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1
Zhang, Weifeng (Gordon), et Dennis J. McGillicuddy. « Warm Spiral Streamers over Gulf Stream Warm-Core Rings ». Journal of Physical Oceanography 50, no 11 (novembre 2020) : 3331–51. http://dx.doi.org/10.1175/jpo-d-20-0035.1.
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AbstractThis study examines the generation of warm spiral structures (referred to as spiral streamers here) over Gulf Stream warm-core rings. Satellite sea surface temperature imagery shows spiral streamers forming after warmer water from the Gulf Stream or newly formed warm-core rings impinges onto old warm-core rings and then intrudes into the old rings. Field measurements in April 2018 capture the vertical structure of a warm spiral streamer as a shallow lens of low-density water winding over an old ring. Observations also show subduction on both sides of the spiral streamer, which carries surface waters downward. Idealized numerical model simulations initialized with observed water-mass densities reproduce spiral streamers over warm-core rings and reveal that their formation is a nonlinear submesoscale process forced by mesoscale dynamics. The negative density anomaly of the intruding water causes a density front at the interface between the intruding water and surface ring water, which, through thermal wind balance, drives a local anticyclonic flow. The pressure gradient and momentum advection of the local interfacial flow push the intruding water toward the ring center. The large-scale anticyclonic flow of the ring and the radial motion of the intruding water together form the spiral streamer. The observed subduction on both sides of the spiral streamer is part of the secondary cross-streamer circulation resulting from frontogenesis on the stretching streamer edges. The surface divergence of the secondary circulation pushes the side edges of the streamer away from each other, widens the warm spiral on the surface, and thus enhances its surface signal.
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Peacock, Simon M. « Advances in the thermal and petrologic modeling of subduction zones ». Geosphere 16, no 4 (5 juin 2020) : 936–52. http://dx.doi.org/10.1130/ges02213.1.
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Abstract In the two decades since Subduction: Top to Bottom was published in 1996, improved analytical and numerical thermal-petrologic models of subduction zones have been constructed and evaluated against new seismological and geological observations. Advances in thermal modeling include a range of new approaches to incorporating shear (frictional, viscous) heating along the subduction interface and to simulating induced flow in the mantle wedge. Forearc heat-flux measurements constrain the apparent coefficient of friction (μ′) along the plate interface to <∼0.1, but the extent to which μ′ may vary between subduction zones remains challenging to discern owing to scatter in the heat-flux measurements and uncertainties in the magnitude and distribution of radiogenic heat production in the overriding crust. Flow in the mantle wedge and the resulting thermal structure depend on the rheology of variably hydrated mantle rocks and the depth at which the subducting slab becomes coupled to the overlying mantle wedge. Advances in petrologic modeling include the incorporation of sophisticated thermodynamic software packages into thermal models and the prediction of seismic velocities from mineralogic and petrologic models. Current thermal-petrologic models show very good agreement between the predicted location of metamorphic dehydration reactions and observed intermediate-depth earthquakes, and between the predicted location of the basalt-to-eclogite transition in subducting oceanic crust and observed landward-dipping, low-seismic-velocity layers. Exhumed high-pressure, low-temperature metamorphic rocks provide insight into subduction-zone temperatures, but important thermal parameters (e.g., convergence rate) are not well constrained, and metamorphic rocks exposed at the surface today may reflect relatively warm conditions in the past associated with subduction initiation or ridge subduction. We can anticipate additional advances in our understanding of subduction zones as a result of further testing of model predictions against geologic and geophysical observations, and of evaluating the importance of advective processes, such as diapirism and subduction-channel flow, that are not captured in hybrid kinematic-dynamic models of subduction zones but are observed in fully dynamical models under certain conditions.
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Lee, Mei-Man, et A. J. George Nurser. « Eddy Subduction and the Vertical Transport Streamfunction ». Journal of Physical Oceanography 42, no 11 (1 novembre 2012) : 1762–80. http://dx.doi.org/10.1175/jpo-d-11-0219.1.
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Abstract Subduction—the transport of fluid across the base of mixed layer—exchanges water masses and tracers between the ocean surface and interior. Eddies can affect subduction in a variety of ways. First, eddies shoal the mixed layer by restratifying water columns through baroclinic instabilities. Second, eddies induce an isopycnic transport that leads to the entrainment of warm waters and subduction of cold waters, which effectively counters the wind-driven overturning circulation. In this study, the authors use an idealized model to examine these two mechanisms by which eddies influence subduction and to discuss how eddy subduction may be better approximated using the concept of vertical transport streamfunction than the conventional meridional transport streamfunction.
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Bouilhol, Pierre, Valentina Magni, Jeroen van Hunen et Lars Kaislaniemi. « A numerical approach to melting in warm subduction zones ». Earth and Planetary Science Letters 411 (février 2015) : 37–44. http://dx.doi.org/10.1016/j.epsl.2014.11.043.
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Lee, Changyeol, et YoungHee Kim. « Role of warm subduction in the seismological properties of the forearc mantle : An example from southwest Japan ». Science Advances 7, no 28 (juillet 2021) : eabf8934. http://dx.doi.org/10.1126/sciadv.abf8934.
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A warm slab thermal structure plays an important role in controlling seismic properties of the slab and mantle wedge. Among warm subduction zones, most notably in southwest Japan, the spatial distribution of large S-wave delay times and deep nonvolcanic tremors in the forearc mantle indicate the presence of a serpentinite layer along the slab interface. However, the conditions under which such a layer is generated remains unclear. Using numerical models, we here show that a serpentinite layer begins to develop by the slab-derived fluids below the deeper end of the slab-mantle decoupling interface and grows toward the corner of the mantle wedge along the interface under warm subduction conditions only, explaining the large S-wave delay times in the forearc mantle. The serpentinite layer then allows continuous free-fluid flow toward the corner of the mantle wedge, presenting possible mechanisms for the deep nonvolcanic tremors in the forearc mantle.
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Johnston, Fraser K. B., Alexandra V. Turchyn et Marie Edmonds. « Decarbonation efficiency in subduction zones : Implications for warm Cretaceous climates ». Earth and Planetary Science Letters 303, no 1-2 (15 février 2011) : 143–52. http://dx.doi.org/10.1016/j.epsl.2010.12.049.
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Honsberger, I. W., J. Laird et J. E. Johnson. « A Laurentian margin subduction perspective : Geodynamic constraints from phase equilibria modeling of barroisite greenstones, northern USA Appalachians ». GSA Bulletin 132, no 11-12 (20 avril 2020) : 2587–605. http://dx.doi.org/10.1130/b35456.1.
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Abstract Phase equilibria modeling of sodic-calcic amphibole-epidote assemblages in greenstones in the northern Appalachians, USA, is compatible with relatively shallow subduction of the early Paleozoic Laurentian margin along the Laurentia-Gondwana suture zone during closure of a portion of the Iapetus Ocean basin. Pseudosection and isopleth calculations demonstrate that peak metamorphic conditions ranged between 0.65 GPa, 480 °C and 0.85 GPa, 495 °C down-dip along the subducted Laurentian continental margin between ∼20 km and ∼30 km depth. Quantitative petrological data are explained in the context of an Early Ordovician geodynamic model involving shallow subduction of relatively young, warm, and buoyant Laurentian margin continental-oceanic lithosphere and Iapetus Ocean crust beneath a relatively warm and wet peri-Gondwanan continental arc. A relatively warm subduction zone setting may have contributed to the formation of a thin, ductile metasedimentary rock-rich channel between the down-going Laurentian slab and the overriding continental arc. This accretionary channel accommodated metamorphism and tectonization of continental margin sediments and mafic volcanic rocks (greenstones) of the Laurentian margin and provided a pathway for exhumation of serpentinite slivers and rare eclogite blocks. Restricted asthenospheric flow in the forearc mantle wedge provides one explanation for the lack of ophiolites and absence of a well-preserved ultra-high-pressure terrane in central and northern Vermont. Exhumation of the subducted portion of the Laurentian margin may have been temperature triggered due to increased asthenospheric flow following a slab tear at relatively shallow depths.
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van Keken, Peter E., Ikuko Wada, Geoffrey A. Abers, Bradley R. Hacker et Kelin Wang. « Mafic High-Pressure Rocks Are Preferentially Exhumed From Warm Subduction Settings ». Geochemistry, Geophysics, Geosystems 19, no 9 (septembre 2018) : 2934–61. http://dx.doi.org/10.1029/2018gc007624.
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Smit, Matthijs A., et Philip A. E. Pogge von Strandmann. « Deep fluid release in warm subduction zones from a breached slab seal ». Earth and Planetary Science Letters 534 (mars 2020) : 116046. http://dx.doi.org/10.1016/j.epsl.2019.116046.
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Topuz, Gültekin, Aral I. Okay, Rainer Altherr, Winfried H. Schwarz, Gürsel Sunal et Lütfi Altınkaynak. « Triassic warm subduction in northeast Turkey : Evidence from the Ağvanis metamorphic rocks ». Island Arc 23, no 3 (29 mai 2014) : 181–205. http://dx.doi.org/10.1111/iar.12068.
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Soret, Mathieu, Philippe Agard, Benoît Ildefonse, Benoît Dubacq, Cécile Prigent et Claudio Rosenberg. « Deformation mechanisms in mafic amphibolites and granulites : record from the Semail metamorphic sole during subduction infancy ». Solid Earth 10, no 5 (23 octobre 2019) : 1733–55. http://dx.doi.org/10.5194/se-10-1733-2019.
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Abstract. This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and accreted below the upper plate mantle wedge during the first million years of intraoceanic subduction, when the subduction interface is still warm. Their formation and intense deformation (i.e., shear strain ≥5) attest to a systematic and transient coupling between the plates over a restricted time span of ∼1 Myr and specific rheological conditions. Combining microstructural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deformation, water activity, mineral change, grain size reduction and phase mixing. Results indicate that increasing pressure and temperature conditions and slab dehydration (from amphibolite to granulite facies) lead to the nucleation of mechanically strong phases (garnet, clinopyroxene and amphibole) and rock hardening. Peak conditions (850 ∘C and 1 GPa) coincide with a pervasive stage of brittle deformation which enables strain localization in the top of the mafic slab, and therefore possibly the unit detachment from the slab. In contrast, during early exhumation and cooling (from ∼850 down to ∼700 ∘C and 0.7 GPa), the garnet–clinopyroxene-bearing amphibolite experiences extensive retrogression (and fluid ingression) and significant strain weakening essentially accommodated in the dissolution–precipitation creep regime including heterogeneous nucleation of fine-grained materials and the activation of grain boundary sliding processes. This deformation mechanism is closely assisted with continuous fluid-driven fracturing throughout the exhumed amphibolite, which contributes to fluid channelization within the amphibolites. These mechanical transitions, coeval with detachment and early exhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscosity contrast and mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the overlying peridotites. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling between mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascadia), in lower continental crust shear zones and oceanic detachments.
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Behr, Whitney M., Alissa J. Kotowski et Kyle T. Ashley. « Dehydration-induced rheological heterogeneity and the deep tremor source in warm subduction zones ». Geology 46, no 5 (22 mars 2018) : 475–78. http://dx.doi.org/10.1130/g40105.1.
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Condit, Cailey B., Victor E. Guevara, Jonathan R. Delph et Melodie E. French. « Slab dehydration in warm subduction zones at depths of episodic slip and tremor ». Earth and Planetary Science Letters 552 (décembre 2020) : 116601. http://dx.doi.org/10.1016/j.epsl.2020.116601.
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Wilson, R. A., C. R. van Staal et S. Kamo. « Lower Silurian subduction-related volcanic rocks in the Chaleurs Group, northern New Brunswick, CanadaGeological Survey of Canada, Contribution No. 2008-0166.Contribution to Natural Resources Canada’s Targeted Geoscience Initiative 3 (2005–2010). » Canadian Journal of Earth Sciences 45, no 9 (septembre 2008) : 981–98. http://dx.doi.org/10.1139/e08-051.
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Early Silurian volcanic and subvolcanic rocks are preserved in the lower part of the Chaleurs Group at two locations in northern New Brunswick. At Quinn Point, mafic to intermediate rocks are hosted by sedimentary rocks of the Weir Formation, and at Pointe Rochette, a bed of felsic tuff occurs near the base of the Weir. These rocks are interpreted as the first evidence in New Brunswick of magmatism associated with Late Ordovician – Early Silurian subduction of Tetagouche–Exploits back-arc oceanic crust. At Quinn Point, mafic rocks include a thick basaltic flow or sill and intermediate to mafic cobbles in overlying conglomerate beds. The in situ mafic rocks and the conglomerate clasts are chemically alike and display subduction-related affinities on tectonic discrimination diagrams. At Pointe Rochette, fine-grained felsic tuff contains elevated Th and U and depleted high-field-strength elements, consistent with a subduction-influenced setting, although rare-earth element (REE) abundances are low and the REE profile is relatively flat. A U–Pb (zircon) age of 429.2 ± 0.5 Ma was obtained from the tuff, consistent with the late Llandovery to early Wenlock age of the overlying La Vieille Formation and coinciding with the latter stages of development of the Brunswick subduction complex. Volcanic rocks were emplaced in the arc to arc-trench gap region, probably reflecting local step-back of the magmatic axis due to accretion of continental back-arc ribbons. The low volume of Early Silurian subduction-influenced rocks is probably related to the relatively narrow width of the back-arc basin and the young, “warm” character of back-arc crust.
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Peacock, S. M. « Seismic Consequences of Warm Versus Cool Subduction Metamorphism : Examples from Southwest and Northeast Japan ». Science 286, no 5441 (29 octobre 1999) : 937–39. http://dx.doi.org/10.1126/science.286.5441.937.
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Thomas, Leif N., et Terrence M. Joyce. « Subduction on the Northern and Southern Flanks of the Gulf Stream ». Journal of Physical Oceanography 40, no 2 (1 février 2010) : 429–38. http://dx.doi.org/10.1175/2009jpo4187.1.
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Abstract Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by ∼90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.
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Furman, Tanya, Barry B. Hanan, Megan Pickard Sjoblom, Biltan Kürkcüoğlu, Kaan Sayit, Erdal Şen, Pinar Alıcı Şen et Tekin Yürür. « Evolution of mafic lavas in Central Anatolia : Mantle source domains ». Geosphere 17, no 6 (4 novembre 2021) : 1631–46. http://dx.doi.org/10.1130/ges02329.1.
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Abstract We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas, Develidağ, Erciyes, and Erkilet volcanic complexes in central Turkey and Tendürek in eastern Turkey to evaluate the mantle sources for volcanism in the context of the geodynamic evolution of the Anatolian microplate. Early Miocene through Quaternary volcanism in Western Anatolia and latest Miocene through Quaternary activity in Central Anatolia were dominated by contributions from two distinct source regions: heterogeneous metasomatized or subduction-modified lithosphere, and roughly homogeneous sublithospheric ambient upper mantle; we model the source contributions through mixing between three end members. The sublithospheric mantle source plots close to the Northern Hemisphere reference line (NHRL) with radiogenic 206Pb/204Pb of ∼19.15, while the other contributions plot substantially above the NHRL in Pb isotope space. The lithospheric source is heterogeneous, resulting from variable pollution by subduction-related processes likely including direct incorporation of sediment and/or mélange; its range in radiogenic isotopes is defined by regional oceanic sediment and ultrapotassic melts of the subcontinental lithospheric mantle. The geochemical impact of this contribution is disproportionately large, given that subduction-modified lithosphere and/or ocean sediment dominates the Pb isotope signatures of mafic Anatolian lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked crustal shortening, took place throughout the region until ca. 16–17 Ma, after which time broad delamination of the thickened lower crust and/or the Tethyan slab beneath Central Anatolia allowed for sediment and/or mélange and slab-derived fluids to be released into the overlying evolving modified mantle. Aggregation of melts derived from both mantle and lithospheric domains was made possible by upwelling of warm asthenospheric material moving around and through the complexly torn younger Aegean-Cyprean slab that dips steeply to the north beneath southern Anatolia.
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Downes, Stephanie M., Nathaniel L. Bindoff et Stephen R. Rintoul. « Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models ». Journal of Climate 23, no 24 (15 décembre 2010) : 6526–41. http://dx.doi.org/10.1175/2010jcli3620.1.
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Abstract A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m−3, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv ≡ 106 m3 s−1). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO2.
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Lewis, T. J., A. M. Jessop et A. S. Judge. « Heat flux measurements in southwestern British Columbia : the thermal consequences of plate tectonics ». Canadian Journal of Earth Sciences 22, no 9 (1 septembre 1985) : 1262–73. http://dx.doi.org/10.1139/e85-131.
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Measured heat fluxes from previously published data and 34 additional boreholes outline the terrestrial heat flow field in southern British Columbia. Combined with heat generation representative of the crust at 10 sites in the Intermontane and Omineca belts, the data define a heat flow province with a reduced heat flow of 63 mW m−2 and a depth scale of 10 km. Such a linear relationship is not found or expected in the Insular Belt and the western half of the Coast Plutonic Complex where low heat fluxes are interpreted to be the result of recent subduction. The apparent boundary between low and high heat flux is a transition over a distance of 20 km, located in Jervis Inlet 20–40 km seaward of the Pleistocene Garibaldi Volcanic Belt.The warm, thin crust of the Intermontane and Omenica Crystalline belts is similar to that of areas of the Basin and Range Province where the youngest volcanics are more than 17 Ma in age. Processes 50 Ma ago that completely heated the crust and upper mantle could theoretically produce such high heat fluxes by conductive cooling of the lithosphere. But it is more likely that the asthenosphere flows towards the subduction zone, bringing heat to the base of the lithosphere. Since the reduced heat flow is high but constant, large differences in upper crustal temperatures within this heat flow province at present are caused by large variations in both crustal heat generation and near-surface thermal conductivity. The sharp transition in heat flux near the coast is the result of the combined effects of convective heating of the eastern Coast Plutonic Complex, pronounced differential uplift and erosion across a boundary within the Coast Plutonic Complex, and the subducting oceanic plate.
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Brown, Michael, et Tim Johnson. « Metamorphism and the evolution of subduction on Earth ». American Mineralogist 104, no 8 (1 août 2019) : 1065–82. http://dx.doi.org/10.2138/am-2019-6956.
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AbstractSubduction is a component of plate tectonics, which is widely accepted as having operated in a manner similar to the present-day back through the Phanerozoic Eon. However, whether Earth always had plate tectonics or, if not, when and how a globally linked network of narrow plate boundaries emerged are matters of ongoing debate. Earth's mantle may have been as much as 200–300 °C warmer in the Mesoarchean compared to the present day, which potentially required an alternative tectonic regime during part or all of the Archean Eon. Here we use a data set of the pressure (P), temperature (T), and age of metamorphic rocks from 564 localities that vary in age from the Paleoarchean to the Cenozoic to evaluate the petrogenesis and secular change of metamorphic rocks associated with subduction and collisional orogenesis at convergent plate boundaries. Based on the thermobaric ratio (T/P), metamorphic rocks are classified into three natural groups: high T/P type (T/P > 775 °C/GPa, mean T/P ~1105 °C/GPa), intermediate T/P type (T/P between 775 and 375 °C/GPa, mean T/P ~575 °C/GPa), and low T/P type (T/P < 375 °C/GPa, mean T/P ~255 °C/GPa). With reference to published thermal models of active subduction, we show that low T/P oceanic metamorphic rocks preserving peak pressures >2.5 GPa equilibrated at P–T conditions similar to those modeled for the uppermost oceanic crust in a wide range of active subduction environments. By contrast, those that have peak pressures <2.2 GPa may require exhumation under relatively warm conditions, which may indicate subduction of young oceanic lithosphere or exhumation during the initial stages of subduction. However, low T/P oceanic metamorphic rocks with peak pressures of 2.5–2.2 GPa were exhumed from depths where, in models of active subduction, the slab and overriding plate change from being decoupled (at lower P) to coupled (at higher P), possibly suggesting a causal relationship. In relation to secular change, the widespread appearance of low T/P metamorphism in the Neoproterozoic represents a “modern” style of cold collision and deep slab breakoff, whereas rare occurrences of low T/P metamorphism in the Paleoproterozoic may reveal atypical localized regions of cold collision. Low T/P metamorphism is not known from the Archean geological record, but the absence of blueschists in particular is unlikely to reflect secular change in the composition of the oceanic crust. In addition, the premise that the formation of lawsonite requires abnormally low thermal gradients and the postulate that oceanic subduction-related rocks register significantly lower maximum pressures than do continental subduction-related rocks, and imply different mechanisms of exhumation, are not supported. The widespread appearance of intermediate T/P and high T/P metamorphism at the beginning of the Neoarchean, and the subsequent development of a clear bimodality in tectono-thermal environments are interpreted to be evidence of the stabilization of subduction during a transition to a globally linked network of narrow plate boundaries and the emergence of plate tectonics.
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Park, Yong, Sejin Jung et Haemyeong Jung. « Lattice Preferred Orientation and Deformation Microstructures of Glaucophane and Epidote in Experimentally Deformed Epidote Blueschist at High Pressure ». Minerals 10, no 9 (11 septembre 2020) : 803. http://dx.doi.org/10.3390/min10090803.
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To understand the lattice preferred orientation (LPO) and deformation microstructures at the top of a subducting slab in a warm subduction zone, deformation experiments of epidote blueschist were conducted in simple shear under high pressure (0.9–1.5 GPa) and temperature (400–500 °C). At low shear strain (γ ≤ 1), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the (010) poles were subnormally aligned with the shear plane. At high shear strain (γ > 2), the [001] axes of glaucophane were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain between 2< γ <4, the (010) poles of epidote were in subparallel alignment with the shear direction, and the [100] axes were subnormally aligned with the shear plane. At a shear strain where γ > 4, the alignment of the (010) epidote poles had altered from subparallel to subnormal to the shear plane, while the [001] axes were in subparallel alignment with the shear direction. The experimental results indicate that the magnitude of shear strain and rheological contrast between component minerals plays an important role in the formation of LPOs for glaucophane and epidote.
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Lee, Dong-Kyu, et Peter Niiler. « Influence of warm SST anomalies formed in the eastern Pacific subduction zone on recent El Niño events ». Journal of Marine Research 68, no 3 (1 mai 2010) : 459–77. http://dx.doi.org/10.1357/002224010794657191.
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Zhang, Weifeng G., et Jacob Partida. « Frontal Subduction of the Mid-Atlantic Bight Shelf Water at the Onshore Edge of a Warm-Core Ring ». Journal of Geophysical Research : Oceans 123, no 11 (novembre 2018) : 7795–818. http://dx.doi.org/10.1029/2018jc013794.
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Quinquis, M. E. T., et S. J. H. Buiter. « Testing the effects of basic numerical implementations of water migration on models of subduction dynamics ». Solid Earth 5, no 1 (26 juin 2014) : 537–55. http://dx.doi.org/10.5194/se-5-537-2014.
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Abstract. Subduction of oceanic lithosphere brings water into the Earth's upper mantle. Previous numerical studies have shown how slab dehydration and mantle hydration can impact the dynamics of a subduction system by allowing a more vigorous mantle flow and promoting localisation of deformation in the lithosphere and mantle. The depths at which dehydration reactions occur in the hydrated portions of the slab are well constrained in these models by thermodynamic calculations. However, computational models use different numerical schemes to simulate the migration of free water. We aim to show the influence of the numerical scheme of free water migration on the dynamics of the upper mantle and more specifically the mantle wedge. We investigate the following three simple migration schemes with a finite-element model: (1) element-wise vertical migration of free water, occurring independent of the flow of the solid phase; (2) an imposed vertical free water velocity; and (3) a Darcy velocity, where the free water velocity is a function of the pressure gradient caused by the difference in density between water and the surrounding rocks. In addition, the flow of the solid material field also moves the free water in the imposed vertical velocity and Darcy schemes. We first test the influence of the water migration scheme using a simple model that simulates the sinking of a cold, hydrated cylinder into a dry, warm mantle. We find that the free water migration scheme has only a limited impact on the water distribution after 1 Myr in these models. We next investigate slab dehydration and mantle hydration with a thermomechanical subduction model that includes brittle behaviour and viscous water-dependent creep flow laws. Our models demonstrate that the bound water distribution is not greatly influenced by the water migration scheme whereas the free water distribution is. We find that a bound water-dependent creep flow law results in a broader area of hydration in the mantle wedge, which feeds back to the dynamics of the system by the associated weakening. This finding underlines the importance of using dynamic time evolution models to investigate the effects of (de)hydration. We also show that hydrated material can be transported down to the base of the upper mantle at 670 km. Although (de)hydration processes influence subduction dynamics, we find that the exact numerical implementation of free water migration is not important in the basic schemes we investigated. A simple implementation of water migration could be sufficient for a first-order impression of the effects of water for studies that focus on large-scale features of subduction dynamics.
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Zhang, Liping, Lixin Wu et Jiaxu Zhang. « Coupled Ocean–Atmosphere Responses to Recent Freshwater Flux Changes over the Kuroshio–Oyashio Extension Region ». Journal of Climate 24, no 5 (1 mars 2011) : 1507–24. http://dx.doi.org/10.1175/2010jcli3835.1.
Texte intégralRésumé :
Abstract Observations have indicated a trend of freshwater loss in the global western boundary current extension regions over several recent decades. In this paper, the coupled ocean–atmosphere response to the observed freshwater flux trend [defined as evaporation minus precipitation (EmP)] over the Kuroshio–Oyashio Extension (KOE) region is studied in a series of coupled model experiments. The model explicitly demonstrates that the positive EmP forcing in the KOE region can set up a cyclonic gyre straddling the subtropical and the subpolar gyre, which induces anomalous southward cold advection in the west and northward warm advection in the interior. This leads to the formation of a temperature dipole in the midlatitudes with a cooling in the west and a warming in the east. With the positive EmP forcing in the KOE, the response of the extratropical atmospheric circulation in the North Pacific sector is characterized by an equivalent barotropic low originating primarily from the western tropical Pacific changes and countered by the extratropical SST forcing. The positive EmP forcing also strengthens the tropical zonal SST gradient and thus ENSO through several competing processes including the surface-coupled wind–evaporative–SST (WES) mechanism, subduction of extratropical warm anomalies, and spinup of the density-driven meridional overturning circulation. Applications to recent Pacific climate changes are discussed.
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Wen, Qin, et Haijun Yang. « Investigating the Role of the Tibetan Plateau in the Formation of Pacific Meridional Overturning Circulation ». Journal of Climate 33, no 9 (1 mai 2020) : 3603–17. http://dx.doi.org/10.1175/jcli-d-19-0206.1.
Texte intégralRésumé :
AbstractThe effects of the Tibetan Plateau (TP) on the Pacific Ocean circulation are investigated using a fully coupled climate model. Sensitivity experiments are designed to demonstrate that the presence of the TP is the reason for the lack of strong deep water formation in the subpolar North Pacific, because removing the TP in the model would enable the establishment of the Pacific meridional overturning circulation (PMOC). The processes involved are described in detail as follows. Removing the TP in the model would excite an anomalous high pressure over the subpolar North Pacific, causing anomalous Ekman downwelling that enhances surface water subduction north of 40°N. Removing the TP would also lead to less freshwater flux into the western Pacific, increasing sea surface salinity over the region. The high-salinity surface water can then be advected northward and eastward by the Kuroshio and its extension, subducting along the 26–27σθ isopycnal surfaces to the deeper ocean, which enables the formation of deep water in the North Pacific and the setup of the PMOC. Afterward, more high-salinity warm water would be transported from the tropics to the extratropics by the Kuroshio, leading to the establishment of the PMOC. The role of the Rocky Mountains is also examined in this study. We conclude that the Rocky Mountains may play a trivial role in modulating the meridional overturning circulations in both the Pacific and Atlantic Oceans.
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Wang, Bin, Wei Tian, Bin Fu et Jia-Qi Fang. « Channelized CO2-Rich Fluid Activity along a Subduction Interface in the Paleoproterozoic Wutai Complex, North China Craton ». Minerals 11, no 7 (9 juillet 2021) : 748. http://dx.doi.org/10.3390/min11070748.
Texte intégralRésumé :
Greenschist facies metabasite (chlorite schist) and metasediments (banded iron formation (BIF)) in the Wutai Complex, North China Craton recorded extensive fluid activities during subduction-related metamorphism. The pervasive dolomitization in the chlorite schist and significant dolomite enrichment at the BIF–chlorite schist interface support the existence of highly channelized updip transportation of CO2-rich hydrothermal fluids. Xenotime from the chlorite schist has U concentrations of 39–254 ppm and Th concentrations of 121–2367 ppm, with U/Th ratios of 0.11–0.62, which is typical of xenotime precipitated from circulating hydrothermal fluids. SHRIMP U–Th–Pb dating of xenotime determines a fluid activity age of 1.85 ± 0.07 Ga. The metasomatic dolomite has δ13CV-PDB from −4.17‰ to −3.10‰, which is significantly lower than that of carbonates from greenschists, but similar to the fluid originated from Rayleigh fractionating decarbonation at amphibolite facies metamorphism along the regional geotherm (~15 °C/km) of the Wutai Complex. The δ18OV-SMOW values of the dolomite (12.08–13.85‰) can also correspond to this process, considering the contribution of dehydration. Based on phase equilibrium modelling, we ascertained that the hydrothermal fluid was rich in CO2, alkalis, and silica, with X(CO2) in the range of 0.24–0.28. All of these constraints suggest a channelized CO2-rich fluid activity along the sediment–basite interface in a warm Paleoproterozoic subduction zone, which allowed extensive migration and sequestration of volatiles (especially carbon species) beneath the forearc.
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CAWOOD, PETER A., CHARLES A. LANDIS, ALEXANDER A. NEMCHIN et SHIGEKI HADA. « Permian fragmentation, accretion and subsequent translation of a low-latitude Tethyan seamount to the high-latitude east Gondwana margin : evidence from detrital zircon age data ». Geological Magazine 139, no 2 (mars 2002) : 131–44. http://dx.doi.org/10.1017/s0016756801006276.
Texte intégralRésumé :
Ion microprobe analyses of detrital zircons in the Te Akatarawa Terrane, New Zealand, reveal that the age of unfossiliferous turbidites overlying a fusuline- and coral-bearing limestone block olistostromal mélange is no older than 255±4 Ma (Late Permian). This is approximately 15 m.y. younger than the Kungurian age of the fusulinid limestone. We interpret this to indicate collapse of a Permian oceanic seamount as it entered a subduction zone along the Pacific margin of Gondwana. These turbidites differ markedly in composition from adjoining Permian to Middle Triassic sand-stones of the Torlesse Terrane. Detrital zircon age data indicate predominantly Permian and Carboniferous ages for source rocks supplying the Te Akatarawa turbidites, but also reveal significant earlier Palaeozoic and Proterozoic components, ranging back to 1.9 Ga. The warm-water setting of limestone blocks and the short 15 m.y. time period between sedimentation and accretion onto a continental margin require the limestone to have formed in a low-latitude position probably off the northeast Australian (New Guinea) margin of Gondwana. Zircons within the sample underwent re-crystallization at around 230±11 Ma which may be related to alteration during accretion in a subduction zone environment. Over a period of 100 to 150 m.y. from 255 Ma the terrane underwent more than 5000 km translation along the continental margin southward to its current location as an exotic mini-terrane enclosed within the New Zealand Torlesse Terrane.
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Jullion, Loic, Karen J. Heywood, Alberto C. Naveira Garabato et David P. Stevens. « Circulation and Water Mass Modification in the Brazil–Malvinas Confluence ». Journal of Physical Oceanography 40, no 5 (1 mai 2010) : 845–64. http://dx.doi.org/10.1175/2009jpo4174.1.
Texte intégralRésumé :
Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.
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Hynes, Andrew, et Toby Rivers. « Protracted continental collision — evidence from the Grenville OrogenThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent. » Canadian Journal of Earth Sciences 47, no 5 (mai 2010) : 591–620. http://dx.doi.org/10.1139/e10-003.
Texte intégralRésumé :
The Grenville Orogen in North America is interpreted to have resulted from collision between Laurentia and another continent, probably Amazonia, at ca. 1100 Ma. The exposed segment of the orogen was derived largely from reworked Archean to Paleoproterozoic Laurentian crust, products of a long-lived Mesoproterozoic continental-margin arc and associated back arc, and remnants of one or more accreted mid-Mesoproterozoic island-arc terranes. A potential suture, preserved in Grenvillian inliers of the southeastern USA, may separate rocks of Laurentian and Amazonian affinities. The Grenvillian Orogeny lasted more than 100 million years. Much of the interior Grenville Province, with peak metamorphism at ca. 1090–1020 Ma, consists of uppermost amphibolite- to granulite-facies rocks metamorphosed at depths of ca. 30 km, but areas of lower crustal, eclogite-facies nappes metamorphosed at 50–60 km depth also occur and an orogenic lid that largely escaped Grenvillian metamorphism is preserved locally. Overall, deformation and regional metamorphism migrated sequentially to the northwest into the Laurentian craton, with the youngest contractional structures in the northwestern part of the orogen at ca. 1000–980 Ma. The North American lithospheric root extends across part of the Grenville Orogen, where it may have been produced by depletion of sub-continental lithospheric mantle beneath the long-lived Laurentian-margin Mesoproterozoic subduction zone. Both the Grenville Orogen and the Himalaya–Tibet Orogen have northern margins characterized by long-lived subduction before continental collision and protracted convergence following collision. Both exhibit cratonward-propagating thrusting. In the Himalaya–Tibet Orogen, however, the pre-collisional Eurasian-margin arc is high in the structural stack, whereas in the Grenville Orogen, the pre-collisional continental-margin arc is low in the structural stack. We interpret this difference as due to subduction reversal in the Grenville case shortly before collision, so that the continental-margin arc became the lower plate during the ensuing orogeny. The structurally low position of the warm, extended Laurentian crust probably contributed significantly to the ductility of lower and mid-crustal Grenvillian rocks.
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Hoareau, G., B. Bomou, D. J. J. van Hinsbergen, N. Carry, D. Marquer, Y. Donnadieu, G. Le Hir, B. Vrielynck et A. V. Walter-Simonnet. « Did high Neo-Tethys subduction rates contribute to early Cenozoic warming ? » Climate of the Past Discussions 11, no 4 (8 juillet 2015) : 2847–88. http://dx.doi.org/10.5194/cpd-11-2847-2015.
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Abstract. The 58–51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9–50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric pCO2, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO2 at arc volcanoes ("carbonate subduction factory"). To analyze the impact of Neo-Tethys closure on early Cenozoic warming, we have modeled the volume of subducted sediments and the amount of CO2 emitted at active arc volcanoes along the northern Tethys margin. The impact of calculated CO2 fluxes on global temperature during the early Cenozoic have then been tested using a climate carbon cycle model (GEOCLIM). We first show that CO2 production may have reached up to 1.55 × 1018 mol Ma−1 specifically during the EECO, ~ 4 to 37 % higher that the modern global volcanic CO2 output, owing to a dramatic India–Asia plate convergence increase. In addition to the background CO2 degassing, the subduction of thick Greater Indian continental margin carbonate sediments at ~ 55–50 Ma may also have led to additional CO2 production of 3.35 × 1018 mol Ma−1 during the EECO, making a total of 85 % of the global volcanic CO2 outgassed. However, climate modelling demonstrates that timing of maximum CO2 release only partially fit with the EECO, and that corresponding maximum pCO2 values (750 ppm) and surface warming (+2 °C) do not reach values inferred from geochemical proxies, a result consistent with conclusions arise from modelling based on other published CO2 fluxes. These results demonstrate that CO2 derived from decarbonation of Neo-Tethyan lithosphere may have possibly contributed to, but certainly cannot account alone for early Cenozoic warming, including the EECO. Other commonly cited sources of excess CO2 such as enhanced igneous province volcanism also appear to be up to one order of magnitude below fluxes required by the model to fit with proxy data of pCO2 and temperature at that time.
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Hoareau, G., B. Bomou, D. J. J. van Hinsbergen, N. Carry, D. Marquer, Y. Donnadieu, G. Le Hir, B. Vrielynck et A. V. Walter-Simonnet. « Did high Neo-Tethys subduction rates contribute to early Cenozoic warming ? » Climate of the Past 11, no 12 (18 décembre 2015) : 1751–67. http://dx.doi.org/10.5194/cp-11-1751-2015.
Texte intégralRésumé :
Abstract. The 58–51 Ma interval was characterized by a long-term increase of global temperatures (+4 to +6 °C) up to the Early Eocene Climate Optimum (EECO, 52.9–50.7 Ma), the warmest interval of the Cenozoic. It was recently suggested that sustained high atmospheric pCO2, controlling warm early Cenozoic climate, may have been released during Neo-Tethys closure through the subduction of large amounts of pelagic carbonates and their recycling as CO2 at arc volcanoes. To analyze the impact of Neo-Tethys closure on early Cenozoic warming, we have modeled the volume of subducted sediments and the amount of CO2 emitted along the northern Tethys margin. The impact of calculated CO2 fluxes on global temperature during the early Cenozoic have then been tested using a climate carbon cycle model (GEOCLIM). We show that CO2 production may have reached up to 1.55 × 1018 mol Ma−1 specifically during the EECO, ~ 4 to 37 % higher that the modern global volcanic CO2 output, owing to a dramatic India-Asia plate convergence increase. The subduction of thick Greater Indian continental margin carbonate sediments at ~ 55–50 Ma may also have led to additional CO2 production of 3.35 × 1018 mol Ma−1 during the EECO, making a total of 85 % of the global volcanic CO2 outgassed. However, climate modeling demonstrates that timing of maximum CO2 release only partially fits with the EECO, and that corresponding maximum pCO2 values (750 ppm) and surface warming (+2 °C) do not reach values inferred from geochemical proxies, a result consistent with conclusions arising from modeling based on other published CO2 fluxes. These results demonstrate that CO2 derived from decarbonation of Neo-Tethyan lithosphere may have possibly contributed to, but certainly cannot account alone for early Cenozoic warming. Other commonly cited sources of excess CO2 such as enhanced igneous province volcanism also appear to be up to 1 order of magnitude below fluxes required by the model to fit with proxy data of pCO2 and temperature at that time. An alternate explanation may be that CO2 consumption, a key parameter of the long-term atmospheric pCO2 balance, may have been lower than suggested by modeling. These results call for a better calibration of early Cenozoic weathering rates.
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Nakanishi, Toshimichi, Wan Hong, Mitsuhiro Kuwahata, Shinji Sugiyama, Shoichi Shimoyama, Ken’ichi Ohkushi, Tatsuhiko Yamaguchi, Jung-Hun Park, Gyujun Park et Futoshi Nanayama. « Radiocarbon age offsets of Plants and Bioclasts in the Holocene Sediments from the Miyazaki Plain, Southeast Coast of Kyushu, Southwest Japan ». Radiocarbon 61, no 6 (24 octobre 2019) : 1939–50. http://dx.doi.org/10.1017/rdc.2019.114.
Texte intégralRésumé :
AbstractTo investigate the relationship between paleoenvironmental changes and marine reservoir effects, the radiocarbon ages of marine bioclasts and terrestrial plants from the same horizons of a sediment core in the Holocene Epoch were measured. This core, with a length of approximately 9 m, was obtained from the southern part of the Miyazaki Plain southeast of Kyushu Island, which faces the Kuroshio warm current. This drilling site is located in an uplift area associated with the subduction of the Philippine Sea Plate. Based on analyses of lithology, molluscan and foraminifera assemblages, and radiocarbon dating, we interpreted four sedimentary units in order of age: tidal flat, inner bay, Kikai-Akahoya volcanic ash, and delta plain. These paleoenvironmental changes were mainly associated with the sea-level rise during the deglacial period. The reservoir ages of nine pairs from the tidal flat to inner bay facies were found to be from the time span of 7300–8200 cal BP. The chronological changes in the reservoir effect are correlated with those seen in Holocene sediments of the other coastal area in East Asia.
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Park, Yong, et Haemyeong Jung. « Seismic velocity and anisotropy of glaucophane and epidote in experimentally deformed epidote blueschist and implications for seismic properties in warm subduction zones ». Earth and Planetary Science Letters 598 (novembre 2022) : 117822. http://dx.doi.org/10.1016/j.epsl.2022.117822.
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Inagaki, Fumio, Kai-Uwe Hinrichs et Yusuke Kubo. « IODP Expedition 337 : Deep Coalbed Biosphere off Shimokita – Microbial processes and hydrocarbon system associated with deeply buried coalbed in the ocean ». Scientific Drilling 21 (27 juin 2016) : 17–28. http://dx.doi.org/10.5194/sd-21-17-2016.
Texte intégralRésumé :
Abstract. The Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser-drilling technology with the drilling vessel Chikyu. The drilling Site C0020 is located in a forearc basin formed by the subduction of the Pacific Plate off the Shimokita Peninsula, Japan, at a water depth of 1180 m. Primary scientific objectives during Expedition 337 were to study the relationship between the deep microbial biosphere and a series of ∼ 2 km deep subseafloor coalbeds and to explore the limits of life in the deepest horizons ever probed by scientific ocean drilling. To address these scientific objectives, we penetrated a 2.466 km deep sedimentary sequence with a series of lignite layers buried around 2 km below the seafloor. The cored sediments, as well as cuttings and logging data, showed a record of dynamically changing depositional environments in the former forearc basin off the Shimokita Peninsula during the late Oligocene and Miocene, ranging from warm-temperate coastal backswamps to a cool water continental shelf. The occurrence of small microbial populations and their methanogenic activity were confirmed down to the bottom of the hole by microbiological and biogeochemical analyses. The factors controlling the size and viability of ultra-deep microbial communities in those warm sedimentary habitats could be the increase in demand of energy and water expended on the enzymatic repair of biomolecules as a function of the burial depth. Expedition 337 provided a test ground for the use of riser-drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of deep scientific ocean drilling.
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Delisle, G. « Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka ». Solid Earth Discussions 3, no 1 (12 mai 2011) : 453–76. http://dx.doi.org/10.5194/sed-3-453-2011.
Texte intégralRésumé :
Abstract. Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age – detected ~30 years ago in the course of several Russian heat flow surveys – was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (> 200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly – as our bathymetric data show – is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure, initiated during descend of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.
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Sun, Guoqiang, Meng Wang, Jiajia Guo, Yetong Wang et Yongheng Yang. « Geochemical Significance of Clay Minerals and Elements in Paleogene Sandstones in the Center of the Northern Margin of the Qaidam Basin, China ». Minerals 10, no 6 (31 mai 2020) : 505. http://dx.doi.org/10.3390/min10060505.
Texte intégralRésumé :
The average thickness of Paleogene sandstones reaches about 3000–4000 m at the northern margin of the Qaidam Basin. However, the provenance and sedimentary environment of these sandstones are uncertain; thus, more comprehensive research is needed. Integrated research is conducted on the provenance and weathering process based on petrographic characteristics, clay minerals, and geochemical compositions of sandstones in the center of the northern Qaidam Basin. The results of lithofacies analysis show that the Paleogene sandstones were mainly derived from an active continental magmatic arc, subduction accretion, or a fold-thrust belt. The average illite content in the Paleogene clay minerals is more than 50%, followed by chlorite and smectite, which reflect climatic and environmental characteristics that were arid to semi-arid, whereas the characteristics of carbon–oxygen isotopes reveal a mainly freshwater sedimentary environment. The corrected chemical index of alteration (CIAcorr) is between 56.3 and 75.7, with an average value of 66.5. These results indicate that the provenance of the Paleogene sandstones in the center of the northern Qaidam Basin mainly formed under cold and dry climatic conditions and experienced limited chemical weathering with a small amount that underwent intermediate chemical weathering under warm and humid conditions.
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Delisle, G. « Positive geothermal anomalies in oceanic crust of Cretaceous age offshore Kamchatka ». Solid Earth 2, no 2 (26 septembre 2011) : 191–98. http://dx.doi.org/10.5194/se-2-191-2011.
Texte intégralRésumé :
Abstract. Heat flow measurements were carried out in 2009 offshore Kamchatka during the German-Russian joint-expedition KALMAR. An area with elevated heat flow in oceanic crust of Cretaceous age – detected ~30 yr ago in the course of several Russian heat flow surveys – was revisited. One previous interpretation postulated anomalous lithospheric conditions or a connection between a postulated mantle plume at great depth (>200 km) as the source for the observed high heat flow. However, the positive heat flow anomaly – as our bathymetric data show – is closely associated with the fragmentation of the western flank of the Meiji Seamount into a horst and graben structure initiated during descent of the oceanic crust into the subduction zone offshore Kamchatka. This paper offers an alternative interpretation, which connects high heat flow primarily with natural convection of fluids in the fragmented rock mass and, as a potential additional factor, high rates of erosion, for which evidence is available from our collected bathymetric image. Given high erosion rates, warm rock material at depth rises to nearer the sea floor, where it cools and causes temporary elevated heat flow.
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Bussy, François, Jean Hernandez et Jürgen Von Raumer. « Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges-Mont Blanc Massifs, Western Alps) ». Earth and Environmental Science Transactions of the Royal Society of Edinburgh 91, no 1-2 (2000) : 221–33. http://dx.doi.org/10.1017/s0263593300007392.
Texte intégralRésumé :
High Precision U-Pb zircon and monazite dating in the Aiguilles Rouges–Mont Blanc area allowed discrimination of three short-lived bimodal magmatic pulses: the early 332 Ma Mg–K Pormenaz monzonite and associated 331 Ma peraluminous Montées Pélissier monzogranite; the 307 Ma cordierite-bearing peraluminous Vallorcine and Fully intrusions; and the 303 Fe-K Mont Blanc syenogranite. All intruded syntectonically along major-scale transcurrent faults at a time when the substratum was experiencing tectonic exhumation, active erosion recorded in detrital basins and isothermal decompression melting dated at 327-320 Ma. Mantle activity and magma mixing are evidenced in all plutons by coeval mafic enclaves, stocks and synplutonic dykes. Both crustal and mantle sources evolve through time, pointing to an increasingly warm continental crust and juvenile asthenospheric mantle sources. This overall tectono-magmatic evolution is interpreted in a scenario of post-collisional restoration to normal size of a thickened continental lithosphere. The latter re-equilibrates through delamination and/or erosion of its mantle root and tectonic exhumation/erosion in an overall extensional regime. Extension is related to either gravitational collapse or back-arc extension of a distant subduction zone.
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Sass, J. H., L. A. Lawver et R. J. Munroe. « A heat-flow reconnaissance of southeastern Alaska ». Canadian Journal of Earth Sciences 22, no 3 (1 mars 1985) : 416–21. http://dx.doi.org/10.1139/e85-040.
Texte intégralRésumé :
Heat flow was measured at nine sites in crystalline and sedimentary rocks of southeastern Alaska. Seven of the sites, located between 115 and 155 km landward of the Queen Charlotte – Fairweather transform fault, have an average heat flow of 59 ± 6 mW m−2. This value is significantly higher than the mean of 42 mW m−2 in the coastal provinces between Cape Mendocino and the Queen Charlotte Islands, to the south, and is lower than the mean of 72 ± 2 mW m−2 for 81 values within 100 km of the San Andreas transform fault, even farther south. This intermediate value suggests the absence of significant heat sinks associated with Cenozoic subduction and of heat sources related to either late Cenozoic tectono-magmatic events or significant shear-strain heating. At Warm Springs Bay, 75 km from the plate boundary, an anomalously high heat flow of 150 mW m−2 can most plausibly be ascribed to the thermal spring activity from which its name is derived. At Quartz Hill, 240 km landward of the plate boundary, a value of 115 mW m−2 might indicate a transition to a province of high heat flow resulting from late Tertiary and Quaternary extension and volcanism.
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Hageskov, Bjørn, et Bente Mørch. « Adakitic high-Al trondhjemites in the Proterozoic Østfold-Marstrand Belt, W Sweden ». Bulletin of the Geological Society of Denmark 46 (25 juin 1999) : 165–79. http://dx.doi.org/10.37570/bgsd-1999-46-14.
Texte intégralRésumé :
This paper investigates the first identified intrusives in SE Norway–W Sweden with the specific signature of adakitic arc magmas, which in recent settings are preferably explained as partial melts extracted from subducted oceanic crust. The studied adakitic high–Al trondhjemites occur as sheets in the Koster archipelago, W Sweden, where they form the oldest recognized granitoids in the metasupracrustals of the Stora Le–Marstrand formation. The trondhjemites were intruded during a short ca. 1.59–1.58 Ga interlude between the early and the main orogenic events of the Gothian orogeny (1.6–1.56 Ga, Åhäll et al. 1998). This interlude is otherwise characterized by ‘ordinary’ calcalkaline magmatism which on Koster is predated by the trondhjemites. The typical adakitic signature suggests that the trondhjemitic magma was extracted from a MORB (Mid Ocean Ridge Basalt) like source, and that a hornblende eclogite restite was left in the region of melting. The restite composition indicates melt extraction at PT conditions in the range of 18–25 kb/800°C to 13-15 kb/950–1050°C. These requirement can only be met by subduction of warm (young or shear heated) oceanic crust beneath a crust including early Gothian metamorphosed and deformed Stora Le–Marstrand formation or by melting of metabasaltic material at a deep crustal level. The latter is a less likely possibility and demands that the Stora Le–Marstrand formation at the time of melt extraction was part of a > 45 km thick crust.
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Li, Jun, Herong Gui, Luwang Chen, Pei Fang, Xiaoping Li, Jie Zhang et Yingxin Wang. « Geochemistry of upper Palaeozoic ‘thin-layer’ limestones in the southern North China Craton : implications for closure of the northeastern Palaeotethys Ocean ». Geological Magazine 159, no 4 (8 novembre 2021) : 494–510. http://dx.doi.org/10.1017/s0016756821001126.
Texte intégralRésumé :
AbstractDuring the late Palaeozoic Era, a series of related marine strata dominated by multi-layer limestones were deposited in the southern North China Craton. In order to gain new insights into the systematic geochemistry of the carbonate succession of the representative formation (Taiyuan Formation), we examined 59 limestone samples collected from the Huaibei Coal Basin (HCB), with a view towards quantitatively determining the major and trace elements and stable isotope compositions. The data obtained can provide essential evidence for reconstruction of the depositional palaeo-environment and tectonic setting of the Taiyuan Formation. Both X-ray diffraction analyses and palaeoredox proxies (e.g. V/Cr, V/(V + Ni) and authigenic U) indicated that the limestone layers were deposited in an oxic–dysoxic zone, with calcite as the main component. Moreover, palaeomagnetic evidence provided support for the conclusion that these limestones were laid down within an epicontinental sea depositional environment under a warm or hot palaeoclimate during the transition between late Carboniferous and early Permian time. Additionally, evidence obtained from our analyses of trace and rare earth elements revealed that the tectonic setting of the Taiyuan Formation (L1–L5) in the HCB transited from an open ocean to a passive continental margin, thereby indicating that this transformation stemmed from the subduction closure of the northeastern Palaeotethys Ocean. The findings of this study would be of interest to those working on the upper Palaeozoic marine strata in the southern North China Craton.
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Scanlon, Darby P., John Bershaw, Ray E. Wells et Ashley R. Streig. « The spatial and temporal evolution of the Portland and Tualatin forearc basins, Oregon, USA ». Geosphere 17, no 3 (21 avril 2021) : 804–23. http://dx.doi.org/10.1130/ges02298.1.
Texte intégralRésumé :
Abstract The Portland and Tualatin basins are part of the Salish-Puget-Willamette Lowland, a 900-km-long, forearc depression lying between the volcanic arc and the Coast Ranges of the Cascadia convergent margin. Such inland seaways are characteristic of warm, young slab subduction. We analyzed the basins to better understand their evolution and relation to Coast Range history and to provide an improved tectonic framework for the Portland metropolitan area. We model three key horizons in the basins: (1) the top of the Columbia River Basalt Group (CRBG), (2) the bottom of the CRBG, and (3) the top of Eocene basement. Isochore maps constrain basin depocenters during (1) Pleistocene to mid-Miocene time (0–15 Ma), (2) CRBG (15.5–16.5 Ma), and (3) early Miocene to late Eocene (ca. 17–35 Ma) time. Results show that the Portland and Tualatin basins have distinct mid-Miocene to Quaternary depocenters but were one continuous basin from the Eocene until mid-Miocene time. A NW-striking gravity low coincident with the NW-striking, fault-bounded Portland Hills anticline is interpreted as an older graben coincident with observed thickening of CRBG flows and underlying sedimentary rocks. Neogene transpression in the forearc structurally inverted the Sylvan-Oatfield and Portland Hills normal faults as high-angle dextral-reverse faults, separating the Portland and Tualatin basins. An eastward shift of the forearc basin depocenter and ten-fold decrease in accommodation space provide temporal constraints on the emergence of the Coast Range to the west. Clockwise rotation and northward transport of the forearc is deforming the basins and producing local earthquakes beneath the metropolitan area.
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Sodoudi, F., A. Brüstle, T. Meier, R. Kind et W. Friederich. « Receiver function images of the Hellenic subduction zone and comparison to microseismicity ». Solid Earth 6, no 1 (4 février 2015) : 135–51. http://dx.doi.org/10.5194/se-6-135-2015.
Texte intégralRésumé :
Abstract. New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23–27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati–Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.
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Altherr, Rainer, Stefan Hepp, Hans Klein et Michael Hanel. « Metabasic rocks from the Variscan Schwarzwald (SW Germany) : metamorphic evolution and igneous protoliths ». International Journal of Earth Sciences 110, no 4 (22 mars 2021) : 1293–319. http://dx.doi.org/10.1007/s00531-021-02016-w.
Texte intégralRésumé :
AbstractIn the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.
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Peng, Hu, Chaoming Xie, Cai Li et Zhongyue Zhang. « Provenance and palaeogeographic implications of detrital zircons from the lower Carboniferous Riwanchaka Formation of the central Tibetan Plateau ». Geological Magazine 156, no 12 (2 juillet 2019) : 2031–42. http://dx.doi.org/10.1017/s0016756819000359.
Texte intégralRésumé :
AbstractThe Longmu Co–Shuanghu suture zone, which divides the Qiangtang terrane into the northern and southern Qiangtang blocks, is regarded as a key locality in reconstructing the evolutionary history of the Palaeo-Tethys Ocean and the break-up of Gondwana. However, although low-temperature – high-pressure metamorphic rocks and ophiolites have been documented within the Longmu Co–Shuanghu suture zone, it remains unclear whether it is an in situ suture zone and represents the relic of the main Palaeo-Tethys Ocean. The uncertainty stems mainly from the limited systematic studies of the provenance, palaeontological evidence and depositional settings of strata on either side of the Longmu Co–Shuanghu suture zone (i.e. northern and southern Qiangtang blocks). Here we report new detrital zircon U–Pb ages and palaeontological data from Lower Carboniferous strata (Riwanchaka Formation) of the northern Qiangtang block, central Tibet. The Riwanchaka Formation contains warm-climate biota with Cathaysian affinities. Provenance analysis reveals that the formation has detrital zircon spectra similar to those from strata of the Yangtze Plate, and it contains a large proportion of zircons with ages (~360 Ma) similar to the timing of synsedimentary magmatic arc activity, implying an active continental margin setting associated with northward subduction of the Palaeo-Tethyan oceanic lithosphere. Conversely, the Carboniferous–Permian strata from the southern Qiangtang block contain cool-water faunas of Gondwanan affinity and exhibit minimum zircon crystallization ages that are markedly older than their depositional ages, suggesting a passive continental margin setting. The differences in provenance, palaeontological assemblages and depositional settings of the Carboniferous to Permian strata either side of the Longmu Co–Shuanghu suture zone indicate the existence of an ancient ocean between the northern and southern Qiangtang blocks. Combining the new findings with previous studies on high-pressure metamorphic rocks, arc magmatism and ophiolites, we support the interpretation that the Longmu Co–Shuanghu suture zone is an in situ suture zone that represents the main suture of the Palaeo-Tethys Ocean.
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Sangana, Peter, Qin Gao et Zilong Li. « The Impact of the Caroline Ridge Subduction on the Geomorphological Characteristics of Major Landforms in the Yap Subduction Zone ». Journal of Marine Science and Engineering 10, no 10 (3 octobre 2022) : 1414. http://dx.doi.org/10.3390/jmse10101414.
Texte intégralRésumé :
The Caroline Ridge (CR) subduction underneath the Philippine Sea Plate brings complex morphotectonic characteristics to the Yap Subduction Zone (YSZ) compared to other normal intra-oceanic subduction systems. However, due to the relative paucity of precise geomorphological information, the detailed morphotectonic settings of the YSZ remain unclear. Therefore, we combine the latest-released bathymetry, marine geomorphometry techniques, and geophysical information to investigate the geomorphological characteristics of landforms in the YSZ and their inter-relationship with the CR subduction. The Parece Vela Basin displays NE-SW oriented fractures which are believed to be influenced by the subduction of CR in the ESE-WNW direction. The north part of the Yap arc exhibits higher Bouguer anomalies, implying the absence of the overlying normal volcanic arc crust. The arc-ward trench shows abnormal higher slope values and reveals two significant slope breaks. The Yap Trench axis reveals varying water depths with an extraordinarily deep point at around 9000 m. The sea-ward trench slope displays higher slope values than normal and shows the presence of grabens, horsts, and normal faults which indicate the bending of the CR before subduction. The CR subduction is observed to be critical in the formation of significant geomorphological characteristics in the YSZ.
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Liu, Hailong, Semyon A. Grodsky et James A. Carton. « Observed Subseasonal Variability of Oceanic Barrier and Compensated Layers ». Journal of Climate 22, no 22 (15 novembre 2009) : 6104–19. http://dx.doi.org/10.1175/2009jcli2974.1.
Texte intégralRésumé :
Abstract A monthly gridded analysis of barrier-layer and compensated-layer width based on observed vertical profiles of temperature and salinity and covering the period 1960–2007 is explored for evidence of subseasonal variability and its causes. In the subtropics and midlatitudes this variability is mostly evident during the local cold season when barrier layers and compensated layers are present. There is significant variability of anomalous (nonseasonal) barrier-layer and compensated-layer width on interannual periods, while in the North Pacific longer-term changes are also detectable. In the winter North Pacific a salinity-stratified barrier layer exists at subpolar latitudes. Farther south along the Kuroshio Extension a compensated layer exists. The width of the barrier layer varies from year to year by up to 60 m while compensated-layer width varies by half as much. During the observation period the barrier-layer width decreased in response to a strengthening of the Aleutian low pressure system, the resulting strengthening of dry northerly winds, and a decrease of precipitation. In contrast, the compensated-layer width increased in response to this pressure system strengthening and related amplification of the midlatitude westerly winds, the resulting increase of net surface heat loss, and its effect on the temperature and salinity of the upper-ocean water masses. The tropical Pacific, Atlantic, and Indian Oceans all have permanent barrier layers. Their interannual variability is less than 20 m but is comparable in magnitude to the time mean barrier-layer width in these areas. In the tropical Pacific west of 160°E and in the eastern tropical Indian Ocean, the barrier-layer width changes by approximately 5 m in response to a 10-unit change in the Southern Oscillation index. It thickens during La Niñas as a result of the presence of abundant rainfall and thins during dry El Niños. Interannual variations of barrier-layer width in the equatorial Pacific are weak east of 160°E with an exception of the area surrounding the eastern edge of the warm pool. Here subduction of salty water contributes to locally stronger variations of barrier-layer width.
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Aygül, Mesut, Aral I. Okay, Bradley R. Hacker et Andrew R. C. Kylander-Clark. « REE behavior in warm and cold subducting oceanic crust ». International Journal of Earth Sciences 111, no 3 (29 janvier 2022) : 905–18. http://dx.doi.org/10.1007/s00531-021-02156-z.
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ΜΟΥΝΤΡΑΚΗΣ, Δ. « Tectonic evolution of the Hellenic Orogen. Geometry and kinematics of deformations ». Bulletin of the Geological Society of Greece 34, no 6 (1 janvier 2002) : 2113. http://dx.doi.org/10.12681/bgsg.16853.
Texte intégralRésumé :
The Hellenic orogen consists of three orogenic belts: 1) the Cimmerian orogenic belt, including Rhodopian, Serbomacedonian, Circum Rhodope, Axios and Pelagonian zones, is the internal belt which has been created in pre-Late Jurassic times as a result of the northward drift of Cimmerian contrinental fragments from Gondwana towards Eurasia. Ophiolites from small ocean basins were mainly emplaced onto the Cimmerian continental margins in Middle Jurassic. 2) the Alpine orogenic belt, including External Hellenides and Pindos-Subpelagonian ophiolites and oceanic sediments (Neo-Tethyan), which has been created in Cretaceous-Paleogene times after the subduction of the Neotethyan oceanic crust beneath the Cimmerian-Eurasian plate and the collision of the Apulian microplate to the later, 3) the Mesogean orogenic belt along the External Hellenic orogenic arc as a result of the Mesogean-African underplate beneath the unique Alpine-Cimmerian-Eurasian plate in Miocen- Pliocene times and the exhumation of the Cretan-Southern Peloponesus tectonic windows. Structural analysis and detailed studies of the geometry and kinematics suggest that during Alpine-Mesogean orogenic process a SW-ward migration of successive complessional and extensional tectonic events took place resulted of successive subductions. Thus, crustal thickening produced by compressional tectonics in each area was followed by an extensional exhumation of underplate rocks as tectonic windows.