Cold Oceanic Lithosphere Research Articles

Modern-style arc magmatism since ca. 2.2 billion years ago revealed by Nb/Ta-Dy/Yb systematics in igneous rocks

The subduction of oceanic lithosphere is a significant mechanism in plate tectonics, which regulates volcanic activity along convergent plate boundaries on the modern Earth. However, while there is evidence for at least episodic, hot subduction in the Archaean, modern-style subduction might have not occurred on the early Earth under a different tectonic regime, potentially due to the high mantle temperature. It remains uncertain when modern-style subduction and its corresponding arc magmatism started operating on a global scale. Here we show that the modern arc intermediate-felsic rocks exhibit a distinct Nb/Ta-Dy/Yb correlation compared to their Archean counterparts due to the contrasting petrogenesis. Our statistical analysis of a worldwide igneous rock database, spanning geological history, indicates the widespread occurrence of intermediate-felsic rocks with modern arc signatures since ca. 2.2 Ga, which coincides with the onset of the supercontinent cycle. Continuous subduction of the cold oceanic lithosphere not only gave rise to arc magmatism along active margins but also accelerated the cooling rate of the mantle, which largely terminated the widespread formations of intermediate-felsic rocks derived from crustal delamination and slab melting.

Assembly of the basal mantle structure beneath Africa.

Plate tectonics shapes Earth's surface, and is linked to motions within its deep interior1,2. Cold oceanic lithosphere sinks into the mantle, and hot mantle plumes rise from the deep Earth, leading to volcanism3,4. Volcanic eruptions over the past 320 million years have been linked to two large structures at the base of the mantle presently under Africa and the Pacific Ocean5,6. This has led to the hypothesis that these basal mantle structures have been stationary over geological time7,8, in contrast to observations and models suggesting that tectonic plates9,10, subduction zones11-14 and mantle plumes15,16 have been mobile, and that basal mantle structures are presently deforming17,18. Here we reconstruct mantle flow from one billion years ago to the present day to show that the history of volcanism is statistically as consistent with mobile basal mantle structures as with fixed ones. In our reconstructions, cold lithosphere sank deep into the African hemisphere between 740 and 500 million years ago, and from 400 million years ago the structure beneath Africa progressively assembled, pushed by peri-Gondwana slabs, to become a coherent structure as recently as 60 million years ago. Our mantle flow models suggest that basal mantle structures are mobile, and aggregate and disperse over time, similarly to continents at Earth's surface9. Our models also predict the presence of continental material in the mantle beneath Africa, consistent with geochemical data19,20.

Rapid cold slab subduction of the Paleo-Tethys: Insights from lawsonite-bearing blueschist in the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock εNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (<10 Ma) between the protolith generation and the high-pressure metamorphic peak further supports a rapid subduction process. Our results suggest that the young oceanic slab was dragged down by the thicker, long-lived, and cold downgoing Paleo-Tethyan Ocean plate, where it experienced blueschist-facies metamorphism. The co-occurrence of lawsonite-bearing epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists and eclogites further points to a cold thermal structure at the convergent plate interface, leading to the interpretation that the CMOB represents a typical oceanic subduction-accretion belt. The results presented in this study provide important insights into the geodynamic evolution of the Paleo-Tethys.

"Diffusive" border of isotopic reservoirs of indian and pacific morb types beneath Kamchatka

The convergence zones of lithospheric plates in the Northwest Pacific are the boundaries of the two main isotopic domains of the Earth - the Indian and Pacific MORB types, separated be cold oceanic lithosphere. This configuration limits of their interaction by special geodynamic environments - the influence of deep plume sources or the distraction of the subducted slab and intrusion of the oceanic asthenosphere into the subcontinental mantle wedge. The latter mechanism is reconstructed in the Central Kamchatka Depression on the basis of geological, geochemical, and high-precision (double-spike) lead isotopic data. The role of the oceanic asthenosphere in magma generation in the zones of convergence of oceanic and continental lithospheres is a poorly studied but not unique phenomenon that must be considered under geodynamic reconstructions and the creation of new, more realistic models of the juvenile continental crust formation.

Collapse of passive margins by lithospheric damage and plunging grain size

The collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction. In order for a new trench to form at the junction between oceanic and continental plates, the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates. Such rates are especially hard to attain in the cold ductile portion of the lithosphere, at which the mantle lithosphere reaches peak strength. The amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress. Stress in a cooling passive margin increases with time (e.g., due to ridge push), and is augmented by stresses present in the lithosphere at the onset of rifting (e.g., due to drag from underlying mantle flow). Increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep, or by decreasing grain size in conjunction with a grain-size sensitive rheology like diffusion creep. While the increasing stress acts to weaken the lithosphere, the decreasing temperature acts to stiffen it, and the dominance of one effect or the other determines whether the margin might weaken and collapse. Here, we present a model of the thermal and mechanical evolution of a passive margin, wherein we predict formation of a weak shear zone that spans a significant depth-range of the ductile portion of the lithosphere. Stiffening due to cooling is offset by weakening due to grain size reduction, driven by the combination of imposed stresses and grain damage. Weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere, making the collapse of a passive margin unlikely in this scenario. However, adding even a small stress-contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere. We posit that rapid grain size reduction in the ductile portion of the lithosphere can enable, or at least significantly facilitate, the collapse of a passive margin and initiate a new subduction zone. We use this model to estimate the conditions for passive margin collapse for modern and ancient Earth, as well as for Venus.

2.09 Ga old eclogites in the Eburnian-Transamazonian orogen of southern Cameroon: Significance for Palaeoproterozoic plate tectonics

Lenses of retrogressed eclogites occur in a 100 km wide zone of the Nyong Complex, a remnant of the Eburnian-Transamazonian orogen, marking a Palaeoproterozoic suture between the Congo and São Francisco Cratons. The eclogites show trace element pattern (depleted in LREE) similar to those of mid-ocean ridge basalts, indicating that the precursor melts formed in a depleted mantle source and the eclogites formed from oceanic crust. Despite numerous plagioclase ‘exsolutions’ up to 25 mol% jadeite component is preserved in omphacite and points to minimum pressures of 16 kbar at c. 800 °C. Pressures may have been 18–20 kbar as indicated by estimated compositions of peak omphacite. The age of eclogite metamorphism has been constrained by U-Pb SHRIMP dating of zircon at 2093 ± 45 Ma. The eclogites are associated with 2.05 Ga old charnockites and mafic granulites containing textures characteristic for near-isobaric cooling. These rocks may represent the plate above a subduction zone in which the eclogites were tectonically emplaced. With an age of 2.09 Ga the eclogites of the Nyong Complex are older than other subduction related Palaeoproterozoic eclogites of the Ubendian (1.88 Ga) and Usagaran belts (2.0 Ga) at the southern border of the Tanzania Craton. They are also older than eclogites in the Belomorian province (1.9 Ga; Russia) and thus represent the oldest known eclogites outcropping in an orogenic belt. The African eclogites (all with MORB chemistry) indicate that during the formation of the Nuna supercontinent the Palaeoproterozoic oceanic lithosphere around the Congo-Tanzania Craton was thick, cold and rigid enough to become subducted similar to cold oceanic lithosphere in the modern plate tectonic regime. However, apparent geothermal gradients of 12–14 °C/km for the Palaeoproterozoic eclogites are higher than those of Neoproterozoic and Phanerozoic eclogites and are interpreted as the result of warm subduction in a hotter Palaeoproterozoic Earth.

The isolated ∼680 km deep 30 May 2015 MW 7.9 Ogasawara (Bonin) Islands earthquake

Deep-focus earthquakes, located in very high-pressure conditions 300 to 700 km below the Earth's surface within sinking slabs of relatively cold oceanic lithosphere, are mysterious phenomena. The largest recorded deep-focus earthquake (MW 7.9) in the Izu–Bonin slab struck on 30 May 2015 beneath the Ogasawara (Bonin) Islands, isolated from prior seismicity by over 100 km in depth, and followed by only a few small aftershocks. Globally, this is the deepest (680 km centroid depth) event with MW≥7.8 in the seismological record. Seismicity indicates along-strike contortion of the Izu–Bonin slab, with horizontal flattening near a depth of 550 km in the Izu region and rapid steepening to near-vertical toward the south above the location of the 2015 event. This event was exceptionally well-recorded by seismic stations around the world, allowing detailed constraints to be placed on the source process. Analyses of a large global data set of P, SH and pP seismic phases using short-period back-projection, subevent directivity, and broadband finite-fault inversion indicate that the mainshock ruptured a shallowly-dipping fault plane with patchy slip that spread over a distance of ∼40 km with a multi-stage expansion rate (∼5+ km/s down-dip initially, ∼3 km/s up-dip later). During the 17 s total rupture duration the radiated energy was ∼3.3×1016 J and the stress drop was ∼38 MPa. The radiation efficiency is moderate (0.34), intermediate to that of the 1994 Bolivia and 2013 Sea of Okhotsk MW 8.3 deep earthquakes, indicating that source processes of very large deep earthquakes sample a wide range of behavior from dissipative, more viscous failure to very brittle failure. The isolated occurrence of the event, much deeper than the apparently thermally-bounded distribution of Bonin-slab seismicity above 600 km depth, suggests that localized stress concentration associated with the pronounced deformation of the Izu–Bonin slab and proximity to the 660-km phase transition likely played a dominant role in generating this major earthquake.

Oxygen isotopes in Pilbara Craton zircons support a global increase in crustal recycling at 3.2 Ga

Oxygen isotopes were measured in zircon crystals from a suite of Paleo- to Mesoarchean igneous and sedimentary rocks from the Pilbara Craton in order to test prevailing models of early Earth tectonic evolution. Our results indicate that igneous zircon crystals older than 3.2Ga in the Pilbara Craton have mantle-like oxygen isotope signatures, whereas zircon grains younger than c. 3.2Ga show, on average, isotopically heavier δ18O values. Stringent tests on the validity of the isotopically heavy oxygen values – in terms of whether they reflect primary properties of the zircon crystals acquired under magmatic genesis, or the result of later alteration – were conducted in some cases by simultaneous measurement of 16O1H/16O, in conjunction with evaluating more traditional indicators of metamictisation. These data demonstrate that whereas some of the heaviest values are anomalous and due to post-crystallisation alteration via incorporation of water into the zircon structure, others record primary values.The data presented here are used to support a previously documented change in tectonic style in the Pilbara Craton at 3.2Ga, from early crustal growth through magmatic accretion above upwelling, hot mantle, to crustal growth that involved significant amounts of crustal recycling arising from the onset of modern-style plate tectonics (steep subduction of old cold oceanic lithosphere). These results align with global datasets of oxygen isotopes, and point to a more general change in the geodynamics of Earth associated with the secular decrease in heat output of our planet.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

AbstractThe Phanerozoic subduction system of the Korean peninsula is considered to have been activated by at least Middle Permian time. The geochemically arc-like Andong ultramafic complex (AUC) occurring along the border between the Precambrian Yeongnam massif and the Cretaceous Gyeongsang back-arc basin provides a rare opportunity for direct study of the pre-Cretaceous mantle wedge lying above the subduction zone. The tightly constrained SHRIMP U–Pb age of zircons extracted from orthopyroxenite specimens (222.1±1.0 Ma) is indistinguishable from the Ar/Ar age of coexisting phlogopite (220±6 Ma). These ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment. The nearly coeval occurrence of a syenite-gabbro-monzonite suite in the SW Yeongnam massif also suggests an extensional tectonic setting along the continental margin side during Late Triassic time. The relatively enriched ɛHf range of dated zircons (+6.2 to −0.6 at 222 Ma) is in contrast to previously reported primitive Sr–Nd–Hf isotopic features of Cenozoic mantle xenoliths from Korea and eastern China. This enrichment is not ascribed to contamination by the hypothetical Palaeozoic crust beneath SE Korea, but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo-Pacific plate. Most AUC zircons show a restricted core-to-rim spread of ɛHf values, but some grains testify to the operation of open-system processes during magmatic differentiation.

The elasticity of lawsonite at high pressure and the origin of low velocity layers in subduction zones

Subduction zones exhibit faster seismic wave velocities compared to the surrounding mantle due to the recycling of relatively cold oceanic lithosphere. In certain subduction zones, however, a 5–10km thick low velocity layer (LVL) has been inferred to exist along the top surface of the subducting slab at depths of up to 250km. Shear-wave velocities, in particular, within these layers have been estimated as up to 10% slower than the surrounding mantle.We have conducted high-pressure ultrasonic interferometric measurements to gain insight into the elastic properties of lawsonite [CaAl2(Si2O7)(OH)2·H2O], a hydrous mineral phase stabilized under cold subduction zone conditions. In addition, we have computed the full elastic constant tensor at elevated pressures and temperature, using static electronic structure calculations and molecular dynamics simulations. The bulk and shear modulus obtained from theory and experiments are in good agreement. We find that lawsonite has an unusually low shear modulus at high pressure and its formation in subducted oceanic crust can explain some seismic evidence for LVL at depths exceeding 100km. To approach estimated LVL velocities requires lawsonite to form in the subducting crust as a result of a fluid influx due to the breakdown of other hydrous minerals such as serpentine. The formation of lawsonite additionally lowers seismic velocities because it forms at the expense of garnet, a mineral with relatively fast seismic velocities. LVL observations may therefore be used to place constraints on the amount of H2O subducted into the deep mantle.

High-temperature, high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian, NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at >10–11kbar and 450–650°C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900°C and at 14 to 30kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22kbar and 1060–1040°C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a >90km×40km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.

Modern mermaids: New floats image the deep Earth

Scientists get a window into deep Earth structures by using a method called seismic tomography. Similar to computed tomography (CT) scans of the brain, seismic tomography uses delays in the arrival times of seismic P waves to make scans and three‐dimensional images of the variations in seismic wave speed in the Earth's interior. Patterns in the delays indicate thermal or compositional anomalies in the Earth's mantle and core, such as those caused by sinking cold oceanic lithosphere or rising hot thermal plumes. A large number of observations of such delays are available for continental regions; the number of observations for the United States is especially high due to a dense deployment of stations currently being installed temporarily and moved across a large area as part of the USArray project, a branch of the U.S. National Science Foundation's EarthScope program. In contrast, no comparable sensor density has been available in the oceans

Mediterranean snapshots of accelerated slab retreat: subduction instability in stalled continental collision

Abstract This review of Mediterranean geodynamics highlights that the Mediterranean region captures at a fortuitous moment in time, a picture of the fate of foundering, old, cold oceanic lithosphere of limited area due to being landlocked in an all-but-stalled continental collision (Africa with Europe). We synthesize the geological spatial and temporal data for stretched crust as well as the 3D distribution of old abyssal plains and new oceanic lithosphere segments in concert with heat flow, palaeomagnetic data, geodetic velocity data, earthquake hypocentre distributions and seismic tomography. We use three Mediterranean subduction system settings (the western Mediterranean, the Hellenic and the Pannonian–Carpathian) that nicely reflect the slab instability and retreat. We assume that mantle slab dynamics best explains the observations. The dispersal and segmentation of the foundering landlocked ocean results in a series of discontinuous subduction zones whose individual lengths gradually diminish while retreat accelerates as slab progressively narrow and tear (i.e. along-strike laterally-propagating slab break-off) due to imminent total consumption of available oceanic lithosphere. We suggest that the Mediterranean region offers a lucid series of snapshots of accelerated slab retreat that, additionally, is globally unique as the only present day example of what we term intra-collisional landlocked ocean subduction.

Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former continental margin. The mafic cumulates and restites, owing to their high densities, eventually foundered into the mantle, leaving behind a more felsic crust. Our grid-based sampling allows us to estimate an unbiased average upper crustal composition for the Peninsular Ranges Batholith. Major and trace-element compositions are very similar to global continental crust averaged over space and time, but in detail, the Peninsular Ranges are slightly lower in compatible to mildly incompatible elements, MgO, Mg#, V, Sc, Co, and Cr. The compositional similarities suggest a strong arc component in global continental crust, but the slight discrepancies suggest that additional crust formation processes are also important in continent formation as a whole. Finally, the delaminated Sierran garnet pyroxenites have some of the lowest U/Pb ratios ever measured for silicate rocks. Such material, if recycled and stored in the deep mantle, would generate a reservoir with very unradiogenic Pb, providing one solution to the global Pb isotope paradox.

Recycling the lid: Effects of subduction and stirring on boundary layer dynamics in bottom-heated planetary mantle convection

[1] The subduction and stirring of cold oceanic lithosphere governs the thermal regime of the Earth's mantle. Whether upwelling mantle plumes are transient isoviscous thermals or long-lived low viscosity plumes depends on the magnitude of the resulting temperature variations in the thermal boundary layer at the base of the mantle. Previous laboratory experiments suggest that low viscosity “Earth-like” plumes occur where the hot thermal boundary layer (TBL) viscosity ratio, λh > O(10). Here, the results from two-dimensional numerical simulations, in which subduction is either forced from above or allowed to arise naturally show that: (1) a morphologic transition from upwellings in the form of isoviscous thermals to cavity plumes occurs where λh ≥ O(10) and is accompanied by a qualitative change in the temporal and spatial dynamics of the hot TBL; (2) this transition corresponds to a condition in which the velocity boundary layer (VBL) is concentrated within the basal part of the TBL for no- and free-slip boundaries; and (3) a regime in which λh ≥ O(10) can only occur if the total viscosity ratio across the convecting system, λT ≥ O(102). Our results support a recent conjecture that low viscosity mantle plumes in the Earth are a consequence of strong mantle cooling by plate tectonics. Moreover, Earth-like plume models may be inappropriate for explaining the origin of surface features on one plate planets such as Mars or Venus.

Global variability of transition zone thickness

We analyze recordings from more than 3000 shallow earthquakes and measure S410S – S660S differential travel times on a global scale. These differential times are obtained using a novel cross‐correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone. Our measurements show absolute perturbations up to 8 s from the global average (242 km), which imply transition zone thickness variations of ±20 km on a length scale of ∼2000 km. The transition zone under major subduction zones, e.g., the western Pacific and South America, is significantly thicker than the global average; this could be associated with large‐scale depressions of the 660‐km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle. A narrow transition zone is observed under the central Pacific and northern Atlantic Oceans. We also observe a modest anticorrelation between S410S – S660S times and the delay times of shear waves (predicted using S12/WM13 [Su et al., 1994]) in the transition zone, which suggests considerable thermal influence on the large‐scale topography of the 410‐ and 660‐km discontinuities. The lack of a stronger anticorrelation suggests that (1) compositional heterogeneities may be present and (2) a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410‐ and the 660‐km discontinuities. A regional analysis shows that the transition zone is, on average, 6–8 km thicker under continents than under oceans; this implies a temperature difference of 50–70°C. However, we do not observe a clear correlation between thickness and the age of the crust.

Onramping of Cold Oceanic Lithosphere in a Forearc Setting: The Southeast Bohol Ophiolite Complex, Central Philippines

Geochemical and mineralogical data show that the Early Cretaceous Southeast Bohol Ophiolite Complex on Bohol island, Central Philippines, was generated in a subduction-related marginal basin. The volcanic-hypabyssal rocks show negative Nb, Zr, and Ti anomalies, suggesting an island-arc affinity. This signature is consistent with the high anorthite contents of the gabbro and norite plagioclases, although the ultramafic rocks contain low XCr [Cr/(Cr + Al) < 0.60] spinels. The ophiolite is thrust on top of a tectonic melange, the Cansiwang Melange, which in turn is thrust over the Alicia Schist, a metamorphosed oceanic crust sliver made up of chlorite schist, amphibolite schist, and quartz-sericite schist. The presence of serpentinite between the ophiolite and the Alicia Schist indicates that the amphibolite schist cannot represent the emplacement-related metamorphic sole of the Southeast Bohol Ophiolite Complex. The composition of the associated serpentinite sole (Cansiwang Melange) of the ophiolite complex suggests that this crust-mantle sequence was emplaced at a relatively low temperature (<500°C). The onramping of the Southeast Bohol Ophiolite Complex as a forearc ophiolite, followed by its collision and suturing with the Alicia Schist, occurred during the Late Cretaceous. This ophiolite could represent a fragment of the Cretaceous proto-Philippine Sea plate.

Fluid-Rock Interactions during High-Pressure and Ultrahigh-Pressure Metamorphism

Fluid inclusions and stable-isotope data of HP and UHP terrains from China and the European Alps are used to evaluate fluid-rock interactions during subduction and continental collision. The choice of the European Alps and China for a comparative study is compelling, because the rocks of the Alps exemplify subduction of a cold (mature) oceanic lithosphere, whereas those of China illustrate the responses of continental crust to subduction under relatively higher temperatures representative of the base of a thickened continental wedge. Our study demonstrates that there is no difference between the fluid-rock behavior of continental and oceanic crust during subduction and metamorphism. In both regions, there is evidence for the presence of a grain-boundary fluid phase during HP metamorphism. Recognition of grain- to hand specimen-scale chemical and isotopic heterogeneity attests to local buffering by mineral assemblages of fluid compositions. Assuming a one-dimensional fluid flow perpendicular to rock-layer contacts, we estimated fluid fluxes of 15 to 465 cm3/cm2 to as low as 1.6 cm3/cm2 in China and Alpine rocks, respectively. The values from China are to be considered as maxima because, in most cases, sampling was at a reconnaissance scale and does not permit evaluation of disequilibrium on a scale of one centimeter. An important finding, characteristic of the entire range of conditions represented by these rocks, is the retention of recognizable vestiges of pre-subduction fluid-rock interactions, be they a geothermal system with a surface-exposure length scale on the order of 100 kilometers in continental rocks of China, or relics of high- and low-temperature hydrothermal alteration features in oceanic material of the Western Alps. A key issue arising from these results concerns the plausibility of transferring a slab component to the source of arc-magma genesis in a low fluid/rock regime. Possible scenarios are discussed in light of several recent innovative experiments that have dispelled some of the speculation obscuring our understanding of fluid-melt-rock interactions during HP and UHP metamorphism.

Percolated mantle peridotites along the Romanche Fracture Zone (Equatorial Atlantic ocean)

Pl-peridotites dredged along the Romanche Fracture Zone (FZ) are peculiar rocks. These peridotites derived from weakly deformed upper mantle rocks widespread percolated by melts with gabbroic composition. Mantle peridotite reacted with hot melt percolating through it and trapped melt which crystallised in pockets and in localised conduits at higher levels in the lithosphere. Reaction and crystallisation caused the peculiar microstructures, modal and chemical compositional variations, both in the bulk rocks and in the residual mineral phases. Electron and ion microprobe analyses suggest that percolating melts, characterised by different geochemical signatures, interacted, at deep levels in the lithosphere, with the Romanche peridotites mostly modifying the chemical compositions of Spl and Cpx. These new data show that impregnated mantle peridotites were emplaced all along the Romanche FZ, also in the western Romanche ridge transform intersection, were only fertile Pl-free peridotites were previously described. The cold equatorial upper mantle belt seems to influence strongly the dynamic of melting, melt migration, extraction and its final emplacement on the seafloor. Melts produced in this cold oceanic lithosphere are not able to segregate and join the overlying oceanic crust, but freeze and remain trapped at depth.

A jadeite–quartz–glaucophane rock from Karangsambung, central Java, Indonesia

High‐pressure metamorphic rocks are exposed in Karangsambung area of central Java, Indonesia. They form part of a Cretaceous subduction complex (Luk–Ulo Complex) with fault‐bounded slices of shale, sandstone, chert, basalt, limestone, conglomerate and ultrabasic rocks. The most abundant metamorphic rock type are pelitic schists, which have yielded late Early Cretaceous K–Ar ages. Small amounts of eclogite, glaucophane rock, garnet–amphibolite and jadeite–quartz–glaucophane rock occur as tectonic blocks in sheared serpentinite. Using the jadeite–garnet–glaucophane–phengite–quartz equilibrium, peak pressure and temperature of the jadeite–quartz–glaucophane rock are P = 22 ± 2 kbar and T = 530 ± 40 °C. The estimated P–T conditions indicate that the rock was subducted to ca 80 km depth, and that the overall geothermal gradient was ∼ 7.0 °C/km. This rock type is interpreted to have been generated by the metamorphism of cold oceanic lithosphere subducted to upper mantle depths. The exhumation from the upper mantle to lower or middle crustal depths can be explained by buoyancy forces. The tectonic block is interpreted to be combined with the quartz–mica schists at lower or middle crustal depths.