Wouter Schellart

Evolution and diversity of subduction zones controlled by slab width.

Subducting slabs provide the main driving force for plate motion and flow in the Earth’s mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250–7,400u2009km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics—the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6–16u2009cmu2009yr-1) only observed close to slab edges (<1,200u2009km), whereas far from edges (>2,000u2009km) retreat velocities are always slow (<2.0u2009cmu2009yr-1). Models with narrow slabs (≤1,500u2009km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width (∼2,000–3,000u2009km) are sublinear and retreat more slowly. Models with wide slabs (≥4,000u2009km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5–10u2009Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges.

Influence of trench width on subduction hinge retreat rates in 3-D models of slab rollback

Subduction of tectonic plates limited in lateral extent and with a free-trailing tail, i.e., “free subduction,” is modeled in a three-dimensional (3-D) geometry. The models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength, width, and thickness. We investigate the time evolution of this progressive rollback subduction, measure the accompanying return flow in the upper mantle, and quantify the plate kinematics. Due to the 3-D geometry, flow is allowed to accompany slab rollback around the lateral edges of the slab (the toroidal component), as opposed to 2-D geometry, where material is forced to flow underneath the slab tip (the poloidal component). A simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate. Our results indicate most of the gravitational energy of the system (i.e., the negative buoyancy of the subducting slab) is converted into a toroidal flow (∼69%), a much smaller amount goes into weakening the plate (∼18%), and the remaining amount goes into driving flow parallel to displacement of the slab (∼13%). For the trench widths (W) we investigate (≤1500 km), a maximum trench retreat rate occurs for trenches 600 km wide, which is attributed to the interaction between a plate of finite width and the induced flow (which has a lengthscale in the horizontal direction). These numerical results quantitatively agree with comparable 3-D laboratory experiments using analogue models with a purely viscous plate material (Schellart, 2004a, 2004b), including correlations between increasing retreat rate with increasing plate thickness, trench width for maximum retreat rate (500 km), and estimated amount of slab buoyancy used to drive rollback-induced flow (∼70%). Several implications for plate tectonics on Earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle (and endothermic phase transition) prior to a flushing event into the lower mantle (mantle avalanche).

Shear test results for cohesion and friction coefficients for different granular materials: scaling implications for their usage in analogue modelling

Abstract Laboratory tests have been carried out on dry granular materials such as quartz sand, glass microspheres and sugar with different grain size, rounding and sphericity. The measurements have been made with a simple shear test machine for different values of normal stress (∼50–900xa0Pa). Shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials. Resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity, while grain size has a less significant influence. Further, the behaviour of the materials for very small normal stresses (∼0–400xa0Pa) is more complex than previously assumed. The fracture envelopes for all materials investigated are convex-outward for this small range and converge towards a straight failure envelope with increasing normal stress. Finally, in extensional faulting experiments, there is no significant change in fault dip with increasing depth. Therefore, the non-linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress. Values for cohesion increase from ∼0xa0Pa (±15xa0Pa) at zero normal stress to 137–247xa0Pa (±15xa0Pa) for normal stresses greater than ∼250–400xa0Pa. The results show that well-rounded, spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well-rounded material, since it has a smaller cohesion and a coefficient of internal friction, which is closer to values of natural rocks.

Kinematics of subduction and subduction-induced flow in the upper mantle

[1]xa0Results of fluid dynamical experiments are presented to model the kinematics of lithospheric subduction in the upper mantle. The experiments model a dense high-viscosity plate (subducting lithosphere) overlying a less dense low-viscosity layer (upper mantle). The overriding lithosphere is not incorporated. Several important features of slab behavior were investigated including the temporal variability of hinge line migration, the kinematic behavior of the slab and the subduction-induced upper mantle flow. Both fixed and free trailing edge boundary conditions of the subducting plate were investigated. Results show that hinge line retreat is a natural consequence of subduction of a negatively buoyant slab. The migration rate increases until the slab approaches the upper-lower mantle discontinuity, resulting in a decrease in migration rate followed by a renewed increase and finally approaching a steady state. Slab retreat results in mantle flow, with material initially located underneath the slab flowing around the lateral slab edges toward the mantle wedge. Experimental results indicate that all rollback-induced flow occurs around the lateral slab edges, forcing the hinge line to attain a convex shape toward the direction of retreat. No signs for poloidal flow underneath the slab tip have been detected. Only a small component of toroidal-type flow was observed underneath slanting slab tips. For a fixed trailing edge, the slab does not sink vertically downward, but sinks at an angle in a regressive manner. For a free trailing edge, slab sinking is oriented more vertically while the surface part of the subducting plate is pulled into the subduction zone.

Kinematics and flow patterns in deep mantle and upper mantle subduction models: Influence of the mantle depth and slab to mantle viscosity ratio

[1]xa0Three-dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models, an upper mantle model and a deep mantle model, and for various subducting plate/mantle viscosity ratios (ηSP/ηM = 59–1375). The models investigate the mantle flow field, geometrical evolution of the slab, sinking kinematics, and relative contributions of subducting plate motion and trench migration to the total rate of subduction. All models show that the subducting plate is always moving trenchward resulting from slab pull. Furthermore, all deep mantle models show trench retreat, as do upper mantle models in the initial stage of subduction before slab tip-transition zone interaction. Upper mantle models with a low ηSP/ηM (66, 217) continue to show trench retreat after interaction. Upper mantle models with a high ηSP/ηM (378, 709) show a period of trench advance after interaction followed by trench retreat. Upper mantle models with a very high ηSP/ηM (1375) show continued trench advance after interaction. The difference in trench migration behavior and associated slab geometries is attributed to both ηSP/ηM and the mantle depth to plate thickness ratio TM/TSP, which both affect the slab bending radius to mantle thickness ratio rB/TM. Four subduction regimes can be defined: Regime I with rB/TM ≤ ∼0.3, trench retreat, slab draping, and a concave trench; Regime II with ∼0.3 < rB/TM < ∼0.5, episodic trench migration, slab folding, and a concave trench; Regime III with rB/TM ≈ 0.5, trench advance, slab rollover geometries, and minor trench curvature; and Regime IV with rB/TM ≥ ∼0.8, trench retreat, slab draping, and a rectilinear trench. In all models, slab-parallel downdip motion induces poloidal mantle flow structures. In addition, trench retreat and rollback motion of the slab induce quasi-toroidal return flow around the lateral slab edges toward the mantle wedge. Rollback-induced poloidal flow around the slab tip is not observed in any of the experiments. Finally, comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1–7 × 102, with a best estimate of 1–3 × 102.

Quantifying the net slab pull force as a driving mechanism for plate tectonics

[1]xa0It has remained unclear how much of the negative buoyancy force of the slab (FB) is used to pull the trailing plate at the surface into the mantle. Here I present three-dimensional laboratory experiments to quantify the net slab pull force (FNSP) with respect to FB during subduction. Results show that FNSP increases with increasing slab length and dip up to ∼8–12% of FB, making FNSP up to twice as large as the ridge push force. The remainder of FB is primarily used to drive rollback-induced mantle flow (∼70%), to bend the subducting plate at the trench (∼15–30%) and to overcome shear resistance between slab and mantle (0–8%).

Overriding plate shortening and extension above subduction zones: A parametric study to explain formation of the Andes Mountains

Mountain building above subduction zones, such as observed in the Andes, is enigmatic, and the key parameter controlling the underlying dynamics remains a matter of considerable debate. A global survey of subduction zones is presented here, illustrating the correlation between overriding plate deformation rate and twelve physical parameters: overriding plate velocity, subducting plate velocity, trench velocity, convergence velocity, subduction velocity, subduction zone accretion rate, subducting plate age, subduction polarity, shallow slab dip, deep slab dip, lateral slab edge proximity, and subducting ridge proximity. All correlation coefficients are low (| R | ≤ 0.39), irrespective of the global reference frame, relative plate motion model, or overriding plate deformation model, except for the trench velocity (0.33–0.68, exact value depends on adopted global reference frame) and subduction velocity, which shows an anticorrelation (0.55–0.57). This implies that no individual parameter can explain overriding plate deformation, except that trench retreat generally corresponds to extension while an approximately stable trench or trench advance generally corresponds to shortening. Understanding of the variety of strain patterns is obtained when slab edge proximity and overriding plate velocity are combined. Orogenesis occurs in overriding plates bordering central regions of wide subduction zones (≥~4000 km) when the overriding plate is moving trenchward at 0–2 cm/yr (e.g., Andes, Japan). Because the center of a wide slab offers large resistance to lateral migration, the overriding plate effectively collides with the subduction hinge, forcing the slab to attain a shallow dip angle (e.g., Nazca and Japan slabs). Overriding plate extension is only found close to lateral slab edges or during overriding plate motion away from the center of a wide subduction zone, but in the latter scenario, maximum extension velocities are much lower than in the former scenario. For subduction settings close to lateral slab edges, overriding plate motion plays no significant role in overriding plate deformation. Thus, for rapid overriding plate extension, the key ingredient is rapid trench retreat, which only occurs close to lateral slab edges, while for overriding plate shortening, the key ingredients are (1) the resistance to rapid trench and hinge retreat, which occurs far from lateral slab edges, and (2) trenchward overriding plate motion.

Asymmetric deformation in the backarc region of the Kuril arc, northwest Pacific : New insights from analogue modeling

[1]xa0The Eocene to middle/late Miocene tectonic evolution of the Kuril arc and backarc region has been simulated with analogue experiments. The experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting Pacific plate. The results show the formation of a N-S to NE-SW-oriented dextral shear zone near the far edge of the retreating boundary analogous to the Sakhalin-Hokkaido dextral shear zone. Contemporaneously, normal faults and grabens form, striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary. This is similar to extensional structures observed in the Kuril Basin and the Sea of Okhotsk. Furthermore, the model shows that the amount of extension progressively decreases away from the retreating boundary. This appears to have also happened in the Kuril-Okhotsk region, as evidenced by crustal thickness variation in the region. Finally, the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation. This can account for the Eocene-early Miocene extension in the Sea of Okhotsk followed by Miocene spreading in the Kuril Basin.

The role of the East Asian active margin in widespread extensional and strike-slip deformation in East Asia

East Asia is a region of widespread deformation, dominated by normal and strike-slip faults. Deformation has been interpreted to result from extrusion tectonics related to the India–Eurasia collision, which started in the Early Eocene. In East and SE China, however, deformation started earlier than the collision (latest Cretaceous to Palaeocene), suggesting that extrusion tectonics is not the (only) driving mechanism for East Asia deformation. It is suggested that the East Asian active margin has influenced deformation in East Asia significantly. Along the margin, Cenozoic back-arc extension took place behind several adjoining arcs, implying eastward rollback of the subducting slab and collapse of the overriding plate towards the retreating hinge-line. We show that extension took place along a c. 7400 km long stretch of the East Asian margin during most of the Cenozoic. Physical models are presented simulating overriding plate collapse and back-arc extension. The models reproduce important aspects of the strain field in East Asia. For geometrical and rheological conditions scaled to represent East Asia, modelling shows that the active margin can be held responsible for deformation in East Asia as far west as the Baikal rift zone, located c. 3300 km from the margin.

Analogue modeling of arc and backarc deformation in the New Hebrides arc and North Fiji Basin

In most backarc basins, extension is perpendicular to the arc. Thus individual spreading ridges extend approximately parallel to the arc. In the North Fiji Basin, however, several ancient and active spreading ridges strike 70°-90° to the New Hebrides arc. These high-angle spreading ridges relocated southward during the asymmetric opening of the North Fiji Basin. We have simulated the structural development of the North Fiji Basin and the New Hebrides arc with scaled analogue models, and the results have inspired us to come to several tentative conclusions. We interpret the orientation of the high-angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge, where they form close to the hinge. Relocation of these spreading ridges is most likely related to subduction of the West Torres Plateau along the New Hebrides Trench. This resulted in localized collision, retarded rollback of the subducting slab along the northwest corner of the trench, and reduced extension and shearing in the northwest corner of the North Fiji Basin. Backarc extension continued in the rest of the North Fiji Basin owing to continued rollback of the southern part of the subducting slab. Here, active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the New Hebrides arc, i.e., the Hazel Holme extensional zone. Moreover, impingement of the dEntrecasteaux Ridge into the overriding plate led to local deformation and fragmentation of the arc.