290 research outputs found
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
Subduction Duration and Slab Dip
The dip angles of slabs are among the clearest characteristics of subduction zones, but the factors that control them remain obscure. Here, slab dip angles and subduction parameters, including subduction duration, the nature of the overriding plate, slab age, and convergence rate, are determined for 153 transects along subduction zones for the present day. We present a comprehensive tabulation of subduction duration based on isotopic ages of arc initiation and stratigraphic, structural, plate tectonic and seismic indicators of subduction initiation. We present two ages for subduction zones, a longâterm age and a reinitiation age. Using cross correlation and multivariate regression, we find that (1) subduction duration is the primary parameter controlling slab dips with slabs tending to have shallower dips at subduction zones that have been in existence longer; (2) the longâterm age of subduction duration better explains variation of shallow dip than reinitiation age; (3) overriding plate nature could influence shallow dip angle, where slabs below continents tend to have shallower dips; (4) slab age contributes to slab dip, with younger slabs having steeper shallow dips; and (5) the relations between slab dip and subduction parameters are depth dependent, where the ability of subduction duration and overriding plate nature to explain observed variation decreases with depth. The analysis emphasizes the importance of subduction history and the longâterm regional state of a subduction zone in determining slab dip and is consistent with mechanical models of subduction
Opposite subduction polarity in adjacent plate segments
Active and fossil subduction systems consisting of two adjacent plates with opposite retreating directions occur in several areas on Earth, as the Mediterranean or Western Pacific. The goal of this work is to better understand the first-order plate dynamics of these systems using the results of experimental models. The laboratory model is composed of two separate plates made of silicon putty representing the lithosphere, on top of a tank filled with glucose syrup representing the mantle. The set of experiments is designed to test the influence of the width of plates and the initial separation between them on the resulting trench velocities, deformation of plates, and mantle flow. Results show that the mantle flow induced by both plates is asymmetric relative to the axis of each plate causing a progressive merging of the toroidal cells that prevents a steady state phase of the subduction process and generates a net outward drag perpendicular to the plates. Trench velocities increase when trenches approach each other and decrease when they separate after their intersection. The trench curvature of both plates increases linearly with time during the entire evolution of the process regardless their width and initial separation. The interaction between the return flows associated with each retreating plate, particularly in the interplate region, is stronger for near plate configurations and correlates with variations of rollback velocities. We propose that the inferred first-order dynamics of the presented analog models can provide relevant clues to understand natural complex subduction systemsPeer ReviewedPostprint (published version
Machine Learning Can Predict the Timing and Size of Analog Earthquakes
Despite the growing spatiotemporal density of geophysical observations at subduction zones, predicting the timing and size of future earthquakes remains a challenge. Here we simulate multiple seismic cycles in a laboratoryâscale subduction zone. The model creates both partial and full margin ruptures, simulating magnitude M_w 6.2â8.3 earthquakes with a coefficient of variation in recurrence intervals of 0.5, similar to real subduction zones. We show that the common procedure of estimating the next earthquake size from slipâdeficit is unreliable. On the contrary, machine learning predicts well the timing and size of laboratory earthquakes by reconstructing and properly interpreting the spatiotemporally complex loading history of the system. These results promise substantial progress in real earthquake forecasting, as they suggest that the complex motion recorded by geodesists at subduction zones might be diagnostic of earthquake imminence
Slab stiffness control of trench motion: Insights from numerical models
Subduction zones are not static features, but trenches retreat (roll back) or advance. Here, we investigate the dominant dynamic controls on trench migration by means of two- and three-dimensional numerical modeling of subduction. This investigation has been carried out by systematically varying the geometrical and rheological model parameters. Our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate, a large viscosity ratio between plate and mantle, and a small density contrast between plate and mantle or an intermediate width (w ⌠1300 km). Advancing slabs dissipate âŒ45% to âŒ50% of the energy in the system. Thin plates with relatively low viscosity or relatively high density, or wide slabs (w ⌠2300 km), on the other hand, promote subduction in the retreating style (i.e., slab roll-back). The energy dissipated by a retreating slab is âŒ35% to âŒ40% of the total dissipated energy. Most of the energy dissipation occurs in the mantle to accommodate the slab motion, whereas the lithosphere dissipates the remaining part to bend and âunbend.â With a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one: plate stiffness. Stiffer slabs cause the trench to advance, whereas more flexible slabs lead to trench retreat. The reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface, but stiff slabs have more difficulty âunbendingâ at depth, when arriving at the 660-km discontinuity. Those bent slabs tend to cause the trench to advance. In a similar way, variation of the viscoplasticity parameters in the plate may change the style of subduction: a low value of friction coefficient weakens the plate and results in a retreating style, while higher values strengthen the plate and promote the advancing subduction style. Given the fact that also on Earth the oldest (and therefore probably stiffest) plates have the fastest advancing trenches, we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration
Slab disruption, mantle circulation, and the opening of the Tyrrhenian basins
Plate tectonic history, geological, geochemical (element and isotope ratios), and
seismological (P-wave tomography and SKS splitting) data are combined with laboratory
modeling to present a three-dimensional reconstruction of the subduction history
of the central Mediterranean subduction. We fi nd that the dynamic evolution
of the Calabrian slab is characterized by a strong episodicity revealed also by the
discrete opening of the Tyrrhenian Sea. The Calabrian slab has been progressively
disrupted by means of mechanical and thermal erosion leading to the formation of
large windows, both in the southern Tyrrhenian Sea and in the southern Apennines.
Windows at lateral slab edges have caused a dramatic reorganization of mantle convection,
permitting infl ow of subslab mantle material and causing a complicated pattern
of magmatism in the Tyrrhenian region, with coexisting K- and Na-alkaline igneous
rocks. Rapid, intermittent avalanches of large amounts of lithospheric material at
slab edges progressively reduced the lateral length of the Calabrian slab to a narrow
(200 km) slab plunging down into the mantle and enhancing the end of the subduction
process
Shear-Velocity Structure and Dynamics Beneath the Sicily Channel and Surrounding Regions of the Central Mediterranean Inferred From Seismic Surface Waves
The evolution of the Sicily Channel Rift Zone (SCRZ) is thought to accommodate the regional tectonic stresses of the Calabrian subduction system. Much of the observations we have today are either limited to the surface or to the upper crust or deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. It is unclear whether the rifting is passive from far-field extensional stresses or active from mantle upwelling beneath. We measure Rayleigh-and Love-wave phase velocities from ambient seismic noise and invert for 3-D shear-velocity and radial anisotropic models. Variations in crustal S-velocities coincide with topographic and tectonic features. The Tyrrhenian Sea has a âŒ10 km thin crust, followed by the SCRZ (âŒ20 km). The thickest crust is beneath the Apennine-Maghrebian Mountains (âŒ55 km). Areas experiencing extension and intraplate volcanism have positive crustal radial anisotropy (VSH > VSV); areas experiencing compression and subduction-related volcanism have negative anisotropy. The crustal anisotropy across the Channel shows the extent of the extension. Beneath the Tyrrhenian Sea, we find very low sub-Moho S-velocities. In contrast, the SCRZ has a thin mantle lithosphere underlain by a low-velocity zone. The lithosphere-asthenosphere boundary rises from 60 km depth beneath Tunisia to âŒ33 km beneath the SCRZ. Negative radial anisotropy in the upper mantle beneath the SCRZ is consistent with vertical mantle flow. We hypothesize a more active mantle upwelling beneath the rift than previously thought from an interplay between poloidal and toroidal fluxes related to the Calabrian slab, which in turn produces uplift at the surface and induces volcanism
A Multidisciplinary Approach to Earthquake Research: Implementation of a Geochemical Geographic Information System for the Gargano Site, Southern Italy
A priority task for correct environmental planning is to evaluate Natural Hazards, especially
in highly populated areas. In particular, thorough investigations based on different Earth
Science techniques must be addressed for the Seismic Hazard Assessment (SHA) in tectonically active
areas. Not only the management but also the multidisciplinary analysis of all the SHA-related data
sets is best performed using a Geographic Information System. In this paper we show how a researchoriented
GIS is built and used in a practical case. The Geochemical Geographic Information System
(G2IS) was developed and applied to the Gargano promontory (southern Italy) in the framework of
an EC research project, the Geochemical Seismic Zonation (GSZ) Project. This multidisciplinary
â multiscaling powerful tool is described in its structure, updating procedures and manipulation
techniques. Preliminary results are presented on the detection of geochemically active fault zones
and their correlation with remote sensing data and other evidences of seismogenic structures.Published255-278JCR Journalreserve
Dynamical effects of subducting ridges: Insights from 3-D laboratory models
We model the subduction of buoyant ridges and plateaus to study their effect
on slab dynamics. Oceanic ridges parallel to the trench have a stronger effect
on the process of subduction because they simultaneously affect a longer trench
segment. Large buoyant slab segments sink more slowly into the asthenosphere,
and their subduction result in a diminution of the velocity of subduction of
the plate. We observe a steeping of the slab below those buoyant anomalies,
resulting in smaller radius of curvature of the slab, that augments the energy
dissipated in folding the plate and further diminishes the velocity of
subduction. When the 3D geometry of a buoyant plateau is modelled, the dip of
the slab above the plateau decreases, as a result of the larger velocity of
subduction of the dense "normal" oceanic plate on both sides of the plateau.
Such a perturbation of the dip of the slab maintains long time after the
plateau has been entirely incorporated into the subduction zone. We compare
experiments with the present-day subduction zone below South America.
Experiments suggest that a modest ridge perpendicular to the trench such as the
present-day Juan Fernandez ridge is not buoyant enough to modify the slab
geometry. Already subducted buoyant anomalies within the oceanic plate, in
contrast, may be responsible for some aspects of the present-day geometry of
the Nazca slab at depth
Seamount Subduction and Megathrust Seismicity: The Interplay Between Geometry and Friction
Subducting seamounts are recognized as one of the key features influencing megathrust earthquakes. However, whether they trigger or arrest ruptures remains debated. Here, we use analog models to study the influence of a single seamount on megathrust earthquakes, separating the effect of topography from that of friction. Four different model configurations have been developed (i.e., flat interface, high and low friction seamount, low friction patch). In our models, the seamount reduces recurrence time, interseismic coupling, and fault strength, suggesting that it acts as a barrier: 80% of the ruptures concentrate in flat regions that surround the seamount and only smaller magnitude earthquakes nucleate above it. The low-friction zone, which mimics the fluid accumulation or the establishment of fracture systems in natural cases, seems to be the most efficient in arresting rupture propagation in our experimental setting
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