[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.

: We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittleviscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscous extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in

International audienceThe Paleozoic French Variscan Belt in Massif Central and Massif Armoricain is a collision belt that provides a good example of a suture zone where ophiolites are rare, and the frontal (i.e., the magmatic arc) part of the upper plate is not present. In the lower plate (or Gondwana), the continental rocks are subdivided into an Upper Gneiss Unit (UGU) and a Lower Gneiss Unit (LGU). The UGU experienced a high-pressure (and likely ultra-high-pressure) metamorphism followed by crustal melting during their exhumation. New chemical U-Th-Pb monazite ages and ion-probe U-Pb zircon ages on migmatites allow us to constrain the P-T-t paths followed by the UGU and LGU. By comparison with thermomechanical experiments, a possible geodynamic evolution scenario can be proposed for the Variscan convergence. The high-compression regime of continental subduction developed during the initial subduction of the northern margin of Gondwana under Armorica in Silurian times. This induced the formation of a new subduction zone in the back-arc basin, which is the youngest, hottest, and thus mechanically the weakest part of the overriding plate. As a result, the arc-back-arc basin domain has been almost totally subducted below Armorica. Only a limited part of the back-arc basin rocks remains exposed in the Devonian St-Georges-sur-Loire Unit. Subsequently, the continental subduction of Gondwana resumed with a steeper dip associated with low-compression regime that in turn allowed the high-pressure rocks to be exhumed and partly melted in Late Devonian times. Such a scheme depicts quite well the complexity of the Variscan Belt

Subduction initiation at passive margins plays a central role in the plate tectonics theory. However, the process by which a passive margin becomes active is not well understood. In this paper we use the southwest Iberia margin (SIM) in the Atlantic Ocean to study the process of passive margin reactivation. Currently there are two tectonic mechanisms operating in the SIM: migration of the Gibraltar Arc and Africa-Eurasia convergence. Based on a new tectonic map, we propose that a new subduction zone is forming at the SIM as a result of both propagation of compressive stresses from the Gibraltar Arc and stresses related to the large-scale Africa-Eurasia convergence. The Gibraltar Arc and the SIM appear to be connected and have the potential to develop into a new eastern Atlantic subduction system. Our work suggests that the formation of new subduction zones in Atlantic-type oceans may not require the spontaneous foundering of its passive margins. Instead, subduction can be seen as an invasive process that propagates from ocean to ocean.

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Statoil ASA, Norway [1] We make use of observations on orogenic strain accumulation and deformation partitioning in the Central Andes to explore the backarc strength evolution at the lithospheric scale. In plan view, the Altiplano-Puna plateaux experienced rapid initial increase of surface area undergoing active deformation during the Cenozoic. Beyond the maximum lateral extent reached around 10-15 Ma (40-50% of entire proto-Andes undergoing deformation) at 10-20% total strain, rapid localization initiated at the eastern flank of the Altiplano (Inter-and Subandean thrust belt) but not at the Puna latitude. Localization was associated with a significant increase in bulk shortening rate. Average fault slip rates equally increased by an order of magnitude following a protracted period of stable average rates. Estimates of strength evolution based on force balance calculations and critical wedge analysis suggest significant backarc weakening driving this change after the Middle Miocene. Strain accumulation led to localization and weakening with development of a detachment propagating through crust and upper mantle. We find that lithosphere-scale failure resulting from strain weakening beyond a critical strain threshold (c. 20%) and fault coalescence with formation of a weak detachment in shales (effective coefficient of friction < 0.1) plays a key role in the evolution of the Andes. Strainrelated lithosphere weakening appears to dominate over the impact of external forcing mechanisms, such as variations of plate convergence, mantle-assisted processes, or erosion. Comparison of these orogen-scale observations with experimental rock rheology indicates substantial similarity of deformation behavior with similar weakening thresholds across a wide range of scales.