On the mechanism of seismic decoupling and back arc spreading at subduction zones

Abstract

To address the problem of the great variability of the mechanical state of subduction zones, we investigate the mechanics of back arc spreading and seismic decoupling. Back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench‐normal tension reaches a critical value. Seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition. Two forces are important in this problem. The first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench. The second is a sea anchor force exerted on the slab that resists its lateral motion, assumed to occur at the upper plate velocity. Both forces contribute to the coupling problem: only the sea anchor force is responsible for back arc spreading. The unbalanced slab pull force is determined from a force balance for subduction, the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid. The model predicts three regimes: seismically coupled compressional arcs with advancing upper plates; seismically decoupled extensional arcs with retreating upper plates, and strongly extensional arcs which also have back arc spreading. This model is applied in detail to the Izu‐Bonin‐Marianas system. It shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1×1013 N m−1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed. It shows why the plate interface near Guam is seismically coupled, while the plate boundary everywhere farther north is decoupled. When applied globally, it successfully predicts the state of seismic coupling and back arc spreading in more than 80% of the world's subduction zones. Of the remaining, half can be seen to contain additional complications not included in the model. About 10% of cases remain unexplained, but some of these may have incorrectly determined seismic coupling coefficients.

The tectonic regime along the Andes: Present‐day and Mesozoic regimes

“…This is correct only in a passive mantle model (Scholz and Campos 1995), because if a global westward motion of the lithosphere is considered these premises are not valid (see discussion in Doglioni et al 2009). If we take into consideration the shortening and stretching rates and the crustal erosion by subduction rate, the relations between the different rates are shown in Figure 2.…”

“…Yañez and Cembrano (2004) interpreted that the arcforeland deformation is controlled by the absolute plate velocity of the continental plate and the resistance at the slip zone. The absolute motion can be expressed by the trench roll-back velocity, which will be retreating when the overridden velocity goes away from the trench as proposed by Daly (1989) and Ramos (1999) in a passive mantle flow model (Scholz and Campos 1995) producing extension.…”

“…In contrast, the Peru-Chile plate boundary is characterized by rapidly converging, younger oceanic crust that is underthrusting a compressional overriding plate at a nearly horizontal dip, accompanied by very large earthquakes (Uyeda and Kanamori 1979). The mechanisms of the seismically decoupled extensional arcs with retreating upper plates, and strongly extensional arcs which also have backarc spreading have been analyzed by Scholz and Campos (1995).…”

“…These last authors were the first to propose an eastward main stream of the mantle, responsible for a velocity gradient in the mantle flow (see recent comments in Doglioni 2009). The present analysis will be restricted to a passive mantle model, in the sense of Scholz and Campos (1995) in which motion in the mantle is taken to be stationary in a hotspot reference frame to avoid the complexity of more advanced models that could be difficult to extrapolate in Mesozoic times ).…”

Bathymetry of Mariana trench‐arc system and formation of the Challenger Deep as a consequence of weak plate coupling

“…It is important to understand that this meaning of coupling differs from ''seismic coupling'' [Pacheco et al, 1993;Peterson and Seno, 1984;Ruff and Kanamori, 1980;Scholz and Campos, 1995] that refers to the resistance of the plates to slip against each other as expressed by occasional strong interplate thrust earthquakes. Although ''shear coupling'' and ''pull-down coupling'' must be related, because both refer to the lithosphere-lithosphere contact along the subduction interface, here we focus only on the vertical detachment forces in order to show its influence on the topography of the trench-arc system.…”