Littérature scientifique sur le sujet « Mantle weakening »

How subduction—the sinking of cold lithospheric plates into the mantle—is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.

Abstract. We report the results of friction experiments on brucite under both dry and wet conditions under various normal stresses (10–60 MPa). The final friction coefficients of brucite were determined to be 0.40 and 0.26 for the dry and wet cases, respectively, independent of the normal stress. Under dry conditions, velocity-weakening behavior was observed in all experiments at various normal stresses. Under wet conditions, velocity weakening was observed at low normal stress (10 and 20 MPa), whereas velocity strengthening was determined at a higher applied normal stress. Microstructural observations of recovered experimental samples indicate localized deformation within a narrow shear band, implying that a small volume of brucite can control the bulk frictional strength in an ultramafic setting. Among serpentinite-related minerals, weak and unstable frictional behavior of brucite under hydrated mantle wedge conditions may play a role in slow earthquakes at the subduction plate interface in the mantle wedge.

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.

AbstractThis book is the result of the first 3 years of the comparative and multidisciplinary Jean Monnet Network, “Crisis-Equity-Democracy for Europe and Latin America”, of senior academics and policy advisors from four European and three Latin American countries, including experts on the European Union and Latin American regionalism. The rationale of the project and the common link is that both Europe and Latin America can learn from their respective experiences on “crisis”, its management and the distributive and democratic implications at national and regional level. The main purposes of the joint research can be summarised as to (1) locate in the current global financial system as one of the very major causes of the financial and debt crises in the EU and Latin America; (2) demonstrate the impact of the paradigm change on global and EU economic governance; (3) analyse key systemic aspects of the global crisis, i.e. climate change, macro-financial instability and the weakening of democracy and their inter-connections; (4) map and evaluate how both regions and individual countries within both regions have tried to manage these crises; (5) discuss the economic, political and social effects of these crises on both regions and individual countries; (6) finally, to make policy suggestions on how to transition from finance capitalism to a more sustainable real capitalism, on how both regions can better manage/govern/respond to such systemic pressures and on how they can increase their cooperation.

ABSTRACT The Basin and Range Province is considered to be one of the most iconic continental rift provinces that postdates a prolonged orogeny. Here, I present evidence that challenges all the assumptions that lead to the long-held notion that gravitational collapse of thickened (55–65-km-thick) continental crust was a major driver of Basin and Range extension. This study focused on integrating the regional tectonic and magmatic history of the northeastern region of the Basin and Range (centered on the Albion–Raft River–Grouse Creek metamorphic core complex) and combines insights from a compilation of data from metamorphic core complexes worldwide to illustrate the effect of accounting for the magmatic histories when estimating pre-extensional crustal thickness. In the region of the Albion–Raft River–Grouse Creek metamorphic core complex, there is evidence of three Cenozoic extensional events and three coeval magmatic events. By taking into account the regional magmatic activity during the Cenozoic (Paleogene, Neogene, and Quaternary magmatism), and the inferred mantle-derived magmatic volume added to the crust during the process of extension, it is shown that the pre-extensional crustal thickness cannot have been more than ~53 km, and it was more likely close to ~46 km. This estimate is consistent with Eocene igneous geochemistry estimates of crustal thickness and with crustal thickness estimates from shortening of ~30-km-thick mid-Jurassic crust. During the Cenozoic evolution of the northeastern Basin and Range, the crust in the area of study thinned from ~46 km to ~32 km, and the elevation of the pre-extensional plateau collapsed from ~2.5 km to its present-day average of ~1.8 km. This study concludes that an alternative mechanism to predominantly gravitational crustal collapse is required to explain the extension in the region of the Albion–Raft River–Grouse Creek metamorphic core complex. I support recent interpretations that this mechanism involved the complex interaction of the removal of the Farallon flat slab (by slab roll-back or delamination of the slab) with the impingement of the Snake River Plain–Yellowstone mantle anomaly. The switch in the stress regime from compression (during the slab subduction) to a complex regime during slab roll-back, followed by extension (in the middle Miocene), and the associated mantle-derived magmatism, led to the thinning of the subcontinental lithospheric mantle, thermal weakening of the crust, and the thinning of the crust during the Cenozoic. This crustal extension is expressed as regional Basin and Range normal faulting and local vertical flow and exhumation of the mobilized middle crust at metamorphic core complexes like the Albion–Raft River–Grouse Creek complex.

Islamism now dates back a hundred years. Concern over members of this political and religious movement relates to their putative and potential radical - or even violent – behavior when confronted with cultural otherness. Such behavior takes root in their assumed wish to redesign the world in their image. From its inception in the 1920s to its more recent manifestations, the Islamist movement strove to lift Muslim societies out of their alleged civilizational lethargy. In so-doing, it has paid substantial attention to the state of international affairs, as well as to potential ways to act on it. If the State remains undeniably Islamist movements’ privileged arena for action, considerations for Muslim countries’ environment; devising strategies aiming at the completion of a “motherland of believers” (al-oumma); thoughts on an interstate order within an Islamic frame of reference - remain prominent concerns to them. From its outset, Islamism has always insisted on the duty to serve religion as a whole - and thus everyone identifying with it. Its end goal therefore overrides geographical, historical and political borders – those being perceived as divisive and weakening the face of Islam. In addition, Islamists consider the current international order as one consciously designed by non-Muslims. In such views, the latter nurse an ontological enmity towards Islam because of its revisionist potential. The Arab revolutions initiated in 2010 have been experimental fields of the oppositional – even revolutionary – dimensions of Islamist ideology. These enable interrogations to be raised on Islamism’s practice and possible evolutions. In other words, how do Islamist movements translate fundamental diplomatic and relational principles into practice with other actors of the international system? If Islamist forces are indeed maintaining special relationships with the outside world mainly driven by the wish to shower the planet with Islam-serving behavior, is it however analytically relevant to identify a specific Islamist practice of international affairs? There are two objectives tied to this presentation. First, it will attempt to shed light on how Islamist activists, leaders and theorists view the world. In so-doing, Islamist speeches and intellectual output will be scrutinized. Then, answers will be provided to the following question: when Islamist officials have had the chance to approach national decision-making arenas - this is the case in some countries that have experienced the Arab Spring – how did they manage to put up a foreign policy agenda centered around an Islamic framework? This question is central for through it one can attempt to measure the empirical outreach of the Islamist ideology.