Oceanic crust

The sources and propagation of VLF (0.2 • 10 Hz) ambient noise on and within the deep ocean crust at Deep Sea Drilling Project (DSDP) Hole 534B in the Blake Bahama Basin are shown to be related to the surface sea state and local lithology. This study represents the first experiment where ambient noise is measured simultaneously at several depths below the seafloor. The low-frequency microseism power spectral density (PSD) peak at 0.3 Hz is nearly invariant with depth between 0 and 100 rn below the seafloor. PSD levels of the peak are 65 and 75 dB (rel 1 (nm/s2)2/Hz) for the vertical and horizontal components, respectively, and both horizontal and vertical components of the ocean bottom seismometer and borehole array compare favorably. Above 0.5 Hz the noise levels decrease with depth and increasing frequency. At 1.0 Hz, 100 rn below the seafloor the noise level is 10 and 20 dB below the levels observed at the seafloor for vertical and horizontal components, respectively. There is evidence that amplification in some frequency bands may make deeper .sites noisier than shallower sites in the same well. Temporal variation of the noise shows nonlinear interaction of local water-borne gravity waves to be the dominant source mechanism and that the passing of a local storm generates interface waves and increases the noise level (•10 dB) from 0.3 to 1.5 Hz and 5 to 64 Hz. Between 1.5 and 5 Hz the spectrum is not strongly affected by the passing storm, indicating that the ocean wave spectrum may be saturated. ing a single triaxial borehole sonde, they found that 5-50 Hz ambient noise was isotropic and nonpolarized and that the amplitude was variable with depth. The nonmonotonic behavior with depth was attributed to local geology and poor clamping. Carter et aI. [1984], using the ocean subbottom seismometer (OSS) in Hole 494A off the western Guatemalan coast, found that the ambient 10-20 Hz noise levels at 194 m below the seafloor were similar to the levels seen on an ocean bottom seismometer (OBS) located within 200 m of the borehole. In the 2-10 Hz band the downhole instrument measured noise levels 3-8 dB below the OBS levels. They attribute the larger surface response to interface waves trapped at the water/ sediment interface (<5 Hz) and shear wave noise (7-10 Hz), which they expect to be greater due to the low shear modulus at the interface. Duennebier et al. [1986, 1987] reported on noise levels measured by the OSS IV in DSDP Hole 58!C near the Hokkaido trench. Noise levels observed 378 m below the seafloor (22 m 11,703 11,704 BRADLEY ET AL.: VERY LOW FREQUENCY NOISE BELOW THE SEAFLOOR subbasement) were extremely quiet between 4 and 15 Hz; between 2 and 20 Hz, OSS IV noise levels were 20 dB below the OBS's levels. Storms (wave breaking), shipping, and biologic sources comprised the major influences on noise above 5 Hz; below 5 Hz, the noise was controlled by local sea state. The observation of a saturation spectrum between 0.5 and 5 Hz on both the OBS and OSS systems for two separate storms indicated that coastal wave reflection was not as important as local nonlinear wave-wave interaction for noise generation. This bandwidth has been identified with wind-driven gravity waves in the most commonly saturated portion of the ocean wave spectrum in deep water. McCreery et al. [1993] have coined the term "Holu" (the Hawaiian word for deep water) spectrum for this band. The above studies using the OSS do not discuss the behavior of the spectra below 1 Hz and, particularly, noise at the microseisin peak near 0.16 Hz. The marine seismic system (MSS), a permanent borehole seismometer like the OSS [Adair et al., 1984] was placed in DSDP borehole 395A on the Mid-Atlantic Ridge and rested unclamped at 516 m subbasement (600 m below the seafloor). Electronic and least count noise were present below 0.3 Hz, and transients contaminated the spectrum above 2.2 Hz. The noise field was stationary on a timescale of i hour but was found to vary over greater timescales (order 10 hours). The MSS recorded noise levels near 60 dB tel ß

A predrill estimate of pore pressure can be obtained from seismic velocities using a velocity-to–pore-pressure transform, but the seismic velocities need to be derived using methods having sufficient resolution for well planning purposes. For a deepwater Gulf of Mexico example, significant differences are found between the velocity field obtained using reflection tomography and that obtained using a conventional method based on the Dix equation. These lead to significant differences in the predicted pore pressure. Parameters ...

We present a new digital crustal model for Moho depth and crustal structure in Europe, Greenland, Iceland, Svalbard, European Arctic shelf, and the North Atlantic Ocean (72W-62E, 30N-84N). Our compilation is based on digitization of original seismic profiles and Receiver Functions from ca. 650 publications which provides a dense regional data coverage. Exclusion of non-seismic data allows application of the database to potential field modeling. EUNAseis model includes Vp velocity and thickness of five crustal layers, including the sedimentary cover, and Pn velocity. For each parameter we discuss uncertainties associated with theoretical limitations, regional data quality, and interpolation. By analyzing regional trends in crustal structure and links to tectonic evolution illustrated by a new tectonic map, we conclude that: (1) Each tectonic setting shows significant variation in depth to Moho and crustal structure, essentially controlled by the age of latest tectono-thermal processes; (2) Published global averages of crustal parameters are outside of observed ranges for any tectonic setting in Europe; (3) Variation of Vp with depth in the sedimentary cover does not follow commonly accepted trends; (4) The thickness ratio between upper-middle (Vp b 6.8 km/s) and lower (Vp N 6.8 km/s) crystalline crust is indicative of crustal origin: oceanic, transitional, platform, or extended crust; (5) Continental rifting generally thins the upper-middle crust significantly without changing Vp. Lower crust experiences less thinning, also without changing Vp, suggesting a complex interplay of magmatic underplating, gabbro-eclogite phase transition and delamination; (6) Crustal structure of the Barents Sea shelf differs from rifted continental crust; and (7) Most of the North Atlantic Ocean north of 55°N has anomalously shallow bathymetry and anomalously thick oceanic crust. A belt of exceptionally thick crust (ca. 30 km) of probable oceanic origin on both sides of southern Greenland includes the Greenland-Iceland-Faeroe Ridge in the east and a similar "Baffin Ridge" feature in the west.

The Serido Group is a deformed and metamorphosed metasedimentary sequence that overlies early Paleoproterozoic to Archean basement of the Rio Grande do Norte domain in the Borborema Province of NE Brazil. The age of the Serido Group has been disputed over the past two decades, with preferred sedimentation ages being either Paleoproterozoic or Neoproterozoic. Most samples of the Serido Formation, the upper part of the Serido Group, have Sm–Nd TDM ages between 1200 and 1600 Ma. Most samples of the Jucurutu Formation, the lower part of the Serido Group, have TDM ages ranging from 1500 to 1600 Ma; some basal units have TDM ages as old as 2600 Ma, reflecting proximal basement. Thus, based on Sm–Nd data, most, if not all, of the Serido Group was deposited after 1600 Ma and upper parts must be younger than 1200 Ma. Cathodoluminescence photos of detrital zircons show very small to no overgrowths produced during ca. 600 Ma Brasiliano deformation and metamorphism, so that SHRIMP and isotope d...

There are three parallel magmatic arcs in the northwest of Iran, of Cretaceous and Eocene-Miocene to Quaternary ages, trending in a NW-SE direction between the Main Zagros Thrust (MZT) in the southwest and the Tabriz Fault in the northeast. In this study, these volcanic belts are referred to as the Sanandaj Cretaceous volcanic (SCV), Sonqor-Baneh volcanic (SBV), and Hamedan-Tabriz volcanic (HTV) belts, respectively. The SCV belt consists mainly of mafic to intermediate submarine rocks with calc-alkaline affinity, and the SBV belt is composed of basalt, gabbro to dioritic bodies, with extrusive to subvolcanic magmatic textures and tholeiitic to alkaline affinity. These extend along the MZT between the Zagros ophiolite in the west and the SCV belt in the east. The HTV belt is part of the Urmieh-Dokhtar Magmatic Arc belt that extends across the Hamedan to Tabriz, and was active in the Miocene to Quaternary. The petrology and geochemistry of the northwestern Iranian volcanic zones indicate that they were generated at an active continental margin. In addition to the volcanic belts, there is a dismembered ophiolite along the MZT from Kermanshah to Turkey, in a NW-SE direction. These ophiolites are remnants of Neo-Tethyan oceanic crust which was obducted over the Arabian passive margin in the late Cretaceous. In this study, we propose that a collision between the Arabian and Iranian plates may have occurred in the middle to late Miocene, and that the Neo-Tethyan oceanic subduction beneath northwestern Iran ceased for a while. As a result, a gap in volcanic activity occurred between the Cretaceous and the Middle Miocene-Quaternary volcanism events. This gap in activity is not observed in southwestern Iran.

The Nehbandan ophiolite complex (NOC) crops out in the Sistan suture zone, which marks the boundary between the Lut and Afghan continental blocks. The NOC is composed of various ophiolitic lithotypes included in a tectono-sedimentary mélange, which are commonly interpreted as remnants of the oceanic lithosphere of the Sistan Ocean. Three different sequences (or associations) of ophiolitic rocks can be recognized in the NOC: (1) mantle peridotites consisting of clinopyroxene-(cpx-) rich harzburgites and depleted harzburgites; (2) olivine websterite-pyroxenite-gabbronorite sequence; (3) wehrlite-troctolitecumulate gabbro-isotropic gabbro-basalt sequence. Petrographic observations, mineral chemistry, wholerock chemistry, and rare earth element (REE) modelling carried out on the different rock associations led to the following conclusions: (1) the wehrlite-troctolite-cumulate gabbro-isotropic gabbro-basalt association represents a portion of oceanic crust generated in a mid-ocean ridge setting; (2) the cpx-rich harzburgites represent the residual mantle after 5-20% removal of mid-ocean ridge basalt-type (MORB) melt. This residual mantle was subsequently enriched in light REE (LREE) by subduction-derived fluids in a suprasubduction zone (SSZ) setting and it compositionally represents the typical mantle source for boninitic melts; (3) the olivine websterite-pyroxenite-gabbronorite association represents a portion of oceanic crust generated in an intra-oceanic arc setting; (4) the depleted harzburgites represent the residual mantle after 10-30% removal of boninitic melts in an intra-oceanic arc setting. The data presented in this paper provide new constraints for the tectonic evolution of the Iranian sector of the Neo-Tethys. In fact, in contrast with previous geodynamic models, the occurrence of SSZ ophiolites in the NOC implies that the phase of convergence between the Lut and Afghan blocks, which led to the closure of the Sistan Ocean, was accompanied by the development of an intra-oceanic arc setting.

Cold springs upwelling through large serpentinite mud volcanoes in the outer half of the Mariana forearc provide a unique window into processes of devolatilization of the subducting Pacific Plate. We have sampled upwelling pore waters with lower chlorinity than seawater from six sites on five serpentinite mud volcanoes, by conventional gravity and piston coring, by push coring from the ROV Jason, by drilling on ODP Legs 125 and 195, and by manned submersible. The sites range from 13°47=N to 19°33=N and 52 to 90 km from the Mariana trench axis, corresponding to approximate depths to the top of the downgoing plate of 16 to 29 km. The composition of the springs varies systematically over this distance: nearer the trench the upwelling waters have much higher Ca and Sr than seawater and much lower carbonate alkalinity, sulfate, Na/Cl, K, Rb, and B. Farther from the trench the waters show the opposite trends relative to seawater. Chlorinity is consistently lower than in seawater and shows large variations that are not systematic with distance from the trench. Cs is consistently higher than in seawater and increases with distance from the trench. All of the waters have high pH and are heavily depleted in Mg, Si, Li, F, and 87 Sr/ 86 Sr relative to seawater. They tend to be enriched in O 18 /O 16. Except for ODP drilling, none of the cores was long enough to produce an asymptotic compositional trend with depth. We have inferred the end-member compositions of the upwelling waters by extrapolation against Mg. At two sites we were able to compare data from gravity cores with data from drill cores or push cores collected at springs to estimate the effects of reactions that occur at shallow depth below the seafloor, on mixing of the upwelling waters with seawater. These effects are different for sites high in dissolved Ca, nearer the trench, vs. those high in alkalinity, farther from the trench. Common to both are large losses from solution of 1) Ca, as CaCO 3 and in exchange for Na; 2) Mg, in exchange for Na or Ca and as brucite; 3) sulfate, probably reduced by microbes or possibly precipitated as gypsum; 4) Sr, Ba, Si, and F. Na is consistently leached from the solids into solution, whereas K and O 18 /O 16 are relatively unreactive. We infer that the upwelling waters are uniformly saturated with CaCO 3 and that the excess H 2 O and the trends in Ca, Sr, alkalinity, and sulfate with distance from the trench result from introduction of H 2 O and dissolved carbonate and sulfate from an external source, the sediment and altered basalt at the top of the subducting plate. The concurrent trends in Na/Cl, B, Cs, and especially K and Rb indicate that these species originate from the top of the subducting plate in response to increasing temperature. These systematic variations across the outer forearc imply that the solutions ascend more or less vertically from the source region and do not travel long distances laterally along the décollement before ascending. Based on leaching of K, the 150°C isotherm is crossed approximately beneath Big Blue Seamount at a depth of ϳ22 km below the seafloor, 70 km behind the trench. By this point it appears that carbonate dissolution has joined dehydration as a significant process at the top of the subducting plate.

The tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood. This paper presents an integrated stratigraphic and tectonic evolution of Late Cretaceous to Recent volcanic arcs and associated basins in the southeastern Caribbean Sea using seismic reflection data, wideangle seismic refraction data, well data, and onland geologic data. We propose a new tectonic model for the opening of the Grenada and Tobago basins and the 50-250-km eastward jump of arc volcanism from the Late Cretaceous Aves Ridge to the Miocene to Recent Lesser Antilles arc in the southeast Caribbean based on the mapping of three seismic megasequences. The striking similarity of the half-graben structure of the Grenada and Tobago basins that flank the Lesser Antilles arc, their similar smooth basement character, their similar deep-marine seismic facies, and their similar Paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single, saucer-shaped, oceanic crust Paleogene forearc basin adjacent to the now dormant Aves Ridge. This single forearc basin continued to extend and widen through flexural subsidence during the early to middle Eocene probably because of slow rollback of the subducting Atlantic oceanic slab. Rollback may have been accelerated by oblique collision of the southern Aves Ridge and southern Lesser Antilles arc with the South American continent. Uplift and growth of the southern Lesser Antilles arc divided the Grenada and Tobago basins by early to middle Miocene time. Inversion of normal faults and uplift effects along both edges of the Lesser Antilles arc are most pronounced in its southern zone of arc collision with the South American continent. The late Miocene to Recent depositional histories of the Grenada and Tobago basins are distinct because of isolation of the Grenada basin by growth and uplift of the Neogene Lesser Antilles volcanic ridge.

The recently discovered quartz-gold veins comprising the Modi Taung-Nankwe gold district in central Myanmar are largely hosted by mudstones of the late Palaeozoic Slate belt. Mesozoic rocks of the Paunglaung-Mawchi zone separate the Slate belt from the Shan Scarp and Shan Plateau to the east. At Modi Taung 5 km of exploration adits indicate that the veins, within steeply dipping oblique reverse-slip shear zones, are displaced by movements along the shears, intruded by late Jurassic calc-alkaline dykes, and offset on conjugate cross-faults. Drill intercepts show that mineralization extends vertically for more than 500 m. Coarse visible gold, book-andribbon texture and stylolitic laminations, and trace metal values are consistent with mineralization at mesothermal depths of several kilometres in the brittle-ductile regime. Our field observations suggest that mineralization took place in the early Jurassic following collision of Myanmar on the passive western margin of a Greater Shan-Thai continental block with an oceanic arc on the overriding plate to the west. Collision generated intra-continental thrusting in and east of the Paunglaung-Mawchi zone, metamorphism of Plateau rocks thrust west beneath the Slate belt to form the Mogok Metamorphics, and ascent of mineralizing fluids expelled from dehydrating underthrust rocks. Reversal in orogenic polarity initiated late Jurassic eastward subduction of oceanic crust beneath Myanmar, and generation of a magmatic arc with dykes cutting the quartz-gold veins. Renewed east-directed thrusting translated the Slate belt over arc clastics, and probably resulted in exhumation of the Mogok Metamorphics and offsets of the veins on conjugate faults. Young K/Ar, Rb 87 /Sr 86 and Ar 40 /Ar 39 ages on the Mogok Metamorphic belt imply renewed Tertiary uplift and possibly intrusion of mid-Tertiary or Palaeogene granites.

We present a new estimate of the Earth's heat loss based on a new global compilation of heat flow measurements comprising 24,774 observations at 20,201 sites. On a 5 ø x 5 ø grid, the observations cover 62% of the Earth's surface. Empirical estimators, referenced to geological map units and derived from the observations, enable heat flow to be estimated in areas without measurements. Corrections for the effects of hydrothermal circulation in the oceanic crest compensate for the advected heat undetected in measurements of the conductive heat flux. The mean heat flows of continents and oceans are 65 and 101 mW m-2, respectively, which when areally weighted yield a global mean of 87 mW m-2 and a global heat loss of 44.2 x 10 •2 W, an increase of some 4-8% over earlier estimates. More than half of the Earth's heat loss comes from Cenozoic oceanic lithosphere. A spherical hat-monic analysis of the global heat flow field reveals strong sectoral components and lesser zonal strength. The spectrum principally reflects the geographic distribution of the ocean ridge system. The rate at which the heat flow spectrum loses strength with increasing harmonic degree is similar to the decline in spectral strength exhibited by the Earth's topography. The spectra of the gravitational and magnetic fields fall off much more steeply, consistent with field sources in the lower mantle and core, respectively. Families of continental and oceanic conductive geotherms indicate the range of temperatures existing in the lithosphere under various surface heat flow conditions. The heat flow field is very well correlated with the seismic shear wave velocity distribution near the top of the upper mantle. implications of the surface heat flow for the temperature structure at shallow depths within the Earth and

Oceanic plateaus, areas of anomalously thick oceanic crust, cover about 3% of the Earth's seafloor and are thought to mark the surface location of mantle plume ''heads''. Hotspot tracks represent continuing magmatism associated with the remaining plume conduit or ''tail''. It is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones, and, therefore: (1) influences the eventual composition of continental crust and; (2) is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs. The Ontong Java Plateau (OJP) of the southwestern Pacific Ocean is the largest and thickest oceanic plateau on Earth and the largest plateau currently converging on an island arc (Solomon Islands). For this reason, this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones. This volume consists of a series of four papers that summarize the results of joint US-Japan marine geophysical studies in 1995 and 1998 of the Solomon Islands-Ontong Java Plateau convergent zone. Marine geophysical data include single and multichannel seismic reflection, ocean-bottom seismometer (OBS) refraction, gravity, magnetic, sidescan sonar, and earthquake studies. Objectives of this introductory paper include: (1) review of the significance of oceanic plateaus as potential contributors to continental crust; (2) review of the current theories on the fate of oceanic plateaus at subduction zones; (3) establish the present-day and Neogene tectonic setting of the Solomon Islands-Ontong Java Plateau convergent zone; (4) discuss the controversial sequence and timing of tectonic events surrounding Ontong Java Plateau-Solomon arc convergence; (5) present a series of tectonic reconstructions for the period 20 Ma (early Miocene) to the present-day in support of our proposed timing of major tectonic events affecting the Ontong Java Plateau-Solomon Islands convergent zone; and (6) compare the structural and deformational pattern observed in the Solomon Islands to ancient oceanic plateaus preserved in Precambrian and Phanerozoic orogenic belts. Our main conclusion of this study is that 80% of the crustal thickness of the Ontong Java Plateau is subducted beneath the Solomon island arc; only the uppermost basaltic and sedimentary part of the crust (f 7 km) is preserved on the overriding plate by subduction-accretion processes. This observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both Precambrian and Phanerozoic orogenic belts.

The operation and extent of modern-style plate tectonics in the Archean and Paleoproterozoic are controversial, although subduction and terrane accretion models have been proposed for most Archean cratons in the world, including both the Yilgarn and Pilbara Cratons of Western Australia. The recognition of ancient island arcs can be used to infer convergent plate margin processes, and in this paper we present evidence for the existence of several intraoceanic island arcs now preserved in Australia. Beginning in the Archean, Australia evolved to its present configuration through the accretion and assembly of several continental blocks, by convergent plate margin processes. In Australia, possibly the best example of an Archean island arc (or primitive continental arc) is preserved within the Mesoarchean (ca. 3130-3112 Ma) Whundo Group in the Sholl Terrane of the West Pilbara Superterrane. Two younger, Neoarchean, island arc terranes, and associated accretion, have also been proposed for the Yilgarn Craton: the Saddleback island arc (ca. 2714-2665 Ma) in the southwest Yilgarn Craton and the Kurnalpi island arc (ca. 2719-2672 Ma) in the eastern Yilgarn Craton. In the early Proterozoic, in the Central Zone of the Halls Creek Orogen, northern Western Australia, the Tickalara Metamorphics (ca. 1865-1850 Ma) have been interpreted to represent an island arc. In the southwest Gawler Craton in South Australia, the St Peter Suite (ca. 1631-1608 Ma), of juvenile I-type calcalkaline tonalite to granodiorite, possibly represents an island arc. In the Musgrave Province in central Australia, age and geochemical constraints are poor due to later overprinting tectonic events, but felsic orthogneisses (ca. 1607-1565 Ma) possibly represent juvenile felsic crust which was emplaced though subduction-related processes into an oceanic island arc. The arcs are volumetrically insignificant, but important, in that they separate much larger tracts of, usually older, continental crust, often of different composition and geological history. The arcs were sutured to continental crust during arccontinent collisional events, which eventually resulted in the assembly of much of present-day Australia. The arcs, thus, indicate lost oceanic crust. The recognition of island arcs in the ancient rock record indicates that subduction processes, similar in many ways to modern day processes at convergent plate margins, were operating on Earth by at least 3100 Ma ago.

Upper Cretaceous units on the Menderes-Tauride Block show a transition from a passive continental margin to a pelagic basin. This transition is related to the emplacement of ophiolitic nappes. The pelagic sediments were studied in the Aydıncık and Arslanköy (Mersin) areas in the central Tauride region, and in the Munzur Mountains in eastern Tauride region. In the Aydıncık area Upper Cretaceous units consist of Upper Campanian carbonate breccias with rudist fragments; Upper Campanian-Lower Maastrichtian slope-to-basinal sediments overlie these carbonate breccias disconformably. In the Arslanköy area the Upper Campanian-Maastrichtian Yavca Formation consists of Globotruncana-bearing biomicrites, siliciclastic turbidites, calciturbidites and debris flows and overlies Jurassic-Cretaceous platform carbonates disconformably. The uppermost levels of the Liassic-Cenomanian platform carbonates in the Munzur Mountains are composed of Cenomanian Hippurites/Orbitolina-bearing reefal limestones and then Turonian-Campanian Globotruncanidae-bearing biomicrites. The stratigraphic sequence in the Menderes-Tauride Block shows that rifting to form a northern Neotethyan basin took place in the Triassic and gave rise to the Inner Tauride Ocean. From Jurassic to Cenomanian times, the northern part of the Menderes-Tauride Platform formed a passive continental margin. In the Late Cretaceous the Inner Tauride Ocean is believed to have commenced intra-oceanic, northward subduction. Inner Tauride oceanic crust-derived nappes were emplaced southward onto the Menderes-Tauride Block. Flexural foreland basins in which pelagic sediments were deposited were formed in the Taurus Mountains related to southward thrusting and the emplacement of the ophiolite nappes.

first deep seismic reflection profiles offshore Brazil were acquired in Campos Basin and processed to 10 s TWT in 1984. Starting in 1989, Petrob& acquired an extensive data set of deep seismic profiles using special acquisition equipment capable of effectively penetrating through the sedimentary layers and imaging the whole crustal architecture. These deep (18 s TWT) seismic reflection profiles extend across the Atlantic-type marginal basins, from the platform to the deepwater province, presently considered frontier regions for petroleum exploration. This work addresses the geological objectives of a deep seismic profile in the Sergipe Basin and discusses the results obtained by integrating regional seismic, gravity and magnetic data. When combined, these data provide evidence that deep seismic reflectors observed in the Sergipe Basin are related to intracrustal-upper mantle structures rather than sedimentary features. The deep seismic reflection profile in the Sergipe Basin also suggests that, rather than a non-volcanic passive margin, the deepwater extension of this basin is marked by several magmatic structures, including thick wedges of seaward-dipping reflectors and volcanic plugs. These magmatic features are associated with basinforming processes resulting from lithospheric extension during the breakup of Gondwana in the Early Cretaceous and subsequent emplacement of oceanic crust. These results are compared to the crustal scale structures observed in the Campos Basin, in the southeastern margin of Brazil. The interpretation of the deep structure of these basins indicates that final separation between the South American and African plates formed passive margins characterized by different patterns of crustal attenuation underlying the rift blocks.

An active low-temperature fluid flow system operates within ~24Ma EPR-generated oceanic crust of the Cocos plate near the Middle American trench, as indicated by anomalously low surface and borehole heat flow values (Langseth and Sliver, GRL, v23, 1996). ODP Legs 170 and 205 examined fluid flow pathways at the Costa Rica margin through coring, downhole logging, and subseafloor monitoring (Kimura et al., Init. Rept. 170, 1997; Morris et al., Init. Rept. 205, 2003). Lateral seawater flow in the upper igneous basement is inferred from basal sediment pore water profiles, which show a return toward seawater chemistry above basement (Silver et al., Geology, 2000). Interpretation of initial results from in situ subseafloor monitors (CORK-IIs) suggests that the upper basement is highly permeable and is hydrologically connected to points of distant recharge (IODP Leg 301T Prel. Rept., 2005). Thermal modeling indicates advective heat extraction in the uppermost 100-600m of basement (Fisher et al., GRL, v30, 2003), an area where lateral permeability may be affected by the abundance of sills identified through correlation of TicoFlux seismic profiles with ODP cores (Silver et al., GRL, v 31, 2004). Legs 170 and 205 cored 180m of low-K tholeiitic gabbros and basalts, consisting of an upper 31m gabbroic sill separated from &gt;148m of a lower igneous unit by ~30m of post-18.2Ma sediment. These units are derived from a homogeneous mantle source enriched relative to EPR MORB; differences in degree of partial melting and crystallization control limited geochemical variation (Dreyer et al., submitted). Low-temperature alteration in the sill is characterized by palagonitization, replacement of primary minerals by clay, zeolitization, and void and vein filling by clays +/- calcite. The lower unit has experienced higher degrees of alteration, though discrete alteration remains low (generally 1-5%, but locally up to 50%). Sparse celadonite and Fe-oxyhydroxides in the recovered rocks suggests reducing conditions prevailed Greater fracture density is observed in cores and borehole logging at depths &gt;510mbsf. Trace elements were measured for 34 igneous rocks from Leg 205 and are within the expected range for variably altered basement. Ratios of fluid-mobile to fluid-immobile incompatible trace elements extend up to ~2x mantle values (Li/Yb, Cs/Rb, Pb/Ce, Ba/La in decreasing order of enrichment). Maximum values generally occur near 513mbsf (Site 1253, Core 24R), coincident with the highest 87Sr/86Sr (0.70516), first downhole appearance of a basalt rather than gabbro, and change in secondary zeolite mineralogy and physical properties. Above this depth, alteration-sensitive trace element ratios increase without changing Sr isotopic ratios and may indicate hydrothermal alteration. Deeper, elevated trace element and Sr isotopic ratios correlate with each other and also with measures of increased fracture density (Pfender and Villinger, submitted) and likely reflect alkali uptake from modified seawater by secondary zeolites. Using seawater compositions, models of open system alteration suggest minimum fluid:rock ratios of ~6 for the most altered sample. These results suggest that Cocos Plate basement cored during Leg 205 records interactions with two distinct fluids over a range in temperatures; CORK-II OsmoSamplers installed in this section of the basement directly sample these fluids.

This may mean that up to 40% of the ocean floor forms in a manner different from what geologists have thought heretofore, with the mantle being emplaced in blocks over large regions of the sea floor where there is little or no crust. The Woods Hole team's investigation was funded by the U.S. National Science Foundation. David Epp, director of the NSF's marine geology and geophysics program, said these research results "will increase our understanding of mid-ocean ridge processes, and that marine geology textbooks will be re-written" based on the findings. The Woods Hole team have published their results in Nature.

Deposition of the Palaeogene to early Neogene calcareous and siliceous deep-water sediments of the Lefkara Formation in Cyprus was succes,;ively dominated by pelagic, turbidity current, a return to pelagic and then bottom-current processes. Sedimentation occurred on the distal parts of a carbonate slope-apron to basin plain setting located in the western arm of the Tethys Ocean. The early phase of sedimentation directly overlying ocean crust, ridge-derived volcaniclastics and chemogenic sediments, was dominated by pelagic deposition of marl, marly chalk and radiolarian-rich calcilutite. This was followed by a gradual increase in the influx of biogenic turbidites (forming both chalk and chert deposits) from the north during the Early and Middle Eocene period, primarily as a response to tectonic uplift of the Kyrenia Range. Slower rates of sedimentation returned during the Late Eocene to Early Miocene, together with significant hiatuses in the sedimentary record. "lbgether these reflect the cessation of turbidity current input, continued pelagic deposition and the onset of bottom-current influence on sedimentation. A combination of subtle features and supporting evidence allows the recognition of contourites in all the sections studied, together with the interaction of turbidity current and bottom-current processes in parts of the Lefkara Formation.

The intraplate Damavand volcanic field that overlies the Alborz Mountains in northern Iran is dominantly comprised of trachyandesite and alkali olivine basalt. Alkali olivine basalt is the most primitive rock type among Damavand lavas and exhibits a number of geochemical characteristics (e.g., high MgO, Ni and Cr abundances) that preclude substantial differentiation and crustal contamination. The lavas are light rare earth element enriched and have small negative Nb-Ta-Ti and positive Ba and Pb anomalies. The initial Nd and Sr isotope compositions of these lavas plot within the mantle array and cluster near bulk silicate Earth-values, while Pb isotope ratios are typical of modern upper crust. Oxygen isotope ratios range from + 6.5 to + 6.9‰. Although previous studies suggest that the mantle source of the Damavand volcanic rocks was metasomatized by the release of fluids during Tertiary subduction of Neo-Tethys oceanic crust under the Iranian Plate, the geochemistry of the alkali olivine basalts (e.g., BSE-like Sr and Nd isotopic compositions; the position of samples on Ti/V vs. Zr/Nb, Ce/Pb vs. Ce, and Th-Ta-Hf diagrams) is not reconcilable with derivation of the parental magma from such a mantle source. In addition, the Mid-Ocean Ridge Basalt (MORB) normalized trace element patterns of the Damavand alkaline olivine basalts most resemble those of Ocean Island Basalt (OIB) that implies the role of deep mantle source in their genesis. It seems that local upwelling of deep mantle materials underneath the Alborz Mountains resulted in the eruption of intraplate Damavand lavas. Compressional stress exerted on the Iranian plate following Neo-Thetys closure was the likely cause of sub-continental lithosphere delamination which led to such local mantle upwelling.

The southern East African Orogen is a collisional belt where the identification of major suture zones has proved elusive. In this study, we apply U -Pb isotopic techniques to date detrital zircons from a key part of the East African Orogen, analyse their possible source region and discuss how this information can help in unravelling the orogen.

Keywords: crustal block metavolcanic amphibolite anorthosite TTG island arc West Greenland Archean The ca. 700 km long, Archean craton of West Greenland consists of six Meso-Neoarchean (ca. 3000-2720 Ma) shear zonebounded crustal blocks that display similar cross-sections; from south to north Ivittuut, Kvanefjord, Bjørnesund, Sermilik, Fiskefjord, Maniitsoq. Each block has a southerly upper and a northerly lower zone, thus each faces upwards to the south. Upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies, whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies. Upper and lower zones consist predominantly of tonalite-trondhjemitegranodiorite (TTG) orthogneisses; geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins. The gneisses contain km-thick metavolcanic amphibolite layers typically bordered by km-thick layers containing anorthosite and leucogabbro. Most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic, andesitic rocks. The two most-prominent metavolcanic belts in the Fiskefjord block at Qussuk (andesitic-volcaniclastic rocks; Garde, A.A., 2007. A mid-Archaean island arc complex in the eastern Akia terrane, Godthåbsfjord, southern West Greenland. Journal of the Geological Society (London) 164, 565-579.) and Ivisaartoq (mafic-ultramafic rocks and anorthosite-leucogabbro from upper and lower parts of a supra-subduction zone system; Polat, A., Frei, R., Appel, P.W.U., Dilek, Y., Fryer, B., Ordóñez-Calderón, J.C., Yang, Z., 2008. The origin and compositions of Mesoarchean oceanic crust: evidence from the 3075 Ma Ivisaartoq greenstone belt, SW Greenland. Lithos 100, 293-321.) have island arc geochemical signatures. The 2 km-thick Fiskenaesset complex (Bjørnesund block) comprises chromite-layered anorthosites, leucogabbros and gabbros, and local pillow-bearing roof pendants from overlying metavolcanic amphibolite. The style of deformation changes downwards within crustal blocks; upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding, and lower zones by kilometre-scale double-triple fold interference patterns. Everywhere TTG protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones, often so extensively that only anorthositic or amphibolitic lenses are preserved. The Meso-Neoarchean crust was thickened by a combination of thrusting, isoclinal folding and continued TTG injection. Dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes. These crustal blocks provide an exceptional example of how continents evolved in the Meso-Neoarchean. Comparable Archean examples in Kapuskasing and Pikwitonei (Canada) and modern analogues in Fiordland (New Zealand), Kohistan (Himalayas), Southern California batholith, Peruvian Andes, and Hidaka (Japan) demonstrate that processes of continental growth from island arc to continental arc magmatism (and by implication to continental collision) were broadly similar throughout most of Earth history.

Pb isotope investigation has been carried out on sediments overlying the Cretaceous oceanic crust at ODP Leg 185 Site 1149B. The sediments (biogenic carbonate and siliceous, clays) contain two horizons with high ''excess'' concentrations of Fe, Zn, Cu, and Pb. These horizons are both characterized by rare Earth element patterns that are concave-upwards and exhibit both negative Ce and positive Eu anomalies; comparable to those from modern hydrothermal plume particles and sediments. However, Pb isotope signatures differ significantly between the two metal-rich horizons. Metal-rich sediments in the lower part of Unit IV have Pb isotopes that lie on a mixing line between seawater and local oceanic crust end-members, which is consistent with a hydrothermal fallout originating from a local oceanic spreading centre. In contrast, sediments from the upper part of Unit IV have much more radiogenic Pb isotope ratios, that cannot be related readily to local end-members (off-axis hydrothermal vent, clays, oceanic crust or large igneous provinces (LIP)). Their age-corrected Pb isotope compositions do, however, overlap with basalts from the Parana-Etendeka LIP. Evidence for related environmental effects include a drastic biotic change, higher oceanic d 13 C values and more radiogenic seawater 87 Sr/ 86 Sr. We estimate that the Parana-Etendeka LIP released $1 Â 10 19 g of carbon into the atmosphere as CO 2 , corresponding to Pb and S degassing fluxes of $1 Â 10 9 g a À1 and $15 Â 10 12 g a À1 , respectively. The study demonstrates that Pb isotopes when combined with other geochemical parameters are ideal tools for detecting and tracing LIP volcanism in the marine geological records.

The phase diagram of a K-free mid ocean ridge basalt-H 2 O system was determined between 4 and 6 GPa to constrain compositions of the liquid phases as liberated from an eclogite during deep subduction and to elucidate mass transfer processes at convergent plate margins. Diamond trap and conventional multi-anvil experiments were combined with a recently developed technique, in which the liquid phases, quenched from high-pressure, high-temperature conditions, are directly laser ablated in a frozen stage and analyzed by ICP-MS. Results show that at 4 GPa a fluid containing~80 wt.% H 2 O coexists with residual eclogite up to 850 8C while a hydrous melt appears at 900 8C, indicating a solidus located between 850 and 900 8C. At 5 GPa the solidus lies between 1000 and 1050 8C but terminates at a second critical endpoint between 5 and 6 GPa. At 6 GPa a supercritical liquid, with a solute content continuously increasing with temperature, forms the volatile bearing phase. Low-temperature H 2 O-rich fluids and supercritical liquids have a nepheline-to quartz-normative, peralkaline character due to incongruent dissolution of clinopyroxene. The hydrous melts and high-temperature supercritical liquids evolve from rhyolitic to trachytic/andesitic with increasing liquid fraction. The melting reaction at the solidus changes from eutectic (4 GPa) to peritectic (5 GPa) with garnet changing from the reactant to the product side. With increasing melt fraction, the system becomes cotectic with about equal amounts of clinopyroxene and garnet consumed, the residual mineralogy being dominated by garnet. Most P-T trajectories calculated for subduction zone environment do not cross the hydrous K-free MORB solidus, therefore, the liquid phase released from the igneous portion of the subducting oceanic crust will change from fluid to supercritical liquid around 6 GPa. This change, however, is most probably associated with the change from fluid-like to melt-like properties of the mobile phase, with important consequences on trace element partitioning that will also change from fluid-to melt-like.

The mainly Permian-Triassic rocks of the Karakaya Complex exposed E-W across Turkey are critical to reconstruction of Tethys in the E Mediterranean region. Their origin remains controversial with both stratigraphic layer-cake and accretionary-type settings being advocated. Suggested tectonic settings range from back-arc rift, to accretionary prism- related to either northward or southward subduction. To test alternatives we have studied the contact relations and the internal fabric of each of the main litho-tectonic units making up the Karakaya Complex and related &quot;basement&quot; in nine outcrop areas across NW Turkey, also taking account of existing chemical and dating evidence. Our results show that the Karakaya Complex was assembled by regional-scale thrust faulting without evidence of layer cake-type stratigraphical contacts, or even of deformed sedimentary contacts separating the major lithotectonic units. In several areas (e.g. Havran) the outcrops of meta-siliciclastic s...

Seismic data acquisition for the MARCONI project took place in 2003 in the eastern half of the Bay of Biscay. The overall goal of the multichannel experiment was to collect seismic data to elucidate the structure and evolution of the southern margin of the Bay where an ...

Kern, H., 1982. Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: the role of the high-low quartz transition and of dehydration reactions. Phys. Earth Planet, Inter., 29: 12-23. The compressional-and shear-wave velocities, V~, and V 5, in typical crustal and mantle rocks were determined in a cubic anvil apparatus over a range of constant confining pressure up to 6 kbar and temperatures ranging from 20-750°C.Samples range between acidic and ultramafic with mean atomic weights between 20.4 and 23.0 and bulk densities of 2.66-3.50 g cm 3 at 6 kbar. Compressional-wave velocities are only roughly related to densities and do not generally follow lines of constant mean atomic weights. However, the rocks typical for the upper mantle, the continental and oceanic crust clearly separate into three distinct areas in the velocity versus density plot. In plutonic and metamorphic rocks velocities are strongly dependent on the mineral composition. Both V~. and Vĩ ncrease with increasing amphibole, garnet, pyroxene and olivine content. High feldspar content produces high Poisson's ratios as a consequence of relatively high compressional wave velocities compared to the respective shear wave velocities. Increasing quartz content results in a decrease of V~,and an accompanying increase of V 5. thereby significantly lowering Poisson's ratio. Elastic-wave propagation is affected to a large degree by flat cracks and pore spaces. High confining pressures act to reduce microcracks; heating causes an increase in porosity. However, microfracturing is increasingly suppressed by the simultaneous action of confining pressure. A pressure increase of at least 10 bar per 1°Cis needed to prevent the opening of cracks. Velocities are highly affected by the a-$ quartz transition. Due to stress concentrations in polycrystalline material the quartz transition temperature is shifted to higher values than might be expected from single crystal measurements. Dehydration reactions in zeolite-beanng basalts cause the velocities to decrease as a consequence of lowering the effective pressure and of a reconstitution of pore geometry. Velocity anisotropies observed in some plutonic rocks, and most metamorphic rocks, at high confining pressures and high temperatures correlate well with preferred lattice orientation of the constituent major minerals. The effect of dimensional orientation and microcracks seems to be of minor importance for the directional dependence of wave velocities under conditions of high pressure. eff c position, as well as the rock nucrostructure, may Every change in the pore-fluid pressure under

Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol 1 ) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite-eclogite is derived from a troctolitic protolith, whereas the epidote-eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO 2 , and high Al 2 O 3 and MgO. The kyanite-eclogite shows inherited cumulate layering. The phengite-eclogite has high TiO 2 , low Al 2 O 3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr-Nd isotope data indicate that the protoliths of both kyanite-eclogite and epidote-eclogite ([ 87 Sr/ 86 Sr] i~0 .703-0.704; ε Nd (T)~5.9-8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower ε Nd (T) value (1.4-4.1) of phengite-eclogite is more or less consistent with an enriched MORB protolith, their high [ 87 Sr/ 86 Sr] i ratio (0.705-0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U-Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician-Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.

Porphyry-type Cu (Mo, Au) deposits have been discovered along the Gangdese magmatic arc in the southern Tibetan Plateau. Extensive field investigations and systematic studies of geochemistry, S-Pb isotopic tracing, together with Re-Os and 40 Ar/ 39 Ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post-collisional crustal relaxation period of the Late Himalayan epoch. These porphyries are petrochemically K-enriched and belong to shoshonitic to high-K calc-alkaline series. They display enrichment of large ion lithophile elements (LILE) Rb, K, U, Th, Sr, Pb and depletion of high field strength elements (HFSE) Nb, Ta, Ti and the heavy rare earth elements (HREE) and Y without Eu anomalies. These characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources. Pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic Pb in the eastern segment of the copper belt to mantle Pb in the western segment. These constitute a mixing line of the Indian Oceanic MORB with Indian Oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components.

Alkalic rocks appear in Iceland in two off-rift areas, the Vestmann Islands (VE) off South Iceland and the Snaefellsnes (SNS) volcanic belt in West Iceland. In South Iceland a transitional rock suite forms a bridge, both chemically and geographically, between the alkalic rocks in the south and the rift zone tholeiites to the north. The two alkalic suites are similar in many ways; however, the SNS suite has a greater compositional range, it is slightly more potassic, and it has higher 87Sr/86Sr and Ce/Sm ratios. Both the alkalic suites and the tholeiites of the rift zones appear to have evolved in equilibrium with olivine, plagioclase, and augite at upper lithospheric and crustal pressures; augite is a common phenocryst in the SNS rocks and in the transitional basalts of South Iceland but not in the VE basalts. The distinguishing features of the alkalic suites are the evolved minor and trace element chemistry of late liquid fractions and, in the case of Snaefellsnes, relatively high Sr isotopic ratios. The combined evidence from mineralogical, chemical, isotopic, and volcano-tectonic criteria points toward the existence of a crustal ne-normative magma component giving rise to the alkalic suites in Iceland, in contrast to the generally accepted mode of origin for oceanic alkaline volcanism. The Icelandic, and oceanic, crust is shown to be mineralogically and chemically differentiated. Interaction is favored by the thickness and tectonics of the Icelandic crust between mantle-derived magmas and the deepest crustal layers which are a potential source of ne-normative liquids derived from the breakdown of alkalic amphibole of crustal origin. Conversely, the upper crustal layers are the source of anomalously high Sr isotope ratios owing to high Rb/Sr ratios in silicic volcanic centers. Resulting from the westerly drift of the North Atlantic plate system relative to the Iceland hotspot, the rift system has been successively relocated to the east during the geologic history of Iceland. While developing, the new rifts 'propagate' through older, chemically and mineralogically stratified crust, giving rise to alkaline volcanism at the leading tip, followed by FeTi basalts and later by tholeiitic rocks. This distinctive temporal and spatial spectrum of compositions results from the interaction of primary mantle-derived ocean tholeiite melt with the stratified crust. Conversely, the Snaefellsnes belt constitutes a volcanic outlier in the western plate, a dying remnant of the relocated rift system. The volcanism is of deep crustal origin: deep as shown by the lack of hydrothermal activity in the area and crustal as shown by Sr isotopes. A reinterpretation along similar lines for Hawaii and the Canary Islands shows the initial alkaline phase of intraplate plume activity (Loihi, Lanzarote) to be tectonically and petrochemically closely resemblant of the tip of a propagating rift, whereas the final transitional to alkaline rock suites of these islands resemble the plate-trapped volcanic centers of Snaefellsnes. Oceanic alkaline volcanism in general is thus interpreted in terms of the interaction of primary mantle-derived tholeiitic magmas and the oceanic crust.

Sapphirine is a metamorphic mineral that forms in the deep crust in rocks with distinctive bulk compositions, in particular high concentrations of Mg and Al. This study investigates a thin, discontinuous layer of sapphirine-bearing garnet-orthopyroxene (SGO) granulite within a Palaeoproterozoic mafic granulite together with metamorphosed mafic and ultramafic rocks, micaceous rocks rich in magnetite and pyrite, and marble. All of these rocks occur within a low-strain zone in the Lewisian complex of South Harris, NW Scotland. Data on mineral compositions and major, trace, and rare earth element (REE) patterns provide evidence for the origin of the precursor of the SGO granulite. The host mafic granulite has a trace element signature and REE pattern comparable with that of E-type MORB. The chemical abundances of Nb, Ta, La, Ce, Nd, Zr, Hf, Ti, and V in the SGO granulite, except for one sample that records total element loss, are similar to those of the host mafic granulite; however, in terms of whole-rock element abundances, the former is relatively enriched in MgO and depleted in CaO, Na 2 O, MnO, Sr, and Eu. Elements within the SGO granulite that were apparently unaffected by hydrothermal alterationdV, Y, Zr, and Crdare within the range of values observed in the host mafic granulite. SmeNd whole-rock isotope systematics suggest that both the host mafic granulite and SGO granulite were metamorphosed at w1.9 Ga, and the Nd initial ratio is consistent with a MORB source at that time. There is no significant difference in the 3Nd values of the two rock types, suggesting that they originally belonged to the same protolith. Chemical trends within a narrow zone between the SGO and host rock granulite suggest that the former was derived from a basaltic precursor of the latter by a process of infiltration metasomatism, comparable with the chemical exchange that takes place when hydrothermal fluids in present-day oceanic crust pass through vents in a ''recharge zone.'' The geological and chemical relations observed in the South Harris rocks are consistent with the following model: hydrothermally altered oceanic basaltic crust was trapped in an accretionary wedge and subducted, followed by granulite-facies metamorphism in a deep continental environment during arcecontinent collision. This is the first report of hydrothermally altered oceanic crust that was converted to sapphirine-bearing granulite deep in the continental crust.

Growing evidence suggests that the mechanism of Palaeozoic continental growth in Central Asia was by subductionaccretion with punctuated collisions that produced ophiolitic sutures between accreted blocks. The Bayankhongor ophiolite is the largest ophiolite in Mongolia and possibly all of Central Asia, and is interpreted to mark the collisional suture between the Baidrag and Hangai continental blocks. New 207 Pb/ 206 Pb zircon evaporation ages for granite plutons and dykes that intrude the ophiolite and its neighbouring lithotectonic units suggest that the ophiolite was obducted at c. 540 Ma at the beginning of a collisional event that lasted until c. 450 Ma. The new data, combined with that of previous studies, indicate regional correlation of isotopic ages north-westward from Bayankhongor to southern Tuva. These data record oceanic crust formation at c. 570 Ma, followed by approximately 30 million years of subduction -accretion that culminated in obduction of ophiolites, collision related metamorphism, and magmatism in the period c. 540 -450 Ma. Correlation of isotopic-age data for the ophiolites of western Mongolia and southern Tuva suggests that the ophiolites define a major collisional suture in the Central Asian Orogenic Belt (CAOB) that defines the southern and western margins of the Hangai continental block. D

The Blanco trough (Juan de Fuca ridge) exposes a continuous section of the upper part of the oceanic crust (basalts and sheeted dykes) hydrothermally altered in a transform fault context. Coupled microthermometric measurements on fluid inclusions and stable oxygen isotope compositions of secondary phases from the diabase unit were performed to investigate the evolution of hydrothermal fluids. The existence of a dense fracture network directly linked to the geotectonic transform fault environment has maintained a still hot ...

Using gravity, geoid, topography and surface heat flow data, we have modeled the density and temperature distribution in the lithosphere along three profiles crossing Iran in SW-NE direction from the Arabian foreland in the SW to the South Caspian Basin and the Turan Platform to the NE. We find thin lithosphere (100-120 km) underneath central Iran, whereas thick lithosphere (up to 240 km) is found underneath Arabia, the South Caspian Basin and the Turan Platform. Crustal thickening is found under the Zagros and Alborz mountains (up to 60 km) and under the Kopet-Dagh Mountains (48 km), whereas the thin crust under the southern Caspian Sea is either an oceanic crust or a highly thinned continental one. Below the South Caspian Sea, the shape of the crust-mantle interface and the base of the lithosphere indicate a subduction of the South Caspian block toward the N-NW. Further east, under the Kopet-Dagh, no evidence for active subduction is visible. This can be explained by a rheologically very strong South Caspian block, surrounded by weaker continental lithosphere.

The very different crustal thicknesses between continental and oceanic crust lead to the establishment of distinct styles of regional seismic wavefield. Observations and numerical simulations of events near subduction zones in Japan allow a direct comparison of seismic wave propagation in oceanic and continental structures. The thick zone of lower seismic velocities in the continental crust acts to trap S-wave energy in particular and generates crustally guided phases such as Pg, Lg. In the oceanic environment there is no efficient S-wave trapping and crustal energy progressively leaks into the mantle. Reverberations in the crust and the water above help to sustain the amplitude of the mantle arrivals Pn, Sn to large distances.

Fluid flow through the axial hydrothermal system at fast spreading ridges is investigated using the Srisotopic composition of upper crustal samples recovered from a tectonic window at Pito Deep (NE Easter microplate). Samples from the sheeted dike complex collected away from macroscopic evidence of channelized fluid flow, such as faults and centimeter-scale hydrothermal veins, show a range of 87 Sr/ 86 Sr from 0.7025 to 0.7030 averaging 0.70276 relative to a protolith with 87 Sr/ 86 Sr of $0.7024. There is no systematic variation in 87 Sr/ 86 Sr with depth in the sheeted dike complex. Comparison of these new data with the two other localities that similar data sets exist for (ODP Hole 504B and the Hess Deep tectonic window) reveals that the extent of Sr-isotope exchange is similar in all of these locations. Models that assume that fluid-rock reaction occurs during one-dimensional (recharge) flow lead to significant decreases in the predicted extent of isotopic modification of the rock with depth in the crust. These model results show systematic misfits when compared with the data that can only be avoided if the fluid flow is assumed to be focused in isolated channels with very slow fluid-rock exchange. In this scenario the fluid at the base of the crust is little modified in 87 Sr/ 86 Sr from seawater and thus unlike vent fluids. Additionally, this model predicts that some rocks should show no change from the fresh-rock 87 Sr/ 86 Sr, but this is not observed. Alternatively, models in which fluid-rock reaction occurs during upflow (discharge) as well as downflow, or in which fluids are recirculated within the hydrothermal system, can reproduce the observed lack of variation in 87 Sr/ 86 Sr with depth in the crust. Minimum time-integrated fluid fluxes, calculated from mass balance, are between 1.5 and 2.6 Â 10 6 kg m À2 for all areas studied to date. However, new evidence from both the rocks and a compilation of vent fluid compositions demonstrates that some Sr is leached from the crust. Because this leaching lowers the fluid 87 Sr/ 86 Sr without changing the rock 87 Sr/ 86 Sr, these mass balance models must underestimate the time-integrated fluid flux. Additionally, these values do not account for fluid flow that is channelized within the crust.

X ray diffraction and transmission electron microscopy on laser-heated diamond cell samples show that with increasing pressure MgA1204 spinel transforms first to A1203 corundum + MgO periclase, then to the CaF%O4-structured phase, and finally to a new phase having the CaTi204 structure above-40 GPa. The CaFe204 and the CaTi204 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe204 to the CaTi204 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle. MgAI:O 4 (4.310 g/cm 3 [Liu, 1978]) is much higher than that of the CaF%O 4 phase (3.937 g/cm 3 [lrifune et al., 1991]), increasing pressure would seem to favor the relative stability of e-MgAl:O4. Pressure uncertainties could be the cause of the apparent discrepancy between these two reports. Alternatively, it is possible that e-MgAl•O4 is metastable because it was synthesized from a reactive gel of MgAI:O 4

The name Ribeira Belt was originally used in a reference to a broad belt of deformed crust, which strikes subparallel to the southeastern Brazilian coastline. It was originally believed to be Neoproterozoic (Brasiliano-Panafrican), but present investigations indicate that this age seems to be mainly related to the extensive arc related plutonism and tectono-metamorphism that affected its most representative formations, whereas the time of deposition of the supracrustals would be significantly older. In this study, the main tectono-stratigraphic features of the Apiaí Domain of the Ribeira Belt are described. The lithostratigraphy is reviewed, as well the paleogeographic, structural, geochemical and radiometric data, and their implications for the tectonic evolution models for the region. The marine sediments of the Apiaí Domain were laid down on the borders of the Paraná and Luís Alves cratons, and on an intervening Mesoproterozoic (1.8-1.1 Ga) ocean crust. They were intensively deformed by ocean closure and collision of the cratonic fragments, and they constitute the majority of the local supracrustals designated as the Açungui Supergroup. Symmetrical continental passive margin deposits are represented to the northwest by a more proximal facies (Itaiacoca Group), probably related to the Paraná paleocontinent, and by the Capiru Formation to the southeast. Between them lies the Central Domain formed by the Votuverava Group, characterized by the remains of gradually deepening carbonatic platform sediments of the Lajeado Subgroup, which grade eastward into distal turbiditic sequences of the Ribeira Subgroup, where ocean floor and immature island arc magmatic rocks are present. During the Neoproterozoic, the Apiaí Domain behaved as a single continental mass into which basic rocks were intruded between 1.1 and 0.85 Ga. Between 0.7 and 0.6 Ga, the Apiaí Domain evolved into a Cordilleran-type magmatic arc, related to the evolution of a subduction zone, possibly situated to the southeast of the Luís Alves cratonic fragment. This subduction caused metamorphism and migmatization of the supra-and infra-crustal rocks of the region, and produced the extensive calc-alkaline granitoid plutonism. Between 0.6 and 0.45 Ga, the majority of the southeastern continental blocks amalgamated during closure of the Adamastor Ocean, followed by development of a system of regional dextral transcurrent shear zones related to oblique collision and/or escape tectonics. Pull-apart basins opened up and were filled with molasse sequences. The region was intruded by post-tectonic granitoids and then underwent regional cooling.

Multibeam bathymetry and bottom imaging (Simrad EM12D) studies on an area of about 9500 km2 were conducted over the Pitcairn hotspot near 25°10′S, 129° 20′W. In addition, 15 dives with the Nautile submersible enabled us to obtain ground-true observations and to sample volcanic structures on the ancient ocean crust of the Farallon Plate at 3500-4300 m depths. More than 100 submarine volcanoes overprint the ancient crust and are divided according to their size into large (>2000 m in height), intermediate (500-2000 m high) and small (<500 m high) edifices. The interpretation of seafloor backscatter imagery accompanied by submersible observations and sampling enabled us to infer that the total volume of submarine lava erupted during hotspot activity is about 5900 km3 within a radius of about 110 km. The most recent volcanic activities occur on both small and large edifices composed of a great variety of lava flows. These flows vary in composition, following a succession from picritic basalt to alkali basalt, trachybasalt, trachy-andesite and to trachyte. Their large range of SiO2 (48-62%), Na2O+K2O (2-11%), Ba (300-1300 ppm), MgO (1-11%), Nb (19-130 ppm), Ni (4-400 ppm) and rare earth elements suggests that crystal-liquid fractionation from basanite and/or picritic melt sources was a major process. The variation in composition between the least evolved basaltic rocks and the other more evolved silicic lava is marked by a difference in their flow morphology (pillow, giant tubes, tabular to blocky flows). The lava composition and field observation indicate that several magmatic pulses giving rise to cyclic eruptions are responsible for the construction of the edifices. The two larger edifices (>2000 m high) show more extensive eruptive events and a wider range in compositional variability than the smaller (<500 m high) ones. Several (five) submersible transects made along the slope of one of the largest edifices (Bounty) enabled us to observe at least nine successive eruptive cycles progressing from pillow and giant tubular basalt to tabular/blocky trachy-andesite and trachyte flows. Pyroclasts and hyaloclastites are often found with these eruptive sequences. The smaller edifices, forming individualized cones, are built mainly of evolved silicic (SiO2>53%) flows consisting essentially of alternating sequences of trachy-andesite and trachyte. The distribution and composition of the small edifices suggest that they are the result of sub-crustal forceful magma injection and channeling supplied from reservoirs associated with the large volcanoes.

The Tianshan (Tien Shan) Range is an important Paleozoic collisional orogenic belt and the key to understand the central Asia tectonic evolution. This paper integrates our research results with the existing Chinese and international literature on sedimentology, geochemistry, isotopic geochronology, paleontonology and paleomagnetism of the Tianshan and Tarim regions to propose that the oblique collision may have played an important role in the late Paleozoic closing of the southern Tianshan oceanic basin. As a result of the Sinian (latest Proterozoic, younger than 800 Ma) continental extension and rifting process, the Tarim and Yili blocks separated from their parent continent in the Late Cambrian-Ordovician. The southern Tianshan oceanic crusts between the two blocks subducted northward beneath the southern margin of the Yili block in the Silurian. During the Devonian-Early Carboniferous, the Tarim block rapidly drifted to the north and rotated about 46º clockwise. This process induced the collision of the Yili micro-continent with the eastern segment (present geographical position) of the Tarim continent in the Late Devonian, and the southern Tianshan oceanic crust evolved to be a west-facing remnant oceanic basin. During the Late Carboniferous-Early Permian, the Tarim block, located within an almost constant latitude range, rotated about 26º clockwise with respect to the Yili micro-continent, which ultimately closed the remnant oceanic basin in a 'scissors-like' manner from east to west and completed the Tarim-Yili collision. Subsequent A-type subduction of the Tarim continental crust and lithosphere-scale sinistral shearing generated a magmatic arc on the southern margin of the Tarim-Yili suture zone. The Late Permian-Early Triassic clastics deposited in a peripheral foreland basin developed above the arc.

Eighteen basic rocks from Ascension Island (South Atlantic) give a mean S7Sr/86 Sr ratio of 0.70311 + 17 for both volcanics and plutonic inclusions. The late-stage differentiated rocks (rhyolites and granitic inclusions) have much higher 87Sr/86 Sr ratios, up to 0.712. All these rocks display the same range of Nd isotopic compositions (~ •a values from 6.9 to 11.1 with a mean on 12 samples of 8.4 + 0.6) implying a cogenetic relation between the two sequences. The D/H systematics lead to the same conclusion. In the Nd-Sr diagram, the data plot close to the mantle array and show a positive correlation. This suggests a mixing between a depleted MORB-type mantle, i.e. the upper mantle, and a hot-spot with less depleted geochemical characteristics, i.e. the OIB mantle source. The total range of 8180 values lies between 4.8%0 for plagioclase cumulates and 6.7%o for the most evolved rocks (peralkaline granites and comendites). The basic rocks have values around 5.3%0, typical of mantle-derived material. These oxygen data indicate that the high 875r/86 Sr ratios in the most evolved rocks (both volcanic and plutonic terms) result from the combination of two different processes: incorporation of slight amounts (< 1%) of high-temperature altered oceanic crust by the magma in the late stages of the differentiation process and then in-situ Rb decay since the time of formation of these rocks. Both processes were very effective because of the high Rb and low Sr contents of these evolved rocks. Oxygen isotope systematics in the Ascension Island granites and rhyolites indicate that a fractional crystallization process alone does not produce 8180 values higher than 6.7%~, i.e. that the ultimate 8180 enrichment, relative to the initial basic magma, is not greater than 1.5%o. * FNRS "' Chercheur Qualifi~".