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The opening of the Tyrrhenian Sea has been punctuated by short-lived episodes of oceanic accretion on separate small backarc basins during early Pliocene (Vavilov basin) and early Pleistocene (Marsili basin) time. These spreading pulses are related to slab rollback and are synchronous with the reduction of the subduction zone width during the formation of the narrow Calabrian arc. Using laboratory models, we investigated the long-term and transient effects of the reduction of slab width on the subduction kinematics. We found that the abrupt reduction in slab width results in a pulse of acceleration of the trench retreat velocity, as the balance between driving and resisting forces acting on the slab is temporarily modified. Our findings also show that the time scale and amplitude of spreading observed in the Tyrrhenian Sea can be experimentally fitted if the scaled viscosity of the uppermost part of the mantle ranges between [10.sup.19] and [10.sup.20] Pa s. doi: 10.1130/G31038.1

Volcanological and structural field data are used to define the tectonic control on the N--S volcanic arc of NE Honshu (Japan) since late Miocene. During late Miocene-Pliocene, bimodal products were mainly erupted from along-arc and NE--SW-aligned and elongated calderas. The deformation pattern mostly consisted of N--S dextral faults and subordinate NE--SW extensional structures produced by NE--SW compression. This pattern, because of the indentation of the Kuril sliver, is similar to that of oblique convergence settings. Magma rose and extruded along NE--SW areas of localized extension created by the dextral faults. These extensional areas were uncoupled with regard to those, ~E--W trending, inferred to have focused the rise of melts from the subducting slab in the mantle. During Quaternary, a larger amount of andesite was mainly erupted from along-arc and ~E--W-aligned and elongated stratovolcanoes. The deformation pattern mostly consisted of N--S thrust faults and subordinate ~E--W extensional structures, produced by ~E--W compression, resulting from orthogonal convergence due to the variation in the absolute motion of the Pacific Plate. The ~E--W extensional structures are the shallowest expression of ~E--W-trending hot mantle fingers, suggesting mantle-crust coupling for the rise of magma. Such a coupling ensures (1) higher extrusion and (2) mixing between a deeper mafic and a shallower felsic magma, generating the andesites. The significantly larger volumes ([Ma.sup.-1] 200 [km.sup.-1] of length of the arc) of magma erupted during Quatemary show that pure convergence conditions do not necessarily hinder the rise and extrusion of magma.

[1] New estimates of trench migration rates allow us to address the dynamics of trench migration and backarc strain. We show that trench migration is primarily controlled by the subducting plate velocity [V.sub.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.sub.t] = 0.5[V.sub.sub] - 2.3 (in cm [a.sup.-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. Citation: Lallemand, S., A. Heuret, C. Faccenna, and F. Funiciello (2008), Subduction dynamics as revealed by trench migration, Tectonics, 27, TC3014, doi:10.1029/ 2007TC002212.

[1] We present lithospheric-scale analog models, investigating how the absolute plates' motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction, possibly resulting in the appearance of flat slab segments, and how it changes the overriding plate tectonic regime. Experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions, part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab. Slab flattening is a long-term process (~10 Ma), which requires the subduction of hundreds of kilometers of buoyant plateau. The overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect. Slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate. The increase of the overriding plate topography close to the trench results from (1) the buoyancy of the plate subducting at trench and (2) the overriding plate shortening. Experiments are compared to the South American active margin, where two major horizontal slab segments had formed since the Pliocene. Along the South American subduction zone, flat slab segments below Peru and central Chile/NW Argentina appeared at ~7 Ma following the beginning of buoyant slab segments' subduction. In northern Ecuador and northern Chile, the process of slab flattening resulting from the Carnegie and Iquique ridges' subductions, respectively, seems to be active but not completed. The formation of flat slab segments below South America from the Pliocene may explain the deceleration of the Nazca plate trenchward velocity. Citation: Espurt, N., F. Funiciello, J. Martinod, B. Guillaume, V. Regard, C. Faccenna, and S. Brusset (2008), Flat subduction dynamics and deformation of the South American plate: Insights from analog modeling, Tectonics, 27, TC3011, doi:10.1029/2007TC002175.

Tectonics and erosion–sedimentation are the main processes responsible for shaping the Earth's surface. The link between these processes has a strong influence on the evolution of landscapes. One of the tools we have for investigating coupled process models is analog modeling. Here we contribute to the utility of this tool by presenting laboratory-scaled analog models of erosion. We explore the erosional response of different materials to imposed boundary conditions, trying to find the composite material that best mimics the behavior of the natural prototype. The models recreate conditions in which tectonic uplift is no longer active, but there is an imposed fixed slope. On this slope the erosion is triggered by precipitation and gravity, with the formation of channels in valleys and diffusion on hillslope that are functions of the analog material. Using digital elevation models (DEMs) and a laser scan correlation technique, we show model evolution and measure sediment discharge rates. We propose three main components of our analog material (silica powder, glass microbeads and PVC powder; PVC: polyvinyl chloride), and we investigate how different proportions of these components affect the model evolution and the development of landscapes. We find that silica powder is mainly responsible for creating a realistic landscape in the laboratory. Furthermore, we find that varying the concentration of silica powder between 40 wt% and 50 wt% (with glass microbeads and PVC powder in the range 35 wt% –40 wt% and 15 wt% –20 wt% , respectively) results in metrics and morphologies that are comparable with those from natural prototypes. [ABSTRACT FROM AUTHOR]

It has long been recognized that sediments subducting along the megathrust influence the occurrence of giant (Mw ≥ 8.5) megathrust earthquakes. However, the limited observation span and the concurrent influence of multiple parameters on megathrust behavior prevent us from understanding how sediments affect earthquake size and frequency. Here, we address these limitations by using two‐dimensional, visco‐elasto‐plastic, seismo‐thermo‐mechanical numerical models to isolate how sediment thickness affects subduction geometry and seismicity. Our results show that increasing sediment thickness on the incoming plate results in a decrease of the slab dip, as the trench retreats due to the seaward growth of the sedimentary wedge that also unbends the slab. This decrease in megathrust dip results in a wider seismogenic zone, so that the maximum magnitude of megathrust earthquakes increases. Concurrently, the recurrence time of characteristic events increases and partial ruptures are introduced. The maximum magnitude estimated for subduction segments with the thickest sediment input (Makran, West‐Aegean, and Calabria) is distinctly higher than the instrumentally recorded magnitude. These segments may thus experience larger than as of yet observed earthquakes, albeit infrequently. Increasing sediment thickness also decreases megathrust normal stresses, as the seismogenic zone is more shallow and overlain by a lighter forearc structure. Thicker incoming plate sediments also favor more splay fault activity, whereas we observe more outer rise events for low sediment thickness. Finally, we demonstrate that modeling long‐term subduction dynamics and sediment subduction is crucial for understanding and quantifying megathrust seismicity and seismic potential of subduction zones. Key Points: Thick sediments reduce slab dip and increase seismogenic zone width and maximum earthquake magnitudeThick sediments also favor splay over outer rise faulting and partial over complete megathrust rupturesSimulation of long‐term subduction dynamics and sediments significantly increases the estimated maximum magnitude of the megathrust [ABSTRACT FROM AUTHOR]

In this work we study the dynamics of double subduction systems with opposite polarity in adjacent segments. A combined approach of numerical and analog experiments allows us to compare results and exploit the strengths of both methodologies. High‐resolution numerical experiments complement laboratory results by providing quantities difficult to measure in the laboratory such as stress state, flow patterns, and energy dissipation. Results show strong asymmetries in the mantle flow that produce in turn asymmetries in the trench and in the downgoing slab deformation. The mantle flow pattern varies with time; the toroidal cells between the plates evolve until merging into one unique cell when the trenches align. In that moment the maximum upward flow is observed close to the trenches. The interaction between the mantle flow produced by each subducting plate makes the rollback processes slower than in a single subduction case. This is consistent with the observed energy dissipation rate that is smaller in the double subduction system than in two single subductions. Moreover, we provide a detailed analysis on the setup and boundary conditions required to numerically reproduce the analog experiments. Boundary conditions at the bottom of the domain are crucial to reproduce their analog counterparts. Numerical results are compared to natural examples of multi‐slab subduction systems in terms of upper mantle seismic anisotropy, relative trench retreat velocities, and composition of subduction‐related magmatism. Key Points: Numerical models of double subduction have been developed to reproduce laboratory experiments and to understand the dynamics of the systemThe interaction between the induced mantle flows slows down the evolution of the system and generates additional deformation of platesIn the horizontal plane mantle flow forms four toroidal cells with symmetry axes that rotate during trench retreat [ABSTRACT FROM AUTHOR]

The roughness of the subduction interface is thought to influence seismogenic behavior in subduction zones, but a detailed understanding of how such roughness affects the state of stress along the subduction megathrust is still debated. Here, we use seismotectonic analogue models to investigate the effect of subduction interface roughness on seismicity in subduction zones. We compared analogue earthquake source parameters and slip distributions for two roughness endmembers. Models characterized by a very rough interface have lower integrated fault strength and lower interseismic coupling than models with a smooth interface. Overall, ruptures in the rough models have smaller rupture area, duration, and mean displacement. Individual slip distributions indicate a segmentation of the subduction interface by the rough geometry. We propose that flexure of the overriding plate is one of the mechanisms that contribute to a heterogeneous stress distribution, responsible for the observed seismic behavior. Plain Language Summary: The largest and most destructive earthquakes on Earth occur along the plate contact in subduction zones, the region where an oceanic plate dives below another plate. The roughness of the downgoing plate, which is a result of the seafloor topography on that plate, is thought to play a role in the occurrence of large subduction earthquakes. With analogue models that include a 3‐D‐printed seafloor, we test the effect of two types of seafloor roughness on the occurrence of earthquakes: a very rough versus a very smooth seafloor. We observe that the rough seafloor geometry generally hinders the occurrence of large earthquakes along the subduction interface. This finding helps us to highlight where large future earthquakes are more likely to occur. Key Points: We perform analogue models to investigate the effect of subduction interface roughness on megathrust earthquakesModels with a very rough subduction interface are characterized by lower integrated fault strength and lower interseismic couplingRuptures in models with a very rough subduction interface are generally smaller in terms of rupture area, duration, and mean displacement [ABSTRACT FROM AUTHOR]

The study of slab‐slab interactions has come to the front of geodynamics researches to explain geological and geophysical observations from tectonically complex areas. Here we aim to better understand the geodynamics of the Central Mediterranean, where the Adria plate subducts on its two opposite sides. Additionally, the slab below the Central South Apennines has been progressively breaking off during the last 3 Myr. The role of a slab window in a single slab or in an outward dipping double‐sided subduction system is addressed by analog models at the scale of the upper mantle, realized using glucose syrup and silicone putty, to model the interaction between the Earth's mantle and the lithosphere. Our results show that the presence of a slab window modifies the pattern of mantle circulation, as well as the trench geometry and kinematics. In particular, the opening of the slab window induces the formation of two arcs flanking the window, while the mantle flows through it and turns toward the arcs, creating a small‐scale toroidal flow. The effect of a slab window is more pronounced on double subduction systems, as the outflow through the window is amplified, while internal deformation is induced in the plate by the opposite slab pull force. These experimental results suggest that the origin of the Apenninic and the Calabrian arcs is the result of the formation of a slab window, providing a new interpretation of the surface deformation and the SKS shear wave splitting pattern of the Adria microplate. Plain Language Summary: The Adria microplate in the Central Mediterranean is an important part of the tectonic puzzle between the converging Africa and Eurasia plates. The heavy lithosphere sinks into the underlying mantle on two sides of the Adria plate, under the Apennines in the west and under the Dinarides in the east. During the most recent few millions of years, the lithosphere started to detach from the surface in the Central South Apennines while sinking into the mantle. This opened a slab window, an additional escape route for the mantle, which is squeezed by the two subducting lithospheres. We scaled down and simplified this setting in order to model and investigate the role of the dual subduction and the opening slab window in the evolution of the Central Mediterranean by means of analog models. Our results show that the two opposite subduction zones have a regional tectonics effect via the complex subduction‐induced mantle circulation. Furthermore, the detaching lithosphere modifies the geometry of the subduction zone, which results in a double‐arc geometry resembling the Northern Apennines and Calabria. Key Points: The combination of slab‐slab interactions and an opening slab window defines the recent tectonic evolution of the Central MediterraneanOutward dipping double‐sided subduction drives the mantle to flow from below the Adria plate toward the Alps and the Pannonian BasinThe opening of a slab window under the Central Apennines changes the trench geometry and the subduction‐induced mantle flow pattern [ABSTRACT FROM AUTHOR]

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 systems. Key Points: Mantle flow induced by opposed adjacent plates is asymmetrical showing a progressive merging of the toroidal cells when trenches approachTrench retreat velocities are not steady and increase/decrease while trenches approach/separateThe first‐order consequences of the dynamics of double subduction systems are relevant in natural scenarios [ABSTRACT FROM AUTHOR]

Abstract: The role of seafloor roughness on the seismogenic behavior of subduction zones has been increasingly addressed over the past years, although their exact relationship remains unclear. Do subducting features like seamounts, fracture zones, or submarine ridges act as barriers, preventing ruptures from propagating, or do they initiate megathrust earthquakes instead? We address this question using a global approach, taking into account all oceanic subduction zones and a 117‐year time window of megathrust earthquake recording. We first compile a global database, SubQuake, that provides the location of a rupture epicenter, the overall rupture area, and the region where the largest displacement occurs (the seismic asperity) for MW ≥ 7.5 subduction interplate earthquakes. With these data, we made a quantitative comparison with the seafloor roughness seaward of the trench, which is assumed to be a reasonable proxy for the subduction interface roughness. We compare the spatial occurrence of megathrust ruptures, seismic asperities, and epicenters, with two roughness parameters: the short‐wavelength roughness RSW (12–20 km) and the long‐wavelength roughness RLW (80–100 km). We observe that ruptures with MW ≥ 7.5 tend to occur preferentially on smooth subducting seafloor at long wavelengths, which is especially clear for the MW > 8.5 events. At both short and long wavelengths, seismic asperities show a more amplified relation with smooth seafloor than rupture segments in general. For the epicenter correlation, we see a slight difference in roughness signal, which suggests that there might be a physical relationship between rupture nucleation and subduction interface roughness. [ABSTRACT FROM AUTHOR]

Abstract: Slab‐slab interaction is a characteristic feature of tectonically complex areas. Outward dipping double‐sided subduction is one of these complex cases, which has several examples on Earth, most notably the Molucca Sea and Adriatic Sea. This study focuses on developing a framework for linking plate kinematics and slab interactions in an outward dipping subduction geometry. We used analog and numerical models to better understand the underlying subduction dynamics. Compared to a single subduction model, double‐sided subduction exhibits more time‐dependent and vigorous toroidal flow cells that are elongated (i.e., not circular). Because both the Molucca and Adriatic Sea exhibit an asymmetric subduction configuration, we also examine the role that asymmetry plays in the dynamics of outward dipping double‐sided subduction. We introduce asymmetry in two ways; with variable initial depths for the two slabs (“geometric” asymmetry), and with variable buoyancy within the subducting plate (“mechanical” asymmetry). Relative to the symmetric case, we probe how asymmetry affects the overall slab kinematics, whether asymmetric behavior intensifies or equilibrates as subduction proceeds. While initial geometric asymmetry disappears once the slabs are anchored to the 660 km discontinuity, the mechanical asymmetry can cause more permanent differences between the two subduction zones. In the most extreme case, the partly continental slab stops subducting due to the unequal slab pull force. The results show that the slab‐slab interaction is most effective when the two trenches are closer than 10–8 cm in the laboratory, which is 600–480 km when scaled to the Earth. [ABSTRACT FROM AUTHOR]

The majority of the largest subduction megathrust earthquakes share the common characteristic of rupturing more than one asperity along strike of the margin. Understanding the factors that control coseismic failure of multiple asperities, and thus maximum magnitude, is central for seismic hazard assessment. To investigate the role of asperities size and spacing on maximum magnitude, seismicity rate, and percentage of synchronized ruptures, we use analog models simulating along-strike rupture behavior of megathrust earthquakes. We found negative correlations between the barrier-to-asperity length ratio Db/Da and maximum magnitude and seismicity rate. Db/Da also controls the process of asperities synchronization along the megathrust. A permanent barrier behavior is observed for Db/Da > 0.5. Comparing our experimental results to the Nankai Trough historical seismicity, we propose that the distribution of megathrust frictional heterogeneities likely explains the diversity of earthquakes which occurred there. [ABSTRACT FROM AUTHOR]