THE ROLE OF FRICTIONAL STRENGTH

[1] At a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc. In this paper, we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins. In numerical models, we vary thickness of a serpentinized layer in the mantle wedge (15 to 25 km) and the frictional strength of both the sediments and serpentinized mantle (friction angle 1° to 15°, or static friction coefficient 0.017 to 0.27) to control the amount of frictional coupling between the plates. With plastic strain weakening in the lithosphere, our numerical models can attain stable subduction geometry over millions of years. We find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths. In the case of low sediment and serpentinite friction, the subduction interface has a shallow dip, while the subduction zone develops an accretionary prism, a broad forearc high, a deep forearc basin, and a shallow trench. In the high friction case, the subduction interface is steep, the trench is deeper, and the accretionary prism, forearc high and basin are all absent. The resultant free-air gravity and topographic signature of these subduction zone models are consistent with observations. We believe that the low-friction model produces a geometry and forearc structure similar to that of accretionary margins. Conversely, models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin. We find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

Subduction dynamics as revealed by trench migration

International audienceNew estimates of trench migration rates allow us to address the dynamics of trench migration and back-arc strain. We show that trench migration is primarily controlled by the subducting plate velocity V-sub, which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: V-t = 0.5V(sub) - 2.3 (in cm a(-1)) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending

Empirical Analysis of Global-Scale Natural Data and Analogue Seismotectonic Modelling Data to Unravel the Seismic Behaviour of the Subduction Megathrust

Subduction megathrusts host the Earth’s greatest earthquakes as the 1960 Valdivia (Mw9.5, Chile), the largest earthquake instrumentally recorded, and the recent 2004 Sumatra-Andaman (Mw9.2, Indonesia), 2010 Maule (Mw8.8, Chile), and 2011 Tohoku-Oki (Mw9.1,Japan) earthquakes triggering devastating tsunamis and representing a major hazard tosociety. Unravelling the spatio-temporal pattern of these events is thus a key for seismichazard assessment of subduction zones. This paper reviews the current state ofknowledge of two research areas–empirical analysis of global-scale natural data andexperimental data from an analogue seismotectonic modelling—devoted to study cause-effect relationships between subduction zone parameters and the megathrustseismogenic behavior. The combination of the two approaches overcomes theobservational bias and inherent sampling limitations of geological processes(i.e., shortness of instrumental and historical data, decreasing completeness andresolution with time into the past) and allows drawing appropriately from multipledisciplines with the aim of highlighting the geodynamic conditions that may favor theoccurrence of giant megathrust earthquakes

On the relationships between slab dip, back-arc stress, upper plate absolute motion, and crustal nature in subduction zones

International audience[1] Statistical analysis of modern oceanic subduction zone parameters, such as the age of a downgoing plate or the absolute plate motions, is performed in order to investigate which parameter controls the dip of a slab and, conversely, what the influence of slab geometry is on upper plate behavior. For that purpose, parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge/plateau/seamount subduction. On the basis of tomographic images, slabs that penetrate through, or lie on, the 670 km discontinuity are also identified. The results of the statistical analysis are as follows: (1) Back-arc stress correlates with slab dip, i.e., back-arc spreading is observed for deep dips (deeper than 125 km) larger than 50°, whereas back-arc shortening occurs only for deep dips less than 30°. (2) Slab dip correlates with absolute motion of the overriding plate. The correlation is even better when the slab lies on, or even more penetrates through, the 670 km discontinuity. (3) Slabs dip more steeply, by about 20° on average, beneath oceanic overriding plates than beneath continental ones. (4) Slabs dip more steeply on average by about 10° near edges. (5) Slab dip does not correlate with the magnitude of slab pull, the age of subducting lithosphere at the trench, the thermal regime of the subducting lithosphere, the convergence rate, or the subduction polarity (east versus west). The present study provides evidence that the upper plate absolute motion plays an important role on slab dip, as well as on upper plate strain. Retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates. One can infer that low slab dips correlate well with compression in continental advancing upper plates, whereas steep dips are often associated with extension in oceanic retreating upper plates. Excess weight of old slabs is often counterbalanced by other forces, probably asthenospheric in origin, such as lateral mantle flow near slab edges or anchor forces, to determine slab dip. Components: 12,676 words, 13 figures, 1 table

IODP Expedition 334: An Investigation of the Sedimentary Record, Fluid Flow and State of Stress on Top of the Seismogenic Zone of an Erosive Subduction Margin

The Costa Rica Seismogenesis Project (CRISP) is an experiment to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Integrated Ocean Drililng Program (IODP) Expedition 334 by R/V JOIDES Resolution is the first step toward deep drilling through the aseismic and seismic plate boundary at the Costa Rica subduction zone offshore the Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. Drilling operations included logging while drilling (LWD) at two slope sites (Sites U1378 and U1379) and coring at three slope sites (Sites U1378–1380) and at one site on the Cocos plate (Site U1381). For the first time the lithology, stratigraphy, and age of the slope and incoming sediments as well as the petrology of the subducting Cocos Ridge have been characterized at this margin. The slope sites recorded a high sediment accumulation rate of 160–1035m m.y.-1 possibly caused by on-land uplift triggered by the subduction of the Cocos Ridge. The geochemical data as well as the in situ temperature data obtained at the slope sites suggest that fluids are transported from greater depths. The geochemical profiles at Site U1381 reflect diffusional communication of a fluid with seawater-like chemistry and the igneous basement of the Cocos plate (Solomon et al., 2011; Vannucchi et al., 2012a). The present-day in situ stress orientation determined by borehole breakouts at Site U1378 in the middle slope and Site U1379 in the upper slope shows a marked change in stress state within ~12 km along the CRISP transect; that may correspond to a change from compression (middle slope) to extension (upper slope)

IODP expedition 334: An investigation of the sedimentary record, fluid flow and state of stress on top of the seismogenic zone of an erosive subduction margin

金沢大学理工研究域地球社会基盤学

Dynamique des zones de subduction : étude statistique globale et approche analogique.

Statistical comparison of subduction zones helps to understand their variability and the respective influences of the acting forces. To allow such a statistical analysis, we have built SubductionZones, a global database which describes all the current oceanic subduction zones around the world.Most of the observations argue for a strong influence of overriding plate and of its motion direction on subduction dynamics. Subducting plate and slab characteristics seem to control plate motions, but their influence on trench motion, overriding plate deformation and slab geometry are likely to be negligible. Some evidences of mantle flows influences have also been enlightened.As it provides the theoretical and dynamical framework for statistical observations interpretation, physical modelling (experimental and numerical) is complementary with statistical approach. Experimental models have thus allowed us to test how overriding plate motion could influence trench motion, slab geometry and overriding plate deformation.Comparer les zones de subductions les unes aux autres permet d'éclairer les raisons de leur diversité et de remonter aux forces qui gouvernent leur fonctionnement. Pour permettre cette comparaison statistique, une base de données globale originale, SubductionZones, qui décrit l'ensemble des zones de subduction océaniques du globe a été construite à partir de bases de données globales préexistantes. Les observations qui découlent de l'analyse statistique globale des zones de subduction, souvent inédites et infirmant parfois les résultats d'études antérieures, impliquent une réévaluation des modèles de subduction en vigueur. Des trois « acteurs » mis en jeu dans le processus de subduction (i.e., la plaque subduite, la plaque supérieure et le manteau), la plupart des observations mettent en avant l'influence de la plaque supérieure et de la direction de son mouvement sur la dynamique de la subduction. Les caractéristiques de la plaque subduite et de son panneau plongeant, en revanche, si elles semblent contrôler le mouvement des plaques, ont vraisemblablement une influence limitée sur le mouvement des fosses, le régime tectonique de la plaque supérieure et le pendage du slab. L'influence du manteau et des flux qui l'animent a par ailleurs été mise en évidence dans quelques cas, notamment à proximité des bordures de slabs.Les modèles physiques (analytiques, numériques ou analogiques), pour leur part, complètent la vision instantanée du processus de subduction que donne l'approche statistique en offrant un cadre théorique et dynamique à l'interprétation des observations. L'approche analogique nous a ainsi permis de tester les modalités de l'influence du mouvement de la plaque supérieure dans le contrôle du mouvement de la fosse, de la géométrie du panneau plongeant et de la déformation de la plaque supérieure

En mode mineur... François Laveau explorateur au Maroni (1887- 1928)

Collomb Gérard, Heuret Arnauld. En mode mineur... François Laveau explorateur au Maroni (1887- 1928). In: Outre-mers, tome 101, n°384-385,2014. Coopérants et coopération en Afrique : circulations d'acteurs et recompostitions culturelles (des années 1950 à nos jours) pp. 301-315

Subduction Zones Parameters

International audienceSubduction zones are characterized by multiple behaviors, dynamics, or geometries. Most of them results from the complex interplay between various parameters. In this article, we define the most common parameters used by scientists and then mention the studies where (set of) parameters are tested empirically, or experimentally. We also focus on the relations among the various parameters including kinematics, geophysical, and seismological properties, geometry, structure, or strain