Subduction across the Scales

Cross-scale model of seismic cycles of great subduction earthquakes

Subduction is substantially multi-scale process where the stresses are built by long-term tectonic motions, modified by sudden jerky deformations during earthquakes, and then restored by following multiple relaxation processes. In collaboration with RU 4, we develop a cross-scale thermomechanical models aimed to simulate the subduction process from ca. 1 minute (earthquake) to million years’ time scale. It generates spontaneous earthquake sequences and recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles.

A surprising prediction of the model is that the viscosity in the mantle wedge drops by more than 3 orders of magnitude during great earthquakes with magnitude above 9. As a result, the postseismic transient deformation after just an hour or day is controlled by visco-elastic relaxation in a several hundred km wide mantle wedge and not by the afterslip localized at the fault as it is currently believed. The model also demonstrates that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in South Chile inferred from long-term geodynamic models and appearance of the largest earthquakes, like the Great Chile 1960 Earthquake.

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Subduction initiation at early Earth and start of Plate Tectonics

The Earth is the only planet in the solar system that exhibits plate tectonics, but how that process started remains an unanswered question. In collaboration with RU1 an advanced 3D numerical modeling work has demonstrated that a hot mantle plume rising to the lithosphere from the deep mantle might have broken the intact outer shell of the early Earth and induced the first large-scale subduction. The study suggests that mantle plumes may play an important role not only in the formation of divergent plate boundaries and oceanic plateaus, but also the initiation of subduction zones and possibly also the start of plate tectonics on Earth.

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New look at classical Wilson Cycle concept

The classical Wilson Cycle concept, describing repeated opening and closing of ocean basins, hypothesizes spontaneous conversion of passive continental margins into subduction zones. The collapse of passive margins is essential in the closing phase of the Wilson Cycle; however, the absence of conversion of any passive margins into active ones since the Cenozoic makes the Wilson Cycle a challenging and debatable topic among geoscientists.

Based on 2-D thermomechanic models we suggest a modified version of the Wilson Cycle concept in which conversion of a passive margin into a subduction zone is triggered by the mantle flow induced by neighboring subduction zones, along with slab remnants of former subduction zones in the mid-mantle. Models suggest that this is a long-term process, thus explaining the lack of Cenozoic examples. We speculate that new subduction zones may form in the next few tens of millions of years along the Argentine passive margin and the US East Coast.

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Plume induced subduction initiation on modern Earth

Despite its importance, many aspects of subduction remain poorly understood, and the most controversial questions involve where new subduction zones initiate and how this process proceeds. 3-d thermo-mechanical models investigate the conditions leading to plume-induced subduction initiation in the modern Earth.

They show four different deformation regimes in response to plume–lithosphere interaction: a) self-sustaining subduction initiation, in which subduction becomes self-sustaining; b) frozen subduction initiation, in which subduction stops at shallow depths; c) slab break-off, in which the subducting circular slab breaks off soon after formation; and d) plume underplating, in which the plume does not pass through the lithosphere and instead spreads beneath it (i.e., failed subduction initiation). The outcomes of the numerical experiments are applicable for subduction initiation in the modern and Precambrian Earth and for the origin of plume-related corona structures on Venus.

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