high temperature mylonitization of quartzofeldspathic gneisses example from the schirmacher hills east antarctica abstract in the schirmacher hills most of the ductile shearing took place under high to medium grade amphibolite facies metamorphism the microstructure of the mylonites shows characteristic features of high temperature deformation and thus gives us an idea of deformation mechanisms of the constituent minerals at great crustal depth the variation in microstructure of the sheared rock is partly due to heterogeneity of the intensity of strain from domain to domain producing protomylonites orthomylonites and ultramylonites however a large part of the microstructural variation has resulted from syn to post tectonic recrystallization and grain growth of constituent minerals both quartz and feldspar have deformed by crystal plastic processes with dominant grain boundary migration the present aspect ratio of the feldspar grains is a result of various degrees of dynamic recrystallization along the grain boundary the ratio varies between 1 5 and 2 presence of exsolution lamellae in perthites and formation of myrmekite at the strained grains of k feldspar suggest diffusion assisted dislocation creep these mylonites are characterized by the presence of weakly strained or unstrained long quartz ribbons the development of quartz ribbons with the absence of significant strain suggests grain recovery and grain growth during high temperature mylonitization the growth of quartz ribbons took place by coalescing neighbouring grains both along and across the ribbon length at the ultramylonite stage the fine grained matrix of quartz and feldspar mostly accommodates the bulk strain

deep crustal ductile extension following thrusting in the southwestern grenville province ontario two lithotectonic assemblages in southern britt domain have different histories of plutonism metamorphism structural development and mafic dyke emplacement these differences are accounted for by postulating that a cryptic grenvillian thrust separates the assemblages amphibolite facies extensional shear along the central britt shear zone cbsz overprinted the thrust obscuring kinematic evidence for its existence the structurally lower bayfield nadeau island assemblage contains orthogneiss suites of disparate age lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss pre grenvillian and grenvillian metamorphic assemblages and at least three mafic dyke suites the overlying ojibway sand bay assemblage contains only younger orthogneiss with grenvillian metamorphic assemblages volumetrically important supracrustal rocks that are younger than the youngest orthogneiss and lacks cross cutting mafic dykes comparable tectono stratigraphic changes are present acros

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

detrital zircon geochronology of jurassic sandstones of western cuba san cayetano formation implications for the jurassic paleogeography of the nw proto caribbean clastic sediments of the early to late jurassic oxfordian san cayetano formation of western cuba are interpreted to reflect syn rift sedimentation coeval with the breakup of pangaea this sedimentary unit is the oldest known in the guaniguanico mountains and cuba u pb shrimp dating of 19 detrital zircon grains from two samples of san cayetano micaceous sandstone provided concordant ages ranging from 398 to 2479 ma the oldest zircon population is of paleoproterozoic age 2479 1735 ma but most zircons have early mesoproterozoic and grenvillian ages 1556 985 ma whereas still younger ages are pan african 561 ma ordovician 451 ma and early devonian 398 ma we discuss the possible origin of these zircons and conclude that the most likely source terrain s are precambrian and early paleozoic massifs in northern south america colombia and or venezuela and the yucatan peninsula in mexico this is compatible with paleogeographic reconstructions of the caribbean that imply that sediments of the san cayetano formation were still part of the disintegrating supercontinent pangea in pre mid oxfordian time

dynamic recrystallization and associated exsolution in perthites evidence of deep crustal thrusting mylonitic gneisses from parry sound ontario are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks 800 c 900 1100 mpa northward onto an amphibolite facies terrane we have examined by optical and electron microscopy microstructures in perthites from these mylonites that preserve the deformation recrystallization cycle which operated during high ductility deformation of the crystals the exsolution and recrystallization microstructures are intimately associated exsolution in the perthites occurred in two stages a calcic plagioclase oligoclase exsolved first and subsequently coarsened and deformed during the single crystal ductility phase of shear zone deformation a progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains the principal mode of recrystallization was subgrain rotation with a component of grain growth that followed the formation of mobile high angle boundaries misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters the second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions the perthites thus record deformation from crustal depths of the order of 34 42 km continental megathrusts such as the parry sound shear zone provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures

a modern structural regime in the paleoarchean 3 64 ga isua greenstone belt southern west greenland abstract the footwall gneisses beneath the western part of the paleoarchean 3 8 ga isua greenstone belt southern west greenland are interpreted here in terms of a 3 64 ga stack of mylonitic crystalline thrust nappes the oldest example known on earth in present coordinates the kinematic history of the thrust nappe stack is couched in terms of initial longitudinal strike parallel thrusting towards the southwest followed by transverse thrusting to the northwest and subsequent extensional collapse of the thickened crust toward the southeast diorite and tonalite that form the western margin of granitoids structurally overlying the western part of the isua greenstone belt and its footwall contain 3 5 ga mafic dykes some of which are deformed and or truncated at fault contacts within the granitoids accordingly a component of the deformation structurally above the isua greenstone belt occurred after 3 5 ga significantly later than the formation of the underlying mylonitic nappes probably during the neoarchean the structural regime of mylonitic thrust nappe stacking is very similar to that in modern mountain belts it would appear that the deformational behaviour rheological constitution and overall strength of paleoarchean and modern continental crust were similar

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

the subsolidus segregation of layer parallel quartz feldspar veins in greenschist to upper amphibolite facies metasediments layer parallel i e parallel to foliation or bedding vein formation in the graywackes and pelites of the quetico metasedi mentary belt occurred during synchronous prograde metamorphism and regional dj com pression in a traverse across metasediments which change in metamorphic grade from green schist to upper amphibolite migmatite facies layer parallel veins show the following trends 1 an increase in thickness and internal corn plexity the latter due to successive boudinage 2 low grade veins are parallel to planes of anisotropy due to the original sedimentary fabric of the host rocks but at higher grades other sites are also used and 3 a systematic increase in plagioclase quartz ratio in the veins towards higher grade adjacent mafic selvedges first exhibit quartz depletion then in the amphibolite facies plagioclase depletion mineralogical zoning is often preserved in a single vein older parts are more quartz rich than younger mass balance calculations and whole rock geochemistry based on veins mafic selvedges and country rock are consistent with a closed system subsolidus segregation origin the layer parallel veins are syntectonic and migration of the mobile components required to form their mineralogy is a stress induced mass transfer the source of these components appears to be dominantly pressure solution of the same minerals in the host rocks although metamor phic reactions may also have contributed veins nucleated first at those sites where initial sedimentary heterogeneites such as fine scale graded bedding provided gradients of normal stress across grain boundaries and hence of chemical potential necessary to drive the sub solidus segregation process the earliest veins are thus parallel to bedding later nucleation of the veins could also occur along more randomly distributed sites within the metasediments and these veins grew parallel to the schistosity rather than bedding if the two were distinct once formed the veins themselves which are more competent than the surrounding rock provide the stress heterogeneity required for their further growth the increasing plagioclase quartz ratio in the veins may be due to a tem perature dependent increase in plagioclase com ponent mobility relative to quartz alterna tively the increasing transfer distances for silica resulting from prior quartz depletion in the inner parts of the mafic selvedge may increase the relative mobility of plagioclase component

fracture propagation associated with dike emplacement at the skaergaard intrusion east greenland abstract extensive magmatic activity including several episodes of dike emplacement was associated with early tertiary continental rifting in eastern greenland exposures of the skaergaard intrusion host a large number of these dikes as well as a variety of hydrothermal veins one vein set has a characteristic outcrop expression and is filled with silica rich chlorites relative timing constraints as well as the distribution and geometric relations of these veins relative to the mafic dikes indicate that at least some of these fractures formed during the emplacement of the dikes that cut the intrusion calculations based on elastic theory permit modeling of dike geometries observed in the field and prediction of fracture geometries that could be associated with the inflation of these dikes extensional fractures may form in the area ahead of the dike termination for a variety of loading conditions the predicted width to length ratio for the dike as well as the size of the predicted fracture zone increases with increasing magma pressure within the dike and or increasing differential between the remote principal stresses for dike geometries similar to those observed in the field the predicted fracture zone is roughly elliptical in cross section and is centered in front of the dike termination the size of the zone perpendicular to the dike trend varies from less than one meter to several meters field observations indicate that dike parallel fractures are locally offset normal to the dike margin by distances of a few centimeters to a few meters our analysis implies that at least some of these fractures initiated and propagated ahead of the dikes as they were emplaced the cooling of these dikes initiated localized hydrothermal systems that formed the vein minerals currently filling the fractures

the influence of crystallographic orientation and grain boundary migration on microstructural and textural evolution in an s c mylonite abstract the analysis of the subgrain and dislocation sub structure present in grains in different crystallographic orientations within a domainal s c mylonite is reported the study reveals a range in subgrain size and development which is related to the crystal orientation of the host grain in the mylonite subgrains in grains with easy slip orientations contain large subgrains while grains in more complex slip orientations contain small subgrains the dislocation densities preserved in the mylonite are extremely variable even within individual subgrains and may be the result of a late stress pulse the analysis indicates that grain boundary migration was an important process in the microstructural evolution initiating the selective removal of some orientations and thus influencing the final crystallographic preferred orientation present in different domains the data collected is also used to assess the potential stability and preservation of microstructures during the non steady state deformation expected at the end of a deformation event

ancient lithospheric source for quaternary lavas in hispaniola fragments of ancient continental lithosphere entrained in the shallow oceanic mantle have been found in a number of locations in the southern hemisphere isotopic analyses of material from quaternary volcanic centres in hispaniola indicate that the lavas are derived from an ancient lithospheric fragment with affinities to the supercontinent gondwana

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

a fracture model for quartz ribbons in straight gneisses abstract polycrystalline composite quartz ribbons in straight gneisses of the grenville province are explained as dilatant quartz filled fractures formed parallel to a strong preexisting gneissosity the ribbons are thin 3 mm but long and continuous and have straight and truncating contacts with bordering grains of other minerals they are similar in morphology to polycrystalline ribbons interpreted in the literature as resulting from high strains in contrast little strain is required if the ribbons form as quartz filled fractures a monoclinic symmetry of the shapes of ribbons and inclusions can be explained by fracture interactions as they propagate the symmetry is consistent with a late extensional south side down sense of shearing which has been documented elsewhere in the grenville province

fault linkage patterns and their control on the formation of the petroleum systems of the erlian basin eastern china abstract fault linkage patterns in rift basins are very common while the process of fault linkage may be very short it changes sedimentation patterns before and after linkage and controls the development of half grabens the propagation and growth of boundary faults can be divided into a simple fault propagation pattern and a fault growth linkage pattern based on structural style sedimentation patterns and oil source correlations in the erlian basin three types of grabens and petroleum systems can be identified the abei graben is controlled by a single boundary fault and forms an independent petroleum system the saihantala graben is controlled by several linked faults which also has an independent petroleum system the wuliyasitai depression controlled by two boundary fault segments which results in two petroleum systems as not all half grabens develop one petroleum system they may therefore have two or more systems the relay ramps between fault segments before linkage are the locations of deposition of sands and conglomerates and consequently are the focus areas for stratigraphic lithologic trap exploration

atypical textures in quartz veins from the simplon fault zone abstract samples of monomineralic quartz veins from the simplon fault zone in southwest switzerland and north italy generally have asymmetric single girdle c axis patterns similar to textures measured from many other regions several samples have characteristically different textures however with a strong single c axis maximum near the intermediate specimen axis y the direction within the foliation perpendicular to the lineation x and a tendency for the other crystal directions to be weakly constrained in their orientation about this dominant c axis maximum this results in streaked pole figure patterns with an axis of rotation parallel to the c axis maximum these atypical samples also have a distinctive optical microstructure with advanced recrystallization and grain growth resulting in a strong shape fabric sb oblique to the dominant regional foliation sa whereas typical samples have a strong sa fabric outlined by very elongate only partially recrystallized ribbon grains the recrystallized grains of the atypical samples are themselves deformed and show strong undulose extinction and a core mantle recrystallization structure the streaked texture is likely to be a direct consequence of lattice bending and kinking during heterogeneous slip on the favoured first order prism 10 1 0 a system the heterogeneity itself being due to problems in maintaining coherence across grain boundaries when insufficient independent easy slip systems are available for homogeneous strain by dislocation glide such bending would be particularly prevalent in very elongate thin ribbon grains resulting in high internal strain energy and promoting recrystallization thus both the texture and the microstructure could be significantly modified by later strain increments affecting quartz grains with an already developed nearly single crystal texture

quartz plastic segregation and ribbon development in high grade striped gneisses quartz microstructures and c axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high grade alem paraiba shear zone in southeastern brazil are documented cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons these ribbons are separated by feldspar rich domains devoid of quartz the stage at which individual stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode which is reflected by an abrupt increase in the smoothness of the ribbon boundaries this change is interpreted to represent a strain softening kink in the stress strain time path progressive ribboning is accompanied by strengthening of the c axis fabric z maximum indicative of continued plastic flow by basal glide operation of basal glide at these high temperature conditions 680 700 c is interpreted to be a consequence of relatively dry deformation conditions a model is then proposed for development of straight quartz ribbons in high grade striped gneisses where scattered quartz grains are continuously stretched and segregated by crystal plastic processes the small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons

crustal deformation measurements in central japan determined by a global positioning system fixed point network a global positioning system gps fixed point network has been operating in the kanto and tokai districts of central japan since april 1988 to detect crustal deformation associated with the convergence of the eurasian pacific north american and philippine sea plates and to monitor the deformation cycles of frequent large interplate and intraplate earthquakes this 10 station network established by the national research institute for earth science and disaster prevention nied is the first continuously monitoring network of its kind we determine deformation within the network using two consecutive days of data every 2 weeks for the first 17 months of operations we use a station and orbit relaxation method which relies exclusively on data collected within the nied network except for 1 week of global gps tracking data which is used to determine initial station positions with respect to the global reference frame we detect relative to a station on the eurasian plate in central japan significant westward motion of 28 mm yr of the northern tip of the philippine sea plate which is subducting beneath the eurasian plate at the suruga trough our results support finite element models of collision of the izu block with the eurasian plate based on earthquake focal mechanisms and plate block motions of the japanese archipelago determined from conventional geodetic measurements over the last century we detect southwestward motion of 18 mm yr of the southeastern tip of the eurasian plate confirming expected surface extension of the subducted plate parallel to the suruga trough axis significant vertical uplift with a velocity of 20 mm yr is suggested at a site inland of the tokai district located in the akaishi uplift zone and at a site on hatsushima island in sagami bay the general tendency of vertical movements of the other sites agrees with vertical velocities obtained from 70 years of geodetic leveling and with quaternary vertical displacements determined from geomorphological and other geological evidence we detect no significant crustal motion across the fossa magna tectonic zone in central japan often considered a plate boundary across the tokyo metropolitan area nor across the sagami trough associated with the subduction of the philippine sea plate beneath northeast japan our results demonstrate the power of regionally based continuously monitoring gps networks for obtaining temporally dense measurements of small horizontal and vertical crustal movements across plate boundary zones

quartz ribbons in high grade granite gneiss modifications of dynamically formed quartz c axis preferred orientation by oriented grain growth quartz ribbons form a well defined ls fabric in granitic gneisses sheared and metamorphosed in the upper amphibolite facies boundaries of quartz grains are smooth and sub perpendicular to ribbon walls suggesting post deformational growth whereas c axes display asymmetric girdle patterns consistent with the sense of shear indicated by mesoscopic fabrics subdivision of the microfabrics according to the proportion of ribbon length occupied by individual grains indicates that the c axes of relatively short grains exhibit across girdle pattern similar to that of the whole sample longer grains reproduce elements of the pattern of the short grains suggesting that oriented grain growth occurred grain boundary mobility which clearly ended in a post deformational static regime probably began during dynamic recrystallization owing to space limitations imposed by the feldspathic matrix grain growth in the ribbons ceased before the initial dynamic fabric was erased in summary this study shows that the c axes pattern of longer grains within quartz ribbons reproduces part of the dynamically formed pattern of the shorter grains reflecting an oriented grain growth which concluded in a static episode although possibly initiated in the dynamic regime

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

evaluation of the 1984 alaska global positioning system campaign with the bernese gps software during the summer of 1984 five ti 4100 global positioning system gps receivers were used to measure an eight station network in alaska and canada previously measured with mobile very long baseline interferometry vlbi systems with baselines ranging from 300 km to almost 2700 km the observations have been processed in the network mode with the bernese gps software using orbit improvement techniques the comparisons of the gps and vlbi solutions established through a seven parameter helmert transformation show an agreement of the order of 0 1 ppm for all solutions no significant scale factor between gps and vlbi coordinates could be detected these results demonstrate for the first time the power of orbit improvement in conjunction with network processing of gps phase observations

generation of pseudotachylite under granulite facies conditions and its preservation during cooling discontinuous ultramylonite zones cut proterozoic granulite facies gneisses in macrobertson land east antarctica and preserve evidence of ductile non coaxial flow and reverse sense of shear cross cutting relationships indicate that ultramylonite deformation involved overthrusting to the east but progressively rotated to involve overthrusting to the north rotation of the principal compressive stress axes is inferred extensive pseudotachylite developed during ultramylonitization the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation neoblastic sillimanite indicates ultramylonitization occurred at 520 c on the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at 700 c and 7 3 0 5 kbar at ah2o 0 3 and low aco2 comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the rayner complex experienced a late increase in pressure of 1 2 kbar during ultramylonitization the p t ah2o conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates most of the pseudotachylite veins are undeformed the mechanism s of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite further strain having been mostly accommodated by a new or subsidiary shear zone renewed stress at reduced strain rates or renewed stress in zones in which the proportion of pseudotachylite was significantly higher could have led to the rare occurrences of deformed pseudotachylite the preservation of fine grained pseudotachylite is dependent on it remaining dry

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

ductile thrusting at mid crustal level southwestern grenville province the northwestern boundary zone of the central metasedimentary belt grenville province in the haliburton area ontario is a stack of alternating tonalitic and syenitic crystalline thrust sheets transported toward the northwest on out of sequence upper amphibolite facies ductile thrust zones during the grenvillian orogeny at 1060 ma approximately 100 ma after the initiation of thrusting in the underlying central gneiss belt kinematics of the deformation are complex predominant northwestward thrusting was at least partly coeval with subordinate northeastward thrusting late synmetamorphic extensional shears cut both thrusts and thrust sheets minor late thrusting on discrete ductile shear zones postdates the extensional structures belts of mechanically weak pelite appear to have localised the thrust sheets highly mobile marble behaved as a relatively low viscosity fluid during transport able to intrude and erode more competent wall rock

temperature constraints on microfabric patterns in quartzofeldsphatic mylonites ribeira belt se brazil abstract based on samples from the major high temperature tres rios alem paraiba padua transpressive shear zone in the ribeira orogenic belt brazil we discuss the applicability of titaniq geothermometry to constrain peak temperature conditions during high temperature mylonitization of quartzofeldspatic rocks and explore the microfabrics formed at these conditions we discuss various aspects of the titaniq method and conclude that deformation occurred at temperatures ranging from 612 to 740 20 c in the studied segment of the shear zone this high temperature deformation resulted in relatively large grain size quartz ribbons and abundant intracrystalline deformation however the cpo fabrics are weak and microstructures suggest that quartz deformation was accommodated by dynamic recrystallization involving grain boundary migration with subsequent grain growth and later some subgrain rotation during exhumation we relate the weak fabrics to diffusion processes during or immediately after dynamic recrystallization and dislocation creep and to the effect of competing slip systems during deformation in terms of rheology evidence for dauphine twinning in our samples suggest strain softening during mylonitization and we suggest that such twinning may add to the rheologically weak nature of quartzo feldspatic portions of hot middle to lower crust

the effect of composition and strain on quartz fabric intensity in pebbles from a deformed conglomerate twenty eight quartz c axis fabrics from individual pebbles from a single outcrop of deformed conglomerate have been measured these pebbles vary in composition and have undergone different amounts of strain contrary to expectations based on published results of computer simulated deformation by intracrystalline slip of pure quartzites it is observed that fabrics are not stronger in the more highly deformed pebbles instead the fabric strength is related to the quartz content of the pebbles pure quartzites show the strongest fabrics as the percentage of non quartz minerals increases up to 25 the strength of the fabrics rapidly decreases though the influence of composition becomes less marked when impurities exceed 25 these observations can be explained if the deformation of the impure quartzites included a contribution from a non intracrystalline mechanism such as grain boundary sliding

the origin of ribbon lineation within the southern adirondacks u s a abstract multiply deformed metamorphic rocks of the southern adirondacks exhibit a pronounced linear fabric consisting of elongate flat ribbons oriented parallel to early fold axes and lying within foliation planes the ribbons which are often monomineralic consist of quartz feldspar and mafic minerals as seen in transition from least to most deformed rocks these ribbons appear to be the result of elongation of grains or grain aggregates in response to a regional rotational strain which also rotated early fold axes into parallelism with the lineation a consistent sense of asymmetry of feldspar tails with respect to foliation suggests that simple shear was the dominant component of strain the long dimension of these ribbons is believed to mark the maximum elongation direction x of the finite strain ellipsoid

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

global positioning system network analysis with phase ambiguity resolution applied to crustal deformation studies in california a 5 year campaign is in progress in central and southern california to monitor tectonic motions using observations of the global positioning system gps satellites evidence from geological and space geodetic data indicates that significant horizontal deformations of order 10 mm yr are occurring west of the san andreas fault along the california margin we demonstrate using data from initial epoch measurements in january 1987 that the goal of measuring horizontal deformations with an accuracy of 5 mm yr over the duration of the campaign should be readily achievable the gps network was designed to span a wide range of baseline lengths within california and across north america to aid in phase ambiguity resolution an important factor in achieving highest accuracy baseline determinations we present an efficient algorithm for multisession network analysis of gps data with simultaneous orbit determination and ambiguity resolution using this algorithm with the california data we demonstrate the improvement obtained in station position and satellite orbit estimation when the phase ambiguities are resolved we demonstrate several millimeters 1 part in 108 short term repeatability over five single session daily analyses for horizontal baseline components and length and several tens of millimeters for the vertical comparison with the independently determined very long baseline interferometry baseline between mojave and owens valley suggests that the accuracy is at the same level as the repeatability we show that simultaneous multisession analyses with ambiguity resolution improve the baseline and satellite orbit estimates compared to the single session analyses with the single session analyses we were able to resolve all phase ambiguities on baselines within california with the multisession analyses we were able to resolve the phase ambiguities for most of the network over regional and continental scales

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

carrier phase ambiguity resolution for the global positioning system applied to geodetic baselines up to 2000 km the global positioning system gps carrier phase data are biased by an integer number of cycles a successful strategy has been developed and demonstrated for resolving these integer ambiguities for geodetic baselines of up to 2000 km in length resulting in a factor of 3 improvement in baseline accuracy and giving centimeter level agreement with coordinates inferred by very long baseline interferometry in the western united states for this experiment a method using pseudorange data is shown to be more reliable than one using ionospheric constraints for baselines longer than 200 km an automated algorithm exploits the correlations between the many phase biases of a gps receiver network to enable the resolution of ambiguities for very long baselines a method called bias optimizing has been developed which unlike traditional bias fixing does not require an arbitrary confidence test bias optimizing is expected to be preferable to bias fixing for poorly configured networks in order to enable ambiguity resolution for long baselines it is recommended that future gps networks have a wide spectrum of baseline lengths ranging from 1000 km and that gps receivers be used which can acquire dual frequency p code data

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

the ereendavaa range north eastern mongolia an additional argument for mesozoic extension throughout eastern asia mesozoic rift basins locally bounding metamorphic core complexes have been recognized in transbaikalia and northern china numerous basement outcrops located between these two regions in eastern mongolia are considered as pre palaeozoic in age one of these the ereendavaa range appears as a gneissic core marked by amphibolite facies metamorphic conditions the range is overlain to the nw by the unmetamorphosed mesozoic onon basin below the basin the upper part of the range consists of a gently nw dipping shear zone associated with top to the nw motion the structural pattern is consistent with syn extensional exhumation of the range preliminary geochronological data indicate that the shear zone is late jurassic to early cretaceous in age coeval with the onon basin these new data from eastern mongolia constitute a link between transbaikalia and northern china indicating that nw se extensional mesozoic tectonics occurred throughout the entire region

preliminary investigation results on fabrics and related physical properties of an anisotropic gneiss abstract in order to evaluate the fabric dependent anisotropy of a particular gneiss type we assessed the quartz lattice and shape preferred orientations as well as the microcrack pattern statistics in the rock mechanics laboratory several strength and deformability tests on drilled rock samples were run and the seismic wave propagation properties along the rock s principal strain axes were determined in the mechanical tests it turned out that despite a distinct stretching lineation with initially extreme grain elongation the rock performs for the larger part as a transversely isotropic material with the schistosity as plane of isotropy the anisotropy in seismic wave propagation within the plane of schistosity is attributed primarily to a highly anisotropic microcrack pattern the distinct gneissose banding and to a lower degree to the lpo of quartz

matrix mosaics brittle deformation and elongate porphyroclasts granulite facies microstructures in the striding athabasca mylonite zone western canada abstract the late archean striding athabasca mylonite zone western canadian shield is a 400 km long linked system of granulite facies mylonite belts developed in the continental crust the granulite facies mylonites 800 1000 c at 0 8 1 1 gpa developed in subsets of the anhydrous mineral assemblage clinopyroxene orthopyroxene garnet plagioclase quartz hornblende comparison of the microstructures of the striding athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the striding athabasca mylonites in addition to dislocation creep and dynamic recrystallisation extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism the microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain scale and aggregate scale diffusion grain boundary mobility and mass transfer at relatively slow strain rates despite their annealed appearance the granoblastic matrix mosaics developed synkinematically deformation induced dilation may enhance metamorphic reactions where products are more voluminous than reactants in the porphyroclast population highly elongate 20 1 monocrystalline orthopyroxenes appear to be fragments of dismembered kinked parent grains rather than stretched porphyroclasts granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites in particular it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained high temperature shear zone rocks

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

central and south america gps geodesy casa uno in january 1988 scientists from over 25 organizations in 13 countries and territories cooperated in the largest global positioning system gps campaign in the world to date table 1 43 gps receivers collected approximately 590 station days of data in american samoa australia canada colombia costa rica ecuador new zealand norway panama sweden united states west germany and venezuela the experiment was entitled casa uno an acronym for central and south america and uno is the spanish word for one designating first epoch measurements the casa uno experiment was the first civilian effort implementing a global gps satellite tracking network scientific goals of the project include measurement of strain in the northern andes measurement of subduction rates for the cocos and nazca plates beneath central and south america and measurement of relative motion between the caribbean plate and south america a second set of measurements are planned in 1991 casa dos and should provide preliminary estimates of crustal deformation and plate motion rates in the region the casa series of experiments are intended to be carried out over at least one decade

simulation of fabric development in recrystallizing aggregates ii example model runs abstract this study investigates the role of the coupling of dynamic recrystallization and lattice rotations in fabric development a new two dimensional computer simulation of polycrystalline deformation is used which combines homogeneous straining internal lattice rotations and dynamic recrystallization processes five example runs of the simulation are described here which compare the progressive development of fabrics resulting from different recrystallization regimes and different straining geometries it is found that these fabrics can evolve significantly with progressive strain from one strong fabric pattern to another the effect of recrystallization is not only to create point maxima concentrations of c axes but also to modify and create girdle distributions correlations are made between the fabrics generated by this model and previously reported quartz and ice fabrics

interplay between folding and ductile shearing in the proterozoic crust of the muskoka parry sound region central ontario grenville gneiss of the central georgian bay region was subjected to ductile deformation that produced narrow mylonite zones as well as three sets of superimposed folds differing greatly in structural signature size and orientation some mylonite zones are concordant to gneissosity and are repeatedly folded others cut gneissosity and postdate the folding gneissosity was generated as a regionally subhorizontal feature either by crustal thinning or like the early mylonite zones by low angle reverse shearing an attempt is made to account for the initially subhorizontal gneissosity the mylonite zones and the folds in a regime of large scale reverse shearing that strikes parallel to the grenville front upright northwest southwest to north south buckle folds dominate the map pattern and are subperpendicular to the reverse grenville front boundary fault these set 2 folds cannot be attributed to reverse simple shearing but require a large component of east west compression such stress could have been

precise regional baseline estimation using a priori orbital information a solution using gps measurements acquired during the casa uno campaign has resulted in 3 4 mm horizontal daily baseline repeatability and 13 mm vertical repeatability for a 729 km baseline located in north america the agreement with vlbi is at the level of 10 20 mm for all components the results were obtained with the gipsy orbit determination and baseline estimation software and are based on five single day data arcs spanning the 20 21 25 26 and 27 of january 1988 the estimation strategy included resolving the carrier phase integer ambiguities bias fixing utilizing an optimal set of fixed reference stations fiducials and constraining gps orbit parameters by applying a priori information derived from initial multi day trajectory fits to broadcast ephemeris data a multi day january 20 27 gps orbit and baseline solution has yielded similar 2 4 mm horizontal daily repeatabilities for the same baseline consistent with the constrained single day arc solutions the application of weak constraints to the orbital state for single day data arcs produces solutions which approach the precise orbits obtained with unconstrained multi day arc solutions this work suggests that the general availability of nominal ephemerides based on many days of global tracking data would be very valuable and practical for regional geodesy allowing for high precision with single day network solutions when combined with a continental scale fiducial network

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

a model for the development of a domainal quartz c axis fabric in a coarse grained gneiss abstract quartz c axis fabrics in high grade coarse grained layered gneisses reveal a distinctive pattern with three perpendicular maxima which can be interpreted in terms of syntectonic recrystallization and growth mapping of the distribution of c axes within a single thin section demonstrates that the fabric is domainal fabric domains are on the scale of millimetres and are associated with individual quartz rich patches no single domain exhibits the overall fabric of the complete thin section within each domain weighting of the c axis fabric by the area of each quartz grain demonstrates that the largest grains have their c axes preferentially at the maxima a model is presented to account for the development of this fabric grains which can keep their stored strain energy to a minimum by the use of a single glide system oriented approximately parallel to the plane and direction of maximum resolved shear stress grow at the expense of other grains the largest most successful grains deform by glide on only one of either the basal a or the prism a systems depending on the crystallographic orientation of the grain each domain develops its own glide direction by co operation of the glide systems of its constituent grains the successful grains in a domain are those which can accommodate that direction the bulk strain of the rock is achieved by the addition of the strains distinct to each domain

evolution of stretching lineations in granulite hosted ductile shear zones eastern ghats province india role of temperature strain rate and pre existing stretching lineations abstract shear zone kinematics are interpreted from sections x z parallel to the stretching lineation x direction and perpendicular to the mylonitic foliation x y the stretching lineation is a product of finite strain defined by minerals or mineral aggregates that deform plastically in quartzofeldspathic rocks deformed below 600 c rheological contrasts between quartz and feldspar are amplified and the mesoscopic stretching lineation in the shears may be indistinct in outcrop as quartz ribbons anastomose around relatively competent feldspar clasts in high strain rate zones rheological contrasts between these minerals are reduced and a stretching lineation suitable for kinematic interpretation develops in late amphibolite and greenschist facies shear zones transecting pre lineated quartzofeldspathic granulites of the eastern ghats province india the stretching lineation represents the resultant of pre existing stretch and superposed increments of later shearing until the stretch accumulated in the x direction of the later deformation exceeds the earlier finite stretch the pre existing stretching lineation progressively rotates towards the later x direction with decreasing aspect ratio these intermediate lineation orientations have no kinematic relevance to either earlier or later deformations this example highlights the importance of temperature strain rate and orientation of pre existing stretching lineations in the appearance and kinematic interpretation of stretching lineations in ductile shear zones