mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

preferred orientation of plagioclase neutron diffraction and u stage data abstract pole figures of anorthosite mylonite an 65 from san juan bautista california were determined with neutron diffraction using a 23 position sensitive detector this novel technique enables us to deconvolute the complex diffraction spectrum of this triclinic mineral into fifteen separate peaks hkl which are measured simultaneously all pole figures display strong preferred orientation with complicated triclinic distributions from which it appears that 001 poles are concentrated normal to the schistosity plane of the specimen the triclinic mineral is well suited to discuss some fundamental issues of texture representation if crystals are measured individually e g on the u stage positive and negative ends of directions can be unequivocally identified and the whole sphere rather than a hemisphere is necessary to represent the distribution of positive axes this is illustrated for a plagioclase rich amphibolite from orno huvud sweden

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

the skaergaard layered series part ii magmatic flow and dynamic layering re examination of textural and structural features of the skaergaard intrusion has clarified the process responsible for various forms of layering two general types are distinguished one produced by magmatic flow and another by non dynamic processes such as compaction and varied nucleation and growth of crystals our concern here is mainly with layering of the first type magmatic flow results in contrasting layers and strong foliation accompanied by a linear orientation of elongated minerals it is associated with slumping and cross bedding in a zone near the margins of the intrusion rocks in the interior of the layered series are foliated and commonly layered but have little or no lineation their layering cannot be due solely to deposition from currents the principal structural elements produced by dynamic processes in the magmatic state foliation lineation shear zones folds and current structures define parallel belts mainly around the western and northern margins their distribution suggests that the strongest component of flow was for the western side and that its strength decreased toward the east shear flow trajectories deduced from lineations and kinematic indicators confirm this asymmetry and point to mass movement toward the southeastern part of the floor

submarine lava flow direction revealed by neutron diffraction analysis in mineral lattice orientation the ocean crust is formed by the rising of magma from mid ocean ridges and voluminous 1 30 km3 flows of lava away from ridge axes however our understanding of the emplacement kinematics of submarine lava is often limited to plan view geometries of near axis lava drilled cores provide in situ access to the intact internal structure of submarine lavas we used neutron diffraction to study off axis lava flows drilled into the uppermost crust of odp iodp site 1256 cocos plate we provide quantitative insights into submarine lava microstructures and strong evidence for a secondary lava injection into the interior of a solidifying flow of lava along the nw se direction parallel to the paleo ridge axis of the east pacific rise the dynamics of lava inflow are controlled by crystal abundance and the temperature of the lava crystal mixture rather than by local seafloor topography we provide a description of an in situ shear within submarine lavas revealed by composite shape and lattice preferred orientations accounting for a dominant laminar nonuniform type flow

a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

the lower zone critical zone transition of the bushveld complex a quantitative textural study the lower zone critical zone boundary of the bushveld complex is an intrusion wide major stratigraphic transition from ultramafic harzburgite and pyroxenite in the lower zone to increasingly plagioclase richpyroxenitesandnoritesinthecriticalzone quantitative textural and compositional data for 29 samples through this transition show the following lower zone orthopyroxene grains are larger have higher aspect ratios are better foliated and have a lower trapped liquid component than those of the critical zone the larger grain size of the lower zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the critical zone although all rocks show differing amountsoffoliation mineral lineations are weak to absent these data are consistent with significant compaction driven recrystallization in the study section numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the lower zone critical zone transition may have formed plagioclase is rare in the lower zone because compaction removes interstitial liquid before it reaches plagioclasesaturation however asthecrystalpilegrows plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites producing more abundant but still modest amounts of plagioclase characteristic of the lower critical zone it is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization primary magmatic textures are not preserved

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

duality of magmatism in the plutonic sequence of the sumail nappe oman abstract detailed field mapping of the crustal sequence in several areas of the sumail ophiolite nappe mainly in the fizh the salahi and the haylayn blocks and comparisons with other well studied areas have shown that the crustal sequence was constructed by two distinct although nearly contemporaneous magmatic series the first a gabbroic series was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit the high level isotropic gabbro unit and their by products the sheeted dyke complex and the lower extrusives fine grained homogeneous planar laminated gabbros commonly occur at the top of the layered gabbro sequence in some specific areas wadi haymiliyah and wadi rajmi these are strongly differentiated fe ti rich gabbronorites and norites which coincide with large scale discontinuities within the plutonic sequence interpreted as axial discontinuities of the spreading magma chambers the second series an ultramafic wehrlitic series is locally rooted in the upper mantle i e the impregnated dunitic transition zone this has intruded the whole crustal sequence up to the pillow lavas countless discordant and concordant wehrlitic bodies intruded the plutonic sequence making up 20 40 of it in volume we describe the various settings and forms of these intrusions which were largely underestimated in previous studies we also describe their mineralogy and textures and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems independently from each other we infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion at the base of the crust however indications of immiscibility between gabbros and wehrlites are common suggesting that both magmatic series were quite contemporaneous the wehrlites are commonly isotropic and may exhibit an internal layering of the dunite wehrlite clinopyroxenite type sometimes appearing at the upper or lower margins of the intrusions in this case the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros these plagioclase free ultramafic layered cumulates therefore are not interbedded within the gabbro cumulates but actually belong to the wehrlitic intrusive series picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives which are for the main part cogenetic with the wehrlites the time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre we believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

magma flow exsolution processes and rock metasomatism in the great messejana plasencia dyke iberian peninsula magma flow in dykes is still not well understood some reported magnetic fabrics are contradictory and the potential effects of exsolution and metasomatism processes on the magnetic properties are issues open to debate therefore a long dyke made of segments with different thickness which record distinct degrees of metasomatism the messejana plasencia dyke mpd was studied oriented dolerite samples were collected along several cross sections and characterized by means of microscopy and magnetic analyses the results obtained show that the effects of metasomatism on rock mineralogy are important and that the metasomatic processes can greatly influence anisotropy degree and mean susceptibility only when rocks are strongly affected by metasomatism petrography scanning electron microscopy sem and bulk magnetic analyses show a high temperature oxidation exsolution event experienced by the very early ti spinels during the early stages of magma cooling which was mostly observed in central domains of the thick dyke segments exsolution reduced the grain size of the magnetic carrier multidomain to single domain transformation thus producing composite fabrics involving inverse fabrics these are likely responsible for a significant number of the abnormal fabrics which make the interpretation of magma flow much more complex by choosing to use only the normal fabric for magma flow determination we have reduced by 50 per cent the number of relevant sites in these sites the imbrication angle of the magnetic foliation relative to dyke wall strongly suggests flow with end members indicating vertical dominated flow seven sites and horizontal dominated flow three sites

understanding the emplacement of martian volcanic rocks using petrofabrics of the nakhlite meteorites in order to validate calculated ages of the martian crust we require precise radiometric dates from igneous rocks where their provenance on the martian surface is known martian meteorites have been dated precisely and quantitatively but the launch sites are currently unknown inferring the formation environment of a correlated suite of martian meteorites can constrain the nature and complexity of the volcanic system they formed from the nakhlite meteorites are such a suite of augite rich rocks that sample the basaltic crust of mars and as such can provide unique insights into its volcanic processes using electron backscatter diffraction we have determined the shape preferred and crystallographic preferred orientation petrofabrics of four nakhlites governador valadares lafayette miller range 03346 and nakhla in order to understand the conditions under which their parent rocks formed in all samples there is a clear link between the shape preferred orientation spo and crystallographic preferred orientation cpo of augite phenocrysts this relationship reveals the three dimensional shape of the augite crystals using cpo as a proxy for 3d spo and also enables a quantitative 3 dimensional petrofabric analysis all four nakhlites exhibit a foliation defined by the cpo of the augite axis in a plane although individual meteorites show subtle textural variations nakhla and governador valadares display a weak cpo lineation within their axis foliation that is interpreted to have developed in a combined pure shear simple shear flow regime indicative of emplacement of their parent rock as a subaerial hyperbolic lava flow by contrast the foliation dominated cpo petrofabrics of lafayette and miller range 03346 suggest formation in a pure shear dominated regime with little influence of hyperbolic flow these cpo petrofabrics are indicative of crystal settling in the stagnant portion of cooling magma bodies or the flattening area of spreading lava flows the cpo foliation of lafayette s is substantially weaker than miller range 03346 probably due to its higher phenocryst density causing grain grain interactions that hindered fabric development the cpo petrofabrics identified can also be used to determine the approximate plane of the martian surface and the line of magma flow to within 20 our results suggest that the nakhlite launch crater sampled a complex volcanic edifice that was supplied by at least three distinct magmatic systems limiting the possible locations these rocks could have originated from on mars

petrology and geochemistry of the lower ocean crust formed at the east pacific rise and exposed at hess deep a synthesis and new results 1 the geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle how these melts mix and crystallize and the role of hydrothermal circulation in this portion of the crust unfortunately lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes at hess deep crust formed at the east pacific rise epr is exposed due to the propagation of the cocos nazca spreading center westward here we review our state of knowledge of the petrology of lower crustal material from hess deep and document new mineral major and trace element compositions amphibole plagioclase thermometry and plagioclase crystal size distributions samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive moderate cr high mg orthopyroxene and very calcic plagioclase because primitive mid ocean ridge basalts morbs are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle these apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction the occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal morb in the axial magma chamber amc but rather that some melts partially crystallize in isolation within the lower crust however evidence that primitive melts fed the amc along with steep fabrics in shallow gabbros from near the base of the dyke complex provides support for models in which crystallization within the amc followed by crystal subsidence is also an important process in lower crustal accretion more evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust amphibole plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high temperatures in the shallow gabbros tave 713 c and show that the temperature of amphibole formation was similar in deeper gabbros tave 722 c this thermometry also suggests that fracture and grain boundary permeability for seawater derived fluids was open over the same temperature interval

emplacement of the whistle dike the whistle embayment and hosted sulfides sudbury impact structure based on anisotropies of magnetic susceptibility and magnetic remanence the sudbury impact structure is one of the few multi ring meteorite impact structures recognized on earth the meteorite impact occurred ca 1850 ma resulting in the production of impact melts derived from the crust possibly with a mantle component the impact melts formed the sudbury igneous complex and also were emplaced as radial and concentric dikes within the target rocks the whistle dike one of the radial offset dikes in the sudbury impact structure is connected to the sudbury igneous complex by the whistle embayment structure which is composed of noritic cumulates we present a study of the emplacement flow patterns in the whistle dike and the embayment using magnetic anisotropy methods to measure the petrofabrics that record emplacement petrographic and scanning electron microscope observations combined with thermal demagnetization measurements and tests of the acquisition and demagnetization of anhysteretic magnetic remanence indicate that the primary sources of the magnetic signals in the sample suite are coarse grained soft coercivity magnetite and pyrrhotite that are interpreted to be primary igneous phases some samples also contain hard coercivity phases of possible metamorphic origin the anisotropy of magnetic susceptibility ams the anisotropy of an hysteretic remanent magnetization aarm and the anisotropy of partial anhysteretic remanent magnetization aparm were measured for the samples collected at 20 sites within the whistle dike and the embayment structure including an exposed chalcopyrite rich massive sulfide body the ams method provided well defined interpretable fabrics for many of the sample sites however the ams and aarm methods were unsuccessful in providing interpretable fabric measurements at some sample sites due to contributions from the hard coercivity ferromagnetic minerals for those samples the aparm method was successful in isolating the sub fabric defined by the primary magnetite pyrrhotite crystals the fabric maps are consistent with lateral horizontal injection of the whistle offset dike and subsequent sinking of molten massive sulfides within the still ductile dike sinking of the massive sulfides is recorded by steeply plunging magnetic lineations the magnetic fabrics in the cumulate norites of the embayment structure show that the norites were deposited in a dynamic environment that resulted in a well oriented lineation which may record flow of gravity or thermal currents channelled along the embayment axis in the embayment the lineations plunge toward a massive sulfide body suggesting the flow that led to the formation of the lineation may also have led to emplacement of the massive sulfide ore it is suggested that steeply plunging lineations elsewhere in the whistle offset dike and possibly in other sudbury offset dikes might indicate the trails of sunken massive sulfides in the subsurface it is also suggested that magnetic fabrics might be useful to locate massive sulfide bodies hosted in embayment structures

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

structures in the canyon mountain ophiolite indicate an island arc intrusion abstract lithosphere created in an interarc basin is expected to be characterized by features distinguishing it from normal oceanic lithosphere apart from island arc geochemical affinities and from the occurrence of hydrous high t parageneses in the mantle and deep crustal sequences it is expected that due to a low rate of spreading vertical transport prevails over lateral drifting the canyon mountain complex located in an island arc environment of permo triassic age offers a remarkable illustration of these expected geological characteristics in particular mantle diapirism is deduced from the structural study smaller diapirs are formed in crustal formations the intrusions took place at variable temperatures 1300 800 c and were accompanied by multistage melting in hydrous conditions

microstructural and experimental constraints on the rheology of partially molten gabbro beneath oceanic spreading centers abstract flow laws for high temperature creep of olivine plagioclase and diabase are used to place constraints on the rheology of partially molten lower oceanic crust this analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene extrapolation of experimental flow laws indicates that at temperatures above 1100 c and stresses less than 10 mpa olivine may be the weakest phase in rocks with gabbroic composition by accounting for variations in the melt fraction 0 10 and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep calculated effective viscosities range from 10 15 to 10 19 pa s based on observations of the geometry of the partially molten zone beneath the east pacific rise and the microstructural and experimental constraints we favor a value of 10 18 pa s this value approaches estimates for the viscosity of the upper mantle beneath ridge axes but is significantly higher than previously suggested for the partially molten lower crust such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution however the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2d sheet like fashion

shape preferred orientation of object populations automatic analysis of digitized images abstract an automatic technique is presented for systematic shape preferred orientation spo analysis of crystal fabrics in rocks this technique is based on digitization of an image with a ccd camera and separation of an object population from its matrix the image is analysed globally using a rotating grid that is superimposed on the square pixel grid of the computer screen the parametric method is based on counting intercepts in all directions the interceps rose provides information on both the orientation and strength of the spo problems of hardware related counting noise anisotropy are avoided using an original method of filtering the intercept function with a linear sliding mask on the counting grid the technique was tested both on single objects of different shapes and on different populations of identical objects a very high degree of accuracy was obtained for orientation measurements fabric shape parameters are introduced which are specific to this intercept technique they compare favourably with shape parameters used in axial fabric analysis

assessing spo techniques to constrain magma flow examples from sills of the karoo igneous province south africa abstract shape ellipsoids that define the petrofabrics of plagioclase in jurassic karoo dolerite sills in kwazulu natal south africa are rigorously constrained using the long axis lengths of plagioclase crystals and ellipse incompatibility this has been undertaken in order to determine the most effective technique to determine petrofabrics when using the spo 2003 programme launeau and robin 2005 the technique of segmenting an image for analysis is scrutinised and as a process is found redundant a grain size threshold is defined to assist with the varying grain sizes observed within and between sills where grains exceed the 0 2 mm size threshold images should be acquired at a high magnification i e 10 magnification petrofabrics are determined using the foliation and the lineation of the ellipsoid as defined by the maximum and minimum principal axes respectively of the resultant ellipsoid samples with strongly prolate fabrics are isolated allowing further constraint on the petrofabric to be made once the efficacy of the petrofabric determination process has been determined the resultant foliations and lineations then elucidate the most accurate petrofabric attainable the most accurate petrofabrics will be determined by using the correct magnification when the images are obtained and to run the analyses without segmenting the image the fabrics of the upper and lower contacts of the karoo dolerite sills are analysed in detail using these techniques and the fabrics are used as a proxy for magma flow

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

compositional and lithological controls on the pge bearing sulphide zones in the selukwe subchamber great dyke a combined equilibrium rayleigh fractionation model the platinum group element pge bearing main and lower introduction sulphide zones of the selukwe subchamber of the great dyke are an increasing number of mafic layered intrusions are made up of a series of subzones within which the ratios pd pt are reported to have distributions of platinum group elements effectively constant whereas these ratios vary significantly between pge au and base metals characterized by vertically the subzones fractionation of pd with respect to pt varies by a offset profiles offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients the link with sulphide is great dyke prendergast keays 1989 wilson et al indisputable and the control is likely to have been the degree of 1989 naldrett wilson 1990 wilson tredoux oversaturation of pge micro nuggets in the magma the apparent 1990 the phenomenon has since been recognized in partition coefficients for pt and pd between silicate and sulphide the munni munni intrusion of western australia barnes liquid are dependent on the degree of oversaturation and thereby et al 1990 the skaergaard intrusion bird et al 1991 exhibit spurious correlation with the pge content of the sulphide and the rincon del tigre complex of eastern bolivia modelling replicates the pt and pd distribution and ratios only by prendergast 2000 pge are also postulated to be transdramatically changing the effective partition coefficients pyroxene ported in fluids emanating from expulsion of intercumulus compositions including tio2 are shown to be strongly dependent liquid from the crystal pile and there is strong evidence on the incompatible element content of the whole rock and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the pge of layered intrusions such as the skaergaard intrusion subzones the overall control is rayleigh fractionation but constancy andersen et al 1998 and in the stillwater complex of the ratio pd pt and the initial pyroxene composition before re boudreau mccullum 1986 1992 in contrast the equilibration with trapped liquid within the subzones is indicative mineralized zones in the great dyke and munni munni of equilibrium crystallization this layered structure may have been intrusion are located in pyroxenite and primary magderived from liquid layers in the magma chamber matic processes have been invoked to explain the min

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

patterns of magma flow in segmented silicic dikes at summer coon volcano colorado ams and thin section analysis abstract a complex pattern of magma flow is found in two silicic dikes of a radial swarm at summer coon an eroded stratovolcano in southern colorado the two intrusions are broken into multiple segments that suggest vertical dike propagation however anisotropy of magnetic susceptibility ams measurements and thin section observations suggest that magma flow was often subhorizontal and away from the center of the volcano segments that are proximal to the central intrusion are characterized by magma that flowed steeply upward at the proximal segment extremity then laterally along the segment and finally downward at the distal end of the segment magma flow in offset segment tips located far from the volcano center was subhorizontal towards the adjacent segment implying lateral propagation of segment tips towards one another this observation suggests relatively high driving pressure in distal dike segments as supported by dike thickening with radial distance from the center of the volcano the present study indicates that radial dike evolution at stratovolcanoes is dominated by lateral flow of magma and dike segmentation is a poor magma flow indicator a horizontally propagating radial dike has the potential to cause an eruption low on the flank of a composite cone which poses a significant yet largely unrecognized hazard to population centers and infrastructure that may surround the volcano

magnetic and plagioclase linear fabric discrepancy in dykes a new way to define the flow vector using magnetic foliation in basaltic dykes the magnetic lineation k1 maximum magnetic susceptibility axis is generally taken to indicate the flow direction during solidification of the magma this assumption was tested in tertiary basaltic dykes from greenland displaying independent evidence of subhorizontal flow the digital processing of microphotographs from thin sections cut in k1 k2 planes yields the preferred linear orientation of plagioclase which apparently marks the magma flow lineation in up to 60 of cases the angular separation between k1 and the assumed flow direction is greater than 45 this suggests that the uncorroborated use of magnetic lineations in dykes is risky a simple geometrical method is proposed to infer the flow vector from ams in dykes based solely on magnetic foliations

strain insensitive preferred orientation of porphyroclasts in mont mary mylonites abstract the shape preferred orientation spo of porphyroclasts was determined in high temperature mylonites the porphyroclasts approach rhomboidal sillimanite or elliptical garnet plagioclase sillimanite shapes and exhibit aspect ratios r as high as 11 particles with r 3 are dominantly rhomboidal the long axis of the best fit ellipse defines a very strong spo inclined at 5 10 to the mylonitic foliation in an antithetic sense with respect to the shear direction this angle is independent of r the inclination of the long sides of rhomboidal sillimanite increases from 10 to 20 with decreasing r in contrast the short sides have a constant orientation of 15 to 17 irrespective of r and are parallel to extensional crenulation cleavage low aspect ratio mainly elliptical objects show low intensity spo close to the shear plane the two spos appear strain insensitive in the case of r r 3 a stable position is acquired this is not explained by any of the current theoretical and experimental models of spo

structures and melt fractions as indicators of rheology in cordierite bearing migmatites of the bayerische wald variscan belt germany a melt but the degree of their shape preferred orientation increases the relation between melt volume fraction melt segregation and with increasing melt segregation and differential stress the mideformation mechanisms of minerals in migmatites and the concrostructures suggest that deformation mechanisms and hence the trolling effects of all these factors on the bulk mechanical behaviour bulk mechanical behaviour of migmatites change in space and time were investigated using rocks from the bayerische wald bohemian during partial melting the observed complex interplay of melt massif variscan belt germany biotite dehydration melting at volume fraction melt segregation bulk strength contrasts mechanical 800 850 c and 0 5 0 7 gpa was the migmatite forming process properties of different minerals and time cannot be described by a in this area four migmatite types were distinguished that alternate single flow law detailed mapping of migmatite areas along with on the scale of several decimetres to several tens of metres type microstructural observations deformation experiments considering mig1 is massive and undeformed types mig2 and mig3 are heterogeneous melt distribution and numerical models that integrate both stromatic leucosome mesosome interlayering but differ in the different flow laws for various stages of partial melting may serve degree of deformation type mig4 has an interlayering of meto derive quantitative models for the bulk mechanical behaviour of lanosome and leucosome and is strongly deformed melt volume crustal sections in the future fractions found by volume estimation of pure melt leucosomes on outcrop hand specimen and thin section scales are 20 40 vol and coincide fairly well with melt volume fractions produced in dehydration melting experiments with similar bulk compositions at the relevant pressure and temperature conditions 20 30 vol the degree of melt removal from mesosomes and melanosomes melt

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

asymmetric shape preferred orientations as shear sense indicators abstract mineral shape preferred orientations can be characterized by plotting aspect ratios against grain length orientation and such plots commonly display asymmetric patterns in xz sections perpendicular to foliation and parallel to the direction of maximum elongation in most cases the asymmetries cannot be discerned by superficial examination of thin sections the sense of asymmetry can be defined by a the shape of the highest aspect ratio per grain length orientation curve and b the relative orientations of the most elongate grain and the mean or median of the population as a whole observed senses of asymmetry display consistent relationships to known senses of shear and they therefore provide a useful alternative to or check on other methods of shear sense determination examples of quartz plagioclase microcline amphibole and biotite shape preferred orientations are described and they include recrystallized mosaics as well as populations of discrete mineral grains the examples include mineral preferred orientations that evolved by plastic deformation anisotropic crystal growth and mechanical rotation of inequidimensional grains

textural character of gabbroic rocks from pito deep a record of magmatic processes and the genesis of the upper plutonic crust at fast spreading mid ocean ridges

ultra high temperature and subsolidus shear zones examples from the poe mountain anorthosite wyoming abstract ultra high temperature shear zones formed with interstitial melts present during the emplacement of the poe mountain anorthosite wyoming the shear zones are characterized by coarse grained plagioclase with flat extinction and rare deformation microstructures dissected grain microstructures suggest that plagioclase underwent recrystallization by fast grain boundary migration orthopyroxene crystallized from interstitial melts as strain free foliation parallel tablet grains plagioclase has a magmatic petrofabric characterized by poles to 010 normal to the shear foliation and 100 parallel to the foliation and movement direction poles to 001 have a broad distribution with a point maximum roughly parallel to the foliation and normal to the movement direction orthopyroxene petrofabrics result from oriented growth parallel to the shear foliation 100 defines a point maximum perpendicular to the shear foliation and 010 and 001 have great circle distributions approximately parallel to the shear foliation microstructures of the ultra high temperature shear zones contrast with those of granulite grade shear zones cutting across the intrusion the granulite grade shear zones initiated as fractures which evolved into shear zones by dynamic recrystallization deformed primary plagioclase and orthopyroxene recrystallized by grain boundary rotation into smaller polygonal grains

petrology and mineral compositions of the middle banded series of the stillwater complex montana the two olivine beating zones of the middle banded series of the stillwater complex are characterized by an increase in the number of cumulus minerals with height in each anorthosite and anorthositic troctolite dominate the lower part whereas olivine gabbro and gabbronorite form much of the upper portions electron microprobe analyses of cumulus minerals indicate little or no variation of average mineral compositions with height in addition no significant lateral variations in cumulus mineral compositions occur along 8 km of section plagioclase from throughout these zones shows the complex reverse and oscillatory zpnation patterns also seen in plagioclase from the thick 500 m anorthosites that sandwich the zones the data suggest that the entire middle banded series is genetically related and therefore requires models for the origin of the thick anorthosites to also explain the olivine bearing rocks between them however textural features such as discordant troctolites pegmatoids slump structures and variably developed mineral laminations and chemical features such as zpnation in clinopyroxene produced by intergranular exchange with orthopyroxene and large within sample variations in the mg number of olivine in lowolivine troctolites indicate that significant modification of these rocks by postcumulus processes has taken place thereby obscuring evidence of their genesis

mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust the sondalo magmatic system n italy

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

evidence from the oman ophiolite for active mantle upwelling beneath a fast spreading ridge it is commonly accepted that as oceanic lithosphere cools it slides away from the spreading centre under its own weight1 it has been suggested2 5 that even at the spreading axis this process controls the dynamics causing asthenospheric mantle material to rise passively before being dragged away from the ridge by the overlying lithosphere alternatively the upwelling of the asthenosphere at the ridge might be active caused by the buoyancy of partial melt6 10 or of the less dense peridotite left behind by such melting11 12 at slow spreading ridges bull s eye gravity anomalies13 14 have been interpreted as suggesting focused accre tion and hence active mantle diapirs at fast spreading ridges however these anomalies are weaker and less clearly three dimensional15 which may reflect passive mantle upwelling16 here we show that structural data from the oman ophiolite which is thought to have been created at a fast spreading ridge17 18 are consistent with active mantle upwelling beneath the ridge and not with models of passive flow the absence of well marked bull s eye anomalies at fast spreading ridges may be explained by their relatively constant crustal thickness and near isostatic equilibrium

crustal growth in the mesoarchean plutonic belt from the memve ele area ntem complex southwestern cameroon evidence of early earth transpressional tectonics abstract the mesoarchean crust of the plutonic belt of the memve ele area lies in the southwestern part of the ntem complex ntc cameroon emplacement of these rocks resulted from both gravity and tectonic driven vertical flow perpendicular to the direction of shortening at the time of partial melting and magmatic accretion such deformation patterns suggest the occurrence of a long lived hot orogen and may explain a nearly isobaric retrograde pressure temperature p t path recorded within high t low to medium p metamorphic rocks however in the memve ele shear zone msz a metamorphic gradient runs from granulite to green schist facies structural analysis reveals four continuous ductile deformations the first stage is characterized by the emplacement of dome and basin structures the second stage is marked by coeval diapiric movements and a transpressional shear zone the third stage is marked by both transpressional and lateral flow regimes and the last stage is marked by horizontal constrictional strain these features indicate that the mesoarchean crust is affected by a long lived horizontal shortening deformation d1 d3 coeval with vertical movements in which concentric cleavages occur around domes thus the studied area can be considered as exemplifying an early earth style tectonics the structural patterns in the msz contain zone of high strain rocks belonging to the deep and shallow mid crusts that have recorded transpressional tectonics the compressional strain regime in which results coeval steep to shallow dipping structural features and dome and basin geometries associated to msz activities contributed to maintaining a stable crustal thickness in the memve ele area during mesoarchean orogeny

bursting the bubble of ballooning plutons a return to nested diapirs emplaced by multiple processes a popular model for the emplacement of roughly spherical plutons is that of ballooning or in situ inflation of a magma chamber in a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent while the hotter tail of magma continues to rise and expand the already crystallized outer margin this expansion forms a concentric gneissic to mylonitic foliation and flattening type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures our reexamination of three supposedly ballooned plutons ardara ireland cannibal creek australia papoose flat california and evaluation of published descriptions of many others indicate that this model is largely incorrect deflections of country rock structures strains and porphyroblast matrix relationships indicate that only minor to moderate expansion usually 30 or less occurred during emplacement that other emplacement mechanisms must have occurred and that regional deformation continued during and after emplacement internal structures indicate that when magma chamber expansion did occur it did so by flow of magma that magmatic foliations and lineations formed late in the magma chamber evolution that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion and that only minor internal subsolidus deformation results from emplacement this study indicates that many plutons previously interpreted as post tectonic ballooning plutons are better viewed as syntectonic nested diapirs emplaced by a variety of country rock material transfer processes this nested diapir model implies that magma ascent may occur by rise of large magma batches instead of transport in dikes followed by ballooning that magma chamber dynamics differ from that in the ballooning model and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt

the gangotri granite garhwal himalaya laccolithic emplacement in an extending collisional belt the gangotri miocene leucogranite is composed of several laccoliths 6 7 km long 1 5 2 km thick which can be divided into two sets of lenses the southern lenses intruded in the lower part of the high himalaya sedimentary cover and the northern lenses intruded in an older porphyritic biotite bearing granite in both cases the magmatic fabric is commonly weak with no dominant stretching direction although a rough e w trend is present in the northern lenses the magnetic fabric is characterized by the fact that drastic changes in the direction in the magnitude of the magnetic parameters or in the fabric type planar or linear may take place over very short distances measurements of preferred orientations in thin sections indicate that the strain regime was largely dominated by a coaxial component for both sets of lenses in agreement with the large dispersion observed in both the field and magnetic lineations the comparison of the granite and host rock structures shows that the leucogranite emplacement dates the onset of the extensional tectonism in the high himalaya range and is not related to a southward directed thrust event associated with the main central thrust this is exemplified by the vertical attitude of the feeder dikes that intrude the metasedimentary rocks beneath die southern lenses the presence of these dikes indicates in turn that magma ascent occurred by fracture propagation the spatial disposition of the southern lenses could have resulted either from the disruption of a single laccolithic intrusion by crustal scale boudinage due to a northern gravity backslide of the top of the tibetan slab or from the intrusion of independent laccoliths the use of the elastic bending theory of pollard and johnson 1973 shows that in both hypotheses the current laccolith sizes are compatible with a laccolithic mode of magma emplacement however neither the density contrast between the magma and its enclosing rocks nor the lithological boundary between the tibetan slab and the overlying tibetan metasedimentary series controlled the level of magma emplacement rather flat lying collapse structures which intersected the upward propagating magma dikes are the most likely causes of magma arrest such a mechanism was favored by the schist rich lithology of the metasedimentary host rocks in addition field relationships indicate that the melt supply through the dike system was a continuous rather than pulsed process existing numerical treatments on the rates of magma transport through fractures show that in such a case the laccoliths could have been built in less than 100 years this short time of emplacement the small size of the laccoliths and their peripheral disposition relative to the badrinath granite suggest that the gangotri lenses may represent the initial stage of pluton accretion in the high himalaya which ultimately gave rise to a much larger massif such as the manaslu granite

the structure and geochemistry of the gabbro zone of the annieopsquotch ophiolite newfoundland implications for lower crustal accretion at spreading ridges the annieopsquotch ophiolite exposes a c 5 5 km thick section of tholeiitic gabbros sheeted dykes and pillow basalts based on the along strike consistency in thickness of the major crustal units and lack of significant throw on spreading related normal faults the annieopsquotch ophiolite is interpreted to have formed at an intermediate to fast spreading ridge the upper c 400 m of the gabbro zone is composed of massive texturally heterogeneous gabbros with compositions that approach those of the sheeted dykes and basalts below this is c 1 6 km of 10 30 m thick planar intrusive sheets or sills the lowermost part of the gabbro zone is composed of gabbroic rocks with relict troctolite and troctolitic gabbro enclaves which are veined and partly replaced by gabbro and pyroxenite sill contacts within the central sill dominated part of the gabbro zone are sub parallel to boundaries between the major ophiolite lithostratigraphic units the upper and lower contacts of individual gabbro sills may have finer grain sizes indicating that the intrusions cooled from the top and bottom locally well preserved comb structures crescumulates indicate downward growth supporting a sill interpretation the sills are composed of weakly or un deformed plagioclase clinopyroxene olivine cumulates trace element modeling suggests that the parental magmas of these cumulates had compositions very similar to the overlying sheeted dykes and basaltic lavas as do dykes emplaced within the gabbro zone model liquids calculated from the gabbroic sills generally become more evolved up section indicating that magma evolution in the annieopsquotch ophiolite was dominated by fractionation in lower crustal conduits below the level of a putative axial melt lens aml the model liquids sheeted dykes and basalts preserve a similar wide range of compositions which may indicate that aggregation homogenization and fractionation in an aml was inefficient similar intra conduit fractionation of mantle derived melts might also contribute to morb evolution at mid ocean ridges

preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the insubric line western alps abstract preferred lattice orientations of plagioclase measured by the universal stage from deformed amphibolite and greenschist facies rocks near the insubric line western alps reflect the type and orientation of the strain system either 001 or 010 are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction by the preferred orientation patterns dominant 010 001 slip is indicated and additionally slip on 001 parallel to the a direction possibly at somewhat higher temperatures and or at a higher an content of the plagioclase it is suggested that obliquities of the symmetry planes of the orientation patterns against the principle axes of strain are related to the sense of shearing

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

tilting and transpression of an archaean anorthosite in northern ontario abstract an archaean anorthosite complex crops out over more than 300 km 2 at the junction between an archaean metasedimentary accretionary prism quetico belt and a volcanic greenstone terrain to the north wabigoon belt the anorthosite is feebly deformed and weakly foliated however anisotropies of low field susceptibility ams and anhysteretic remanence aarm define consistent magnetic fabric directions extending over subareas 25 km 2 in extent whereas magnetic fabrics cannot define strain magnitudes they normally provide reliable indications of finite strain orientations even where the fabrics developed by stress controlled nucleation during a progressive noncoaxial strain history the field schistosity s 1 being the oldest foliation rotated closest to the shear plane ams foliation due to preferred crystallographic orientation of silicates and pyrrhotite as well as preferred dimensional orientation of magnetite lies at a greater angle to the shear plane because it formed late then s 1 had less time to rotate the few measurements of the youngest aarm foliation caused by magnetite and pyrrhotite suggest that it may lie at a still greater angle to the shear plane thus s 1 and succeeding magnetic foliations provide a cryptic shear sense indicator they reveal relative motions along the terrain boundary that include transpression due to nw se shortening causing dextral e w shear with a north side up component palaeomagnetic data confirm a progressive northwards steepening of the anorthosite sills that is consistent with northwards subduction of the accretionary prism to the south

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

plagioclase preferred orientation and induced seismic anisotropy in mafic igneous rocks fractional crystallization and crystal segregation controlled by settling or floating of minerals during the cooling of magma can lead to layered structures in mafic and ultramafic intrusions in continental and oceanic settings in the lower crust thus the seismic properties and fabrics of layered intrusions must be calibrated to gain insight into the origin of seismic reflections and anisotropy in the deep crust to this end we have measured p and s wave velocities and anisotropy in 17 plagioclase rich mafic igneous rocks such as anorthosite and gabbro at hydrostatic pressures up to 650 mpa anorthosites and gabbroic anorthosites containing 80 vol plagioclase and gabbros consisting of nearly equal modal contents of plagioclase and pyroxene display distinctive seismic anisotropy patterns vp z vp y 1 and vp z vp x 1 for anorthosites while 0 8 vp y than the gabbros laminated anorthosites with vp x vp y vp z display a strong crystal preferred orientation cpo of plagioclase whose 010 planes and 100 and 001 directions parallel to the foliation for the gabbros and amphibolites characterized by vp x vp y vp z and vp x vp y vp z respectively pyroxene and amphibole play a dominant role over plagioclase in the formation of seismic anisotropy the poisson s ratio calculated using the average p and s wave velocities from the three principal propagation polarization directions x y and z of a highly anisotropic anorthosite cannot represent the value of a true isotropic equivalent the cpo induced anisotropy enhances and decreases the foliation normal incidence reflectivity at gabbro peridotite and anorthosite peridotite interfaces respectively

magma emplacement into the lemptegy scoria cone chaine des puys france explored with structural anisotropy of magnetic susceptibility and paleomagnetic data the lemptegy volcano is a small monogenetic scoria cone located in the chaine des puys auvergne france which erupted about 32 000 years ago a first edifice lemptegy 1 formed during a trachybasalt eruption as a group of satellite vents of the puy de gouttes scoria cone a second trachyandesitic edifice lemptegy 2 formed soon after and completely covered lemptegy 1 with an 80 m high breached cone since 1946 the lemptegy volcano has been quarried for scoria and today offers unprecedented three dimensional exposure of the subvolcanic plumbing system to map the internal flow architecture of the plumbing system and to study the subvolcanic deformation of lemptegy 2 structural mapping petrographic observations anisotropy of magnetic susceptibility ams rock magnetic and paleomagnetic data were collected field structural mapping and thin section study of tension gashes riedel shears striations as well as ductile shear zones and bubbles allow the direction and sense of the magma flow to be determined twenty ams sites were established in ten dikes one to four sites in each dike with 504 specimens analyzed and 479 specimens used to infer magma flow patterns structural data the maximum susceptibility axis k 1 and the imbrication of the magnetic foliation k 1 k 2 planes indicate both upward and downward sense of flow as well as flow toward and away from the central vent rock magnetic experiments reveal that a cubic fe ti oxide phase likely low ti titanomagnetite is the principal magnetic phase carrying both the remanence and anisotropy paleomagnetic data from some sites yield statistically distinct at the 95 confidence level remanence directions while at other sites the data are indistinguishable at the 95 confidence level the paleomagnetic results observed steeply tilted scoria layers internal unconformities and faults show that as each dike was emplaced it displaced earlier dikes evidencing subvolcanic deformation the lemptegy 2 volcano shares similarities in terms of inferred eruption style and structures with other scoria cones such as cerro negro nicaragua and thus provides an excellent field laboratory to investigate active scoria cones world wide

on the measurement of plagioclase lattice preferred orientations abstract there are two major difficulties in measuring plagioclase lattice preferred orientations lpos by u stage the first which is caused by dependence of the lpo determination on anorthite content cannot be overcome using the u stage method at present the second difficulty which is caused by the dependence on orientation of thin sections can be overcome by measuring all twinned grains in any two orthogonal sections compared with previous methods published in the literature the present method using two rather than three orthogonal sections is more statistically meaningful and time saving in practice this method was used for measuring lpos of plagioclase an 50 from a typical anorthosite mylonite deformed under the granulite facies conditions in the morin ductile shear zone within the canadian grenville tectonic province

using ams combined with mineral shape preferred orientation analysis to understand the emplacement fabrics of the apiai gabbro norite ribeira belt se brazil the apiai gabbro norite is a massive fine grained neoproterozoic intrusion emplaced in a core of synformal structure that deforms low grade marine metasedimentary rocks of the ribeira belt of south eastern brazil the lack of visible magmatic layering or any internal fabric has been a major limitation in deciding whether the emplacement occurred before or after the regional folding to assist in the tectonic interpretations we combine low field anisotropy of magnetic susceptibility ams and silicate shape preferred orientation spo to reveal the internal structure of the mafic intrusion magnetic data indicate a mean susceptibility of about 10 2 si and a mean anisotropy degree p of about 1 08 essentially yielded by titanomagnetite the magnetic and silicate foliations for p 1 10 are parallel to each other while the lineations tend to scatter on the foliation plane in agreement with the dominant oblate symmetry of the ams and spo ellipsoids for lower p values the magnetic and silicate fabrics vary from coaxial to oblique and for p 1 05 their shapes and orientations can be quite distinct the crystal size distribution csd of plagioclase for p 1 05 is log linear in agreement with a bulk simple crystallisation history these results combined show that for a strong spo corresponding to a magnetic anisotropy above 1 10 ams is a reliable indicator of the magmatic fabric they indicate that the apiai gabbro norite consists of sill like body that was inclined gently to the north by the regional folding

microstructures deformation mechanisms and seismic properties of a palaeoproterozoic shear zone the mertz shear zone east antarctica abstract the mertz shear zone msz is a lithospheric scale structure that recorded mid crustal deformation during the 1 7 ga orogeny we performed a microstructural and crystallographic preferred orientation cpo study of samples from both mylonites and tectonic boudins that constitute relics of the terre adelie craton tac the deformation is highly accommodated in the msz by anastomosed shear bands which become more scattered elsewhere in the tac most of the msz amphibolite facies mylonites display similar cpo thermal conditions intensity of deformation and dominant shear strain preserved granulite facies boudins show both coaxial and non coaxial strains related to the previous 2 45 ga event this former deformation is more penetrative and less localized and shows a deformation gradient later affected by a major phase of recrystallization during retrogression at 2 42 ga both msz samples and granulite facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted cpo of minerals quartz feldspar biotite amphibole and orthopyroxene in particular we highlight the development of an uncommon cpo in orthopyroxene from weakly deformed samples characterized by 010 planes oriented parallel to the foliation plane 001 axes parallel to the stretching lineation and clustering of 100 axes near the y structural direction lastly we computed the seismic properties of the amphibolite and granulite facies rocks in the msz area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the msz our results reveal that i the low content of amphibole and biotite in the rock formations of the tac and ii the interactions between the cpo of the different mineralogical phases generate a seismically isotropic crust thus the seismic anisotropy recorded by the seismic stations of the tac including the msz must be due to mantle rather than crustal structures

polymineralic orientation analysis of magmatic rocks using electron back scatter diffraction implications for igneous fabric origin and evolution abstract electron back scatter diffraction ebsd which provides an easy way of acquiring large numbers of individual crystallographic orientation data from different phases has been applied to the study of magmatic fabrics using this technique the crystallographic preferred orientation cpo of plagioclase biotite orthopyroxene hornblende and quartz in natural tonalites and quartzdiorites from the santa olalla igneous complex sw iberia deformed during the magmatic stage have been determined plagioclase is the coarser phase defining the main fabric in each sample whereas biotite can display either the same fabric as plagioclase or a completely different one the differences between these two phases occur because 1 smaller phases interact with the larger ones to produce more random orientations 2 under simple shear finer phases can completely rotate giving a girdle included in the xz plane 3 finer phases can more easily preserve relict fabrics while the coarser phases are completely reoriented by the last stress tensor the last phases to crystallize show weak to completely random cpos hornblende or completely random distributions quartz the study was completed with a shape preferred orientation analysis using the intercept method in order to detect weak magmatic lineations and numerical modelling simulations of theoretically equivalent simple shear situations for each sample

ephemeral magma chambers in the trinity peridotite northern california abstract the trinity massif comprises the major lithologies of an ophiolite as defined at the 1972 penrose conference previous studies have shown however that it differs from the semail oman or table mountain newfoundland ophiolitic massifs particularly because its crustal section is thin and because its mantle section has vertical plastic flow planes these features have led to an interpretation of the trinity massif as a fragment of slow spreading oceanic lithosphere le sueur et al 1984 boudier and nicolas 1985 in this paper we show that the trinity gabbros occur in discontinuous kilometre sized pockets intrusive into the mantle peridotites the internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short lived magma chambers these ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle when it had cooled down to lithospheric temperatures we discuss the possibility that these small and ephemeral magma chambers formed at a slow spreading oceanic ridge

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

intrinsic and scattering attenuation of the mt fuji region japan summary mount fuji is the focus of intense study because of its potential hazard signaled by seismic geologic and historical activity based on extensive seismic data recorded in the vicinity of mt fuji coda quality factor q 1c using a single scattering model hypothesis and intrinsic and scattering quality factor q 1i and q 1s using the multiple lapse time window mltw method were measured to focus the study on the magmatic structure below mt fuji the data were separated into two groups a near fuji region of rays traversing an area with radius 5 km around the summit r 20 km this classification shows the largest discrepancy of q 1c at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km further the spatial division shows significant difference of q 1i and q 1s at hypocentral distance of 80 km the large difference of q 1s in bandwidths 2 4 and 4 8 hz indicates lithospheric heterogeneity beneath mt fuji with a characteristic heterogeneity scale length of about 1 km the results have a small error range due to the large data sample showing that all q 1 values in the near fuji area are greater than those of the far fuji area and q 1i for both the near and far fuji areas is higher than q 1s at high frequencies the q 1i and q 1s values for far fuji are in the range of values for typical non volcanic areas the q 1i values of the near fuji area are lower than those of other volcanic areas considered where as values of q 1s are not the low q 1i for the volcanic region of near fuji suggests that the magmatic activity indicated by percent partial melt in the vicinity of mt fuji is not as active as hot spot volcanoes such as kilauea hawaii

the ardara pluton ireland deflating an expanded intrusion abstract the ardara pluton north western ireland is a circular monzodiorite granodiorite intrusion with a tail it has been regarded as a typical example of a pluton emplaced entirely by expansion balloning after initial intrusion most of the foliation in the pluton is magmatic with weak superimposed solid state features apart from a thin rind of strong solid state deformation the solid state deformation in the pluton appears to be largely if not entirely of post intrusive tectonic origin flattened microgranitoid enclaves a concentric magmatic flow foliation and compositional zoning in the pluton are consistent with some expansion of magma prior to crystallization however strains in the wall rocks indicate that only a small amount 25 38 of the volume of the pluton can be ascribed to ballooning this relatively small amount of expansion could account for a observed structural concordance and strong syn metamorphic deformation in the aureole b flattening of microgranitoid enclaves magma globules in the pluton and possibly c a small amount of solid state deformation in the pluton during lock up of magmatic flow evidence of solid state deformation superimposed on a strong magmatic flow foliation in the tail of the pluton implies that a fault was active during emplacement and may have assisted the intrusion further space for the pluton may have been made by a complex interaction of processes many of which cannot be constrained from a two dimensional cross section through the area these possibilities highlight the limitations of attempting to constrain emplacement mechanisms on the sole basis of structures adjacent to plutons

the origin of laminated and massive anorthosite sept iles layered intrusion quebec canada well laminated plagioclase rich rocks anorthosite occur as layers in mineralogically similar but massive rocks in the sept iles intrusion several phases of crystallization of poikilitic clinopyroxene have preserved the intermediate stages in the production of these rocks plagioclase nucleated and initially grew in random orientations in the stationary part of the boundary layer there in the absence of other events crystallization produced the massive anorthosite the laminated anorthosite may have been produced by episodic increases in the velocity of the magma adjacent to the boundarylayer induced by magmatic density currents such currents would have caused the zone of simple shear of the boundary layer to migrate outwards into the previously stationary part this simple shear would then have rotated the existing plagioclase crystals as well as crystals that nucleated at that time towards the shear plane some rocks show evidence of several periods of enhanced flow velocities the production of the lamination may have weakened the crystal mush and enabled slumping to occur producing folds and contorted layers finally compaction liberated large quantities of intercumulus magma of which some was trapped as granophyric segregations and some escaped by overturing and churning the crystal pile earlier formed granophyric segregations and laminated layers were disrupted during this process to produce a range of more complex rocks including igneous breccias

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

gabbro layering induced by simple shear in the oman ophiolite moho transition zone we investigate the origin of modal layering in gabbro lenses of the moho transition zone in the oman ophiolite with a microstructural study gabbro lenses exhibit a shape preferred orientation of plagioclase crystals that are euhedral and devoid of any intracrystalline deformation this texture and field kinematic indicators show a strong simple shear deformation in magmatic conditions the parallelism of the lineation in gabbros and the plastic lineation of the host dunite indicate that their development is contemporaneous and that the magmatic features are passively coupled to the solid plastic flow of the host mantle we also found undeformed isotropic and weakly deformed roughly layered gabbros the host rock is plagioclase and clinopyroxene impregnated dunite it contains euhedral plagioclase suggesting that some grains crystallized in suspension there is no clear boundary between the gabbro and the host rock on the contrary the limit is diffuse with increasing plagioclase and clinopyroxene content from the dunite to the gabbro over tens of meters this suggests that the gabbro corresponds to melt impregnated dunite we defined 4 gabbro types from unlayered type 1 to well layered type 4 with progressively more continuous and distinct layers we characterized deformation with crystal shape and crystallographic preferred orientation cpo measurements unlayered samples have a random fabric type 2 have a very weak shape fabric and planar cpo defined by olivine and plagioclase 010 planes type 3 have a weak fabric with a lineation defined by the 001 olivine axis and the 100 plagioclase axis which reflects the shape fabric type 4 have a strong shape fabric and cpo with a lineation defined by the 100 olivine and plagioclase axis we interpret the progression from isotropic to clear s l fabrics as a result of increasing deformation imposed by the flowing host mantle the type 2 gabbros must result from compaction with increasing simple shear the lineation is developed in type 3 samples finally with lower melt content suspension flow switches to plastic deformation in the olivine rich layers leading to plastic olivine fabrics in type 4 samples we deduce that layering originates from simple shear which other studies show induces strong segregations in suspended particles and plastically deformed rocks to build the isotropic stage melt must accumulate faster than the deformation suggesting that melt is extracted by dikes

textures of orthopyroxenites from the burgersfort bulge of the eastern bushveld complex republic of south africa a suite of 30 samples was collected from the burgersfort bulge area of the eastern bushveld complex for a detailed quantitative textural study the studied section represents a stratigraphic column approximately 1 000 m thick of mainly orthopyroxenites from the lower zone lz and lower critical zone crystal size distribution csd plots and quantitative fabric analyses reveal a history of crystal aging and compaction as evident from the loss of smaller sized crystals and the development of foliation with little evidence of mineral lineation compared with the thicker section from jagdlust 80 km north the lz orthopyroxenites at burgersfort are consistently finer grained foliation is equally well developed at both locations and is consistent with a relatively thin compaction zone at the top of the mush column grain size is largely a function of local cooling history with a grain size growth exponent estimated to be between 2 0 and 3 4

the structures of two ophiolite massifs bay of islands newfoundland a model for the oceanic crust and upper mantle abstract for a detailed analysis of their main structures accumulation planes foliation planes and lineations the peridotite gabbro units of table mountain and blow me down mountain bay of islands newfoundland have been divided into several structural units each one being characterized by specific microstructures of deformation or accumulation a fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it is postulated for the structure of the lithosphere formed at a spreading center the tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle and the direction of the ridge is inferred from the lineation trend in the peridotites the structures in the cumulates are related to subsidence of the magma chamber floor during spreading at the base of the massifs the tectonic structures in the mafic metamorphic rocks are parallel to those observed the latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites from these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere the direction of thrusting is given by the lineation trend and its sense by the analysis of shear all these data are combined into a paleogeographic scheme for the newfoundland caledonides

the significance of microgranitoid enclave shapes and orientations abstract enclaves are often incorrectly used to measure magmatic strains in plutons we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons 1 adjacent enclaves may form at different times and different places and initially have non spherical shapes with axial ratios up to 2 7 2 the final shapes and orientations of enclaves are a complex function of a initial shape and temperature of enclaves b subtle changes in composition melt percents volatiles grain sizes and thus temporally variable viscosity contrasts between the enclave and magma c a competition between strain and interfacial energies and d deformation path which may include internal strain and rigid rotations caused by magma flow during ascent convection expansion chamber boundary processes and tectonism 3 enclaves spend much of their time in magma as relatively rigid objects and thus rigidly rotate and potentially break apart rather than strain at matrix strain rates 4 in some instances enclaves do not record or track finite strain because of the above final enclave populations are heterogeneous and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses on the other hand a comparison of the preserved characteristics of igneous layering mineral fabrics and carefully evaluated enclave fabrics including internal mineral alignment in enclaves may provide qualitative data on the changing magnitude and kinematics of magmatic strains

evidence from gabbro of the troodos ophiolite for lateral magma transport along a slow spreading mid ocean ridge the lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid ocean ridge segmentation and variations in crustal thickness direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust but such studies can be made on ophiolites fragments of oceanic crust accreted to a continent in the oman ophiolite small scale radial patterns of flow have been mapped along what is thought to be the relict of a fast spreading mid ocean ridge here we present evidence for broad scale along axis flow that has been frozen into the gabbro of the troodos ophiolite in cyprus thought to be representative of a slow spreading ridge axis the gabbro suite of troodos spans nearly 20 km of a segment of a fossil spreading axis near a ridge transform intersection we mapped the pattern of magma flow by analysing the rocks magnetic fabric at 20 sites widely distributed in the gabbro suite and by examining the petrographic fabric at 9 sites we infer an along axis magma flow for much of the gabbro suite which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow spreading mid atlantic ridge

wavelet transform a future of rock fabric analysis abstract although twenty years ago fabric was defined as the complete spatial and geometrical configuration of all those components that make up a deformed rock fabric was mainly synonymous with lattice preferred orientation very little attention has been paid to the multi scale geometrical and spatial relationships of the rock components fabric quantification in terms of size shape orientation and location at all scales is now performed in two dimensions using anisotropic wavelets as a first example the wavelets are applied to the famous colour plate of sander and the results compared to his axial distribution analysis applied to the k feldspars of a rock section from the sidobre granite pluton montagne noire france the wavelet analysis shows i alignment of grains resulting from mechanical interactions between grains and shows that shearing occurred within the crystalline frame ii preferred grain orientation the classical grain shape fabric parallel to the overall mineral lineation and iii small tensional domains elongate perpendicular to the lineation infilled by the residual melt just before total crystallisation attesting to the stretching nature of the mineral lineation the future of rock fabric analysis will come from new steps in understanding the processes acting during fabric development along with a further development of wavelet analysis using high resolution three dimensional fabric data

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow