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International audience; Based on 2D seismic profiles, multibeam and seabed grab cores acquired during the Garanti cruise in 2017, 1–5 km wide seabed giant polygons were identified in the Grenada basin, covering a total area of ∼55,000 km2, which is the largest area of outcropping polygonal faults (PF) ever found on Earth so far. They represent the top part of an active 700–1,200 m thick underlying polygonal fault system (PFS) formed due to the volumetric contraction of clay- and smectite-rich sediments, initiated in the sub-surface at the transition between the Early to Middle Pliocene. The short axes of the best-fit ellipses obtained from a graphical center-to-center method were interpreted as the local orientation of a preferential contraction perpendicular to the creep deformation of slope sediments. In the North Grenada Basin, the polygons are relatively regular, but their short axes seem to be parallel to a N40°E extension recently evidenced in the forearc, possibly extending in the backarc, but not shown in the study area. They are most probably related to a progressive burial due to a homogeneous subsidence. In the South Grenada Basin, the polygons are more elongated and their axes are progressively rotating southeastward toward the depocenter, indicating a creep deformation toward the center of the basin created by a differential subsidence. Seabed polygons and underlying PF could thus be indicative of the deformation regime of shallow sediments related to main slopes controlled by two different basin architectures.

International audience

International audience

International audience

International audience

International audience

International audience; The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake catalogue for the period 1900–2007. The new catalogue provides homogenized M′W magnitudes and shows that several M′W≥ 7.0 earthquakes occurred offshore Hualien and Suao cities. We then focused on the 1920 June 5 earthquake (reported surface wave magnitude 8.1) previously located beneath the accretionary prism. The revised moment magnitude has been estimated in our catalogue at 7.7 ± 0.2. It is the biggest earthquake ever recorded in the Taiwan area but the fault that has produced this earthquake has not yet been identified with confidence. We relocated this event using regional phases (seismological bulletins archived at the Central Weather Bureau of Taiwan) about 50 km NNE and shallower of its former location, that is, within the Ryukyu Arc basement. According to earthquake bulletin information, revised magnitude, new hypocentral determination and known regional faults, we propose four potential active faults as candidates for the slip associated to this event: (1) the interplate seismogenic zone (ISZ), (2) an out-of-sequence thrust cutting through the forearc and branching on the ISZ, (3) a NS strike-slip fault cutting through the Ryukyu arc and (4) a N–S, westward dipping thrust fault, affecting the Philippine Sea Plate east of the Luzon Arc. The best compromise is to consider a rupture along the ISZ with a shallow nucleation possibly along a splay-fault followed by a downward and lateral propagation of the rupture that would explain the lack of significant seafloor motion and subsequent tsunami. We also estimate the maximum seismic coupling of the ISZ in the region east of Taiwan to about 0.4. In parallel, the evidences of aseismic slip occurring along the ISZ allow us to conclude that this region should only be affected by M < 8 earthquakes.

International audience

International audience

Ouvrage de la Société géologique de France

DOI ouvrage : https://doi.org/10.1130/SPE358; International audience; We performed two-dimensional finite-element numerical modeling of subduction to study the deformation and failure pattern of an overriding plate. This plate has elasto-plastic rheology with strain weakening and contains a "volcanic arc" represented in the model as a thinned and rheologically weakened zone. The subducting plate is not modeled directly. Its effect on the overriding plate is simulated by the normal and tangential stresses along the interplate surface, which correspond to the interplate pressure and interplate friction, respectively. Both stresses cause nonhydrostatic (tectonic) compression on this plate. The first approximation for the interplate pressure is derived from previous experimental modeling of subduction of oceanic lithosphere. Numerical modeling shows that the overriding plate fails under the arc along a trenchward-dipping fault. We then vary both the interplate pressure and the interplate friction and show that the failure direction does not change. Subduction of the continental margin modifies (increases) the interplate pressure because of the buoyancy of thick continental crust. The compressive stress in the overriding plate increases. Failure of this plate can occur in the same way as indicated or in the opposite direction along a fault that dips under the arc (away from the trench and toward the ocean. The failure mode depends in particular on the depth of the margin's subduction prior to lithosphere rupture. Failure along a fault dipping toward the continent results in a subduction reversal, whereas failure along a fault dipping toward the ocean is followed by subduction of a forearc block beneath the arc. Both scenarios seem to have natural analogues.We consider one analogue, the ongoing arc-continent collision in Taiwan, and argue that this process occurs according to the second scenario, which involves forearc subduction.

International audience; A variety of atypical plume-like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed, and seismological imaging has shown ubiquitous small-scale convection in the uppermost mantle in regions such as the Mediterranean region, the western US, and around the western Pacific. We argue that the three-dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics. The testable surface expressions of this process are the outside-arc alkaline volcanism, topographic swell, and low-velocity seismic anomalies associated with partial melt. Using three-dimensional, simplified numerical subduction models, we show that focused upwellings can be generated both ahead of the slab in the back-arc region (though -five times further inward from the trench than arc-volcanism) and around the lateral edges of the slab (in the order of 100 km away from slab edges). Vertical mass transport, and by inference the associated decompression melting, in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities. The upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation. We discuss representative case histories from the Pacific and the Mediterranean where we find possible evidence for such slab-related volcanism.

The seismic hazard presented by slow subduction zones is not well known. While there is a widely accepted apparent relation between “fast-young plate” subduction and great earthquake generation (e.g., Chile, 1960), the seismic record indicates that slow subduction zones are also capable of generating mega-thrust earthquakes (M > 8.2). Available data on the recurrence interval for slow subduction margins, suggests that repeat times are longer than for more rapid convergence margins (on the order of several hundred to a few thousand years). For several of these margins, however, no shallow dipping thrust earthquake focal mechanisms are observed and no mega-thrust earthquakes known either.

International audience; Recent global kinematic studies reveal that most of the trenches roll back but a significant number of them advance toward the upper plate. Those advancing trenches are mostly located in the Western Pacific and correspond to the subduction of very old, Mesozoic oceanic lithosphere. While retreating trenches are commonly explained by the slab pull action of the descending lithosphere, the origin of advancing trenches is still debated. Since this relationship is dependent upon the adopted reference frame, we select region where geological studies show the variability of trench migration style with time. The Izu-Bonin-Mariana (IBM) region represents a key example. The detailed reconstruction of the trench migration of the IBM subducting system reveals that after a long episode of asymmetric rollback, the IBM trench recently started advancing. We propose that this change from retreating to advancing trench mode results from the subduction of progressively older and stiffer lithospheric material. We test this hypothesis by means of two-dimensional (2-D) numerical models, reproducing the effects of the lithospheric aging during subduction. The result of our numerical tests shows that the entrance of old and stiff lithosphere forces the trench to advance because the increasing stiffness of the slab prevents the slab to unbend once it has subducted. We adapt this physical result to the IBM evolution, showing that the switch from trench retreat to trench advance can reconcile the shape of IBM slab as previously suggested. Finally we discuss the possibility that the subduction of the Cretaceous lithosphere might have triggered the change of the motion of the Pacific plate around 5 Ma.

A combination of statistical studies on present-day subduction zones and three-dimensional (3D) laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones. In 3D laboratory models, the analogue of a two layer linearly viscous lithosphere–upper mantle system is achieved by means of silicon putty glucose syrup tank experiment. The subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics. Both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior: (1) trenches migrate at a rate close to the overriding plate motion, but always move slower than the overriding plates. The mechanism at work is a direct consequence of “slab anchoring” opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability. (2) An overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench. (3) Shallow and steep dips are found if the overriding plate moves toward and away from the trench, respectively.

The motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle. These models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation. Here, using a recent compilation of a global data set, we found that the motion of trenches (either advancing or retreating with respect to upper plates) scales with their corresponding subducting plates motion. Based on simple experimental tests, we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships. We deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle.