting of a single point maximum oriented either parallel or oblique to the normal to the shear plane experimental simple shear calcite c axis fabrics have been divided by previous workers into low lt and high temperature ht fabrics twinning and intracrystalline r slip have been identified as the dominant deformation mechanisms in low temperature experiments whereas intracrystalline r and f slip dominate in high temperature experiments lt fabrics have a single point maximum which is oriented approximately parallel to the principal stress axis and is oblique to the normal to the shear plane ht fabrics have three point maxima which are asymmetrical both in orientation and intensity with respect to the normal to the shear plane the transition between lt and ht fabrics occurs at temperatures lower than 400 c in fine grained limestones and lower than 700 c in marbles at laboratory strain rates natural calcite c axis fabrics in agreement with study almost invariably consist of a point maximum oriented either normal or slightly oblique to the shear plane because their fabrics are similar to experimental lt fabrics they have been interpreted as lt fabrics by comparison with experimental and computer simulated lt fabrics the obliquity of their c axis point maximum with respect to the shear plane normal has been used as a shear sense indicator and as a measure of the degree of non coaxiality of the deformation two lines of evidence mitigate against calcite c axis fabrics as shear sense and strain path indicators in mylonite zones first the obliquity of the c axis point maximum in natural examples is not consistent for a given sense of shear secondly mylonite microstructures reported here and in the literature supports the contention that dynamic recrystallization and intracrystalline slip are responsible for the strong c axis fabrics of most calcite mylonites therefore natural calcite mylonite c axis fabrics do not generally correlate with either lt or ht experimental fabrics instead natural calcite mylonites have similar microstructures and fabrics to marbles that were deformed experimentally at very high temperatures 800 850 c and underwent pervasive dynamic recrystallization by grain boundary migration consequently c axis fabrics of calcite mylonites cannot be used as a measure of the degree of non coaxiality of the deformation nor can they be used as shear sense indicators

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples preliminary results abstract following previous successful paleo stress determinations from numerically generated aggregates and naturally deformed samples preliminary results about stress reconstructions based on inversion of calcite twin data from experimentally deformed monophase samples are reported this new study carried out on samples of carrara marble and crinoidic limestone from eastern france provides a test of the validity of stress reconstructions using inverse methods which had not been properly calibrated before the percentages of correlated twins are discussed for each stress tesnor and the range of uncertainties on the determination of stress orientations and differential stress magnitudes is evaluated

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

tectonics of the punta bianca promontory insights for the evolution of the northern apennines northern tyrrhenian sea basin the structural setting of the eastern headland of the la spezia gulf punta bianca promontory provides some constraints on the contractional and extensional framework of the northern apennines northern tyrrhenian sea basin geodynamic system and allows a detailed reconstruction of a complex evolutionary path out of sequence thrusting is supposed in the inner side of the chain two different extensional events are identified the older early middle miocene may be related to the imbrication of the deeper metamorphic units of the apenninic thrust wedge younger extension fits very well into an eastward asymmetrical faulting framework related to the opening of the northern tyrrhenian sea basin from the late miocene to the pliocene pleistocene well exposed low angle normal faults developed during this event extensional structures were then overprinted and reactivated by a mainly strike slip regime in the quaternary

late pleistocene paleolake evolution in the hetao basin inner mongolia china abstract the paleolake that once existed in the hetao basin china represents an important stage of the yellow river drainage system evolution in the region but it has not been studied in detail to date in particular the formation and driving forces of the paleolake evolution since the late pleistocene is still poorly understood in this study optically stimulated luminescence and radiocarbon dating were combined with multiproxy analysis of lacustrine sediments in the central hetao basin to reconstruct the paleolake evolution proxies included carbonate 18 o and 13 c isotopes grain size magnetic susceptibility and color in combination with stratigraphic records the proxies indicated the existence of a lake environment in the hetao basin between 150 and 18 ka bp the paleolake between 150 and 130 ka bp was an open system between 130 and 54 ka bp the paleolake basin closed with high lake levels at the beginning of the period and then a gradual decrease after 54 ka bp the paleolake began to outflow and gradually shrank in size and after 18 ka bp an alluvial fan covered the region and the paleolake disappeared we propose a combination of tectonic uplift and climate changes as the main drivers of paleolake formation and evolution since the late pleistocene

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

the high temperature static recovery and recrystallization behaviour of cold worked carrara marble the static recovery and recrystallization behaviour of carrara marble has been investigated as a function of pre strain 8 27 and annealing temperature 500 700 c quantitative estimates of the rate of recovery were obtained by comparing the stress strain curves generated at a temperature of 426 c a confining pressure of 170 mpa and a strain rate of 3 10 4s 1 before and after a period of isostatic annealing curves of fractional recovery r as a function of log time t show a characteristic sigmoidal shape in which the rate of recovery dr d logt is initially small increases rapidly and finally becomes small again the acceleration of recovery occurs at shorter times both with increasing pre strain and with increasing annealing temperature microstructural examination of the annealed material shows that the acceleration of recovery corresponds to the onset of recrystallization and that prior to the onset of recrystallization recovery is controlled by the development of a subgrain structure whereas after recrystallization has gone to completion it is controlled by the intensification and growth of this substructure the results have been interpreted within the perspective of hart s state variable description of inelastic deformation this analysis shows that the decay of the mechanical state of the marble during annealing can be treated as a two stage static recovery process substructure formation substructure coarsening with the recrystallization marking the transition between these two stages

regional distribution of al silicates and metamorphic zonation in the low grade verrucano metasediments from the northern apennines italy abstract mineralogical and petrological studies of triassic verrucano metasediments of the northern apennines are reported the widespread occurrence of al silicates allows the delineation of four metamorphic zones with increasing metamorphic grade 1 kaolinite zone well perugia 2 umbria 2 kaolinite pyro phyllite zone monte argentario and part of the verrucano of the monticiano roccastrada area and monti leoni 3 pyrophyllite zone monti pisani iano monti leoni the monticiano roccastrada area and some wells in the larderello region 4 kyanite zone massa area and some wells in the larderello area the four metamorphic zones correspond to temperatures ranging from 300 c to about 450 c on the basis of the si content of muscovite and geological arguments pressures of between 3 and 5 kbar are estimated the metamorphic zones are located more or less parallel to the bent north west south east trending structural zonation of the northern apennines with the concave side towards the tyhrrenian sea during the alpine orogeny the verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity s1 s2 s3 during the first folding phase the verrucano sediments were buried increasingly deeply within the crust from east to west the climax of alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between massa and larderello the inferred paleo temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone and surrounded by the pyrophyllite zone a similar pattern is still present in the tuscan crust as indicated by a series of geothermal anomalies passing through the northern apennines

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

the effect of second phase particles on stable grain size in regionally metamorphosed polyphase calcite marbles samples of the calcite rich shelburne marble collected at the pfizer quarry in adams massachusetts show an order of magnitude variation in grain size calcite grain size ranges from 94 to 1101 m because these calcite marbles share the same pressure temperature and strain histories some other factor must be responsible for the grain size variation grain size appears to be controlled by the concentration of impurity or second phase particles large calcite grain size occurs where the volume fraction of second phase particles is low and grain size decreases as second phase volume fraction increases the relationship between calcite grain size d second phase grain size d and second phase volume fraction f can be described by the power law d d 1 4 f 0 36 a result that is consistent with models based upon short term hours or days laboratory experiments with metals and ceramics and computer simulations of grain growth grain growth appears to be greatly restricted by as little as a few per cent of second phase particles with a transition from highly restricted to almost unrestricted grain growth occurring at 5 volume of second phase particles these results indicate that second phase particles exercise an important control on grain size and can effectively inhibit grain growth in metamorphic rocks the behaviour of second phases in short term laboratory experiments may closely approximate the behaviour of second phases in grain growth lasting several orders of magnitude longer in the metamorphic environment