Dependency of slab geometry on absolute velocities and conditions for cyclicity: insights from numerical modelling

Abstract

SUMMARY The aim of this study is to quantify the relationship between the kinematics of subduction, deformation in the overriding plate and the evolution of slab geometry. A 2‐D finite element numerical code is used, and a first objective consists in benchmarking previously published analogue models. Far‐field plate velocities are applied, and once the subducting plate reaches the 660 km discontinuity, modelled as a rigid base, we obtain two different forms or styles of subduction that depend on the overriding plate velocity vop: if vop > 0, the slab lies forwards on the 660 km discontinuity (style 1), and if vop≤ 0, the slab lies backwards on the discontinuity (style 2). We also obtain a cyclic pattern with the slab folding on itself repeatedly when vsp > 0 and 2vop+vsp > 0 (where vsp is the subducting plate velocity). These conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied. When the slab periodically folds on the 660 km discontinuity, periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate. Folding periodicity is controlled by the slab viscosity and subduction velocity. When a low‐viscosity zone is included in the overriding plate, the trench motion is effectively decoupled from the overriding plate velocity, therefore allowing it to be directly controlled by the deep dynamics of the slab. For the cyclic style 2 corresponding to forward folding of the slab, the low‐viscosity region in the overriding plate increases the stress amplitudes oscillations, the trench motion and the folding periodicity with time. Therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction. Using the Nazca and South American plate velocities we produce models of cyclic folding with a period of ca. 22 Ma and a minimal dip angle of ca. 10°. Episodic folding of the slab on the 660 km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the Andes.

Relationship between slab dip and topography segmentation in an oblique subduction zone: Insights from numerical modeling

“…Slab folding generates variations in time of the subduction dip, changing the state of stresses in the overriding plate and consequently also the surface topography. In 2‐D models, Gibert et al [] and Cerpa et al [] showed that periods of low slab dip generate periods of compression in the overriding plate, consistent with observations in natural subduction zones [ Lallemand et al , ]. In contrast, the OP stretches when the slab sinks before its first contact with the 660 km discontinuity, and during periods of slab rollback.…”

“…Gibert et al [] have determined a kinematic condition that predicts slab folding with 2‐D subduction models. If | v op |< v s ( v s = v op + v sp is the subduction velocity when internal overriding plate deformation is neglected), then the subducting plate folds when depositing over the 660 km discontinuity.…”

“…Two‐dimensional numerical models have explored the dynamics of subduction systems driven by plate motions where the subducting plate anchors at 660 km depth. Results from these experiments have shown that within a given range of applied far‐field plate motion, the SP is forced to fold [ Gibert et al , ; Cerpa et al , ]. Slab folding generates variations in time of the subduction dip, changing the state of stresses in the overriding plate and consequently also the surface topography.…”

Overriding plate deformation and variability of fore‐arc deformation during subduction: Insight from geodynamic models and application to the Calabria subduction zone

“…If the overriding plate is fixed, F SU will be larger with respect to the setting with a free overriding plate. Furthermore, the magnitude of F SU_HOR depends on the subduction interface dip angle (α; F SU_HOR = F SU sin(α), see Figure ), which in turn is controlled by the trench velocity (a decrease in trench velocity causes an increase in dip angle) [ Schellart , ; Griffiths et al ., ; Gibert et al ., ]. The value of α in the fixed overriding plate models is larger than that in the free overriding plate models due to a lower trench velocity (Figures a–d and 4).…”

“…In the models with fixed overriding plates, the slab developed a recumbent fold after arriving at the bottom of the tank (Figures c and d). This geometric feature can be attributed to the relatively low velocity of the trench (Figures c and d), as suggested in previous works [e.g., Kincaid and Olson , ; Christensen , ; Čížková et al ., ; Schellart , ; Gibert et al ., ; Duarte et al ., ]. A slow trench retreat velocity results in an increase of the slab dip and eventually in an overturned slab.…”