a history of cleavage and folding an example from the goldenville formation nova scotia temporal relationships between cleavage formation and folding in sedimentary rock sequences can vary from either phenomena preceding the other to both occurring simultaneously knowledge of this relationship however is required to understand fully the relationship of cleavage orientation to finite strain and the origin of cleavage refraction across competent incompetent layers evidence from the goldenville formation provides an example of cleavage formed before folding by 50 pressure solution removal of material resulting in cleavage planes normal to bedding during active folding this layer parallel shortening was accompanied by excess pore pressure that in some cases exceeded lithostatic pressure causing formation of bedding parallel quartz veins with crack seal textures folds developed by a mechanism of differential flexural flow with no loss of coherence across bedding surfaces en echelon veins formed sequentially across some beds during active folding and subsequently were deformed by various increments of bedding parallel shear after folds reached 80 limb dips continued horizontal shortening was accommodated by additional pressure solution but no recognizable new cleavage was formed because cleavage in the goldenville formation formed perpendicular to bedding before folding and was passively rotated and extended by bedding parallel simple shear during fold amplification the resulting cleavage planes cannot lie in the xy plane of the finite strain ellipsoid differential shear between metagray wacke and slate beds accounts for the observed cleavage refraction and the bedding cleavage angle provides a good estimate of the shearing across these beds during active folding if cleavage can be shown to postdate folding in other examples it is possible that cleavage planes will lie in the xy plane of the finite strain ellipsoid

strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

stratigraphy and ductile structure of the presidential range new hampshire tectonic implications for the acadian orogeny the presidential range of new hampshire with its unique high relief contains exceptionally well exposed rocks of the acadian orogenic hinterland these rocks are within the central maine terrane a belt of complexly metamorphosed and deformed silurian and devonian metasedimentary rocks the presidential range lies between regions to the south dominated by refolded nappes and synkinematic high grade regional metamorphism and regions to the northeast dominated by overlapping multiple deformation and contact metamorphism produced by late and post acadian plutons to better understand these complex tectonic variations and in particular to clearly and accurately determine the effects of the devonian acadian orogeny in a well exposed region the bedrock geology in the alpine zone of the presidential range was mapped in great detail the results of the stratigraphic and ductile structural analyses are presented here five metasedimentary formations have been recognized and constitute a conformable stratigraphy which from oldest to youngest are the silurian rangeley perry mountain smalls falls and madrid formations and the devonian littleton formation the littleton formation has been subdivided into fifteen different members and three submembers based on variations in bedding style of the schists and quartzites and other lithologic peculiarities the madrid smalls falls and perry mountains formations are extremely thin consistent with their deposition near the silurian tectonic hinge the rangeley formation is interpreted as an olistostromal melange and has been subdivided into three different members and six submembers on the basis of lithologic variations in the gneiss and subordinate units the presidential range preserves three phases of ductile folding f1 f2 and f3 and a single phase of thrust faulting t1 well constrained southeast east or northeast facing directions and consistent asymmetry of f1 nappe limbs suggest that nappe vergence was easterly severe refolding of some nappes resulted during f2 collapse of the nappe pile the west vergent greenough spring thrust fault truncated f1 and f2 structures and juxtaposed nonmigmatized and migmatized rocks f3 east vergent folding dominates the alpine zone and represents the final tightening of acadian structures which occurred after early granitic plutonism and prior to the emplacement of postkinematic granitic sheets the structural and sedimentological evidence supports the existence of a subduction system that was active beginning in the early silurian on the east flank of the bronson hill anticlinorium with a west dipping subducting slab geometry this system persisted throughout the acadian

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

integrated seismic studies of the baltic shield using data in the gulf of bothnia region in the autumn of 1989 a co operative experiment involving 12 research institutions in northwestern europe collected 2268 km of deep seismic reflection profiles in the gulf of bothnia and the baltic

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

modelling the bulk viscosity of two phase mixtures in terms of clast shape abstract the rheological behaviour of many rock types will depend on how mixtures of different minerals and rock clasts behave together this paper develops two dimensional modelling of the bulk viscosity of two phase viscous mixtures with different shape fabrics in pure shearing based on self consistent mechanics and behaviour of inclusion matrix systems an equation is presented for the bulk viscosity in terms of the phase viscosity contrast phase fractions and clast shape results are given for idealised two dimensional mixtures of circles squares or ellipses with a variety of phase viscosity contrasts in pure shearing parallel perpendicular to the shape fabric the normalised bulk viscosities of all these mixtures fall between the theoretical upper and lower viscous bounds given by homogeneous strain rate and homogeneous stress respectively but nearer to the upper bound with increasing shape fabric mixtures with a particular geometry are shown to have a constant relative horizontal distance between these two bounds as proposed by p d bons using graphs for the normalised bulk viscosities of mixtures with variably elliptical aligned shape fabrics the results for fabrics orthogonal to pure shearing can be extrapolated to a diagonal fabrics in pure shearing and b orthogonal or diagonal fabrics in simple shearing these reveal the bulk viscous anisotropy of mixtures with shape fabric in two dimensions it is speculated that during a progressive finite deformation two phase mixtures might undergo changes in bulk viscosity over time with steady stiffening during pure shear and perhaps cyclic stiffening and softening during simple shear the two dimensional models provide an approximation to the three dimensional properties of mixtures with shape fabrics with potential applications to rocks including conglomerates

fluid enhanced deformation transformation of granitoids to banded mylonites western sierra nevada california and southeastern australia abstract deformation of tonalite and adamellite from the western sierra nevada california and southeastern australia has produced mylonite zones that show a broad range of microstructural development varying from strongly banded and or foliated to locally non foliated accompanying these fabrics are new mineral assemblages commonly rich in quartz and epidote whereas the banded foliated varieties appear to develop from progressive deformation of the granitic host they also occur in close proximity cm m to domains of non banded and or weakly to non foliated fabrics of similar mineralogy suggesting a complex timing and partitioning of deformation fluid flow and metamorphism in the mylonite zone whole rock major and trace element analyses of the tonalites show large enrichments of sio 2 cao fe 2 o 3 total fe and sr and strong depletions in na 2 o k 2 o feo mgo ba and rb with increasing stages of deformation although substantial chemical change appears to occur under static conditions locally chemical changes in the adamellite are much less dramatic all suites show systematic decrease in 18 o and convergence of oxygen isotopic compositions of whole rock and quartz with increasing deformation minimum fluid rock is estimated to lie between 0 1 and 10 increasing with intensity of deformation and depending on host composition isotopically exchanged seawater or connate water are the most plausible fluids chemical data from a number of mylonite zones in various orogenic belts show widely varying behavior gain loss of major and minor elements eleven factors control which and how much elements are gained lost during mylonitization of the granitoids the most influential of which are primary composition mineralogy and primary texture of the host as well as several aspects of fluid interaction active during deformation these influences may be a function of local rather than regional geological conditions

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

estimates of upper crustal heterogeneity in the baltic shield from seismic scattering and borehole logs abstract constraints on small scale heterogeneity in the upper crust of the baltic shield were obtained from stochastic modelling of a variety of seismic data from the babel and siljan ring surveys a non linear least squares inversion scheme was applied to wavefield fluctuations in seismic reflections to obtain statistical parameters of the medium through which the wave energy propagated the reflections are from dolerite sills intruded at depths of 6 12 km into the palaeoproterozoic upper crust of the central baltic shield analysis of the babel data set reveals that the upper crust is a weakly scattering stochastic medium with velocities characterised by an exponential autocorrelation function with a correlation length of 150 50 m and rms velocity perturbation of 1 5 0 5 further inversions show that most of the scattering occurs within the upper few kilometres of the crust analysis of the sijan seismic data set reveals heterogeneity with length scales of 210 50 m and a velocity perturbation of about 1 0 the autocovariance and power spectrum of the sonic log and studies of frequency dependent q in vsp data from the gravberg 1 borehole give consistent estimates of basement heterogeneity the upper crust in this region is characterised by a correlation distance of 35 5 m and rms velocity perturbation of 4 6 in the uppermost 1 2 km of crust associated with near surface fracturing and a correlation distance of 140 50 m and rms velocity perturbation of 1 5 0 5 in the uppermost 6 8 km associated with compositional heterogeneity

advances in magneto optical imaging applied to rock magnetism and paleomagnetism 1 we present results of the magneto optical imaging technique applied to rock samples this technique measures the magnetic flux threading a magneto optically active film which rotates the polarization direction of transmitted light faraday rotation and is directly placed on the sample through the analyzer of a reflected light microscope the vertical component of the surface magnetic field of the sample is observed and can be quantified through a specific calibration procedure owing to the thin magneto optically active film 5 m and the small sample to film distance 1 m stray fields produced by magnetic grains in rocks carrying saturation isothermal remanent magnetization are successfully imaged with a spatial resolution of 10 m we can also image the surface field distribution of rocks carrying natural remanent magnetizations by modulating the analyzer angle in addition to its high spatial resolution this technique offers a direct comparison between magnetic and reflected light images therefore this new technique appears to be a powerful tool to map and identify the carriers of magnetic remanence in rock samples

estimation of viscosity contrast and finite strain from deformed elliptical inclusions a method of strain analysis is described which takes into account a possible competence difference between the strain markers and the rest of the rock using data consisting of the sectional shapes and orientations of groups of inclusions from conglomerate like rocks limits are placed on the possible effective viscosity contrast between the inclusions and the rock as a whole as well as on the bulk finite strain suffered by the rock these results are calculated using a computer program based on the theory for the deformation of elliptical cylinders the method can be considered an extension of the rf method but provides more information and involves less restrictive assumptions

magnetic field visualization of magnetic minerals and grain boundary regions using magneto optical imaging 1 magneto optical imaging based on the faraday effect has been used to characterize magnetic minerals embedded in a nonmagnetic matrix we have studied magnetite grains and magnetite magnetite grain boundary regions in samples of skarns and serpentinites distributions of the remanent magnetic field were measured across at the surface of polished thin sections kept at room temperature the magneto optical images resolve directly magnetic structures on length scales ranging from millimeter down to micrometer thereby revealing the shape and arrangement of the magnetite grains and allow determination of the grain magnetization we find that 1 for the skarns the intergrain interactions do not affect the magnetic properties of magnetite grains within 0 6 60 m mo f each other while the saturation remanence decreases weakly with increasing grain size from 40 mm to 0 6 mm and 2 for the serpentinites the magnetic properties of the stripes are size dependent due to variations in chemical composition

scaling of fault displacements from the badajoz cordoba shear zone sw spain faulting occurs over a large range of scale parts of which are sampled by various techniques e g microscopy outcrop measurement mapping seismic reflection and other forms of remote sensing use of a single technique to measure displacement or strain will not sample faults at all scales and hence will give a biased estimate in order to assess this bias a knowledge of the distribution over all scales is needed many samples of fault displacement appear to follow a power law distribution with departures which can be attributed to sampling effects the number of faults with a displacement u is given by n u cu d the power law distribution of displacement is consistent with similar distributions of other fault parameters and earthquake magnitudes when sampling along a line e g a bedding trace on a map or section a self similar fault population would have d 1 whereas self affine geometries yield d 1 displacement and extension are dominated by small faults when d 1 and by large faults when d 1 when sampling over areas or volumes these critical values are 2 and 3 respectively a set of strike slip faults from the badajoz cordoba shear zone spain were sampled at two different scales using 1 50000 maps and outcrop measurements displacement ranges over 6 orders of magnitude these and other fault populations typically have d ranging from 0 6 to 1 5 the power law relationship may be integrated to yield estimates of the displacement or extension for faults which lie beyond the resolution of the sampling system for example a knowledge of d allows the extension measured on a map or seismic section to be corrected for faults whose displacement is below the resolution of the survey based on an overall estimate of d 0 9 for the badajoz cordoba data only some 40 of the extension would be recorded by map scale faults a corrected extension of 41 along the shear zone is estimated which if typical for the entire 300 km zone represents some 87 km of along strike extension thus work suggests that significant displacement occurs on faults which are too small to be interpreted from conventional seismic profiles and geological maps

zonation patterns of skarn garnets records of hydrothermal system evolution chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower paleozoic sedimentary rocks within the contact aureole around the drammen granite in the oslo rift southern norway major and trace element zonation profiles the latter obtained using a scanning high resolution proton microprobe reveal early infiltration controlled growth of relatively grossular rich garnets the major and trace elements compositions being buffered by local mineral dissolution subsequent rapid epitaxial growth of andradite rich garnet on grossular rich cores marks the onset of vigorous and focused fluid flow along high permeability zones during this later stage the hydrothermal fluid composition was to a large extent externally controlled and the andradite precipitating from these fluids was characterized by high as and w contents the zonation patterns of the andradite rich garnets record at least five intermittent growth periods with rapid andradite precipitation from fluid batches with high f o 2 and progressively decreasing as and w contents thin layers poor in fe as and w but relatively high in al and mn represent periods of slow growth rates between the major pulses of hydrothermal fluids the marked rimward decrease in the as and w contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling controlled distillation processes at depth

deformation induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses tectonic and metamorphic implications a sequence of at least three al2sio5 bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the willimantic window connecticut the ductile deformation localized at and near the boundary between the putnam nashoba terrane and underlying avalon terrane is characterized by a network of anastomozing shear zones that outline metre scale tectonic blocks of migmatitic kfs sil gt bi pg qtz ilm ru gneiss these assemblages record acadian or older metamorphic conditions of 6 kbar 700d c mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss the reconstituted rocks exhibit relict ky st grt pl bt qtz rt ilm assemblages and require a minimum pressure for the ky str grade metamorphism of 8 5 kbar kyanite in block margins is widely replaced by sillimanite and locally by andalusite during a period of post alleghanian ductile deformation the interiors of blocks do not record this sequence of polymorphs the pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane bounding fault zone strain provided the activation energy for recrystallization and retrograde mineral reactions the p t conditions of post alleghanian ductile deformation evolved from 600d c and 6 kbar to 550d c and 3 kbar the occurrence of ky str bearing assemblages overprinting acadian kfs sil bearing assemblages and subsequently overprinted by alleghanian sillimanite and andalusite bearing assemblages along with reset hornblende 40ar 39 ar mineral ages from mississippian to permian requires a prograde alleghanian metamorphism of rocks previously metamorphosed during the acadian thus mineral assemblages from gneisses in the willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high grade acadian orogenesis tectonic loading resulting from alleghanian collision of avalon with north america and tectonic exhumation in permo triassic time the c 3 kbar pressure decrease between prograde and retrograde alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the putnam nashoba terrane cover sequence following alleghanian orogenesis in south eastern new england

strain analysis and fold shape in a limestone layer and implications for layer rheology abstract folds are developed in thin limestone layers within slates of the mckay formation exposed to the east of the rocky mountain trench british columbia canada they possess geometrical characteristics expected of development by buckling strain in the profile plane of a selected fold is similar to that predicted by tangential longitudinal strain except that magnitudes are too low for the observed curvature this is attributed to inhomogeneity of strain on the scale of measurement largely because of pressure solution material removed by pressure solution from the inner arc of the fold appears to form veins perpendicular to the hinge a direction of tectonic stretching bedding parallel stylolites developed diagenetically prior to tectonism layer parallel shortening during the initiation of buckling was less than 20 and probably less than 10 the mean arclength thickness ratio is 6 5 and 7 1 with a dispersion of 0 48 and 0 37 for local and regional populations of 29 and 212 folds respectively application of buckling theory to this data suggests that folding followed a non linear flow law the viscosity contrast between limestone and slate would be higher and the power law exponent lower if initial irregularities in the layers were in the form of a constant amplitude spectrum rather than one of white roughness the data do not allow a choice of initial amplitude spectrum to be made nor do they closely constrain estimates of n the power law exponent and viscosity contrast deformation in the limestone layers was accommodated by intracrystalline flow twin gliding pressure solution and extensional veining the last two linked by diffusive mass transfer the first two dominated deformation in the profile plane of the fold and the last in association with fracturing allowed for extension parallel to the hinge experimental and theoretical considerations suggest that deformation by a combination of these processes should be non linear the non linear flow law deduced from buckling analysis is consistent with expectations based on observations of active deformation mechanisms

the rheology of polymineralic rocks an approach deduced from empirical and theoretical work a model is proposed for the rheology of bi and polyphase rocks based on the rheology of the components their volume fractions and their geometrical distribution the model is formulated for the common case where some minerals deform by crystall plasticity at steady state and others stronger ones deform in the vincinity of the brittle ductile transition

tectonics of the acadian orogeny in new england and adjacent canada paleogeographic analysis of post taconian rocks in new england and adjacent parts of canada has revealed the existence of two volcanic arcs which shut off at the time of the acadian orogeny one was built on arc basement previously accreted to north america during the taconic orogeny the other on precambrian continental basement of avalonia in the intervening merrimack fredericton trough metamorphosed and poly deformed turbidites and black shales record deep water conditions in silurian following mckerrow and ziegler 1971 this tract is interpreted as the site of an ocean which closed in siluro devonian by simultaneous subduction beneath both continental margins in the molucca sea in indonesia a comparable arc arc collision is in an early stage of development moore et al 1982 suggested that an accretionary prism built against one arc is overthrusting its counterpart which developed on the other side of the ocean an identical geometry with an avalonian accretionary prism overriding the conve

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

amount of extension on small faults an example from the viking graben extension estimates based on palinspastic restoration of only the largest faults in a deformed terrane may be significantly deficient due to neglect of small faults i e unsampled faults most of which have smaller displacements than do sampled faults small faults individually account for little extension but there may be large numbers of them the cumulative extension on small faults can be calculated from the fractal size distributions of fault displacements independent extension estimates support the inference from fault population statistics that small faults account for significant amounts of extension for example restorations of mesozoic faults interpreted from seismic reflection profiles across the viking graben in the north sea yield extension estimates that are only 40 75 of independent estimates the fractal size distribution of fault displacements in the viking graben is consistent with the hypothesis that the remaining 25 60 of extension resulted from displacements on small faults

analogies between the high temperature deformation of polyphase rocks and the mechanical behavior of porous powder metal abstract in some rock types one or more mechanically similar minerals may constitute a loadcarrying framework while other subordinate phases carry little load if incompetent phases are more than one or two orders of magnitude weaker than the competent phases the load carrying framework behaves much like a porous solid the deformational properties of porous powder metal provide analogies for the deformation of such porous like rock types 1 the yield strength of porous powder metal is analogous to the power law flow stress for rock deformed at high temperatures 2 the correspondence principle relating elastic and viscoelastic deformation may be used to relate the young s modulus of porous powder metals to the flow stress for nabarro herring creep in rock where the incompetent fraction is 15 of the rock there should be a 50 reduction in power law flow stress for an incompetent fraction of 40 the power law stress is reduced by an order of magnitude and the nabarro herring creep stress is reduced by 80

mullions in the high ardenne slate belt belgium numerical model and parameter sensitivity analysis in the high ardenne slate belt belgium pre existing layer perpendicular quartz veins in psammite layers acted as mechanical boundaries due to the difference in competence between vein quartz and psammite this caused the formation of mullions during layer parallel shortening in this paper the process of mullion formation is modelled using finite element techniques a parameter sensitivity analysis of the mullion model demonstrates that the stress exponent of psammite the total horizontal shortening of the psammite layers the initial aspect ratio of the psammite segments between the veins and the competence contrast between psammite and vein quartz are the controlling parameters for the shape of the mullions our results suggest that the morphology of the mullions can be used to constrain the rheology of psammite deforming in the middle crust moreover the parameter sensitivity analysis illustrates the range of layer parallel shortening extension structures associated with layer perpendicular quartz veins that can be expected in nature e g dogbones inverted mullions and creates future perspective for the use of the model as a paleorheological gauge for these structures

silicon diffusion in a natural quartz aggregate constraints on solution transfer diffusion creep abstract silicon bulk diffusion rates were determined in a fine grained 1 2 m diameter natural quartz aggregate novaculite from hot springs ar the diffusion experiments were done at 600 800 c and 150 mpa confining pressure hydrothermal and at 800 1100 c and 1 atm in a stream of dry n2 the technique employed an isotopic tracer 30si and standard step scan analysis using an ion microprobe sims the temperature dependence of silicon bulk diffusion in the novaculite is described by the arrhenius parameters d0 bulk 3 7 10 10 m2 s and q 137 18 kj mol and d0 bulk 6 2 10 9 m2 s and q 178 38 kj mol for the hydrothermal and dry experiments respectively at 800 c the silicon bulk diffusion rate at 150 mpa hydrothermal is about a factor of 10 greater than at 1 atm dry for an assumed effective grain boundary width of 2 nm the silicon grain boundary diffusion rates in novaculite are a factor of two to three less than oxygen diffusion rates for similar hydrothermal conditions and more than 12 orders of magnitude greater than anhydrous silicon volume diffusion rates extrapolated to the temperature of these experiments the results of this study provide valuable constraints on the role of diffusion accommodated deformation in quartz rich rocks

flow laws of multiphase rocks calculated from experimental data on the constituent phases abstract an attempt has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases the flow strengths predicted by the proposed model for granite diorite diabase and lherzolite are in good agreement with previously determined experimental values the study allows one to calculate to the first approximation the flow laws of a large number of rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates

direct imaging of nanoscale magnetic interactions in minerals the magnetic microstructure of a natural finely exsolved intergrowth of submicron magnetite blocks in an ulvospinel matrix is characterized by using off axis electron holography in the transmission electron microscope single domain and vortex states in individual blocks as well as magnetostatic interaction fields between them are imaged at a spatial resolution approaching the nanometer scale the images reveal an extremely complicated magnetic structure dominated by the shapes of the blocks and magnetostatic interactions magnetic superstates in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization are observed directly

measurement and characterisation of spatial distributions of fractures abstract a variety of line sample 1 d and map 2 d datasets of faults and joints has been used to investigate the spatial distributions of fractures and to test techniques of fractal analysis the natural fracture datasets have been supplemented by synthetic datasets with known characteristics the fault datasets investigated range in scale from regional to outcrop and the joint datasets are derived from outcrop 1 d datasets were analysed by the spacing population interval counting and fracture number interval counting techniques 2 d datasets were analysed by box counting and fracture number box counting techniques results indicate that fracture spacing can be characterised in line samples using either the 1 d interval counting technique or more simply by measuring the spacing population as the cumulative frequency distribution of spaces between adjacent fractures tectonic faults frequently show a power law spacing population indicating fractal dimensions of between 0 4 and 1 0 but except for outcrop data truncation effects degrade the analysis unrefined joint datasets commonly show negative exponential or lognormal spacing cumulative frequency distributions however single orientation joint sets are characterised by a regular spacing and are therefore non fractal for both fault and joint map data box counting techniques do not yield the power law relationship between box size and the number of boxes that is expected of fractal geometries it is concluded that 2 d box counting techniques are too insensitive to characterise the many different parameters of a fracture array the fracture density technique appears not to discriminate between very different fracture patterns and therefore does not provide useful results a fracture pattern incorporates several attributes e g size distribution orientation linkage density roughness each of which may scale independently and each of which may be fractal with its own fractal dimension full characterisation of fracture patterns will therefore require independent analysis of each of these attributes

bulk flow strength of forsterite enstatite composites as a function of forsterite content creep experiments have been conducted to investigate the effect of varying forsterite content vfo on the bulk flow strength of dry forsterite enstatite fo en aggregates in order to evaluate the applicability of existing theoretical models to two phase rocks as well as to understand the rheology of polyphase systems in general the experiments were performed at temperatures of 1423 1593 k stresses of 18 100 mpa oxygen fugacities of 10 14 10 2 5 mpa and 0 1 mpa total pressure the fine grained fo 10 17 mm en 14 31 mm composites of various fo volume fractions vfo 0 0 2 0 4 0 5 0 6 0 8 and 1 were synthesized by isostatically hot pressing in a gas medium apparatus at 1523 and 350 mpa our experiments show that flow strength contrasts between fo and en are in the range of 3 8 at the given experimental conditions with fo as the stronger phase the measured stress exponent n and activation energy q values of the fo en composites fall between those of the endmembers the n values show a nearly linear increase from 1 3 to 2 0 while the q values display a non linear increase from 472 to 584 kj mol with en volume fraction from 0 to 1 0 there is no clear dependence of creep rates on oxygen fugacity for the fo en composites the mechanical data and tem microstructural observations suggest no change in deformation mechanism of each phase when in the composites compared to when in a single phase aggregate the en deformed mainly by dislocation creep while the fo deformed by dislocation accommodated diffusion creep for our grain sizes and experimental conditions comparisons between the measured composite strengths and various theoretical models indicate that none of the existing theoretical models can give a precise predication over the entire vfo range from 0 to 1 however the theoretical models based on weak phase supported structures wps yield a good prediction for the flow strengths of the composites with vfo 0 6 no model gives a good prediction for the bulk strength of two phase composites in the transitional regime vfo 0 4 0 6 applications of the wps and sps based models in the transitional regime result in under and over estimations for the composite flow strength respectively thus the effect of rock microstructure should be taken into consideration in modeling the bulk flow strengths of the crust and upper mantle using laboratory determined flow laws of single phase aggregates d 2001 elsevier science b v all rights reserved

oxygen bulk diffusion measurements and tem characterization of a natural ultramylonite implications for fluid transport in mica bearing rocks oxygen bulk diffusion rates were experimentally determined in a natural ultramylonite sample c 5 m grain size 15 20 biotite 20 quartz 60 65 feldspars and minor fe oxides from the gerrish island shear zone se maine usa the diffusion experiments were performed at 250 550 c and 100 mpa water pressure oxygen bulk diffusion rates were determined both parallel and perpendicular to the strong foliation of the sample the arrhenius parameters for transport parallel to the foliation are dbulk0 2 0 10 11 m2 s 1 and q 30 6 kj mol 1 the bulk diffusivity perpendicular to the foliation is about a factor of 3 5 less than that parallel to the foliation with the same activation energy the values of bulk diffusivity and activation energy obtained are consistent with ionic diffusion through a static aqueous fluid suggesting that an interconnected fluid exists in the ultramylonite even under hydrostatic conditions the microstructure of the ultramylonite was characterized using transmission electron microscopy tem the nature and distribution of the interconnected fluid cannot be completely resolved from the tem analysis however the low percentage of three grain channels and open grain interphase boundaries suggests that the fluid resides as a thin film on the grain surfaces the results of this study have direct applications in many important geological settings and provide valuable insights into the observed rapid diffusion rates strong lithological control and pervasive nature of fluid transport in mica bearing rocks

statistical inversion of controlled source seismic data using parabolic wave scattering theory summary statistical parameters describing heterogeneity in the proterozoic basement of the baltic shield were estimated from controlled source seismic data using a statistical inversion based on the theory of wave propagation through random media wprm derived from the parabolic wave approximation synthetic plane wave seismograms generated from models of random media show consistency with wprm theory for forward propagation in the weak scattering regime whilst for two way propagation a discrepancy exists that is due to contamination of the primary wave by backscattered energy inverse modelling of the real seismic data suggests that the upper crust to depths of 15 km can be characterized subject to the range of spatial resolution of the method by a medium with an exponential spatial autocorrelation function an rms velocity fluctuation of 1 5 0 5 per cent and a correlation length of 150 50 m further inversions show that scattering is predominantly occurring in the uppermost 2 km of crust where rms velocity fluctuation is 3 6 per cent although values of correlation distance are well constrained by these inversions there is a trade off between thickness of scattering layer and rms velocity perturbation estimates with both being relatively poorly resolved the higher near surface heterogeneity is interpreted to arise from fractures in the basement rocks that close under lithostatic pressure for depths greater than 2 3 km

the sampling of fault populations in dolerite sills of central sweden and implications for resolution of seismic data abstract geological mapping of faults affecting dolerite sills was carried out along the baltic coast in central sweden and scaling relationships for the faults analysed in order to assess the seismic imaging of the sills the sills are 250 400 m thick and dip at low angle

the relationship between foliation and strain an experimental investigation reply abstract a multilayered salt mica specimen with embedded strain markers was shortened to produce a fold and the distribution of strain was subsequently mapped out over the profile plane on a fine scale the initial foliation which is parallel to the undeformed layers is folded by tight kinks to produce two new foliations one is defined by the preferred orientation of kink boundaries and the other by the preferred orientation of 001 of mica in the hinge region of the fold the first of these new foliations is parallel to the local 1 2 principal plane of strain whereas the preferred orientation of mica is bimodal and is symmetrical about the 1 2 plane elsewhere the two new foliations are not parallel to the principal plane of strain and angular divergencies of up to 30 35 are measured if a march model with initial random mica orientation is assumed for the development of mica preferred orientation then the correct value of strain is predicted but the orientation of the principal plane of strain can be grossly in error a theoretical analysis of the angular relationships to be expected between kink boundaries and the 1 2 plane of strain confirms that for the type of geometries experimentally developed large divergences of up to 35 should be common in rocks where the foliation has developed by processes similar to those recorded here large angular divergencies between the foliation and the 1 2 principal plane of strain should be expected as the rule

sandbox experiments on basin inversion testing the influence of basin orientation and basin fill analogue modelling experiments using brittle materials are performed to study the inversion of extensional structures asymmetric grabens of two different orientations are first created during a phase of extension and progressively filled they are subsequently shortened in the same direction the aim of our experiments is to determine factors affecting the style of deformation during inversion we specifically investigate variations in thickness and distribution of strong and weak layers constituting the graben fill and in initial basin orientation the main advantage of our experimental set up is that we have a complete control on graben location width infill and orientation before inversion the experiments show that shortening results only in limited reactivation of pre existing normal faults in general forward thrusts and backthrusts cut across normal faults into the footwall of the graben the forward thrusts either propagate parallel to the enveloping surface of faulted blocks or they cut across basin limiting normal faults at various angles the graben fill is mechanically extruded by displacement along forward thrusts that accommodate most of the shortening both pre existing faults and weak graben fill act as zones of weakness during inversion and determine the orientation and location of both backthrusts and forward thrusts the results of our experiments conform well to natural examples of inverted graben structures

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

rheological information from geological structures the contrast in rheological properties between layers of different composition or texture and between stiff inclusions and their matrix gives rise to perturbations in flow that result in structures theory and modeling allow us to understand the conditions necessary for such structures to form and conversely we can use the form of the structures to infer possible rheological conditions for the rocks during natural deformation we review here several structures and their use as indicators of rheological behavior based on theory numerical and experimental models and observations on natural structures

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

deformation partitioning during folding and transposition of quartz layers abstract banded iron formation bif from the quadrilatero ferrifero southeastern brazil shows a compositional layering with alternating iron rich and quartz rich layers this layering was intensively folded and transposed at a centimeter millimeter scale through a component of bedding parallel shear related to flexural slip at middle to high greenschist facies conditions 400 450 c the microstructure and c axis fabrics of normal limbs inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy sem and digital image analysis in the normal limbs recrystallized quartz grains show undulose extinction relatively dry grain boundaries c axes at high angle to foliation and a pervasive grain shape fabric gsf indicating operation of crystal plastic processes in the inverted limb quartz grains show more serrated and porous wet grain boundaries the gsf is similar to that of the normal limb but c axes are oriented at 90 to those of the normal limb we interpreted these characteristics as reflecting operation of solution precipitation deformation in inverted limbs as a consequence of grains having been rotated to an orientation that was hard to basal a glide but easy to dissolution precipitation creep this deformation partitioning between crystal plasticity and solution transfer during folding transposition of quartz may explain the common occurrence of layered quartz rocks where individual layers show alternating c axis fabrics with opposite asymmetries but a consistent gsf orientation such characteristics may reflect an earlier event of pervasive folding transposition of a preexisting layering

the effects of brittle plastic transitions in basement involved foreland belts the central southern alps case n italy abstract the central southern alps csa a 100 km wide basement involved fold and thrust belt formed during the europe africa convergence from the cretaceous onward two main phases of deformation have been recognized in the central southern alps the first is a pre late oligocene thrusting of pre alpine basement units upon triassic cover sequences which is sealed by the adamello intrusion 42 30 ma in the northern sector of the belt the folds and thrust faults of this phase are locally overprinted by a post adamello deformation phase possibly corresponding to the middle late miocene 16 6 ma milano phase recorded by the oligo miocene sediments buried under the plio pleistocene padane plain several lines of evidence indicate that compressive deformations originating from the current relative convergence between eurasian and adriatic plates are still occurring within the southalpine wedge which can therefore be modelled as a critical wedge a simple brittle rheology is not adequate to explain the slope and plateau topographic features of the belt which compare more favourably with those predicted by a critical taper model including the effects of temperature dependent deformations low angle surface slopes typical of frontal sectors of wedges deforming with a purely brittle behaviour are interrupted by relatively steeper slopes where the wedge base crosses the brittle plastic b p transition and starts creeping while the basal decollement deforming at higher strain rates still behaves frictionally further to the rear where both the wedge base and the decollement are plastic the surface slope flattens plateau calculated brittle plastic transition depths along the basal decollement fit well with observed topographic changes the location of slope to plateau topographic changes in the csa is strongly influenced by surface heat flow distribution and the occurrence within the wedge and in particular at the wedge base of rock types characterised by different plastic rheological parameters is responsible for repeated topographic changes each related to a different b p transition the transition from brittle to plastic behaviour at the base of the wedge is responsible for a significant weakening within the basal decollement which in turn has dramatic consequences on the dynamics and hence on the kinematics of the foreland fold and thrust belt the maintenance of the critical state of a wedge with a weak plastic basal decollement necessitates a lower amount of internal deformation than that required by completely brittle wedges moreover these deformations are strongly localised in the lower weak portions of the wedge and do not pervasively affect the upper brittle portions these characteristics have been ascertained in the rear part of the csa b p transitions commonly develop not only within wide mountain belts e g the andes but also in narrow ones e g csa this means that b p transitions must be taken into account when modelling the mechanics of foreland fold and thrust belts at all scales the occurrence of b p transitions must be tested particularly when topographic features suggest their existence since only an appropriate rheological behaviour can account for the structural evolution and kinematics derived from field studies

studies of strain and rheology of conglomerates abstract conglomerates can show significant strain variations among their component rock types previous studies have exploited this feature as a method of quantifying the effective viscosity ratios among different rock types using inclusion matrix models that we review here we further investigate the rheology of conglomerates via models of multiphase mixtures of spherical clasts with varying phase fractions using idealised models with two to four phases we can quantify the influence of phase viscosities and volume fractions on the bulk viscosity and reveal how this controls the strain partitioning among the different phases we consider practical methods of analysing conglomerates and choose r f analysis for measuring clast strain and fry analysis for whole rock strain the methods are illustrated through case studies of two very different conglomerates the port askaig tillite scotland and cesson conglomerate brittany we define the important rock phases in each and assess their strain variations although providing strain measurements of regional interest our main purpose is to quantify the viscosity ratios among the component rocks we find that among the most common rock types semipelite quartzite psammite volcanics granite the viscosity ratios span about one order of magnitude agreeing with previous studies these small numerical values for viscosity ratios among many rock types in many conglomerates lead us to conclude that these rocks each deformed at approximately constant viscosity i e as linearly viscous rather than according to a power law

magnetic microstructures of metal grains in equilibrated ordinary chondrites and implications for paleomagnetism of meteorites meteorites are a primary source of information about past magnetic field in the solar system yet the small scale magnetic properties of feni metals which are the magnetic carriers of most meteorites are poorly known we study here the magnetic microstructures of feni metals in two equilibrated chondrites two types of tetrataenite bearing microstructures are revealed 1 zoned taenite particles that consist of a cloudy zone 20 250 nm large tetrataenite precipitates embedded in ni poor matrix and a 1 10 m thick tetrataenite rim 2 zoneless plessite particles that consist of large tetrataenite grains 1 m embedded in a kamacite matrix magneto optical imaging of saturation remanence shows that the submicron sized tetrataenite islands in cloudy zone carry a much stronger remanence than the m sized tetrataenite crystals in the tetrataenite rims and plessite micron scale mapping of coercivity of remanence bcr shows that the center part of the cloudy zone has finer tetrataenite grains 20 nm and higher bcr values 1 t than the outer part 250 nm and 400 mt respectively these results suggest that the micron sized tetrataenite is in a multi domain state whereas the submicron sized tetrataenite in the cloudy zone are in a single domain like state and may be regarded as a potentially good paleomagnetic recorder in meteorites the stability of the remanent magnetization in ordinary chondrites is a function of the amount of the cloudy zones of the zoned taenite grains rather than the bulk amount of tetrataenite the fe ni phase diagram indicates that precursor of tetrataenite is paramagnetic when the metamorphic temperature was above 350 400 c therefore the remanent magnetization of tetrataenite cannot be an evidence of early magnetic activity on the parent body e g dynamo activity during the first 10 to 50 myr after the peak of metamorphism assuming 900 c peak temperature and 50 100 c myr cooling rate our tem observations show that tetrataenite has a homogeneous crystallographic orientation in an individual zoned taenite grain in low or null field tetrataenite may acquire a spontaneous magnetization whose direction is controlled by this crystallographic orientation which varies from grain to grain the small scale heterogeneity of remanence observed in equilibrated chondrites may imply that no significant magnetic field e g dynamo field was present during cooling below 350 400 c

heat flow in the tectonic provinces crossed by the southern segment of the european geotraverse abstract several published and unpublished surface heat flow density hfd data sets have been reviewed to obtain a valid hfd estimate for the main tectonic provinces crossed by the european geotraverse southern segment egt s and to produce a regional hfd map for this area because the quality and areal distribution of the data are rather heterogeneous particular care has been devoted to data validation and processing we used a geostatistical approach to estimate the regional range of influence of the hfd data in the various tectonic provinces the hfd map has been obtained using an automatic interpolation technique kriging hfd values generally higher than 100 mw m 2 are observed in the post eocene rifting areas such as the rhine graben the tyrrhenian ligurian provencal and balearic basins the younger part of the sardinia rift the central part of the sardinia tunisia channel and the pantelleria rift system the alps show values ranging between 50 and 90 mw m 2 and a mean value of about 70 mw m 2 whereas the molasse basin lying on their northern front exhibits a mean hfd significantly more elevated 87 mw m 2 the northern apennines separate the low hfd about 40 mw m 2 foreland po basin from the internal zones of the chain which shows higher hfd values and a tensional stress regime values higher than 200 mw m 2 pertain to areas recently affected by magmatic and volcanic activity such as the tuscany latium geothermal area and the oceanic tyrrhenian basin the atlas zone shows an hfd trend with values around 75 mw m 2 whereas the pelagian block and the northern edge of the saharan platform display higher values the hercynian crust of sardinia and southern corsica is characterized by a value of about 60 mw m 2 the higher values found along the western corsica and sardinia margins are probably due to a thermal event related to recent tectonic phases of the ligurian provencal and sardo balearic rifting a remarkably good correspondence between the hfd field and the bouguer gravity anomaly strongly supports the existence of transient thermal regimes in the recent deformational areas of the egt s as opposed to the steady regime mostly affecting the central and northern segments of the egt

some comments on the rheologically critical melt percentage abstract the concept of rheologically critical melt percentage rcmp originally proposed by arzi tectonophysics 44 1978 173 184 for partially molten granitic rocks is re examined it is shown that there is no experimental support to show the presence of rcmp the published experimental data suggest that the effective viscosity of partially molten granitic rocks is reduced rapidly and continuously with increasing melt fraction it is also shown that the experimental data may be modeled by means of the upper bound behavior the voigt bound of two phase material by assuming a melt localization which implies that there is no partitioning of strain between the solid and the melt

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

influence of lithological heterogeneity mechanical anisotropy and magmatism on the rheology of an arc north cascades washington abstract many aspects of crustal dynamics are dependent on changes in rheology and strength with depth in the lithosphere several of the controlling factors for rheology are difficult to study experimentally particularly lithological heterogeneity mechanical anisotropy and magmatism and we focus on these in a study of the deformation patterns in a thick crustal section 5 to 40 km paleodepth through the cretaceous cascades core of the nw cordillera this magmatic arc consists of metamorphosed oceanic and arc terranes intruded by magmatic bodies ranging from heterogeneous brittle deformation marked by serpentinite melange characterizes the shallowest part of the crustal section and the remainder of the section is characterized by heterogeneous fold dominated ductile deformation early tight to isoclinal recumbent folds and associated axial planar fabrics are refolded by one or more cycles of nearly coaxial open to isoclinal upright to overturned folds layering played a mechanically active role during folding at all levels as indicated by cleavage refraction boudinaged layers and kinematic indicators that record fold related shear ductile deformation intensifies in the narrow structural aureoles of plutons and sw directed reverse shear was partitioned into some of the aureoles the poor strain memory of these magmatic bodies makes it difficult to determine if deformation was focused in the pluton magma chambers before they reached the solidus as commonly predicted all of the plutons have magmatic foliations that at least in part reflect regional strains and these foliations are strong in the deeper plutons the thinner sheets acted as competent bodies during folding and boudinage after they reached the solidus but generally did not cause marked strain gradients in their hosts a relative strength profile constructed for the cascades crustal section shows an overall decrease in strength with depth for the ductile part of the arc that fits idealized strength profiles however in more detail relative strengths are markedly variable units were able to accumulate large ductile strains but even small variations in the physical properties of interlayered rock types exerted a strong influence on deformation patterns throughout the mid to deep crustal part of the profile this profile thus emphasizes the complex vertical rheological stratification of arcs at the crustal to thin section scale and should be applicable to many other magmatic arcs

scaling systematics of fault sizes on a large scale range fault map abstract a large scale high quality fault map was produced for an intensively mined area of the u k south yorkshire coalfield this database has been used to investigate both the scaling properties of fault systems and the robustness of techniques for the prediction of sub resolution faulting the mapped area is 87 km 2 and contains 2257 individual fault traces with on average one throw reading available for every 82 m of fault trace recorded throw values range from 10 cm to 180 m and fault trace lengths from 10 m to 12 km two sets of normal faults nw and ne striking are present together with local wnw striking dextral strike slip zones size populations are power law over at least 2 5 orders of magnitude and the exponents of the throw maximum throw and fault trace length population curves are interrelated and taking into account the effects of resolution on trace length populations are internally consistent derivatives of the primary map reproduce resolutions typical of offshore seismics and illustrate the limitations of seismic data power law fault size distributions provide a basis for sub seismic fault prediction for this data set a fault throw resolution of better than 10 m would be required to provide an accurate prediction of the throw population down to ca 0 6m

a theory of finite strain variation through contrasting layers and its bearing on cleavage refraction abstract a theory of finite strain variation in contrasting viscous layers is presented the theory is applicable to layers which are oblique to two principal strains but parallel to the third and is not restricted to plane strain results may be obtained algebraically or by use of the mohr diagram for strain solutions are given for various examples of layering attitude and viscosity ratio measured with respect to a reference layer it is shown that the finite strain ellipsoid changes in shape and orientation across contrasting layers and in some cases the principal axes may be exchanged low strain is indicated in relatively more viscous layers and high strain with extension at a smaller angle to the layering in less viscous layers the latter may approximate simple shear parallel to layering if the strain is sufficiently high the geological implications of the theory are strong strain variations in layered rock sequences it is suggested that in general strain will be inhomogeneous from layer to layer both in orientation and amount the relationship of strain and cleavage are reviewed in the introduction to establish the validity of predicting cleavage patterns from strain data qualitative comparisons between the theoretical results of strain variation and natural cleavage refraction in layered rocks would appear to justify the assumption that cleavages of varied morphologies are sub parallel to the xy planes of strain with this assumption some three dimensional features of planar and linear fabric refraction in contrasting rocks are predicted

are viscosity ratios of rocks measurable from cleavage refraction abstract in this questions contribution i consider some of the accepted methods of measuring viscosity ratios in rocks and pose a new one for a wide range of strain and orientations it can be shown that the bedding normal is immeasurably close to the xy plane and this has applications to the relationship between cleavage and strain i therefore propose that with some limitations cleavage refraction can provide a measure of effective viscosity ratios in layered rocks examples show that cleavage refraction across competence contrasts yields surprisingly small viscosity ratios this method might also provide a way of distinguishing newtonian from non newtonian behaviour of rocks over time and space

fault populations and their relationship to the scaling of surface roughness the relationship between the scaling properties of faulted geological surfaces and parameters describing the underlying fault population are investigated using simulations of a dip slip faulted surface analysis of multiple simulations of sections through the surface allowed the construction of a statistical relationship between the parameters defining the fault population and the fractal dimension of the surface the results indicate a direct if complex relationship between the fault population and the scaling of the surface roughness the main determining factor is the displacement distribution with spacing and dip having only a minor contribution this relationship is tested against examples from the moray firth scotland and the central indian ocean

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

the influence of structural lithic units in fault related folds seminoe mountains wyoming u s a abstract analysis of basement involved folds in the seminoe mountains south central wyoming reveals that lithological contrasts within the sedimentary section created a mechanical anisotropy that influenced both fault geometry and the relative rates of fault propagation and fault slip two structural lithic units are identified a competent lower unit and an overlying incompetent unit the upper unit is made up of moderate ductility rocks that were thinned ahead of the hurt creek fault offset formations in the upper unit have small relative stretch r values and are interpreted to have represented an impediment to fault growth in contrast fault propagation was rapid relative to fault slip in the competent rocks of the lower structural lithic unit which have a correspondingly higher relative stretch the black canyon fault is oriented at a low angle to bedding in the lower structural lithic unit and is layer parallel near the base of the upper unit the red spring fault is a thin skinned thrust fault and is interpreted to be linked to the black canyon fault to generate a triangle zone geometry similar structures can be identified elsewhere in the rocky mountain foreland and this configuration may represent a previously unrecognized indicator of low angle basement faults changes in the deformation style between structural lithic units must be reflected in changing fold form thus preventing the direct application of geometric and kinematic models that predict a uniform fold profile

evidence for temporally changing mechanical stratigraphy and effects on joint network architecture previous studies of jointing in sedimentary rocks have shown that joint network archi tecture is controlled by mechanical stratigraphy which is described by 1 the thickness and rigidity of stratigraphic units and 2 the nature of interfaces between beds using joint pattern to infer rigidity contrast between beds is complicated by the possibility that material properties of the beds may change between fracturing events observation of both an early bed contained joint set and a differently oriented late throughgoing joint set in the same beds of growth strata in the oliana anticline in the spanish pyrenees may reflect changing mechanical properties i e via diagenesis during the folding process using a schmidt hammer we assess the rigidity contrast between the individual units and show that the present day contrast is not great enough to terminate joints at interfaces this result is consistent with an interpretation that the late stage throughgoing joints formed in strata with conditions similar to those of the present day and that the early bed contained joints formed when the rigidity contrast between beds was significantly greater than that of the present day for example differential diagenesis rates between layers of differing grain size would produce temporally changing mechanical stratigraphy we propose that changes in mechanical stratigraphy may have previously been unrec ognized in other stratigraphic sections and that these changes affect joint network archi tecture and subsequent fluid flow pathways