on the rheology of partially molten synthetic granite abstract in an attempt to obviate some of the difficulties inherent in using natural rocks to study experimentally the rheology of partially molten granitic rocks we report some preliminary experiments using hot pressed synthetic aggregates with quartz as the solid phase and a quartz albite glass to form the melt phase undrained constant displacement rate and stress relaxation experiments were performed at 1100 c and 300 mpa total confining pressure using nominally 16 and 28 wt anhydrous melt and at 980 c with 12 wt of water saturated melt strain rate varies non linearly with stress corresponding to a stress exponent of 3 6 for the anhydrous melt tests and 1 9 with the wet melt strain rate is enhanced about 160 fold through doubling the melt fraction from 16 wt intracrystalline plasticity appears to contribute insignificantly to the observed deformation and the preservation of relatively equant grains implies a granular flow process microcracking occurs during isostatic hot pressing and also contributes to subsequent non isostatic deformation at high stresses and strain rates models of flow of partially molten rock that assume diffusive transfer accommodation of granular flow predict linear viscosity but only at lower strain rates than accessed experimentally it is suggested that the non linear granular flow observed in these experiments involved the episodic unsticking of sintered contacts between grains by a rate dependent cracking and or dislocation accommodation process

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

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

the xigaze ophiolite tibet a peculiar oceanic lithosphere the xigaze ophiolite which outcrops along the yarlung zangbo river southern tibet locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north to fresh cr diopside rich harzburgites to the south in contrast with other ophiolites the mafic part of the sequence is particularly thin it is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava flows or pillow lavas the ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh cr diopside rich harzburgites the origin of this peculiar ophiolite of oceanic crust is discussed

the role of strain heating in the evolution of megathrusts on the basis of elementary energy considerations and the nonlinear temperature dependence of experimental flow laws a model is developed for the interplay of thermal and mechanical processes along megathrust systems the model is based on the observation that thrust systems involving large portions of the lithosphere have a two part history during the early history of movement dry lower crustal upper mantle rocks are deformed in a shear zone where because of the nonlinear effect of temperature dependent diffusion strain heating rates are directly controlled by the initial thermal state that is heating is insignificant if initial temperatures are high but where temperatures are low large temperature increases are possible as a result strain heating acts as a thermal buffer such that upon arrival of upper crustal rocks at depths of 20 km or more the temperatures along the thrust would be controlled largely by the rheology of hanging wall rocks rather than the initial thermal state moreover the relatively high melting temperatures for dry lower crustal upper mantle rocks implies that the temperatures achieved along the thrust should be relatively high as thrusting progresses this early history is successively eliminated as wet upper crustal rocks are emplaced beneath the preheated hanging wall the granitic melts that characterize many natural megathrust systems are probably generated when these wet metasedimentary rocks move beneath the hanging wall either by hydrous fluxing of the hot hanging wall or by melting in the highest grade footwall rocks rapid uplift implied by both metamorphic studies and preservation of the inverted metamorphic sequence is interpreted as a late phase event created by tectonic pileup beneath the thrust thorough testing of the model will require thorough assessment of the time dependent problem however one dimensional steady state models together with energy considerations indicate that the general model is allowable the concept of thermal buffering by strain heating is important because it implies that megathrust systems are a natural system where metamorphic temperatures are controlled by rock rheology hence they represent real world systems where a thermal and mechanical system are intimately linked this rationale is used to extend the model by indicating a method in which metamorphic conditions along exhumed megathrusts might be used as a natural constraint on experimentally determined flow laws

equatorial and low latitude ionosphere thermosphere system response to the space weather event of august 2005 1 the response of the equatorial and low latitude ionosphere thermosphere system to the geomagnetic storm during 23 26 august 2005 is investigated the study is carried out using the vertical total electron content vtec measured by gps receivers along the 77 78 e longitude and the o n2 ratio obtained from the global ultraviolet imager instrument on board the timed satellite the equatorial ionization anomaly eia shows a poleward latitudinal expansion on 24 august probably associated with an eastward prompt penetration electric field the equatorial and low latitudes show an increase in vtec during the main phase of the storm on 24 august the prompt penetration eastward electric field along with an increase in the o n2 ratio could be responsible for this observed positive phase the vtec variations on 24 august also reveal the signature of a large scale acoustic gravity wave traveling atmospheric disturbance propagating with a horizontal velocity of 750 m s suppression of the eia is observed during the storm recovery phase on 25 august which probably is associated with the westward disturbance dynamo electric field and the equatorward expansion of the neutral composition changes o n2 depletion

generation and obduction of ophiolites constraints from the bay of islands complex western newfoundland constraints provided by the bay of islands ophiolite complex boic western newfoundland suggest that its generation as well as its obduction is related to collision of the dunnage zone island arc system with the irregular continental margin of eastern north america the constraints include the supra subduction zone setting of the boic its extensional environment of generation the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the north american continental margin and the restriction of the boic as well as other appalachian ophiolites and associated taconian allochthons to reentrants in the north american margin during the early phases of collision of the dunnage arc with the north american margin subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation the first develops through trench rollback leading to ophiolite generation in the overriding arc plate outboard of the reentrant whereas the second develops in a transtensional environment with spreading centers offset by strike slip faults which propagate through the overriding plate away from the indenter promontory the configuration of the boic spreading ridge normal to the continental margin and the direction of initial obduction parallel to the margin favor generation of the ophiolite in an overall strike slip regime with complete subduction of the oceanic remnant extension in the overriding plate ceases and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the dunnage zone and for termination of arc volcanism along the north american margin

influences of chemical degradation on mechanical behaviour of a limestone this paper is devoted to the experimental investigation of chemical degradation effects on mechanical behaviour of a porous limestone hydrostatic and triaxial compression tests are first performed respectively on sound and chemically degraded samples it is found that the chemical degradation induces the diminution of pore collapse limit stress elastic modulus and material cohesion coupled triaxial creep chemical degradation tests are also performed the chemical degradation enhances time dependant deformation and can generate significant increase in permeability through the formation of wormholes networks

geochemical and geomechanical effects of scco2 and associated impurities on physical and petrophysical properties of permotriassic sandstones germany an experimental approach effects of impurities sox nox and o2 from the extraction processes of the power plants in the scco2 fluids are studied with an experimental approach combining geochemical alteration in high p medium t autoclaves compressional tests in a heated triaxial testing device as well as geochemical and mineralogical analyses before and after the influences of distinct scco2 fluids on 140 70 mm sized samples of reservoir sandstones from the north german basin with carbonate cementation silicate cementation and a combined silicate carbonate cementation on the background of variable lithological compositions the attempt was made to normalize the scattering geomechanical data by the pure co2 behavior measurement data to achieve a direct impression of any differences due to the impurities changes in the chemical composition of the saline fluids were systematically measured during the scco2 alteration experiments in the autoclave systems at distinct pt conditions the development of the silica concentrations and of the ph value in the fluid solution during alteration showed a clear dependency on the fluid composition through systematical analysis of changes on the mineral surfaces using sem microscopy images before and after scco2 impurities alteration experiments or triaxial compression tests corrosion effects were detected but no differences which could be directly related to differences in the fluid composition could be identified in all the types of sandstones visible or measurable changes on mineral surfaces and pore fluid composition respectively take place as an interaction of rock with fluids of scco2 with and without impurities changing deformability and compressive strength of the rocks are the most apparent consequences due to these compositional changes it is supposed that the chemical fluid differences are responsible for these changes also a specific distribution of the different fluids in the ambient pore space is indicated with even undetected consequences for the fluid influence on stress transmission and distribution within the grain structure all work was performed within the project co2 purity for capture and storage cooral supported by the german federal ministry for economic affairs and energy

mylonitic ductile shear zones within tectonites and cumulates as evidence for an oceanic transform fault in the antalya ophiolite s w turkey summary in the coastal ophiolite belt of antalya especially in the massifs of adrasan and cirali tekirova tectonites and cumulates have preserved their primary structures tectonites display a first high temperature low deviatoric stress deformation s1 linked to the diapiric uplift of the asthenosphere below an east west orientated spreading centre a second mylonitic deformation produced under high deviatoric stress and lower temperature conditions produced shear zones of millimetric to kilometric scale throughout the tectonites and even in the cumulates their mean orientation n 150 e is near to perpendicular to the ridge axis and the associated lineation indicates horizontal essentially sinistral strike slip displacement this deformation must have taken place during the final crystallisation of the cumulates as indicated by its relationship to intrusive gabbroic dykelets representing residual liquids of the cumulates the s2 deformation is attributed to a transform fault situated sw of the coastal complexes later tectonic events include thrusting and wrench faulting again sinistral in a north south trending direction

time dependent brittle creep in darley dale sandstone 1 the characterization of time dependent brittle rock deformation is fundamental to understanding the long term evolution and dynamics of the earth s crust the chemical influence of pore water promotes time dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short term failure strength here we report results from a study of time dependent brittle creep in water saturated samples of darley dale sandstone initial porosity 13 under triaxial stress conditions results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress a reduction of only 10 in differential stress results in a decrease in strain rate of more than two orders of magnitude however natural sample variability means that multiple experiments must be performed to yield consistent results hence we also demonstrate that the use of stress stepping creep experiments can successfully overcome this issue we have used the stress stepping technique to investigate the influence of confining pressure at effective confining pressures of 10 30 and 50 mpa while maintaining a constant 20 mpa pore fluid pressure our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures with the creep strain rate reduced by multiple orders of magnitude the influence of doubling the pore fluid pressure however while maintaining a constant effective confining pressure is shown to influence the rate of stress corrosion within the range expected from sample variability we discuss these results in the context of microstructural analysis acoustic emission hypocenter locations and fits to proposed macroscopic creep laws

heat flow heat production and crustal structure in peri atlantic regions abstract a linear relationship is commonly observed between heat flow and heat generation it provides information about the distribution of radioelements with depth over large regions the linear relationship is no more tenable but points still cluster within an elongated trend in which a regression analysis provides the bulk reduced heat flow and a gross estimate of the heat production distribution with depth over the whole considered region when geological provinces share an identical heat flow heat generation relation they form a geothermal province data measurements in western europe suggest that at least two provinces should be considered one can be related to the hercynian crust and the second to the older caledonian crust we suggest that the depth obtained from thermal data corresponds to the depth of the mafic granulite layer western france south and central england share identical geothermal parameters which can be correlated with the depth of the granulitic layer down to 17 18 km data from scotland do not fit the relationship of south and central u k and present a lower slope indicating that the radiogenic enriched layer is shallow 8 10 km this correlates with the seismic structure of the caledonian crust where the granulite layer is observed at this depth data from scotland can be compared to those from norway newfoundland new hampshire and eastern appalachian crust where the same crustal composition is observed

synthesis of palaeozoic deformational events and terrane accretion in the canadian appalachians plate tectonic theory predicts that most deformation is associated with subduction and terrane accretion with some deformation associated with transform transcurrent movements deformation associated with subduction varies between two end members 1 where the tectonic regime is dominated by subduction of oceanic lithosphere containing small terranes a narrow surface zone of accretionary deformation along the subduction zone starts diachronously on the subducting plate at the trench as material is transferred from the subducting plate to the over riding plate and 2 where continent continent collision is occurring a wide surface zone of accretionary deformation starts synchronously or with limited diachronism palaeozoic deformational events in the canadian appalachians correspond to narrow diachronous events in the ordovician and silurian whereas devonian carboniferous and permian deformational events are widespread and broadly synchronous along the western side of the canadian appalachians the taconian deformational event starts diachronously throughout the ordovician and corresponds to the north north west accretion of the notre dame ascot weedon st victor and various ophiolitic massifs volcanic arc and peri arc terranes over cratonic north america within the eastern half of the central mobile belt the late cambrian early ordovician penobscotian deformational event corresponds to the south easterly accretion of the exploits subzone various volcanic are and peri arc terranes over the gander zone continental rise in the centre of the orogen the late ordovician silurian beothukan deformational event corresponds to the south easterly accretion of the notre dame over the exploits gander subzones along the south eastern side of the central mobile belt the silurian ganderian deformational event corresponds to the north north east sinistral transcurrent accretion of the avalon composite terrane microcontinent over the gander exploits zones along the south eastern half of the orogen the late silurian middle devonian acadian deformation event corresponds to the westerly accretion of the meguma terrane intradeep or continental rise over the avalon composite terrane affecting the entire orogen the late devonian carboniferous and permian acadian alleghanian deformational events correspond to the east west convergence between laurentia and gondwana continent continent collision

cufenis mineral assemblages in upper mantle peridotites from the table mountain and blow me down mountain ophiolite massifs bay of islands area newfoundland their relationships with fluids and silicate melts abstract plastically deformed ultramafic rocks in the table mountain and blow me down mountain ophiolites comprise a basal unit of slightly depleted iherzolites an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids thirty five samples including all of the above rock types have been investigated on 100 polished thin sections for cu fe ni s mineral assemblages most of them contain traces of cu fe ni sulfides native metals and locally ni arsenide compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present cu fe ni s minerals derive from an upper mantle sulfide component through extensive subsolidus re equilibration down to 100 c the primitive component predominant pentlandite minor pyrrhotite and chalcopyrite is preserved as sulfide inclusions in chromites of the transition zone due to a subsolidus re equilibration in a closed system on the contrary sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur deficient sulfides such as heazlewoodite and mackinawite and native metals native copper and awaruite microscale variations of redox conditions and the removal of fe from the silicate during serpentinization may account for the peculiar grain by grain equilibrium state of intergranular assemblages in spite of low temperature alteration a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone up to 0 2 by volume the first pattern is consistent with the low melting nature of the sulfide component in mantle melting processes microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone the sulfide enrichment pattern is thus ascribed to the percolation of a sulfur saturated basaltic magma into residual dunites

hyaloclastites and the slope stability of hawaiian volcanoes insights from the hawaiian scientific drilling project s 3 km drill core abstract core samples recovered during the hawaiian scientific drilling project hsdp drilling project reveal that the upper 1 km of the submarine flank of mauna kea is comprised mainly of hyaloclastites progressive very low temperature alteration of these hyaloclastites has been accompanied by systematic transformations in physical properties of these deposits hyaloclastite deposits which directly underlie ca 1 km of subaerially emplaced lavas are very poorly consolidated but over a depth interval of ca 500 m compaction and especially precipitation of zeolitic pore filling cements associated with palagonitization of sideromelane have eliminated porosity as well as promoted the consolidation of these hyaloclastites the latter is reflected in unconfined compressive strengths which increase from mean values respectively of 2 5 and 4 6 mpa in weakly consolidated smectite rich hyaloclastites from the incipient 1080 to 1335 mbsl and smectitic 1405 1573 mbsl alteration zones to a mean value of 10 0 mpa in the more highly consolidated hyaloclastites of the palagonitic zone of alteration from 1573 mbsl to the bottom of the drill hole conversely overlying intercalated and underlying lava flows are generally much less altered and have mean compressive strengths which are 1 to 2 orders of magnitude greater then hyaloclastites at equivalent depths the shear strengths of the hyaloclastites also increase with depth and grade of alteration but are uniformly and substantially lower in the lavas those hyaloclastites exhibiting the highest grade of alteration i e palagonitic also exhibit the highest measured strengths and thus the alteration of hyaloclastites appears to strengthen as opposed to weaken the flanks of the edifice however the contrast in strength between hyaloclastites and lavas may be a primary factor in localizing destabilization and the zones of weak and poorly consolidated hyaloclastites may facilitate slumping by servings as hosts for shallow detachment surfaces on the unsupported flanks of these volcanoes

the peridotites of the kukes ophiolite albania structure and kinematics abstract structural study of the kukes ultramafic massif which belongs to the albanian mirdita zone is of great interest for reconstruction of alpine tectonic environments from bottom to top the following lithological succession is observed in the ultramafic mafic section harzburgites harzburgites with dunitic lenses dunites an intermediate zone and gabbros the ultramafic rocks are apparently weakly deformed the textures indicate a high temperature deformation and slight locally superimposed low temperature deformation this is confirmed by olivine lattice preferred orientation which indicates that 010 100 was the activated glide system in olivine i e at high temperature the kinematics of deformation have been determined from the lattice preferred orientation of olivine an inversion of the shear sense occurs at the base of the dunite zone and is interpreted as being due to asthenospheric flow a simple geometric reconstruction indicates that the kukes ultramafic massif could correspond to the eastern flank of a nnw sse orientated paleo ridge

structure of the xigaze ophiolite yarlung zangbo suture zone southern tibet china genetic implications three massifs in the xigaze area southern tibet china have been preserved from the intense deformation observed everywhere else in the yarlung zangbo suture zone these massifs have similar lithologies and display a remarkable homogeneity in the attitude of most magmatic and tectonic structures such as lava flow planes dolerite dikes and sills and foliation planes in peridotites this indicates that these ophiolites are only slightly dismembered inasmuch as they locally display complete sequences from pillow lavas at the top to fresh cr diopside bearing harzburgites at the base gabbroic rocks are rare and are present as screens between dolerite intrusives or in small sized pockets at the limit between mafic and ultramafic rocks one of the most important feature of this ophiolite is the occurrence of dolerite sills and dikes throughout the whole sequence indicating that this ophiolite results from to two successive magmatic events with 1 creation of the crustal part of the oceanic lithosphere with deposition of minor cumulates in small magma chambers over mantle residual rocks and 2 intrusion of dolerite sills and dikes throughout the whole sequence

kinematics of oceanic thrusting and subduction from basal sections of ophiolites the structural analysis of high stress deformation at the base of the peridotite section in many ophiolites in suture zones enables the kinematics of oceanic convergence responsible for the suture to be defined this deformation results from the thrusting over oceanic crust of a young lithosphere involved in a subduction process flow plane flow line sense of shear related to the thrusting are geometrically deduced from the study of mineral preferred orientations in the basal peridotite and in the quartzites belonging to the amphibolite sole

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

metamorphic evolution and fluid composition of garnet clinopyroxene amphibolites from the tatra mountains western carpathians relics suggesting an early high pressure eclogite facies stage have been identified in the garnet and clinopyroxene bearing amphibolites of the tatra mountains in the variscan basement of the western carpathians slovakia in these rocks primary omphacite cpx i has been wholly converted to symplectites of diopside cpx ii and plagioclase apart from minor inclusions in the garnet cores amphibole and plagioclase are secondary minerals formed in the kelyphitic rims between garnet and clinopyroxene several generations of amphibole pargasite hornblende cummingtonite actinolite are evidence of a transformation down to greenschist facies conditions thermobarometric calculations from mineral inclusions in the garnet cores yield 670 700 c and 10 15 kbar recording the initial path from upper amphibolite to eclogite facies conditions the attainment of the eclogite facies stability field is inferred from the composition of a reconstructed omphacite jd36 implying a minimum peak pressure of 15 16 kbar conditions of around 650 c and 8 10 kbar record the post eclogite breakdown and partial re equilibration in the amphibolite facies region the earliest fluid inclusions contain high density nitrogen dominated water absent fluid younger are polyphase brines and two phase h2o n2 ch4 inclusions with signs of heterogeneous entrapment of coexisting gas rich and water rich immiscible phases pure nitrogen 0 5 mol ch4 is considered to have been the major component during the high pressure metamorphism in contrast the later aqueous inclusions are interpreted to represent retrogression related fluids the brines have originated by leaching and re entrapment of saline melt inclusions observed in tonalitic trondhjemitic layers of the amphibolite enclosing the eclogitic relics a similar mechanism is assumed for the origin of the h2o n2 ch4 inclusions these represent relics of the primary eclogite facies related n2 inclusions re equilibrated and re entrapped during retrogression under increasing water activity a possible source of n2 and cl rich fluids was primary amphibole which decomposed during the prograde metamorphism from the amphibolite to the eclogite facies the metamorphic pt path is generally clockwise reflecting a steep increase in pressure and temperature during burial followed by decompression and cooling the retrograded eclogites occur in boudins within an allochthonous unit of an inverted metamorphic sequence their exhumation was facilitated by tectonic transport along a ductile shear zone during the variscan orogeny

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

the base of a sheeted dyke complex oman ophiolite implications for magma chambers at oceanic spreading axes the sheeted dyke complex in the maydan syncline area of the oman ophiolite was formed during a single sea floor spreading episode in some areas individual dykes from this sheeted dyke complex have been seen to root in the high level gabbro and others are truncated by high level gabbro diorite and trondhjemite however elsewhere dykes penetrate laterally into the layered part of the plutonic sequence this situation suggests that active magma chambers and zones of dyke injection were discontinuous along the spreading axis at any one time

upper mantle peridotites in the bay of islands ophiolite newfoundland formation during the final stages of a spreading centre abstract the structure of upper mantle rocks in ophiolites reflects the high temperature flow beneath an oceanic spreading centre locally overprinted by detachment related strain the structure of the mantle section in the ophiolitic table mountain massif bay of islands complex boic newfoundland was investigated in detail it is correlated with other massifs in the boic and the spreading related mantle flow history is reconstructed six structural units can be recognized on the basis of changes in the attitude of the high temperature mineral stretching lineation the relative age relationships between the structural units can partly be established they support the plate thickening model for oceanic lithosphere formation the structural history of the mantle section suggests that early plate driven flow was followed by forced flow underplating of an off axis diapir to the base of the lithosphere and detachment this complex history is explained in a model of a decreasing spreading rate which tracks the waning stages of spreading before detachment of the ophiolite during this final history the rate of spreading was decoupled from the rate of mantle upwelling so that a large forced flow field could build up in the blow me down bmd massif of the boic all spreading related flow structures are parallel to the ridge suggesting that the structures and the massif were frozen in near the ridge itself the presence of several kilometres of massive dunites in the bmd massif might indicate that in the final stages of spreading melt production was higher than could be accommodated by spreading so that melt ponded at the base of the crust lherzolites occurring at the base of the boic are considered as product of non adiabatic mantle upwelling prevalent in the final stages of spreading

geologie und bau des sithonia ophioliths chalkidiki ne griechenland anmerkungen zur bildung ozeanischer krusten der oberjurassische sithonia ophiolith der chalkidiki ne griechenland bietet die moglichkeit die an der bildung fossiler ozeanischer krusten beteiligten prozesse ausfuhrlich zu untersuchen

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

evaluation of upper mantle microstructures in the table mountain massif bay of islands ophiolite within the undisrupted 6 km thick mantle section of the table mountain massif bay of islands ophiolite newfoundland several distinct peridotite microstructures are recognized in the uppermost mantle section very high temperature t 1200 high strain microstructures with evidence for syntectonic melt infiltration are preserved remarkable among these is an orthopyroxene impregnated dunite at an intermediate level of the mantle section peridotites have been affected by a weak lower temperature t 1000 1100 c overprint they are underlain by a thin sliver of low strain high temperature peridotites all these peridotites are interpreted to have formed during spreading related strain lower temperature t 1000 c high stress microstructures with high accumulated strain are locally preserved at intermediate levels of the mantle section and are regionally present at its base they culminate in metasomatic ultramafic ultramylonites and are related to detachment of the ophiolite with the exception of the basal ultramylonitic peridotites the activated olivine slip system was 0kl 100 in the highest temperature peridotites a weak preference for the activation of distinct slip planes 010 001 011 was noted based on piezometry and olivine flow laws strain rate estimates for the basal detachment fabrics are 10 12 s 1 this strain rate is consistent with the observed width of detachment overprint in the ophiolite and reasonable plate tectonic convergence rates strain rate estimates for the very high temperature peridotites are even higher recent data about grain growth rates in olivine indicate that in order to preserve the peridotite microstructures olivine grain growth must have been highly impeded olivine flow law data suggest that even peridotites with lower temperature microstructures t 900 950 c could form within the rheological asthenosphere the detachment or spreading related origin of peridotites with a lower temperature overprint may thus only be concluded from the regional geological context

structural classification of chromite pods in southern new caledonia a structural classification of podiform chromite orebodies from southern new caledonia results in a division of deposits into three major types discordant subconcordant and concordant with penetrative structures foliation and lineation in the enclosing peridotite discordant deposits are very irregular in shape and clearly crosscut banding and foliation subconcordant deposits are generally tabular in shape and lie within 10 degrees to 25 degrees in strike and or dip to the foliation concordant deposits are also tabular and lie parallel to foliation in peridotite pyroxene lineation in host peridotite always indicates the elongation direction of these deposits within subconcordant and discordant deposits chromite lineations are often oblique to those in the surrounding rocks they follow local variations in the orebody shape and indicate possible deposit extensions at depth the classification of chromite bodies corresponds to three stages of increasing deformation as evidenced by chromite ore textures discordant deposits which are the least deformed are characterized by primary textures such as the nodular one the foam texture the chromite net and the occluded silicate texture within subconcordant and mainly concordant deposits massive disseminated and antinodular ores show evidence of strong deformation the lack of geochemical distinctions between the different deposit types supports such an hypothesis these chromite deposits in ophiolitic harzburgites are thought to have been formed beneath an oceanic spreading ridge if the discordant pods are considered as representative of the original situation it is proposed that the chromite has crystallized and has been dynamically concentrated along steep conduits traversing the enclosing harzburgite and feeding a magma chamber next the chromite enriched pipes are caught up by plastic deformation in the mantle and tectonically reoriented toward the foliation the chromite deposits in the massif du sud are located within a domain about 1 5 km thick in the harzburgites and dunite zones this domain is limited upward by the first cumulates and gabbros and downward by the transition to a different plastic flow regime in the harzburgites finally some guides to chromite prospecting and exploration are cited applicable at different geologic scales and based on lithologic and structural criteria

the nature and rheology of the cordilleran upper mantle of british columbia inferences from peridotite xenoliths abstract peridotite xenoliths in alkalic basaltic rocks from eight localities within the british columbian cordillera include metamorphic tectonites metacumulates and cumulates chemical analyses of these show that the xenoliths encompass the range of both depleted and primitive undepleted mantle application of two pyroxene thermobarometric techniques yield geotherms characteristic of normal oceanic mantle and diapiric mantle wherein upper mantle stratigraphic profiles are either homogeneously stratified or heterogeneously mixed respectively stress estimates as a function of depth are obtained for peridotite xenoliths from each of the localities by applying recrystallised grain size paleopiezometry and pyroxene thermobarometry stress and temperatures obtained are combined with a representative creep flow law to calculate strain rate and viscosity profiles that appear to be physically reasonable these profiles are juxtaposed to present a dynamic model of a mantle diapir convecting through a stratified mantle section and these two different types of mantle stratigraphic profile are related to current large scale tectonic theories of the cordillera

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma