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

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

Mantle convection: clues from lithosphere sinking at subduction zones and numerical modelling.

Subduction zones show a worldwide asymmetry that can be observed in slab dip, kinematics of the subduction hinge, morphology, structural elevation, gravity anomalies, heat flow, metamorphic evolution, subsidence and uplift rates, depth of the decollement planes, mantle wedge thickness, magmatism, backarc development or not, etc. This asymmetry could be easily explained if related to the geographic polarity of the sinking slabs (Doglioni & Panza, 2015). In fact, geophysical and kinematics constraints show that all the plates move “westward”. This preferential flow of plates would suggest a relative “eastward” mantle flow. If we look then to subduction dynamics within this set of conditions, this “eastward” mantle flow should have an important role in influencing subduction dynamics itself. Furthermore, along W-subduction zones slabs sink with a higher velocity with respect to the “easterly or northeasterly” directed ones. The faster “westerly” directed slabs determine that the volume of lithosphere recycled into this kind of subduction is larger than that along the converse ones. This should determine a more vigorous counterflow within the mantle below W-directed subductions with respect to the one below E-directed ones. Starting from these observations we attempted to estimate volumes of lithosphere that are currently subducting below the principal subduction zones: our results show that there are about 288 km3/yr of lithosphere currently subducting below W-directed subduction zones, while only about 78 km3/yr of lithosphere are currently subducting below E- to NE-directed subduction zones. Then we tried to demonstrate quantitatively the consequent difference in mantle circulation between the two subduction settings using numerical modelling tools (Gerya, 2010), using data coming from our volumetric calculation as input data for the models. Moreover, we tried to look at these volumes with respect to the latitude of subduction zones, being plates velocities strongly linked to the Earth’s rotation. In fact, seismicity is latitude dependent and decreases with increasing latitude (Riguzzi et al., 2010; Varga et al., 2012). Our results show that most of the volume currently subducting below worldwide principal subduction zones is concentrated between 30° and -30° of latitude

Variable plate kinematics promotes changes in back-arc deformation regime along the north-eastern Eurasia plate boundary

The stretching of the lithosphere leading to back-arc basins formation generally develops behind arc-trench systems and is considered the consequence of slab retreat relative to the upper plate. Here, we examine the deformation regime evolution within the overriding plate due to subduction processes, using thermo-mechanical numerical simulations. We explore the north-eastern Eurasia plate boundary and the mechanisms of subducting Pacific plate since 57 Ma. During this time interval, several extensional basins formed along the Eurasia margin, such as the East China Sea, the Japan Sea, and the Kuril basin. Here, we increased the simulation complexity, with the inclusion of (i) the kinematic variability of the Pacific plate over the geological past with respect to a fixed Eurasia, incorporating time-dependent (i.e., temporally evolving) velocities computed from plate motion reconstructions; (ii) a Low-Velocity Zone within the asthenosphere, and (iii) a horizontal eastward mantle flow. Our results show a crucial role of the mantle flow for the development of lithospheric extension and back-arc basin opening, and a main kinematic control of the subduction trench position, which advances and retreats, into distance intervals in the order of ∼ 100 km, and providing stages of compression and extension in a back-arc basin.ISSN:2045-232

Asymmetric convergent margins: what controls the hinge motion in subduction zones?

Looking at subduction zones worldwide, a striking asymmetry can be identified: W- to SW-directed subduction zones (e.g., Marianas, Tonga, Sandwiches) present the lowest topography in the world, while on the E- to NE-directed subduction (e.g., Andes) and collision zones (e.g., Alps, Himalaya) are characterized by the highest mountains worldwide. This asymmetry appears primarily controlled by the slab polarity with respect to the westward drift of the lithosphere due to a global-scale eastward mantle flow (Doglioni et al., 2007). However, the potential influence of a polarized mantle flow on the hinge motion in subduction zones has never been tested quantitatively, thus leaving significant gap in understanding of this key plate tectonic phenomenon. Here, we explore the effects of a priori defined mantle flow on the slab dynamics and response at the surface by means of self-consistent two-dimensional thermomechanical numerical experiments in which an oceanic plate sinks beneath a continental plate under the control of realistic visco-plastic rock rheologies. Results show that the motion of the subduction hinge toward or away relative to the upper plate is the simple kinematic control for the occurrence of two different subduction styles: (1) When the subduction hinge converges toward the upper plate, the upper plate is shortened and a double vergent belt, such as the Alps, forms. On the contrary, (2) when the slab and the related hinge retreat relative to the upper plate, the upper plate is stretched (a backarc basin opens) and a single vergent belt develops, such as along the western Pacific margin. The two settings also show different type of rocks involved in the mountain building process. In the first setting, in fact, the orogen is principally composed by sedimentary cover, i.e., young and shallow rocks coming from superficial erosion of the plates involved in the subduction process. This occurs because the basal decollement of the subducting plate is never connected to the surface but is rather folded and swallowed down inside the subduction zone, being thus unable to feed the accretionary prism with rocks coming from high depths. On the other hand, the second subduction setting show orogens involving older and deeper rocks because of the deeper décollement planes, being thus able to involve the basement of the subducted plate. These findings are supported by a quantitative agreement with observations derived from the global-scale models, which indicate that mantle flow would be the leading feature influencing slab-dip, subduction rate and motion of the slab hinge: E- or NE-directed subductions have shallower slabs, with low dip angles (24° on average), while Wor SE-directed slabs are deeper and steeper (61° on average, Ficini et al., 2017). These results, thus, mimic the asymmetry that can be recognized along the subduction zones worldwide. These models, combined with the observations, support the hypothesis of an asymmetric pattern of the mantle convection strongly driven by the easterly-polarized mantle flow

Back-Arc Spreading Centers and Superfast Subduction: The Case of the Northern Lau Basin (SW Pacific Ocean)

The Lau Basin is a back-arc region formed by the subduction of the Pacific plate below the Australian plate. We studied the regional morphology of the back-arc spreading centers of the Northern Lau basin, and we compared it to their relative spreading rates. We obtained a value of 60.2 mm/year along the Northwest Lau Spreading Centers based on magnetic data, improving on the spreading rate literature data. Furthermore, we carried out numerical models including visco-plastic rheologies and prescribed surface velocities, in an upper plate-fixed reference frame. Although our thermal model points to a high temperature only near the Tonga trench, the model of the second invariant of the strain rate shows active deformation in the mantle from the Tonga trench to ~800 km along the overriding plate. This explains the anomalous magmatic production along all the volcanic centers in the Northern Lau Back-Arc Basin

MINERALOGY AND OXYGEN ISOTOPE PROFILE OF PELECYORA GIGAS (VENERIDAE, BIVALVIA) FROM TUSCAN PLIOCENE

a specimen with joined valves of Pelecyora gigas, an extinct species, was collected in a sandy layer of Early Pliocene age in the northern part of the Siena Basin (Tuscany, Italy). XRD data demonstrated that the original mineralogical composition of the specimen was aragonitic and it maintained substantially the original structure and composition. Neglecting possible changes in sea salinity during different seasons, we can estimate, using oxygen isotope composition a maximum seasonal temperature differences of ca. 9 °C experienced during the life of the individual. An approximate estimation of past sea water composition allows to calculate an average temperature of 23.0±2.7 °C for the water where the shell lived, whereas calculated temperature extremes are 18.5 °C for the colder season and 27.6 °C for the warmer. These data are in a good agreement with those proposed on the basis of the Pliocene Mediterranean taxa nowadays living along the western African shores

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-Liquine-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.Centro de Investigaciones Geológica

Do Solid Earth Tides have a local behaviour? First results from the analysis of 20-year GNSS timeseries

This work presents the first results of the project: Tidal Interplate Lithospheric Deformation of Earth (TILDE) funded by ESA under the NAVISP program (NAVISP-EL1-047). The goals of TILDE project are the estimation of Local Solid Earth Tides (LSET); i.e., models which depends on the geographical position of the selected sites. Furthermore it will investigate possible correlations between LSET and geological/geophysical events, such as tectonic plates movements, earthquakes and volcanic activities. Finally, we test if the adoption of LSET modelling can improve the quality of GNSS geodetic solutions. 73 GNSS stations have been selected all over the world for which a stack of data of 20-year long at least were available. We have processed the data providing the solutions (namely coordinates) with a sampling rate in turn of 1 day (1D) and 3 hours (3H). For each solution, we have in turn switched off (OFF) and switched on (ON) the SET. The solutions with sampling rate of 1D and the OFF mode were used to estimate long periodic constituents (LPC) of LSET; while 3H solutions will be used for shorter ones. We will present the first results achieved working on LPC. A relationship has been found between Love and Shida numbers and the absolute values of the latitudes of the GNSS stations to which they refer to. Their relationship is a convex parabola which has the maximum just close to tectonic equator which has an inclination of about 28.5 degrees, corresponding to the ecliptic angle increased by the moon inclination of five degrees