cryptic variation within the cumulate sequence of the oman ophiolite magma chamber depth and petrological implications the cumulate sequence of the oman ophiolite consists of layered mafic and ultramafic cumulates that represent the products of a dynamic long lived crustal magma chamber that underlay a late cretaceous spreading ridge regional variations in cumulate sequence stratigraphy reflect not only contrasting degrees of mantle fractionation but also contrasting primary magma compositions microprobe data are presented from a 600 m section of modally layered olivine gabbros coherent cryptic cyclic variation is shown with both normal and reversed geochemical gradients present these are explained in terms of fractionation mixing and eruption in an open system magma chamber the height of the liquid column required to precipitate the observed cyclic units is calculated to be about 100 m the implications of this result are discussed a contrast between the fo an co variation between olivine and plagioclase in the oman cumulate sequence and that observed for morb votcanics is noted this may reflect compositional stratification in spreading ridge magma chambers or signifcant differences between the petrogenesis of the oman ophiolite and morb

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

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

probable low pressure intrusion of gabbro into serpentinized peridotite northern california the castle lake gabbroic body intrudes the trinity ultramafic sheet in northern california the extent of concentric reaction zones of talc tremolite and chloritic blackwall on included blocks of peridotite correlates with increasing proportions of igneous hornblende in the gabbroic host consequently the reaction zones probably formed during initial cooling of the gabbro the principal source of water in the gabbroic magma and in the reaction zones was probably a hydrothermal system which existed during and after crystallization of the magma blocks of pyroxenite in a succession of intrusive sheets of gabbroic rock indicate repeated intrusion of magma into its previously solidified margins an increasing proportion of hornblende in the younger sheets correlates with a marked increase in the grain size variable distribution of dissolved and exsolved water and transitory minor decompression events can explain the grain size layering in the younger sheets vugs as much as 5 mm in diameter in the pegmatitic hornblende gabbro are interpreted to be igneous in origin and together with the patchiness of the metamorphism suggest that the gabbro crystallized at a pressure of 2 kbar or less igneous hornblende near solidus and rare garnet are consistent with such a low pressure the lack of sodium rich plagioclase in the patchily metamorphosed gabbros suggests that the hydrothermal metamorphic fluids were poor in sodium nonmarine or distilled marine because of the correlation between these igneous and metamorphic features it is likely that the castle lake gabbro intruded its own hydrothermal envelope of serpentinized peridotite

volcanics and structure of the famous narrowgate rift evidence for cyclic evolution amar 1 a near bottom photographic survey on the mid atlantic ridge from the famous region south to transform b reveals structural and volcanologic evidence for an alternating widening and narrowing rift valley extension wedges appear to be propagating south from transform a and north from transform b these merge at mt mars where the rift valley is narrowest and shallowest narrowgate giving the valley a symmetrical hourglass shape three major volcanic en echelon lineaments trend n10 e 10 oblique to the strike of the inner walls faulting and fissuring are antisymmetrical across the rift valley in contrast to the major extension this contrasting antisymmetry reflects the surficial distribution of stress across crust of varying strengths whereas the large scale wedge phenomenon determines the periodicity of transition from a narrow valley to a wide valley using vector analysis the extension wedges are propagating at about 9 cm yr toward each other at this rate a narrow rift valley will be fully transformed into a wide rift in 0 64 m y

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

evidence of sea level rise at the early ostionan coralie site gt 3 c ad 700 grand turk turks and caicos islands abstract soil and sediment analyses were conducted on samples from the early ostionan coralie site c ad 705 60 on grand turk turks and caicos islands the earliest dated settlement site in the bahama islands the focus of these analyses was a sand layer mantling archaeological features discovered on a buried land surface the tapering wedge shaped configuration of the sand body prompted the hypothesis that it was deposited by water either a storm surge or a more gradual rise in sea level this hypothesis was tested using soil morphological descriptions particle size distribution analysis and grain characteristics contents of organic carbon and total phosphorus and identification of invertebrate organisms found in the soil samples a fining upward trend in particle size round and rolled grain shapes and a rich shallow water micro invertebrate infauna indicated a gradual marine transgression over the site as opposed to either a catastrophic storm event or aeolian deposition these results explain the incremental inland shift of settlement activity over time at coralie

structure of an atypical ophiolite the trinity complex eastern klamath mountains california the trinity complex consists of an ophiolite section displaying some distinctive characteristics the mantle section is represented mainly by a relatively fertile plagioclase iherzolite and the crustal section is rather thin owing to the limited extent of the mafic cumulates which may explain the occurrence of hydrothermal reactions as far down as the uppermost peridotites the high temperature flow plane in the peridotites is vertical and the flow line horizontal parallel to the ridge orientation as defined by the sheeted dikes similar attitudes have been encountered elsewhere in other plagioclase iherzolite bodies this particular type of lithosphere may result from a small degree of partial melting associated with mantle diapirism the ophiolite may have been formed at a spreading center characterized by a slow spreading rate the trinity complex represents a fragment of ordovician oceanic lithosphere possibly trapped in a marginal sea environment during the devonian just before its accretion by northwestward thrusting onto the central metamorphic belt

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

cryptic mineral chemistry variations in a detailed traverse through the cumulate ultramafic rocks of the north arm mountain massif of the bay of islands ophiolite newfoundland

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

large scale response of the pacific ocean subarctic front to momentum transfer a numerical study abstract a numerical four layer hydrodynamical model is implemented to investigate the role the atmosphere exerts on the dynamics of the subarctic front while the effect of the westerly winds causes a southward shift of the front the effect of a negative curl shows convergence at the front this convergence strongly favors frontogenesis the salient features at the wake of a strong cyclone can be summarized as follows i upwelling of 20 m of the four interfaces occurs ii the water remains upwelled for several weeks iii the e folding time scale of the vertical oscillations induced by the storm s passage is of the order of 10 days iv a region of relative maximum convergence is observed in the lower layers below a region of relative maximum upwelling in the upper layer and v the along and cross front velocity fields are in quadrature in time with the cross front velocity lagging the along front velocity with the exception of the cyclone case it is established that the effects of the atmos

the subarctic frontal zone in the north pacific characteristics of frontal structure from climatological data and synoptic surveys the subarctic front is a thermohaline structure across the north pacific separating colder fresher water to the north from warmer saltier water to the south levitus s 1982 data and 72 conductivity temperature depth salinity temperature depth sections are used to show the spatial and seasonal variations of the climatological frontal zone and the characteristics of the frontal structure in synoptic surveys the temperature gradient in the mean frontal zone is stronger in the western pacific and decreases eastward while the salinity gradient has less variation across the pacific the temperature gradient also has larger seasonal variation with a maximum in spring than the salinity gradient the synoptic surveys show that the frontal zone is narrower and individual fronts tend to be stronger in the western pacific than in the eastern pacific density gradients tend to be more compensated at the strongest salinity fronts than at the strongest temperature fronts a horizontal minimum of vertical stability is found south of the subarctic halocline outcrop the northern boundary of the north pacific intermediate water merges with the frontal zone west of 175 w and is north of the northern boundary of the subarctic frontal zone in the eastern pacific the shallow salinity minima start within the subarctic frontal zone in the eastern pacific

the simulated response of an upper ocean density front to local atmospheric forcing four numerical experiments with different atmospheric forcing are considered to investigate a variety of transient responses of an upper ocean density front similar to the maltese front south of sicily in an embedded mixed layer ocean circulation model of particular interest is a case that simulates forcing observed by o m johannessen unpublished report 1975 in may 1971 measurements of the maltese front were being taken in calm weather and then the wind suddenly increased to 10 15 m s 1 down the front for this forcing the model result is consistent with the observations predicting steepening of the frontal interface spreading of the surface isopycnals and very little horizontal displacement of the surface front however the model predicts too much mixed layer deepening and fails to predict the observed strengthening of the along front flow three additional experiments are considered to assess the effects of a change in the wind direction and the inclusion of a surface buoyancy flux the inclusion of a diurnal surface buoyancy flux produces a very different response from the simulations with no buoyancy flux when the surface buoyancy flux is negative the mixed layer reforms at a shallow depth resulting in much higher adverting velocities and subsequently larger displacements of the surface front the negative buoyancy flux can effectively protect the subsurface structure with a shallow stable surface layer it also decouples the deeper frontal processes from the surface mixed layer processes

a nd and sr isotopic study of the trinity peridotite implications for mantle evolution field evidence indicates that the trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center a sm nd mineral isochron for a plagioclase lherzolite yields an age t 427 32 ma and initial e nd 10 4 0 4 which is distinctly higher than that expected for typical depleted mantle at this time this age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite p t field this time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere the sm nd model age of the plagioclase lherzolite total rock is t chur nd 3 4 ae this suggests that the trinity peridotite was derived from a mantle that was depleted rather early in earth history the peridotite contains many generations of pyroxenite dikes and some microgabbro dikes we report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite a microgabbro dike yields a sm nd mineral isochron age of t 435 21 ma and e nd 6 7 0 3 a pyroxenite dike yields an initial e nd 7 3 0 4 the initial e nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite demonstrating that they are not genetically related rb sr data do not give any coherent pattern however some bounds can be put on initial sr values of e sr 21 for the plagioclase lherzolite and e sr 8 7 for the microgabbro dike it is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone major and trace elements and sm nd isotopic data indicate a multiple stage history for the trinity peridotite a small melt fraction was extracted from an undepleted source 3 4 ae or more ago to produce the proto lherzolite a large fraction of melt 12 to 23 was extracted from the proto lherzolite to produce the present rock the lherzolite was then crosscut by dikes from average depleted mantle 0 44 ae ago the data are compatible with the depleted mantle source being formed very early in earth history although most available data indicate that the depleted upper mantle has been relatively well stirred through time the trinity data suggest that very ancient nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid ocean ridge basalts as well as the oceanic lithosphere which they intrude

frontogenesis in the north pacific oceanic frontal zones a numerical simulation abstract a primitive equation model geophysical fluid dynamics laboratory s gfdl s mom 2 with one degree horizontal resolution is used to simulate the seasonal cycle of frontogenesis in the subarctic frontal zone safz and the subtropical frontal zone stfz of the north pacific ocean the safz in the model contains deep greater than 500 m in some places regions with seasonally varying high gradients in temperature and salinity the gradients generally weaken toward the east the stfz consists of a relatively shallow less than 200 m in most places region of high gradient in temperature that disappears in the summer fall the high gradient in salinity in the stfz maintains its strength year round and extends across almost the entire basin the model simulates the location and intensity of the frontal zones in good agreement with climatological observations generally to within two degrees of latitude and usually at the same or slightly stronger intensity the seasonal cycle of the frontal zones al

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

silurian plutonism in the trinity terrane neoproterozoic and ordovician klamath mountains california united states new data indicate that the trinity terrane of northern california is a polygenetic composite terrane rather than a single slice of oceanic lithosphere we suggest approximately one third of the trinity terrane consists of silurian intrusive rocks that represent the roots of a previously unrecognized silurian magmatic arc crosscutting relations and u pb zircon isotopic data document at least one early paleozoic deformation in the trinity terrane of northern california a ductile shear zone between neoproterozoic metagabbro and ordovician harzburgite is intruded by the upper silurian china mountain pluton this evidence indicates a major early paleozoic shear zone formed in the eastern klamath mountains after the middle ordovician but prior to late silurian plutonism

chemical transfer between mantle xenoliths and basic magmas evidence from oceanic magma chambers the trinity ophiolite northern california abstract the trinity ophiolite consists of small magma chambers inside a large mantle body xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism quantitative elemental mass transfers determined from the composition volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major si al ti na ca fe and mg and trace elements cr ni in the open system material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction during a first stage of metasomatism the mantle xenoliths were affected by high temperature reactions at 600 to 925 c they resulted from the interaction between solid mantle lherzolites and basic melts the reactions are 1 1 those forming orthopyroxene magnetite simplectite 2 2 those forming plagioclase magnetite corona 3 3 clinopyroxene spinel i pargasitic hornblende spinel ii chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle the chemically modified magmas within oceanic magma chambers were depleted in ti fe and na this could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow spreading ridges the breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic fe ti oxides between oceanic crust and mantle during the second stage the serpentinization of olivine and the production of talc were superimposed on the products of the first stage these reactions require large amounts of h2o the hydrothermal fluid was probably seawater it circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust all the chemical reactions examined suggest a significant open system element transfer by infiltrating melts or circulating fluids the results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

tectonics of slow spreading mid ocean ridges and consequences of a variable depth to the brittle ductile transition geophysical evidence especially microearthquakes that extend into the upper mantle beneath the inner floor of the mid atlantic and gorda ridges indicates that the axial magma chamber episodically freezes beneath slow spreading ridges freezing of the axial magma chamber will result in a variable thermal structure that has important tectonic consequences most importantly large near axis faults will intersect as the brittle ductile transition deepens causing some of the faults to become locked the topographic effect of fault locking will be a variation in the width of the inner floor from narrow 10 km when a magma chamber is absent the deep microearthquakes also imply that faults at slow spreading ridges extend into the upper mantle the nature of these faults at depth is unknown but the following hypotheses are presented 1 the faults may be planar and pass downward into aseismic ductile shear zones and 2 the faults may sole into a flat detachment fault defining a sharp boundary between brittle faulting and homogeneous ductile flow such low angle normal faults are common in continental extensional domains large scale tilting 50 70 is probably common at slow spreading ridges and implies either listric faulting or rotation of planar faults and fault blocks similar to toppling dominoes calculations of temperatures at the brittle ductile transition are made assuming olivine and diabase rheologies for the mantle and crust respectively the temperature is approximately 800 c when the transition is at 8 km depth and approximately 600 when the transition is at 2 km depth the strength of the lithosphere varies dramatically with the rise and fall of the brittle ductile transition as magma chambers form and subsequently freeze strong lateral temperature gradients will be present when a magma chamber is formed resulting in localization of faulting within thin weak crust above the chamber when the magma chamber freezes lateral temperature gradients will be small and the strength of the lithosphere will be similar over a wide area this results in active faulting occurring over a width of as much as 60 km some ophiolites possibly provide insights to the tectonic processes at slow spreading ridges the entire crustal section of the josephine ophiolite was tilted by approximately 50 prior to deposition of overlying pelagic sediments this tilting probably took place by faulting suggesting that oceanic faults extended into the upper mantle possible oceanic faults have been identified in the josephine ophiolite but their oceanic origin cannot be established beyond doubt

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

paleozoic and lower mesozoic magmas from the eastern klamath mountains north california and the geodynamic evolution of northwestern america abstract the paleozoic to early mesozoic geology of the eastern klamath mountains n california is characterized by three major magmatic events of ordovician late ordovician to early devonian and permo triassic ages the ordovician event is represented by a calc alkalic island arc sequence lovers leap butte sequence developed in the vicinity of a continental margin the late ordovician to early devonian event consists of the 430 480 ma old trinity ophiolite formed during the early development of a marginal basin and a series of low k tholeiitic volcanic suites lovers leap basalt keratophyre unit copley and balaklala formations belonging to intraoceanic island arcs finally the permo triassic event gave rise to three successives phases of volcanic activity nosoni dekkas and bully hill represented by the highly differentiated basalt to rhyolite low k tholeiitic series of mature island arcs the permo triassic sediments are indicative of shallow to moderate depth in an open warm sea the geodynamic evolution of the eastern klamath mountains during paleozoic to early mesozoic times is therefore constrained by the geological petrological and geochemical features of its island arcs and related marginal basin a consistent plate tectonic model is proposed for the area consisting of six main stages 1 1 development during ordovician times of a calc alkalic island arc in the vicinity of a continental margin 2 2 extrusion during late ordovician to silurian times of a primitive basalt andesite intraoceanic island arc suite which terminated with boninites the latter suggest rifting in the fore arc followed by the breakup of the arc 3 3 opening and development of the trinity back arc basin around 430 480 ma ago 4 4 eruption of the balaklala rhyolite either in the arc or in the fore arc ending in early devonian time with intrusion of the 400 ma mule mountain stock 5 5 break in volcanic activity from the early devonian to the early permian and 6 6 development of a mature island arc from the early permian to the late triassic the eastern klamath mountains island arc formations and ophiolitic suite are part of the cordilleran suspect terranes considered to be gondwana margin fragments that have undergone large northward translations before final collision with the north american craton during late mesozoic or cenozoic times these eastern klamath mountains island arcs could be associated with the paleo pacific oceanic plate that led to accretion of these allochthonous terranes to the american margin

emplacement of deep crustal and mantle rocks on the west median valley wall of the mark area mar 23 n abstract exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow spreading mid atlantic ridge nine dives of the submersible nautile explored the western axial valley wall in the northern cell of the mark area mid atlantic ridge kane fracture zone where peridotite and gabbro outcrops had been previously reported in order to constrain the structure and determine emplacement mechanisms the ridge transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl locally overlain by metabasalts and metadolerites capped by slightly weathered basalts the morphology is controlled by ridge parallel faults dipping moderately 40 65 to the east transform parallel scarps present in the northernmost dives become rare toward the south brittle deformation along moderately dipping fault scarps produced a dense microcrack network filled with greenschist facies minerals chlorite actinolite epidote quartz locally overprinting high temperature ductile deformation fabrics the small hill located at 23 20 in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl above 3100 mbsl the summit of the hill is composed of pillow basalts and sediments the peridotite outerops display a strong schistosity dipping 20 40 to the east parallel to striated normal fault planes some steeper east facing fault scarps truncate the lower dipping fault surfaces the serpentinites deriving from a harzburgitic protolith are cut by rare dikelets of highly differentiated gabbro these new data combined with previous results of alvin dives are used to draw a generalized geological map of the western axial valley wall this map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the mark area the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets low angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south

investigating shellfish deposition and landscape history at the natia beach site fiji the relationship between environmental variation and subsistence practices is a central point of discussion in much oceanic archaeology while human predation can significantly reduce prey populations environmental variation also contributes to reductions in prey abundance possibly leading to increased human competition and resource scarcity at the natia beach site nacula island fiji geoarchaeological evidence suggests that coastal progradation began soon after initial occupation of the coastal plain additionally at approximately 650 bp a marked increase in clay and silt deposition occurred changes in coastal geomorphology may be explained by landscape response to regional mid holocene sea level fall combined with human induced soil erosion due to upland settlement smaller scale environmental changes associated with climate variability may have also played a role additionally landscape change appears to have had a measurable impact on local nearshore mollusks that are sensitive to high levels of water turbidity minor evidence of human exploitation is observable in this shellfish assemblage although changes in predation pressure may have allowed shellfish populations to recover increased ceramic diversity and fortified settlements also appear at approximately 650 bp on nacula and other parts of fiji the suite of changes at natia may be explained by processes of regional and local environmental changes and human adaptation in terms of subsistence spatial organization and competition

models of lithospheric scale deformation during plate collision effects of indentor shape and lithospheric thickness abstract analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision models presented here include layers of putty and sand to form four three and two layer lithosphere types craton arc and oceanic equivalent respectively which collide during unidirectional shortening in most cases the stronger thicker four layer lithosphere does not deform but simply acts to bulldoze weaker material aside deformation concentrated in the thinnest layers consistently verges away from the indenting continent irrespective of indentor shape lateral strike slip component deformation occurs in the early stages of most models even with simple unidirectional shortening and is enhanced by highly oblique indentor shapes the effects of extrusion during collision tested by repeating models with and without a free edge which allows extrusion consistently indicate that with the ability to extrude structures are smaller and more numerous without extrusion fewer larger structures tend to develop two geological examples from the precambrian part of the tasman orogenic belt and the modern day collision at the leading edge of australia illustrate aspects of the models

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

stretching of the deep crust at the slow spreading southwest indian ridge abstract the 500 metres of gabbro drilled in the southwest indian ocean during odp leg 118 allow direct observation of early high temperature extensional deformation in the deep crust of a slow spreading rift valley about 30 of the recovered gabbros have been plastically stretched at progressively decreasing temperatures granulite to lower amphibolite facies metamorphic conditions increasing water rock ratio and probably increasing deviatoric stresses the early solid state strain in granulite facies metamorphic conditions produced low dipping foliations and low strain gradients at this stage mylonitic shear zones only formed in oxide rich gabbros strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy in upper to lower amphibolite facies metamorphic conditions the ductile strength of the gabbro was probably higher and deformation concentrated in mylonitic shear bands which may have provided access to seawater derived fluids strain localization at this stage was accompanied by brittle failure and boudinage in the mylonitic foliation of individual porphyroclasts or of thick gabbro intervals the influence of such brittle failure on hydrothermal circulations was critical it allowed for extensive crust seawater interactions at high temperatures upper amphibolite facies conditions in a spreading context where continuous magma supply provides heat and new crustal material to the axial region the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite facies conditions this may lead to the steady state necking configuration described by tapponnier and francheteau 1978 alternatively a discontinuity of the magma supply allowing the lower crust of the axial region to cool below granulite facies temperatures should cause mylonitic shear zones characteristic of amphibolite facies deformation to extend deeper into the axial domain eventually leading to the detachment fault configuration proposed by karson in press as magma supply to slow spreading oceanic ridges is likely to be discontinuous necking and faulting periods should alternate through time

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

extreme nd isotopic variation in the trinity ophiolite complex and the role of melt rock reactions in the oceanic lithosphere peridotites dykes and gabbros from the 470 420 ma trinity ophiolite complex of northern california exhibit large geochemical rare earth element ree and nd isotopic variations on the small scales which are indicative of a complex history the trinity ophiolite which covers an area of 1600 km2 consists of three distinct units 1 a 2 4 km thick sheet of plastically deformed peridotites including various ultrabasic lithologies plagioclase and spinel lherzolite harzburgite wherlite and dunite the peridotite unit is a fragment of mantle lithosphere of oceanic affinity 2 a series of small 1 km diameter undeformed gabbroic massifs 3 several generations of basic dykes the peridotites display the largest geochemical and isotopic variations with nd t values ranging from 10 down to 0 in the gabbroic massifs and intrusive dykes the variation in model nd t values is reduced to 7 nd units 0 to 7 as a general rule peridotites gabbros and dykes with nd t values around 0 or 3 give less depleted l light ree patterns than do those with nd t values in the range 7 to 10 in the peridotites the nd isotopic variations take place over very short distances with jumps as large as 7 nd units occurring on scales of less than 20 m comparison with available age data indicates that the peridotites with nd t 10 could be slightly older than the intrusive gabbro massifs and basic dykes 470 ma vs 420 ma strontium isotopic data used in connection with sm nd results demonstrate that the 10 nd units variation displayed by the trinity peridotite is a primary feature and not an artefact due to ree mobility during seawater interaction the variable nd isotopic signatures and variable lree patterns in the trinity peridotite cannot represent mantle source characteristics as there is evidence that this unit was partially melted when it rose as part of the upwelling convecting mantle field petrographic geochemical and isotopic data rather suggest that the observed heterogeneity is due to local reactions between a 470 ma proto peridotite with nd t 10 and younger 420 ma basaltic melts with lower nd t values i e the gabbroic massifs and the dykes the gabbros and basic dykes of the trinity complex have geochemical and isotopic compositions similar to the arc basalts from the adjacent copley formation so it is proposed that the younger melts are related to arc magmatism

palynological study of a mi kmaw shell midden northeast nova scotia canada before now no palynological studies were made on shell midden deposits in nova scotia despite a long history of archaeological research a test study of samples taken from the shell midden at maligomish indian island on the northumberland strait was conducted to determine if sufficient pollen fern and moss spores and other non pollen palynomorphs npp s including fungal remains algal spores could be recovered to allow interpretation of environmental conditions at the time of deposition several pollen processing methods were tested sieving and swirling chemical treatment with cold hcl 5 koh and brief exposure to 5 nh4oh and treatment with acetolysis solution we recovered variable amounts of pollen spores and npp s from samples taken at 3 5 cm 5 15 cm and 15 25 cm below surface abundant microcharcoal fragments prevented effective use of the swirling technique but sufficient palynomorphs were recovered by chemical treatments to allow assessment of down section changes in assemblages three radiocarbon ages from mya arenaria and crassostrea sp and measurements of sediment grain size moisture organic and inorganic carbon provide a framework for analysing palynological assemblage changes and context for interpreting differential palynomorph preservation despite some oxidation and microbial degradation of palynomorphs in the time interval ca 380 1600 cal yr bp the midden assemblages correlate with pollen zones from marine and lake core sites within ca 50 km of maligomish shifts in abundances of algal spores testate amoebae and aquatic plant pollen also indicate changes in depositional conditions during the time of midden use the study provides the first reference for archaeologists attempting to use palynology for analysis of samples from nova scotian shell bearing contexts

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

geochemical study of an early paleozoic island arc back arc basin system part 1 the trinity ophiolite northern california a complete ophiolitic succession crops out along the easternmost exposure of the eastern klamath mountains trinity ultramafic sheet ultrabasic basic plutonic rocks sheeted dike complex and basaltic pillow lavas the ophiolitic succession is characterized by small amounts of iron enrichment low titanium and incompatible trace element contents and depleted lree patterns the strong island are geochemical affinities of the trinity basatts and the e nd values of the ophiolitic succession ranging between 6 6 and 7 5 suggest that the trinity ophiolite is the remnant of a back arc basin located in the vicinity of an island arc important sr isotopic variations e sr range between 17 3 and 23 6 contrasting with the rather constant e nd values suggest that sea water interaction occurred the calculated water rock ratios ranging between 0 and 10 show that the hydrothermal activity was maximal in the high level gabbros just below the sheeted dike complex

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges