structural reactivation in plate tectonics controlled by olivine crystal anisotropy structures formed during ancient tectonic events are commonly reactivated during subsequent tectonism numerical models point to mechanical anisotropy arising from the inherited orientation of crystals of the mineral olivine in the lithospheric mantle as the cause of this behaviour

slip rate dependent melt extraction at oceanic transform faults crustal thickness differences between oceanic transform faults and associated mid ocean ridges may be explained by melt migration and extraction processes slow slipping transform faults exhibit more positive gravity anomalies than the adjacent spreading centers indicating relative thin crust in the transform domain whereas at intermediate spreading and fast spreading ridges transform faults are characterized by more negative gravity anomalies than the adjacent spreading centers indicating thick crust in the transform domain we present numerical models reproducing these observations and infer that melt can be extracted at fast slipping transforms but not at slow slipping ones melt extraction is modeled as a three step process 1 melt moves vertically through buoyancy driven porous flow enhanced by subvertical dissolution channels 2 melt accumulates in and travels along a decompaction channel lining a low permeability barrier at the base of the thermal boundary layer 3 melt is extracted to the surface when it enters a melt extraction zone a melt extraction width of 2 4 km and a melt extraction depth of 15 20 km are needed to fit the tectonic damages associated with oceanic plate boundaries that reach into the upper mantle our conclusions are supported by the different degrees of magmatic activities exhibited at fast slipping and slow slipping transforms as reflected in geological features geochemical signals and seismic behaviors we also constrain that the maximum lateral distance of crust level dike propagation is about 50 70 km

high pressure metamorphism of the lanzo peridotite and its oceanic cover and some consequences for the sesia lanzo zone northwestern italian alps abstract this paper presents new data on the alpine high pressure evolution of the lanzo peridotite massif and its metasedimentary cover western alps field relations in the northwestern lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate metabasalt calcschist metaquartzite and gneiss remnants of granulite facies mafic gneiss associated with metaophicarbonate are found within the serpentinite major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments bell shaped mn zoning in garnet from metaquartzite indicates a one stage alpine metamorphic evolution while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high grade metamorphic conditions in pre alpine times similar to kinzigites within the neighbouring sesia zone phase equilibria in metaquartzite and mafic gneiss point to 500 50 c at pressures of 0 9 to 1 3 gpa for the alpine metamorphic equilibration that contrasts with the 550 620 c and pressures in excess of 2 0 gpa obtained for kyanite talc chloritoid and talc chloritoid garnet domains in a metagabbro dike from the internal part of the lanzo massif these different p t trajectories can be explained by a much stronger retrograde overprint at the basement cover contact than in the internal part of the lanzo massif

folding of mylonitic zones in cordilleran metamorphic core complexes evidence from near the mylonitic front metamorphic core complexes of north america are asymmetric when viewed perpendicular to the direction of tectonic transport on one side mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper plate transport and the mylonitic rocks have a down dip sense of shear on the opposite side the mylonitic zone rolls over to a dip opposed to the direction of upper plate transport diverges downward from the detachment fault and dies out upward across a mylonitic front new kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization

geochemistry of the merensky reef rustenburg section bushveld complex controls on the silicate framework and distribution of trace elements this whole rock and silicate mineral study focuses on the genesis of the merensky reef sequence as well as the footwall and hanging wall norites at an area of rustenburg platinum mines in a demonstrably normal undisturbed environment continuous sampling provides major and trace element variations and mineral compositions and allows an evaluation of the post liquidus processes which affected the sequence following the formation of liquidus phases three stages are envisaged to have modified the rocks these are a migration of fluid during early compaction of cumulates b circulation of fluids within the crystal mush and c reaction and solidification of trapped liquid liquidus compositions are nowhere preserved in the sequence a strong link is demonstrated between orthopyroxene compositions e g mg and tio2 and the incompatible trace element content of the whole rocks the final amount of trapped liquid is shown to have been variable but never exceeded 10 calculated liquidus pre equilibration orthopyroxene compositions show an up sequence progression of evolving compositions from the footwall norite to the hanging wall norite initial sr isotopic values do not support a simple magma mixing model by which radiogenic main zone magma mixes with that of the critical zone at the level of the merensky reef there is evidence that the hanging wall norite formed from a much more evolved magma these conclusions have implications for the distribution and origin of the pge enriched merensky reef package

role of late magmatic fluids in merensky type platinum deposits a discussion a number of features of platinum group element pge rich layers in layered intrusions provide evidence for the involvement of late magmatic fluids the presence of pegmatitic textures intergrowth of sulfides with intercumulus volatile rich phases high cl content in volatile bearing phases and association with graphite and fluid inclusions have been taken by some authors as evidence for postcumulus transport and deposition of pge by a fluid phase of late magmatic derivation we argue that these apparently hydrothermal features are the result of postcumulus processes superimposed on layers whose extensive uniform pge concentrations were formed by magmatic cumulus processes the association with pegmatites and volatiles arises from the relatively low solidus temperatures of orthocumulate layers which cause these layers to act as traps for fluids migrating through the crystal pile these fluids are mostly derived by vapor exsolution from fractionated intercumulus melt and are consequently enriched in ci redissolution of these fluids into vapor undersaturated trapped liquid in orthocumulate layers causes recrystallization and formation of gabbro pegmatites immiscible sulfide oxide liquid solidifies at near solidus temperatures in the interstitial pore space between early crystallizing silicate minerals and it is forced into areas occupied by late crystallizing ci bearing silicates this accounts for intergrowths of cu and pge rich sulfides with amphiboles and micas a ubiquitous feature of sulfide bearing gabbros further exsolution of a vapor phase during late stages of crystallization causes deuteric alteration formation of fluid inclusions and possible local redistribution of sulfides and pge cooling of this vapor phase may lead to precipitation of graphite at or below the orthocumulate solidus graphite deposition may be catalyzed by sulfides and pge

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

fluid assisted deformation and strain localization in the cooling mantle wedge of a young subduction zone semail ophiolite this study reports on feedback mechanisms between fluid migration ductile deformation and strain localization processes in an incipiently forming mantle wedge the basal banded unit of the semail ophiolite these peridotites were located right above the plate interface during intraoceanic subduction infancy that ultimately led to ophiolite obduction during this stage they were affected by coeval ductile deformation forming proto mylonites at 900 800 c to ultramylonites at 700 c and interaction with subduction fluids from the petrological and microstructural study of these hydrated peridotites and their protolith preserved lenses of porphyroclastic tectonites we show that peridotite interaction with hydrous fluids triggered dissolution precipitation processes dissolution of coarser grains and precipitation of new and smaller ones mainly pyroxenes spinel and amphibole resulted in a drastic grain size reduction phase mixing and a switch from olivine dislocation to grain size sensitive creep in proto mylonites data also evidence a feedback process of fluid focusing in actively deforming shear zones this rapid switch in olivine deformation mechanism driven by subduction fluid peridotite interaction triggered an intense weakening of the peridotites at 900 800 c and a transition from a mechanically decoupled to coupled plate interface as witnessed by the detachment and underplating of a high temperature metamorphic sole to the base of the ophiolite it also explains the intense strain localization 1 km thick shear zone along this ductile portion of the plate interface considering that similar mechanisms take place in mature subduction zones they may explain plate interface coupling at subarc depths in worldwide subduction zones

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

an experimental study of ostwald ripening of olivine and plagioclase in silicate melts implications for the growth and size of crystals in magmas we carried out an experimental study to characterize the kinetics of ostwald ripening in the forsterite basalt system and in the plagioclase an65 andesite system eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution csd with time t the experiments were performed in a 1 atmosphere quench furnace at 1 250 c for plagioclase and 1 300 c for olivine very contrasted coarsening kinetics were observed in the two series in the plagioclase series the mean grain size increased as log t from 3 m to only 8 7 m in 336 h the kinetic law in log t means that ostwald ripening was rate limited by surface nucleation at plagioclase liquid interfaces in the olivine series the mean grain size increased as t1 3 from 3 m to 23 2 m in 496 h a kinetic law in t1 3 is expected when ostwald ripening is rate limited either by diffusion in the liquid or by grain growth dissolution controlled by a screw dislocation mechanism the shape of olivine csds in particular their positive skewness indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism not by diffusion as the degrees of undercooling t or supersaturation involved in ostwald ripening are essentially 1 c the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers we extrapolated our experimental data to geological time scales to estimate the effect of ostwald ripening on the size of crystals in magmas in the case of plagioclase ostwald ripening is only efficient for mean grain sizes of a few microns to 20 m even for a time scale of 105 years it can however result in a significant decrease of the number of small crystals per unit volume and contribute to the development of convex upwards csds for olivine the mean grain size increases from 2 3 m to 70 m in 1 year and 700 m in 103 years a mean grain size of 3 mm is reached in 105 years accordingly the rate of grain size dependent processes such as compaction of olivine rich cumulates or melt extraction from partially molten peridotites may significantly be enhanced by textural coarsening

microstructural evolution and rheology of quartz in a mid crustal shear zone abstract we present microstructural and crystallographic preferred orientation cpo data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization deformation processes and cpo evolution the sample investigated here is a moderately deformed quartz rich mylonite from the blue ridge in virginia we have created high resolution crystallographic orientation maps using electron backscatter diffraction ebsd of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100 recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast we document a systematic decrease in fabric intensity with recrystallization suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre existing cpo relationships between recrystallization fraction and shear strain suggest that complete microstructural re equilibration requires strains in excess of 5 variation in the degree of recrystallization implies that strain was accumulated heterogeneously and that a steady state microstructure and rheology were not achieved

deformation in a partially molten mantle constraints from plagioclase lherzolites from lanzo western alps abstract we studied the deformation in a partially molten mantle through detailed petrostructural analysis of plagioclase lherzolites from the northwestern part of the central lanzo massif these peridotites display a pervasive anastomosed network of millimetre to centimetre scale plagioclase rich bands which grades locally into a planar layering marked by an intercalation of plagioclase rich and olivine rich layers up to tens of centimetres wide both the anastomosed and the planar layering are subparallel to the deformation fabric characterized by a foliation defined by the shape preferred orientation of olivine and by crystal preferred orientation cpo of olivine orthopyroxene and clinopyroxene this parallelism the coexistence of microstructural evidence for melt rock reactions and for deformation by dislocation creep and the predominance of axial 010 olivine cpo patterns characterized by 010 axes clustered normal to the layering indicate that deformation and the reactive melt percolation that formed the layering were coeval strong heterogeneity in mineral composition in the planar layering domains with fe enrichment in olivine and spinel and highly variable ti content in spinel in cm scale plagioclase rich bands implies that the latter interacted with higher melt volumes high ti contents in pyroxenes and spinel in the anastomosed layering domains point to changes in melt composition hinting to less effective melt transport we propose therefore that the planar layering records melt segregation in layers parallel to the shear plane whereas the anastomosed layering results from melt alignment along grain boundaries subparallel to the shear plane without segregation finite strain in the different layers cannot be quantified but comparison of the present observations with data from experiments and from other mantle outcrops displaying shear controlled melt organization suggests that the transition from the anastomosed to the planar layering might record an increase in finite strain that is strain localization associated with variations in the instantaneous melt fraction

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

chloritoid bearing assemblages in eclogitised metagabbros of the lanzo peridotite body western italian alps abstract dykes of metagabbro from the lanzo peridotite body in the western italian alps contain mg chloritoidtalc garnet na pyroxenes chlorite or fe chloritoid glaucophane garnet na pyroxenes eclogitic assemblages of cretaceous age in these metagabbros the metamorphic equilibration is incomplete and controlled by local equilibrium conditions here the chemical gradients which document the magmatic microstructural domains and the diffusion patterns during metamorphism survive the eclogitic crystallisation concentration of elements like al mg and fe near the boundaries between the magmatic domains favours the crystallisation in reaction coronas of mg rich chloritoid in mg metagabbro and of fe rich chloritoid in fe ti metagabbro phase compatibilities in the al2o3 feo mgo sio2 h2o system suggest temperatures around 400 500 c at approximate pressures of 15 18 kbar for the eclogitic metamorphism of the lanzo rocks in spite of the difficulties and of the inaccuracies of these thermobarometric estimates the inferred p t values are in good agreement with determinations involving na pyroxenes and garnets

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

control of shear zone location and thickness by initial grain size variations in upper mantle peridotites abstract the turon de tecouere peridotite north pyrenean fault zone france contains protomylonites grading to ultramylonites grain size reduction to 0 005 0 025 mm took place by reaction during deformation within the protomylonite domain the size of the porphyroclasts suggests an initial grain size in this domain of 2 10 mm in the ultramylonite domain porphyroclasts are polycrystalline with an internal grain size of 0 1 0 2 mm indicating that the ultramylonite domain had a finer initial grain size than the protomylonite domain lattice preferred orientations and misorientations within a polycrystalline porphyroclast in the ultramylonite domain suggest an early episode of dislocation creep accommodated dynamic recrystallization restricted predominantly to the present day ultramylonite domain later grain size reduction by reaction resulting in very fine grained ultramylonites was concentrated in the same domain for reaction involved deformation initial grain size may be crucially important as it controls the surface area available for nucleation of new grains as a result of the finer initial grain size within the present day ultramylonites a critical percentage of fine grained matrix developed in the present day ultramylonite domain earlier than in the present day protomylonite domain resulting in localization of further deformation in the ultramylonites this study suggests that zones of locally smaller grain size can act as heterogeneities that control the location and width of shear zones formed by reaction softening

a simple parameterization of strain localization in the ductile regime due to grain size reduction a case study for olivine we propose a simple parameterization of the transition between dislocation creep and grain size sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine rich rocks through numerical modeling and linear stability analysis we determine the conditions under which this transition takes place and potentially leads to strain localization we pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas we conclude that at constant temperature strain localization can only take place if the rheology of the material is nonlinearly related to grain size that strain localization is facilitated by syndeformation cooling that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place and that grain growth inhibits strain localization at fast cooling rates but may lead to grain growth localization at low cooling rates we draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the earth s surface

grain coarsening maps a new tool to predict microfabric evolution of polymineralic rocks polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments as long as no mineral reactions occur the grain coarsening in such rocks is complex because the different phases influence each other and it is this interaction that controls the rate of grain coarsening of the entire aggregate we present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information based on samples from polymineralic carbonate mylonites that were deformed at different temperatures we demonstrate how the mathematical relation can be calibrated for this natural system using such data sets for other lithologies grain coarsening maps can be generated which allow the prediction of microstructural evolution in polymineralic rocks such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths such as structural geology geophysics geodynamics and metamorphic petrology

adcumulus dunite growth in a laboratory thermal gradient laboratory experiments near 1450 c at 1 bar qfm on komatiite bulk composition show olivine and liquid in cumulus textures which evolve with experiment duration orthocumulus texture with settled olivine crystals separated by liquid matrix is developed within a day experiments quenched after a few days to a week show a progression of textures which include development of columns of olivine crystals separated by channels of liquid olivine grain sizes increase with the cube root of time suggesting that dissolution and reprecipitation of olivine may be involved in the organization into columns and channels experiments quenched after two weeks have well developed adcumulus texture the basal polycrystalline granular olivine aggregate forms from the decay of the olivine columns melt expulsion from the aggregate can be virtually complete leaving 1 or less of the melt originally present

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

anatomy of an extensional shear zone in the mantle lanzo massif italy analysis of the microstructures in the km scale mantle shear zone that separates the northern and the central parts of the lanzo peridotite massif provides evidence of an evolution in time and space of deformation processes accommodating shearing in the shallow mantle within an extensional setting this shear zone displays an asymmetric distribution of deformation facies from south to north gradual reorientation of the foliation of coarse porphyroclastic plagioclase bearing peridotites is followed by development of protomylonites mylonites and mm scale ultramylonite bands a sharp grain size gradient marks the northern boundary early deformation under near solidus conditions in the south is recorded by preservation of weakly deformed interstitial plagioclase and almost random clinopyroxene and plagioclase crystal orientations feedback between deformation and melt transport probably led to melt focusing and strain weakening in the shear zone overprint of melt rock reaction microstructures by solid state deformation and decrease in recrystallized grain size in the protomylonites and mylonites indicate continued deformation under decreasing temperature less enriched peridotite compositions and absence of ultramafic dykes or widespread melt impregnation microstructures north of the shear zone and clinopyroxene and amphibole enrichment in the mylonites and ultramylonites suggest that the shear zone acted as both a thermal barrier and a high permeability channel for late crystallizing fluids these observations together with chemical data indicating faster cooling of central lanzo relative to the northern body corroborate that this shear zone is a mantle detachment fault all deformation facies have crystal preferred orientations consistent with deformation by dislocation creep with dominant activation of the 010 100 and 100 001 systems in olivine and orthopyroxene respectively dynamic recrystallization produces dispersion of olivine cpo but not a change of dominant deformation mechanism evidence for activation of grain boundary sliding is limited to mm scale ultramylonite bands where solid state reactions produced very fine grained polymineralic aggregates except for these latest stages of deformation strain localization does not result from the microstructural evolution the grain size decrease is a consequence of the need to deform a rock volume whose strength continuously increases because of decreasing temperature conditions strain localization in the intermediate levels thus essentially results from the more localizing behavior of both the deep partially molten and shallow parts of this extensional shear zone distribution

the petrological significance of misorientations between grains misorientation analysis quantifies microstructural features in tectonites metamorphic and igneous rocks and allows hypotheses on their formation to be tested the misorientation between two lattices can be expressed by a rotation axis and rotation angle for lattices with symmetry it is conventional to take the minimum angle that enables one lattice to be rotated into the other for a group of lattice measurements two types of misorientation distribution can be calculated selecting random pairs of grains gives the random pair misorientation distribution selecting neighbouring pairs gives the neighbour pair misorientation distribution the forms of both distributions are visualised using histograms or cumulative frequency diagrams they are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry in many rocks the random pair misorientation distribution and neighbour pair misorientation distribution are statistically significantly different quantified using the kolmogorov smirnov test differences between the random pair misorientation distribution and neighbour pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure interactions include 1 reduction in surface energy by lattice alignment we show this may have occurred in garnet clusters in schist and olivine in a cumulate it is well known in metals and may be a common geological process 2 nucleation where those nuclei have influenced the orientation of adjacent nuclei 3 mechanical rotations of facetted grains in compacting crystal mushes so that faces become parallel 4 growth twinning inheritance includes 1 subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes 2 mechanical and transformation related twinning 3 domainal microstructures e g where grains have formed from a few large original grains may give rise to spurious correlations when the orientation data cover more than one domain with this proviso misorientation analysis can be used to investigate many important microstructural processes

trace elements in the merensky reef and adjacent norites bushveld complex south africa trace elements were analysed in rocks and minerals from three sections across the merensky reef in the rustenburg platinum mine in the bushveld complex of south africa whole rocks and separated minerals were analysed by inductively coupled plasma mass spectrometer icp ms and in situ analyses were carried out by ion microprobe and by laser source icp ms merensky reef pyroxenites contain extremely high concentrations of a wide range of trace elements these include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements for major elements and compatible trace elements the measured concentrations in cumulus phases and the bulk rock compositions are similar for highly incompatible elements however concentrations in bulk rocks are far higher than those measured in the cumulus phases in situ analyses of plagioclase have far lower concentrations of th zr and rare earth elements than icp ms analyses of bulk separates of plagioclase a difference that is attributed to the presence of trace element rich accessory phases in the bulk mineral separates we used these data to calculate the trace element composition of the magmas parental to the merensky unit and adjacent norites we argue that there is no reason to assume that the amount of trapped liquid in the merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites we propose that the bushveld complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks the magma that gave rise to the merensky unit was the more contaminated and unusually rich in incompatible trace elements and when it entered the main bushveld chamber it precipitated the unusual phases that characterize the merensky reef the hybrid magma segregated sulphides or platinum group element rich phases during the course of the contamination in the lower chamber these phases accumulated following irruption into the main bushveld chamber to form the merensky ore deposits

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

origin of the pegmatitic pyroxenite in the merensky unit bushveld complex south africa the genesis of the pegmatitic pyroxenite that often forms the base of the merensky unit in the bushveld complex is re examined large 1 cm orthopyroxene grains contain tricuspidate inclusions of plagioclase and chains and rings of chromite grains which are interpreted to have grown by reaction between small primary orthopyroxene grains and superheated liquid this superheated liquid may have been an added magma or be due to a pressure reduction as a result of lateral expansion of the chamber there would then have been a period of non accumulation of grains permitting prolonged interaction with the crystal mush at the crystal liquid interface crystal ageing and grain enlargement of original orthopyroxene grains would ensue only after the pegmatitic pyroxenite had developed did another layer of chromite and pyroxenite with normal grain size accumulate above it immiscible sulphide liquids formed with the second pyroxenite but percolated down as a result of their density contrast even as far as the footwall anorthosite in some cases whole rock abundances of incompatible trace elements in the pegmatitic pyroxenite are comparable with or lower than those of the overlying pyroxenite and so there is no evidence for addition and or trapping of large proportions of interstitial liquid or of an incompatible element enriched liquid or fluid in the production of the pegmatitic rock because of the coarse grained nature of the rock modal analysis especially for minor minerals is unreliable annealing has destroyed primary textures such that petrographic studies should not be used in isolation to distinguish cumulus and intercumulus components geochemical data suggest that the merensky pyroxenite both pegmatitic and non pegmatitic typically consists of about 70 80 cumulus orthopyroxene and 10 20 cumulus plagioclase with a further 10 of intercumulus minerals and could be considered to be a heteradcumulate

shear zones in the upper mantle evidence from alpine and ophiolite type peridotite massifs there is abundant field and microstructural evidence for localization of deformation in alpineand ophiolite type mantle massifs on the basis of field relationships and microstructures we recognize two types of tectonite shear zones mediumto coarseand fine grained as well as two types of mylonitic shear zones anhydrous and hydrous peridotite mylonites in tectonite shear zones softening processes responsible for localization are probably melt related weakening in the medium to coarse tectonites and a change in limiting slip system in the fine grained tectonites in peridotite mylonites the most likely cause for softening and localization is a change in dominant deformation mechanism from dislocation to grain size sensitive creep microstructural and petrological study of mylonite rocks reveals that reactions either continuous net transfer reactions anhydrous and hydrous or melt rock reactions play a key role in the formation of fine grained material that promotes grain size sensitive creep these reactions occur over a broad range of pressure temperature conditions encompassing a large part of the lithospheric upper mantle we conclude that mantle shear zones are widespread and that they reduce the bulk strength of the lithosphere significantly attempts to understand the dynamics of tectonic processes depend critically on quantitative knowledge concerning the mechanical properties of the lithosphere and the relative strength of the crust and upper mantle e g brun 2002 for many years it has been accepted that the upper part of the mantle lithosphere is strong compared to the crust e g kirby 1983 kuznir park 1986 the notion of a strong uppermost mantle is based on extrapolation of laboratory flow laws kohlstedt et al 1995 and the depth distribution of seismicity in the lithosphere chen rc molnar 1983 the inferred high strength of the shallow mantle implicitly refers to homogeneous flow however at large strains deformation of the mantle lithosphere may become localized within weak shear zones fig 1 so the application of low strain flow laws may seriously overestimate the strength of the mantle rutter brodie 1988 handy 1989 vissers et 01 1995 in a recent study on the seismicity and elastic thickness of continental lithosphere magi et al 2000 have suggested that the crust is in fact the strongest layer thus it is emerging that the strength of lithospheric upper mantle may vary considerably over the last decade we have studied a number of peridotite massifs both of orogenic alpine type and ophiolitic origin in order to establish under which conditions and to what extent deformation in mantle rocks is localized in this contribution we present an overview of shear localization features in these peridotite massifs from our data as well as published data from other workers it emerges that under lithospheric conditions localization of deformation in mantle rocks is a common phenomenon fig 1 table i in addition shear localization seems to occur over a wide range of pressure temperature conditions producing a number of distinct shear zone types the range of shear zone types is comparable to types of deformation structures found in crustal rocks e g carreras et al 1980 microstructural work has highlighted the importance of mineral reactions as a softening mechanism in the development of mantle mylonite shear zones handy 1989 drury et al from alsop g i holdsworth r e mccaffrey k j w hand m eds 2004 flow processes in faults and shear zones geological society london special publications 224 11 24 0305 8719 15 00 o the geological society of london 2004 12 a h dijkstra eta l

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

the effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions abstract spinel peridotite xenoliths from persani mountains southeastern carpathians romania were submitted to deformation by dislocation creep accompanied by dynamic recrystallization under variable stress and temperature conditions predominance of low angle boundaries parallel to 100 with well defined 0vw rotation axes in olivine indicates that subgrain rotation is the main recrystallization mechanism and that dislocation glide occurs mainly in 100 0kl systems analysis of olivine crystal preferred orientations highlights that recrystallization results in dispersion of the orientations of recrystallized grains relatively to the parent grains this dispersion may be quantified by a dimensionless dispersion factor defined as the ratio of the random component of the 100 axis distributions between the recrystallized grains and porphyroclasts this factor is largely independent from the overall fabric strength the number of grains analyzed and the recrystallized grain size the olivine cpo strength and anisotropy at the rock scale quantified by j index are largely controlled by the volume of recrystallized grains which depends on finite strain comparison of olivine cpo in coarse and fine grained porphyroclastic peridotites shows that deviatoric stress and temperature conditions play nevertheless a role on the cpo evolution during recrystallization selective grain growth which is favored at high temperature and low stresses does counteract the dispersion produced by the nucleation processes

origin of oceanic ferrodiorites by injection of nelsonitic melts in gabbros at the vema lithospheric section mid atlantic ridge abstract oxide gabbros are a minor but diffuse component of the lower oceanic crust their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models here we report on a study of lower crust gabbros recovered from the vema lithospheric section at 11 n along the mid atlantic ridge where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusives and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in fe ti oxides and apatites their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of fe ti p saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing de compaction mineral resorption and chemical re equilibration melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment we propose a regime of lateral instead of vertical melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of fe ti p melt pockets in oceanic domains

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

enso related variability of the southern hemisphere winter storm track over the eastern pacific atlantic sector abstract the interannual variability associated with the enso of winter storm tracks over the region extending from the eastern south pacific ocean across south america to the south atlantic ocean is described using 39 yr of data from the ncep reanalysis data base tropical sea surface temperature anomalies associated with enso induce large scale atmospheric circulation anomalies over large areas of the southern hemisphere in particular positive height anomalies dominate the bellingshausen sea during warm events and consistently a weakening of the eddy activity at low levels is found to the west of the antarctic peninsula during el nino en the storm track evidences an equatorward shift over the subtropical pacific ocean and a slight strengthening in the central atlantic ocean time lagged correlation analysis applied to anomalies of the 300 hpa meridional wind at selected base points was used to study the structure and propagation characteristics of the waves during en waves emanating from the at

the deformation record of olivine in mylonitic peridotites from the finero complex ivrea zone separate deformation cycles during exhumation mylonitic peridotites from the finero complex are investigated to detect characteristic olivine microfabrics that can resolve separate deformation cycles at different metamorphic conditions the heterogeneous olivine microstructures are characterized by deformed porphyroclasts surrounded by varying amounts of recrystallized grains a well developed but only locally preserved foam structure is present in recrystallized grain aggregates this indicates an early stage of dynamic recrystallization and subsequent recovery and recrystallization at quasi static stress conditions where the strain energy was reduced such that a reduction in surface energy controlled grain boundary migration ultramylonites record a renewed stage of localized deformation and recrystallization by a second generation of recrystallized grains that do not show a foam structure this second generation of recrystallized grains as well as sutured grain and kink band boundaries of porphyroclasts indicate that these microstructures developed during a stage of localized deformation after development of the foam structure the heterogeneity of the microfabrics is interpreted to represent several at least two cycles of localized deformation separated by a marked hiatus with quasi static recrystallization and recovery and eventually grain growth the second deformation cycle did not only result in reactivation of preexisting shear zones but instead also locally affected the host rock that was not deformed in the first stage such stress cycles can result from sudden increases in differential stress imposed by seismic events i e high stress loading rates during exhumation of the finero complex