Modes of subducting slab motion and deformations: insights from analogue and numerical models

Abstract

Understanding the mechanics of convergent plate margins, in form of lithospheric subduction is a key to the study of earth’s geodynamic processes. The effects of different subduction parameters on the temporal evolution and subduction dynamics of a subducting slab is well manifested in the evolution of slab dip and radius of curvature. The dynamics of slab motion influences the deformation in the slab occurring in sub-slab mantle. The slab deformation is governed by the force balance between negative buoyancy force, resisting mantle force and the viscous dissipation of forces during bending. Apart from these, slab deformation gets enhanced due to interaction with the transition zone. When slab deformation becomes much more intense and vigorous compared to the mechanical strength of the subducting slab it may get detached. Slab detachment may occur due to multiple reasons such as: temporal paucity in convergence motion of the subducting slab, thermal weakening in the slab, and presence of weak zone in form of dead ridges or pre-deformed strain weakened slab lithology etc. The aim of this study is to evaluate the modes of slab motion and dynamics in terms of slab dip and curvature and to find the criticality of mode shiftfrom slab deformation to detachment. Two principle types of scaled laboratory experiments were performed: Type A- Homogeneous Slab and Type B- Slab with pre-defined weak-zones. Type A experiment using homogeneous slab, exhibit gradual increase in slab dip and decrease in radius of curvature, with time. With progressive time steps, slab flattening in the transition zone produced a number of subsidiary slab dips with values lower than the primary one. However, the deformation pattern changes when a pre-defined weak zone is introduced in the system. The weak zone is varied withcompetent and incompetent type of material. For experiments with competent weak zone, the slab exhibits steady increase of dip and decrease radius of curvature until folding. After reaching the transition zone the slab experiences profuse folding. The folding localizes at the introduced weak zone with progression of subduction. The slab dip reduces at the instance of folding and increases afterwards.However, for experiments with incompetent weak zone the slab becomes steep and exhibit necking in the weak zone without any detachment at initial stages.At the instance of detachment the slab dip decreases as the slab rebounds as a consequence of sudden reduction of negative buoyancy.Computational fluid dynamic (CFD) experiments were performed to evaluate different slab deformation modes. The experiments were conducted varying the ages of the subducting lithosphere and detachment was induced by paucity in convergence motion, without introducing any pre-defined weak zone within the slabs. Experiments reveal that slabs may get folded but not detached when the convergence rate is constant temporally and the slab is sufficiently young (≤ 60 Ma age). However, paucity in the convergence motion of anold (100 Ma) subducting slab results in slab detachment due to force partitioning between the pull and push component. From experiments it can be seen that the detachment is a continuous process which is accomplished by progressive necking of the slab. Moreover, numerical simulations reveal the essential criticality of negative buoyancyfor detachment in form of slab age. From this study, it is evident that the process of slab detachment is guided by viscous necking. Detachment of subducting slab by necking and vanishing of slab pull force as a consequence is manifested in temporal evolution of parameters like reduction slab dip and radius of curvature