New paper on subduction initiation

Can subduction be initiated at a transform fault? The short answer is probably not. The not so short answers are below and in the linked paper.

 

Initiation of Subduction Along Oceanic Transform Faults: Insights From Three-Dimensional Analog Modeling Experiments

David Boutelier and David Beckett
School of Environmental and Life sciences, University of Newcastle, Newcastle, NSW, Australia

Subduction initiation is a fundamental component of the plate tectonic theory, yet how subduction starts remains controversial. Oceanic transform faults and fracture zones have been proposed as sites of subduction nucleation because they are thought to be mechanically weak and the large buoyancy gradient across these faults because of the difference in the age of the lithosphere, was thought to facilitate foundering. Self-sustaining subduction, defined as subduction driven by the negative buoyancy of the sinking lithosphere might be achieved if at least ~100 to 150 km of convergence can be imposed on an oceanic fracture zone with sufficient buoyancy gradient across the fault.

Previous modelling however did not take into account the fact that the age of the lithosphere and therefore its strength and buoyancy not only varies across the oceanic fault zone, but also along the strike of the fault since many oceanic transform fault link segments of spreading ridges. Here we investigate using three-dimensional analog models how the spatial distribution of strength and buoyancy along and across an oceanic transform fault zone affects the polarity of subduction and whether self-sustaining subduction can be obtained. We designed three-dimensional analog experiments in which two oceanic lithospheric plates are separated by a weak transform fault and convergence is imposed in the horizontal direction perpendicular to the strike of the fault. The spatial distribution of plate thickness and buoyancy are varied along and across the strike of the transform fault, and whether self-sustaining subduction is obtained is assessed using a force sensor.

Cylindrical experiments reveal that subduction polarity is controlled by the buoyancy gradient and the strengths of the plates. With no inclined weak zones, imposed orthogonal compression results in the nucleation of a new fault in the weakest plate leading to the young and positively buoyant plate subducting. However, with an inclined weak zone, the buoyancy contrast controls subduction polarity with the most negatively buoyant plate subducting and a self-sustaining subduction regime obtained after ~300 km of imposed shortening.

Interestingly, this situation is also obtained when including an inverted triangular weak zone on top of the transform fault associated with the serpentinization of the crust and mantle.

In non-cylindrical experiments, taking into account the change along strike of plate strength and buoyancy, the capacity of the transform fault to generate a self-sustaining subduction regime is greatly reduced. Subduction initiates simultaneously with opposite polarity at the two extremities of the transform segment and, at depth, a lithospheric tear is produced that separates the two subducting slabs. In the center of the transform fault, the lack of buoyancy or strength contrast between the two plates leads to multiple thrusts with variable polarities, overlapping each other, and each accommodating too little shortening to become the new plate boundary. This indicates that additional mechanical work is required in the center of the transform fault which prevents the establishment of a self-sustaining subduction regime.

The paper is open access and can be obtained here.

 

Figure 1. Maps of seafloor age (Müller et al., 1997, 2008), plate thickness and buoyancy relative to underlying mantle for fast spreading ridge separating the Pacific and Antarctic plates (A–C), or slow spreading ridge separating the Africa and Antarctic plates (C,E,F). Half spreading rates from GSRM (Kreemer et al., 2003, 2014). Points labeled 1–6 refer to profiles in Figure 2. Plate thickness and relative buoyancy are calculated using the half-space cooling model and the simple plate structure discussed in the text.

 

Figure 8. Successive stages of Exp. 21. Panels (A–E) show the surface view of the deformed model with PIV vectors, and convergence-parallel horizontal shortening rate. Panel (F) shows the evolution of the force measured at the trailing edge of the plate on the left-hand side. The variation of plate thickness and buoyancy along the strike of the transform fault resulted in generation of multiple faults.

About the author

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