Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
1] Recent statistical analysis by Lallemand et al. (2008) of subduction zone parameters revealed that the back-arc deformation mode depends on the combination between the subducting (nu(sub)) and upper (nu(up)) plate velocities. No significant strain is recorded in the arc area if plate kinematics verifies nu(up) = 0.5 vsub - 2.3 (cm/a) in the HS3 reference frame. Arc spreading ( shortening) occurs if nu(up) is greater ( lower) than the preceding relationship. We test this statistical law with numerical models of subduction, by applying constant plate velocities far away from the subduction zone. The subducting lithosphere is free to deform at all depths. We quantify the force applied on the two converging plates to sustain constant surface velocities. The simulated rheology combined viscous (non-Newtonian) and brittle behaviors, and depends on water content. The influence of subduction rate vs is first studied for a fixed upper plate. After 950 km of convergence ( steady state slab pull), the transition from extensional to compressive stresses in the upper plate occurs for vs similar to 1.4 cm/a. The effect of upper plate velocity is then tested at constant subduction rate. Upper plate retreat ( advance) with respect to the trench increases extension ( compression) in the arc lithosphere and increases ( decreases) the subducting plate dip. Our modeling confirms the statistical kinematic relationship between vsub and nu(up) that describes the transition from extensional to compressive stresses in the arc lithosphere, even if the modeled law is shifted toward higher rates of upper plate retreat, using our set of physical parameters ( e. g., 100 km thick subducting oceanic plate) and short- term simulations. Our results make valid the choice of the HS3 reference frame for assessing plate velocity influence on arc tectonic regime. The subduction model suggests that friction along the interplate contact and the mantle Stokes reaction could be the two main forces competing against slab pull for upper mantle subductions. Besides, our simulations show that the arc deformation mode is strongly time dependent.
ISSN: 1525-2027

hal-00412328
https://hal.archives-ouvertes.fr/hal-00412328
https://hal.archives-ouvertes.fr/hal-00412328/document
https://hal.archives-ouvertes.fr/hal-00412328/file/arcayG32008.pdf
DOI : 10.1029/2007GC001875

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress (S-Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S-Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental-continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20 degrees E to N100 degrees E, remains active: the portion ranging from N30 degrees E to N40 degrees E, orthogonal to the regional convergence. (C) 2011 Elsevier Ltd. All rights reserved.
ISSN: 0264-3707

hal-00671033
https://hal.archives-ouvertes.fr/hal-00671033
DOI : 10.1016/j.jog.2010.12.003

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : 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.
ISSN: 1525-2027

hal-01261567
https://hal.archives-ouvertes.fr/hal-01261567
https://hal.archives-ouvertes.fr/hal-01261567/document
https://hal.archives-ouvertes.fr/hal-01261567/file/lallemandG32005.pdf
DOI : 10.1029/2005GC000917

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Based on global earthquake catalogs, the hypocenters, nodal planes, and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007. Assuming that the seismogenic zone coincides with the distribution of 5.5 <= M < 7 earthquakes, the subduction interface seismogenic zones were mapped for 80% of the trench systems and characterized with geometrical and mechanical parameters. Using this database, correlations were isolated between significant parameters to identify cause-effect relationships. Empirical laws obtained in previous studies were revisited in light of this more complete, accurate, and uniform description of the subduction interface seismogenic zone. The seismogenic zone was usually found to end in a fore-arc mantle, rather than at a Moho depth. The subduction velocity was the first-order controlling parameter for variations in the physical characteristics of plate interfaces, determining both the geometry and mechanical behavior. As such, the fast subduction zones and cold slabs were associated with large and steep plate interfaces, which, in turn, had large seismic rates. The subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches. Events with M-w >= 8.5 preferentially occurred in the vicinity of slab edges, where the upper plate was continental and the back-arc strain was neutral. This observation was interpreted in terms of compressive normal stresses along the plate interface. Large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally.
ISSN: 1525-2027

hal-00617681
https://hal.archives-ouvertes.fr/hal-00617681
https://hal.archives-ouvertes.fr/hal-00617681/document
https://hal.archives-ouvertes.fr/hal-00617681/file/heuretG32011.pdf
DOI : 10.1029/2010GC003230

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Physical modeling of oblique subduction is performed to study the mechanism of strain partitioning. The model is two-layer and includes the elasto-plastic lithosphere (the overriding and subducting plates) and the low-viscosity liquid asthenosphere. The subduction is driven by a push force from a piston and a pull force when the density contrast Ap between the subducting plate and the asthenosphere is positive. We vary both Ap and the interplate friction (frictional stresses). Slip partitioning is obtained only in the models with high interplate friction and only when the overriding plate contains a weak zone. This zone in the models corresponds either to locally thinned litho-sphere or to cut (fault). The horizontal, trench-normal component of the interplate friction force [Ffh[ can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force [Fph [ in the subduction zone. Ffh is always negative (compression), while Fph can be either negative (compressional subduction regime) or positive (extensional regime). High friction, which promotes partitioning, can completely cancel the extensional (suction) force Fph. Back arc tension and strike-slip faulting appear thus as conflicting processes, although they can coexist in the same subduction zone, depending on the relative values of relevant forces. It appears that high friction can exist only in compressional subduction zones where partitioning should develop more easily. This conclusion is supported by the comparison of two oblique subduction zones, having similar geometry: the compressional southern Kurile zone (strong partitioning) and extensional southern Ryukyu zone (no lithospheric-scale partitioning). Other factors controlling the strain partitioning are the length of the oblique subductlon zone, the boundary conditions at the transverse limits of the forearc sliver, and of course, the obliquity of subduction.
ISSN: 2169-9275

hal-01261396
https://hal.archives-ouvertes.fr/hal-01261396
https://hal.archives-ouvertes.fr/hal-01261396/document
https://hal.archives-ouvertes.fr/hal-01261396/file/chemendaJGR2000.pdf
DOI : 10.1029/1999JB900332

Éditeur(s) : HAL CCSD Elsevier
Résumé : 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.
ISSN: 0012-821X

hal-00406598
https://hal.archives-ouvertes.fr/hal-00406598
DOI : 10.1016/j.epsl.2007.02.004