Littérature scientifique sur le sujet « Warm subduction »

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.

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.

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.

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.

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.

L’obiettivo di questo studio è quello di caratterizzare il trasferimento di massa tra crosta e mantello. A questo scopo sono stati considerati due terreni metamorfici di alta pressione (HP) dove peridotiti a granato affiorano all’interno di rocce crostali di alto grado, i.e. l’area del Monte Duria (falda Adula-Cima Lunga, Alpi centrali, N Italia) e la zona d’ Ultimo (falda del Tonale, Alpi orientali, N Italia). Nell’area del Monte Duria, peridotiti a granato affiorano in contatto diretto con eclogiti migmatitiche (Borgo). Sia le peridotiti che le eclogiti registrano condizioni di picco in HP a 2.8 GPa e 750 ° C e un riequilibratura statica a 1.0 GPa e 850 ° C. Le peridotiti mostrano abbondanti anfibolo, dolomite, flogopite e ortopirosseno (su olivina), suggerendo che le peridotiti registrano metasomatismo ad opera di agenti crostali arrichiti in SiO2, K2O, CO2 e H2O. Le peridotiti mostrano anche un frazionamento in LREE (La/Nd = 2.4) legato alla presenza di anfibolo e clinopirosseno. Questi minerali sono equilibrio con il granato, indicando che il metasomatismo è avvenuto in HP. Nelle eclogiti, microstrutture di fusione come aggregati microcristallini a Kfs+Pl+Qz+Cpx e Cpx+Kfs sono allineate lungo la foliazione a Zo+Omp+Grt, indicando che le eclogiti hanno subito un evento di fusione parziale in HP. Il contatto tra le peridotiti e le eclogiti di Borgo è marcato dalla presenza di un livello di tremolitite. Boudins di tremolititi si ritrovano anche trasposti lungo la foliazione a granato della peridotite, indicando che il boudinage delle tremolititi è avvento in alta pressione. Le tremolititi mostrano aggregati a Phl+Tc+Chl+Tr interpretati come psudomorfi su granato. Tali pseudomorfi si sviluppano in condizioni statiche post-datando la formazione dei boudins, suggerendo che le tremolititi derivano da precursori a granato. Le tremolititi mostrano Mg# > 0.90 e Al2O3 = 2.75 wt.% tipici di composizioni ultramafiche ma allo stesso tempo presentano arricchimenti in SiO2, CaO, e LREE, indicando che esse rappresentano il prodotto dell’interazione in alta pressione tra le peridotiti e i fusi derivati dalle eclogiti. Per testare questa ipotesi abbiamo sviluppato un modello termodinamico a P = 3 GPa e T = 750 °C. I nostri risultati indicano che l’interazione fuso-peridotite produce una paragenesi a Opx+Cpx+Grt, suggerendo che le tremolititi rappresentano il prodotto di retrocessione di una westerite a granato. Nella zona d’Ultimo, numerose lenti di peridotite affiorano all’interno di rocce crostali di alto grado. Le peridotiti mostrano una transizione da lherzoliti a spinello protogranulari a peridotiti milonitiche a granato e anfibolo. Le pirosseniti trasposte lungo la foliazione della peridotite mostrano un’evoluzione simile, da pirosseniti a spinello a pirosseniti a granato. Questa evoluzione riflette il passaggio indotto dal corner flow del mantello da condizioni in facies a spinello a a granato. Come consguenza, il granato forma corone intorno allo spinello ed essoluzioni all’interno dei porfiroclasti di pirosseno, e cristallizza lungo la foliazione delle pirosseniti e delle peridotiti Evidenze tessiturali e dati cristallografici indicano che la transizione spinello-granato avviene in un contesto deformativo. I porfiroclasti di pirosseno mostrano evidente CPO, alte frequenze delle misorientazioni a basso angolo, e distribuzione non-random degli assi di misorientazione per misorientazioni a basso angolo, indicando che i pirosseni si deformano per dislocation creep. Il dislocation creep è contemporaneo a processi di ricristallizzazione dinamica e alla transizione spinello-granato. Ciò induce una riduzione della grana e una transizione permanente da disclocation creep nei porfiroclasti a grain-size sensitive creep nei grani ricristallizzati che risulta in un forte indebolimento delle pirosseniti e delle peridotiti quando queste vengono tettonicamente accoppiate alle rocce crostali.
In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both crustal and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 °C and post-peak static equilibration at 0.8-1.0 GPa and 850 °C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer, which also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# > 0.90 and Al2O3 = 2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P = 3 GPa and T = 750 °C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that this interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite. In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse spinel-lherzolites to mylonitic garnet-amphibole peridotites. Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole clinopyroxenites. This coupled evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 Gpa) to garnet-facies conditions (850 °C and 2.8 Gpa) likely induced by mantle corner flow. As a consequence, garnet formed coronas around spinel and exsolved from porphyroclastic, high-T pyroxenes, and finally crystallised along the pyroxenite and peridotite foliations. Textural evidences and CPO data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed CPOs, high frequencies of low-angle misorientations, and non-random distribution of the low-angle misorientation axes, indicating that pyroxene porphyroclasts primarily deform by dislocation creep. Dislocation creep is accompanied by reaction-induced dynamic recrystallisation during the spinel to garnet phase transition, which promotes a sudden reduction of the grain size and a shift from dislocation creep in the porphyroclast to grain-size sensitive creep (GSS) in the recrystallised grains. This results in a dramatic rheological weakening of pyroxenites at HP peak conditions when pyroxenites and host peridotites were coupled with crustal rocks.

Seismic signals provide an effective early detection of tsunamis that are generated by earthquakes, and for epicentres in the hard-rock subduction zones there is a robust analysis procedure that uses a global network of seismometers. For earthquakes with epicentres in soft layers in the upper subduction zones the processes are slower and the seismic signals have lower frequencies. For these soft-rock earthquakes a given earthquake magnitude can produce a bigger tsunami amplitude than the same earthquake magnitude in a hard rock rupture. Numerical modelling for the propagation from earthquake-generated tsunamis can predict time of arrivals at distant coastal impact zones. A global network of deep-water pressure sensors is used to detect and confirm tsunamis in the open ocean. Submarine landslide and coastal collapse tsunamis, meteo-tsunamis, and other disturbances with no significant seismicity must rely on the deep-water pressure sensors and HF radar for detection and warning. Local observations by HF radar at key impact sites detect and confirm tsunami time and amplitude in the order of 20–60 minutes before impact. HF radar systems that were developed for mapping the dynamics of coastal currents have demonstrated a capability to detect tsunamis within about 80 km of the coast and where the water depth is less than 200 m. These systems have now been optimised for tsunami detection and some installations are operating continuously to provide real-time data into tsunami warning centres. The value of a system to warn of hazards is realised only when coastal communities are informed and aware of the dangers.