Trench Migration Research Articles

Competing influences of plate width and far-field boundary conditions on trench migration and morphology of subducted slabs in the upper mantle

Seismic tomography of the mantle provides a snapshot view of the present-day structure of subducted tectonic plates (slabs) in the mantle. However, a correct interpretation of how such features developed requires knowledge of both the history of plate motions, in particular the locations of subduction zones over time, as well as an understanding of how slabs deform and interact with the lower mantle. The understanding of seismic tomography requires a detailed knowledge of the motion of the trench with time, as well as the time-evolution of subduction zones and their associated slabs. We use 3-D numerical models to investigate two effects which can strongly influence the kinematics of subduction, in particular the migration of the trench: the width of the plate and the far-field boundary conditions of the subducting plate. These models study the full range of plate widths (300–6000 km) and are subjected to the full range of boundary conditions (“fixed,” “free,” and “pushed”) representative of the tectonic settings found on Earth. Additionally, our results indicate that trailing edge boundary conditions dominate over the effect of plate width. Each boundary condition is identified to produce a particular trench migration behavior with an associated upper mantle slab morphology: (1) fixed edges result in retreating trenches and retrograde motion of the slab which ends up lying flat atop the lower mantle, (2) free-trailing edges tend to exhibit quasi-stationary trenches with slabs developing into folded piles atop the lower mantle, and (3) pushed trailing edges generally result in advancing trenches that overrun previously subducted material to form forward-draping slabs lying prone atop the lower mantle. This range of behavior can be observed in seismic tomography and an example of each is provided. Within each suite of models with similar boundary conditions, plate width has a subsidiary role in the observed slab morphology but three distinct regimes of plate width can be distinguished in regards to trench migration. Thus plate width is an important effect on trench migration and does result in notable differences in slab morphologies between narrow and wide plate widths.

Subduction erosion modes: Comparing finite element numerical models with the geological record

During subduction erosion, the upper plate is tectonically eroded by the subducting plate and carried into the mantle. The geological record suggests that subduction erosion is a fundamental process at subduction margins; however the underlying causes are not well constrained. Finite-element numerical models of ocean–continent subduction are used to investigate the roles of crustal frictional strength, subduction angle, and convergence rate in subduction erosion processes. Subduction erosion occurs in models in which the plate boundary zone is moderately strong, due to either high frictional strength or shallow angle of subduction. The models exhibit two distinct modes of subduction erosion: (1) steady, with slow trench migration rates (< 4 km Ma − 1 ), subsidence in the remaining forearc, and a decrease in the angle of subduction, in which the edge of the continental plate is eroded in small blocks, and (2) unsteady, with fast trench migration rates (⋙ 15 km Ma − 1 ), subsidence at the end of the process, and an increase in the angle of subduction, in which a large block of continental forearc is removed. The unsteady mode is compatible with the sudden migration of the volcanic arc into the continental interior, a concurrent hiatus in arc volcanic production, and geochemical signatures showing crustal (forearc) contamination in the magma source region when arc volcanism renews in its new location. Both modes are inhibited by the presence of a thick sediment layer within the subduction zone, and neither mode requires the presence of topographic asperities on the lower plate. In natural subduction zones, subduction erosion may initially occur through steady erosion at the edge of the continental plate. As material is removed, the subduction angle may gradually decrease, increasing the strength of the plate boundary zone. The increased strength may lead to failure in the continental interior, unsteady removal of a large forearc block and relocation of the subduction zone into the upper plate where the cycle may repeat. The proposed cycle may explain observed patterns in the Andean margin, including steady and unsteady erosion recorded in the geological record and along-strike variation in present-day subduction angles.

Dynamics of slab tear faults: Insights from numerical modelling

Tear resistance at the edge of a slab is an important parameter controlling the evolution of subduction zones. However, compared with other subduction parameters such as plate strength, plate viscosity, plate thickness and trench width, the dynamics of tearing are poorly understood. Here we obtain a first-order understanding of the dynamics and morphology of subduction zones to resistance during tear propagation, by developing and using a novel computational modelling technique for subducting slabs, with side boundaries described by visco-plastic weak zones, developing into tear faults. Our 3D model is based upon a visco-plastic slab that sinks into the less dense mantle, generating poloidal and toroidal flows. The asthenospheric mantle field is static and only develops flow due to the subducting slab. We use the finite element code eScript/Finley and the level set method to describe the lithosphere to solve this fluid dynamics problem. Our results show the importance of tear resistance for the speed of trench migration and for shaping the final geometry of subduction systems. We show that slab tearing along a weak layer can result in a relatively straight slab hinge shape, while increasing the strength in the weak layer results in the curvature of the hinge increasing substantially. High tear resistance at the slab edges may hinder rollback to the extent that the slab becomes stretched and recumbently folded at the base of the domain. Tear resistance also controls whether the subducting lithosphere can experience accelerating rollback velocities or a constant rollback velocity.

On the relation between trench migration, seafloor age, and the strength of the subducting lithosphere

Oceanic lithosphere thickens and strengthens as it grows older. Worldwide databases reveal that the age of the slab to a certain extent controls the subduction style. Old and thick (and consequently strong) slabs show a trench “advance,” while younger, thin (weak) slabs are migrating in retreating style (trench “rollback”). We performed numerical models to show that this configuration could be the result of the dynamic equilibrium between gravity and resisting forces. In particular we show that energy dissipation caused by bending and unbending of the slab, although less important than other resisting forces, could be a primary control on trench migration. Our results fit well with global compilations of kinematic data from modern subduction zones in two reference frames with different amounts of net rotations. Based on the age at which the transition from retreating to advancing style occurs, we propose an effective lithosphere/mantle viscosity of ~200 during bending of the lithosphere into the subduction zone.

Density profiles of oceanic slabs and surrounding mantle: Integrated thermodynamic and thermal modeling, and implications for the fate of slabs at the 660 km discontinuity

We have calculated the mineralogical properties of the Earth's mantle and the lithological units constituting the subducting oceanic slabs within a wide range of P–T conditions within the CaO–FeO–MgO–Al 2O 3–SiO 2 system, except for the basalt top-layer of a slab, for which the system is extended to include Na 2O. The mineralogical data are then converted, using the appropriate P–V–T relations, to bulk densities. The calculated adiabatic density vs. depth profile of the mantle between 200 and 725 km depths is in good agreement with geophysical and experimental data. The density data of the different compositional units are combined with calculated thermal structures for a variety of slab–mantle systems to construct equilibrium density profiles as a function of depth. The mean equilibrium densities of the slabs within the transition zone (400–660 km depth) are found to be ∼0.04–0.05 g/cm 3 greater than those of the ambient mantle within the same depth interval. For the entire upper mantle, density differences between slabs and ambient mantle are slightly less, but the slabs still remain denser than the latter. At 670 km depth, slabs have lower density than the ambient lower mantle because of the commencement of perovskite forming reactions within the mantle, and displacements of these reaction boundaries to higher pressures within the slabs as a consequence of their negative P–T slopes. If perovskite forming reactions within slabs are hindered for kinetic sluggishness, then neutral buoyancy would be achieved when the slabs have penetrated ∼100 km into the lower mantle. However, using the available data on the kinetics of spinel to perovskite plus periclase reaction, we conclude that the reaction would go to completion in a Peru-type young slab (41 Myr), and very likely also in a Tonga-type old slab (110 Myr), before these penetrated 100 km into the lower mantle. Thus, slabs should always remain negatively buoyant, and therefore continue to subduct through the lower mantle once it penetrates through the 660 km discontinuity. Despite a negative buoyancy force, a slab could deflect at the top of the lower mantle (660 km) because of factors resisting subduction, namely viscosity jump, low dip angle, slab roll back, and metastable persistence of olivine in cold slabs. If published scale model experiments represent realistic approximations of the factors affecting plate subduction, then according to our density data, any slab with a dip angle of ≤40–50° would bend at the 660 km discontinuity if there is a viscosity jump of at least by a factor of ∼10 and trench migration. The basalt top-layer of a slab is denser than other slab components and the ambient mantle at all depths to 660 km, and therefore should continue to sink into the lower mantle, especially if a slab directly penetrates the 660 km barrier, instead of peeling off in the transition zone to form a “perched eclogite” or “piclogite” layer, as previously proposed. The harzburgite layer, which is sandwiched between denser basalt and lherzolite layers, faces greater resistance to subduction, especially in a young slab, and thus could significantly contribute to the deformation of a slab near the 660 km discontinuity.

The Geological Evolution of the Tibetan Plateau

The geological evolution of the Tibetan plateau is best viewed in a context broader than the India-Eurasia collision zone. After collision about 50 million years ago, crust was shortened in western and central Tibet, while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western Pacific and Indonesia. Cessation of rapid Pacific trench migration ( approximately 15 to 20 million years ago) coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern Tibet. The latter appear to result from rapid eastward flow of the deep crust, probably within crustal channels imaged seismically beneath eastern Tibet. These events mark a transition to the modern structural system that currently accommodates deformation within Tibet.

Subduction zone trench migration: Slab driven or overriding-plate-driven?

Subduction zones on Earth, and their associated trenches and hinges, migrate with respect to the overriding plate, as indicated by overriding plate deformation (e.g. backarc extension or backarc shortening), and migrate with respect to hotspot and no-net-rotation reference frames (global or “absolute” reference frames). Three geodynamic models exist that attempt to explain how “absolute” trench migration velocity correlates with overriding plate deformation. In one model, trench and hinge migration result from lateral migration of the slab, with trench retreat causing overriding plate extension, and trench advance causing overriding plate shortening. In the second model, trench migration is forced by the overriding plate, with trenchward motion causing trench retreat and overriding plate shortening, and motion away from the trench causing trench advance and overriding plate extension. In the third model, the trench and subduction hinge are thought to be static, while overriding plate extension/shortening is accommodated by landward/trenchward overriding plate motion. In this paper, the conflicting geodynamic predictions made by these three models are tested by using global kinematic calculations of trench migration velocity ( v T ⊥ ), overriding plate velocity ( v OP ⊥ ) and overriding plate deformation in different global reference frames. The dependence between v T ⊥ and overriding plate deformation, as well as v OP ⊥ and overriding plate deformation was investigated. Correlation coefficients ( R) and confidence limits were calculated using a quantitative approach with overriding plate deformation velocity ( v OPD ⊥ ), using a semi-quantitative approach with an overriding plate strain-classification approach, and using a Spearman rank correlation approach. For the quantitative approach the correlation between v T ⊥ and v OPD ⊥ is consistently positive ( R = 0.33–0.68), where trench retreat corresponds to extension, and statistically significant at 95% confidence level for all but one reference frame. For v OP ⊥ and v OPD ⊥ the correlation is consistently negative but much less significant ( R = −0.29 to −0.17) and statistically not significant at 95% confidence level for all reference frames. These findings indicate that trench migration results predominantly from lateral migration of the slab rather than from overriding plate motion. Such lateral slab migration is most likely driven by the negative buoyancy force of the slab itself, as demonstrated by geodynamic models.

Subduction dynamics as revealed by trench migration

New 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 Vsub, 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: Vt = 0.5Vsub − 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.

Reply to comment on “The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle”

Reply to comment on “The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle”

Trench migration, net rotation and slab–mantle coupling

Laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales. Our models are set up with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (mantle). For our study, the lithosphere/upper mantle viscosity contrast has been systematically varied, from ~ 10 to ~ 10 5 in order to explore the parameter space between weak and strong slab dynamics. We found that subduction is characterized by a retreating mode for viscosity ratios > 10 4, by the coexistence of a retreating mode and an advancing mode for viscosity ratios between ~ 10 4 and ~ 10 2, and quasi-stationary, Rayleigh–Taylor like behaviour for ratios < 10 2. By combining our experimental results and kinematic data from current subduction zones in four reference frames which differ in the amount of net rotation, we infer that a lithosphere/upper mantle viscosity contrast of 150–500 is necessary to obtain realistic trench/subducting plate velocity ratios as well as the variability of subduction styles observed in nature.

Comment on “The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle” by W.P. Schellart

The Schellart's [Schellart, W.P., 2007, The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle. Tectonophysics, 445, 363–372.] paper uses slab dip and upper plate extension for testing the westward drift. His analysis and discussion are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones are sensitive also to other parameters such upper plate thickness, geometry and obliquity of the subduction zone with respect to the convergence direction. The deeper (> 125 km) part cannot easily be compared as well because E- or NE-directed subduction zones have seismic gaps between 270–630 km. Moreover the velocity of subduction hinge cannot be precisely estimated and it does not equal to backarc spreading due to accretionary prism growth and asthenospheric intrusion at the subduction hinge. It is shown here that hinge migration in the upper plate or lower plate reference frames supports a general global polarization of the lithosphere in agreement with the westward drift of the lithosphere. The W-directed subduction zones appear controlled by the slab–mantle interaction with slab retreat imposed by the eastward mantle flow. The opposite E-NE-directed subduction zones seem rather mainly controlled by the convergence rate, plus density, thickness and viscosity of the upper and lower plates. Finally, the geological and geophysical asymmetries recorded along subduction and rift zones as a function of their polarity with respect to the tectonic mainstream are not questioned in the Schellart's paper, but they rather represent the basic evidence for the westward drift of the lithosphere.

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

Three‐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 &lt; rB/TM &lt; ∼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.

Global trench migration velocities and slab migration induced upper mantle volume fluxes: Constraints to find an Earth reference frame based on minimizing viscous dissipation

Since the advent of plate tectonics different global reference frames have been used to describe the motion of plates and trenches. The difference in plate motion and trench migration between different reference frames can be substantial (up to 4 cm/yr). This study presents an overview of trench migration velocities for all the mature and incipient subduction zones on Earth as calculated in eight different global reference frames. Calculations show that, irrespective of the reference frame: (1) trench retreat always dominates over trench advance, with 62–78% of the 244 trench segments retreating; (2) the mean and median trench velocity are always positive (retreating) and within the range 1.3–1.5 cm/yr and 0.9–1.3 cm/yr, respectively; (3) rapid trench retreat is only observed close to lateral slab edges (< 1500 km); and (4) trench retreat is always slow far from slab edges (> 2000 km). These calculations are predicted by geodynamic models with a varying slab width, in which plate motion, trench motion and mantle flow result from subduction of dense slabs, suggesting that trench motion is indeed primarily driven by slab buoyancy forces and that proximity to a lateral slab edge exerts a dominant control on the trench migration velocity. Despite these four general conclusions, significant differences in velocities between such reference frames remain. It is therefore important to determine which reference frame most likely describes the true absolute velocities to get an understanding of the forces driving plate tectonics and mantle convection. It is here proposed that, based on fluid dynamic considerations and predictions from geodynamic modelling, the best candidate is the one, which optimises the number of trench segments that retreat, minimizes the trench–perpendicular trench migration velocity ( v T⊥) in the centre of wide (> 4000 km) subduction zones, maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge, minimizes the average of the absolute of the trench–perpendicular trench migration velocity (| v T⊥|) for all subduction zones on Earth, and minimizes the global upper mantle toroidal volume flux ( ϕ To) that results from trench migration and associated lateral slab migration (i.e. slab rollback or slab roll-forward). Calculations show that these conditions are best met in one particular Indo-Atlantic hotspot reference frame, where 75% of the subduction zones retreat, v T⊥ in the centre of wide subduction zones ranges between − 3.5 and 1.8 cm/yr, 83% of the trench segments located within 2000 km of the closest lateral slab edge retreat, the average of | v T⊥| is 2.1 cm/yr, and ϕ To = 456 km 3/yr (lower limit) and 539 km 3/yr (upper limit). Inclusion of all the incipient subduction zones on Earth results in slightly greater fluxes of 465 km 3/yr (lower limit) and 569 km 3/yr (upper limit). It is also found that this reference frame is close to minimizing the total sub-lithospheric upper mantle volume flux ( ϕ K) associated with motion of continental keels located below the major cratons. It is stressed, however, that ϕ K is an order of magnitude smaller than ϕ To, and thus of subordinate importance. In conclusion, the Indo-Atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities.

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 ∼45% to ∼50% 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 ∼35% to ∼40% 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 “unbend.” 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 “unbending” 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.

The Subduction Zone Flow Field from Seismic Anisotropy: A Global View

Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |V(t)|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |V(t)|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity V(c); their relative influence is governed by the relative magnitude of |V(t)| and V(c).

The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle

A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for westward-dipping subduction zones (e.g. Mariana) and predicts overriding plate shortening, oceanward trench retreat and a gentle slab dip for east to northeastward-dipping subduction zones (e.g. Chile). This paper investigates these predictions quantitatively with a global subduction zone analysis. The results show overriding plate extension for all dip directions (azimuth α = − 180° to 180°) and overriding plate shortening for dip directions with α = − 90° to 110°. The wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west-dipping slabs. West-dipping subduction zones are never fixed, irrespective of the choice of reference frame, while east to northeast-dipping subduction zones are both retreating and advancing in five out of seven global reference frames. In addition, westward-dipping subduction zones have a range in trench-migration velocities that is twice the magnitude of that for east to northeastward-dipping slabs. Finally, there is no recognizable correlation between slab dip direction and slab dip angle. East to northeast-dipping slabs ( α = 30° to 120°) have shallow (0–125 km) slab dip angles in the range 10–60° and deep (125–670 km) slab dip angles in the range 40–82°, while west-dipping slabs ( α = − 60° to − 120°) have shallow slab dip angles in the range 19–50° and deep slab dip angles in the range 25–86°. Local mean deep slab dip angles are nearly identical for east and west-dipping slabs, while local mean shallow slab dip angles are lower by only 4.7–8.1° for east to northeast-dipping slabs. It is thus concluded that overall, there is no observational basis to support the three predictions made by the westward drift model, and for some sub-predictions the observational basis is very weak at most. Alternative models, which incorporate and underline the importance of slab buoyancy-driven trench migration, slab width and overriding plate motion, are better candidates to explain the complexity of subduction zones, including the variety in trench-migration velocities, overriding plate deformation and slab dip angles.

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 (250-7,400 km) 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 (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) 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.

Dynamical modeling of trench retreat driven by the slab interaction with the mantle transition zone

Abstract We present the 2-D self-consistent dynamical model of interactions of a subducting slab with the 410-km and 660-km phase boundaries to further our understanding of the relation between the slab stagnation/penetration and the trench migration. Our model takes into account freely-movable plate boundaries and the difference between tensional and compressional yield strengths in the lithosphere. For the case in which the tensional strength is weaker than the compressional one, the negative buoyancy of the subducting slab produces extension of the overriding lithosphere and, accordingly, the trench retreats. Interactions with the 410-km and 660-km phasetransition boundaries further promote the trench retreat, and the dip angle of the slab is substantially decreased. This enhances the resistance of the 660-km phase boundary against the slab penetration. Slab weakening caused by the grain-size reduction in the transition zone may result in a horizontally-lying slab and trench retreat.

Fitting Northland, New Caledonia and dlEntrecasteaux geology into the Late Cretaceous-Cenozoic Southwest Pacific tectonic framework

Since the advent of plate tectonics different reference frames have been proposed to describe the “absolute” motion of plates. The most prominent ones include the hot spot reference frame and the no-net-rotation reference frame. The difference in absolute plate motion between different reference frames can be substantial (up to 4 cm/yr). These differences are truly significant, considering that the velocity of the fast plates is only 6-12 cm/yr in such reference frames. It is thus of primary importance to extract which of these reference frames is most likely to describe the absolute motion of plates. Here we propose that the best candidate representing an absolute reference frame is the one, which minimizes the cumulative amount of trench migration for all subduction zones. From the absolute reference frames proposed thus far, it is found that the cumulative amount of trench migration is lowest in the Indo-Atlantic hot spot reference frame. This implies that the Indo-Atlantic hot spot reference frame is currently the best approximation of an absolute reference frame on Earth.

Trench motion, slab geometry and viscous stresses in subduction systems

SUMMARY A semi-analytic, 3-D model for subduction within a Newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress, trench motion and slab geometry in subduction systems. Although negative slab buoyancy provides the basic driving force for subduction, slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow (<100‐150 km) mantle and undersupported by viscous stresses at greater depth in the upper mantle. These deeper parts of the subduction system act as an ‘engine’ for subduction while shallower parts act as a ‘brake’ on trench motion; trench migration rates and slab geometry reflect a competition between these two effects. During steady-state subduction, trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of ∼2 × 10 20 Pa s above 300 km and ∼5 × 10 20 Pa s below. Steady-state slab dip increases as slab density decreases, especially for very low-density slabs, which dip significantly more steeply than high-density slabs. The horizontal velocity at the top of the lower mantle, measured relative to the foreland, has a very large effect on trench migration rates, rivalling or even exceeding that of slab buoyancy. Slab width, parallel to the trench, also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab. The stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere. Very stiff slabs, with elastic plate thicknesses more that ∼40 km or viscosities in the range of 10 25 ‐10 26 Pa s, subduct significantly more slowly than weak slabs, with trench migration rates in the range of half to a third that of weak slabs. Large, unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate, indicating that local geology can exert important constraints on slab dynamics. During non-steady-state subduction, rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere. In the absence of other driving forces for convergence, trench migration rates can change by a factor of two or more in as little as 2‐3 Myr, for example, from 35 to 70 mm yr −1 when an oceanic piece of slab follows a continental one into the subduction system. Subduction of variable-buoyancy lithosphere is accompanied by changes in slab dip with depth and through time.