Numerical modeling of subduction zones and implications for mantle convection

Le interazioni tra la tettonica delle placche e la convezione del mantello sono continuamente oggetto di investigazione nel campo della geodinamica e tettonica delle placche. Questa tesi di dottorato contribuisce ad ampliare le conoscenze su queste interazioni, integrando la modellazione numerica effettuata in differenti geometrie con dati geofisici e cinematici. Questi dati suggeriscono un forte carattere asimmetrico della tettonica delle placche, sia lungo i margini convergenti che estensionali. Un moto globale verso “ovest” della litosfera potrebbe esserne il principale responsabile, causando interazioni asimmetriche tra la litosfera ed il mantello sottostante. A partire da modelli numerici in geometria piana, è stata simulata l’interazione tra una placca oceanica in subduzione e un flusso orizzontale del mantello. I risultati hanno mostrato come la dinamica di subduzione sia fortemente influenzata da questo flusso orizzontale, riproducendo alcune delle principali caratteristiche di subduzioni attuali, come ad esempio la pendenza dello slab, lo stato di stress della placca a tetto ed il movimento della cerniera della subduzione. La litosfera che rientra nel mantello attraverso la subduzione è uno degli aspetti principali quando si analizzano le interazioni tra la tettonica delle placche ed i movimenti del mantello ed è strettamente legato al tipo di movimento della cerniera dello slab in subduzione. In un sistema di riferimento in cui la placca a tetto è fissa, una cerniera che si muove verso di essa contribuisce a diminuire il tasso di subduzione e viceversa. È stata effettuata quindi una analisi cinematica globale, calcolando i tassi di subduzione ed il volume di litosfera attualmente in subduzione per ogni subduzione. I risultati hanno mostrato tassi di subduzione e volumi di litosfera subdotti maggiori in corrispondenza della maggior parte delle subduzioni con polarità geografica dello slab verso “ovest” rispetto alle subduzioni con polarità opposta. Il tasso di subduzione è un parametro-chiave per la caratterizzazione della dinamica delle subduzioni poiché racchiude la velocità di convergenza della placca in subduzione ed il comportamento della cerniera. Perciò i modelli numerici sono stati migliorati per includere questa velocità come condizione al contorno, usando i tassi di subduzione ottenuti tramite l’analisi cinematica. L’asimmetria delle subduzioni è stata riprodotta dai modelli numerici, supportando l’analisi cinematica. Per verificare infine la dinamica di subduzione in geometria sferica, il moto delle placche è stato simulato in sistemi di riferimento assoluti e relativi: 1) hotspot profondi, 2) hotspot superficiali, 3) placca a tetto fissa. I risultati forniscono materiale di riferimento per futuri lavori di ricerca.Relationships between plate tectonics and mantle convection are under relentless investigation in the field of geodynamics and plate tectonics. This dissertation contributes to the understanding of this interplay, integrating numerical models in different geometries with both geophysical and kinematic data. These data would suggest an asymmetric character of plate tectonics, along both extensional and convergent margins. A global “westward” displacement of the lithosphere with respect to the underlying mantle could be responsible for that, causing asymmetries in lithosphere-mantle interactions. Starting from numerical models in a rectangular box, interactions between subducting oceanic plates and a horizontal mantle flow have been simulated. Results showed that subduction dynamics is strongly affected by the horizontal flow, reproducing some of the main features of present-day subduction zones such as slab dip, state of stress within the upper plate and motion of the subduction hinge. Lithosphere re-entering into the mantle through subduction is one of the main aspects when analyzing the interplay between mantle motion and plate tectonics and it is highly affected by the displacement of the subduction hinge. In the fixed upper plate framework, the hinge moving towards the upper plate contributes to decrease the subduction rate and vice-versa. Thus, a worldwide kinematic analysis was performed, eventually calculating the subduction rate and volumes of subducted lithosphere for each subduction zone. Results showed both faster subduction rates and higher volumes of subducted lithosphere along most of the “westward”-directed subduction zones with respect to the opposite ones. The subduction rate could be considered as a key-parameter for subduction zones dynamics, enclosing both the plate convergence velocity and the motion of the subduction hinge. Therefore, the numerical simulations have been improved to include this velocity as boundary condition, using subduction rate estimations obtained from the kinematic analysis. The numerical models reproduced subduction zones asymmetries, supporting the kinematic analysis. Finally plate reconstructions have been computed to verify subduction dynamics in a spherical domain using both mantle- and relative-reference frameworks, e.g., 1) deep hotspots; 2) shallow hotspots; 3) upper plate fixed. Results provide reference material for future research works

Horizontal mantle flow controls subduction dynamics

It is generally accepted that subduction is driven by downgoing-plate negative buoyancy. Yet plate age –the main control on buoyancy– exhibits little correlation with most of the present-day subduction velocities and slab dips. “West”-directed subduction zones are on average steeper (~65°) than “East”-directed (~27°). Also, a “westerly”-directed net rotation of the lithosphere relative to the mantle has been detected in the hotspot reference frame. Thus, the existence of an “easterly”-directed horizontal mantle wind could explain this subduction asymmetry, favouring steepening or lifting of slab dip angles. Here we test this hypothesis using high-resolution two-dimensional numerical thermomechanical models of oceanic plate subduction interacting with a mantle flow. Results show that when subduction polarity is opposite to that of the mantle flow, the descending slab dips subvertically and the hinge retreats, thus leading to the development of a back-arc basin. In contrast, concordance between mantle flow and subduction polarity results in shallow dipping subduction, hinge advance and pronounced topography of the overriding plate, regardless of their age-dependent negative buoyancy. Our results are consistent with seismicity data and tomographic images of subduction zones. Thus, our models may explain why subduction asymmetry is a common feature of convergent margins on Earth

Development of large scale production of Nd-doped phosphate glasses for megajoule-scale laser systems

Nd-doped phosphate glasses are the preferred gain medium for high-peak-power lasers used for Inertial Confinement Fusion research because they have excellent energy storage and extraction characteristics. In addition, these glasses can be manufactured defect-free in large sizes and at relatively low cost. To meet the requirements of the future mega-joule size lasers, advanced laser glass manufacturing methods are being developed that would enable laser glass to be continuously produced at the rate of several thousand large (790 x 440 x 44 mm{sup 3}) plates of glass per year. This represents more than a 10 to 100-fold improvement in the scale of the present manufacturing technology

The westward lithospheric drift, its role on the subduction and transform zones surrounding Americas. Andean to cordilleran orogenic types cyclicity

We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs. In the Andes, the subduction zone is shallow and with low dip, because the mantle flow sustains the slab; the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen. The Sandwich Arc is generated by a westerly-directed SAM (South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone. In this context, the slab hinge is retreating relative to the upper plate, generating the backarc basin and a low bathymetry single-verging accretionary prism. In Central America, the Caribbean plate presents a more complex scenario: a) To the East, the Antilles Arc is generated by westerly directed subduction of the SAM plate, where the eastward mantle flow is steepening and retreating the subduction zone. b) To the West, the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate, where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau. In the frame of the westerly lithospheric flow, the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate's convergence angle, such as in NAM (North American) plate with the collision with the Pacific/Farallon active ridge in the Neogene (Cordilleran orogenic type scenario). The easterly mantle drift sustains strong plate coupling along NAM, showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate. This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated (San Andreas and Queen Charlotte). The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate, transporting them along the strike slip fault systems as para-autochthonous terranes. This Cordilleran orogenic type scenario, is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene, segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-Liquiñe-Ofqui and Atacama strike slip fault systems, where subduction was terminated and para-autochthonous terranes transported. In the Neogene, the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario

The westward lithospheric drift, its role on the subduction and transform zones surrounding Americas: Andean to Cordilleran orogenic types cyclicity

We investigate the effect of the westerly rotation of the lithosphere on the active margins that surround the Americas and find good correlations between the inferred easterly-directed mantle counterflow and the main structural grain and kinematics of the Andes and Sandwich arc slabs. In the Andes, the subduction zone is shallow and with low dip, because the mantle flow sustains the slab; the subduction hinge converges relative to the upper plate and generates an uplifting doubly verging orogen. The Sandwich Arc is generated by a westerly-directed SAM (South American) plate subduction where the eastward mantle flow is steepening and retreating the subduction zone. In this context, the slab hinge is retreating relative to the upper plate, generating the backarc basin and a low bathymetry single-verging accretionary prism. In Central America, the Caribbean plate presents a more complex scenario: (a) To the East, the Antilles Arc is generated by westerly directed subduction of the SAM plate, where the eastward mantle flow is steepening and retreating the subduction zone. (b) To the West, the Middle America Trench and Arc are generated by the easterly-directed subduction of the Cocos plate, where the shallow subduction caused by eastward mantle flow in its northern segment gradually steepens to the southern segment as it is infered by the preexisting westerly-directed subduction of the Caribbean Plateau.In the frame of the westerly lithospheric flow, the subduction of a divergent active ridge plays the role of introducing a change in the oceanic/continental plate's convergence angle, such as in NAM (North American) plate with the collision with the Pacific/Farallon active ridge in the Neogene (Cordilleran orogenic type scenario). The easterly mantle drift sustains strong plate coupling along NAM, showing at Juan de Fuca easterly subducting microplate that the subduction hinge advances relative to the upper plate. This lower/upper plate convergence coupling also applies along strike to the neighbor continental strike slip fault systems where subduction was terminated (San Andreas and Queen Charlotte). The lower/upper plate convergence coupling enables the capture of the continental plate ribbons of Baja California and Yakutat terrane by the Pacific oceanic plate, transporting them along the strike slip fault systems as para-autochthonous terranes. This Cordilleran orogenic type scenario, is also recorded in SAM following the collision with the Aluk/Farallon active ridge in the Paleogene, segmenting SAM margin into the eastwardly subducting Tupac Amaru microplate intercalated between the proto-Liquiñe-Ofqui and Atacama strike slip fault systems, where subduction was terminated and para-autochthonous terranes transported. In the Neogene, the convergence of Nazca plate with respect to SAM reinstalls subduction and the present Andean orogenic type scenario

Analytical Benchmark Problems for Multifidelity Optimization Methods

The paper presents a collection of analytical benchmark problems specifically selected to provide a set of stress tests for the assessment of multifidelity optimization methods. In addition, the paper discusses a comprehensive ensemble of metrics and criteria recommended for the rigorous and meaningful assessment of the performance of multifidelity strategies and algorithms

La revocatoria degli atti a titolo gratuito: aspetti processuali e sostanziali

La presente tesi si occupa di esaminare a fondo la categoria di atti a titolo gratuito e, in particolare, lo strumento predisposto dall’ordinamento a tutelare i creditori di fronte ad atti compiuti a titolo gratuito dal debitore, vale a dire l’azione revocatoria. L’elaborato, che si divide in quattro capitoli, intende fornire un’analisi dell’evoluzione, dei presupposti e dei procedimenti che riguardano la revocatoria degli atti a titolo gratuito del debitore sia nell’ambito del Diritto civile che concorsuale. This thesis deals with an in-depth examination of the category of gratuitous acts and, in particular, the instrument set up by the law to protect creditors against acts carried out gratuitously by the debtor, which is the revocatory action. The thesis, which is divided into four chapters, intends to provide an analysis of the evolution, conditions and procedures concerning the revocation of the debtor's free-of-charge deeds both in the context of civil and insolvency law