Corner flow (or wedge flow or induced asthenospheric convection)

This model (Fig. 2c) postulates that subduction induces a convective flow in the asthenospheric wedge overlying the slab and that this flow causes extension in the overriding plate (Sleep and Toksoz, 1971; Toksoz and Hsui, 1978; Jurdy and Stefanik, 1983). This last process is an effect of two kinds of force, one is the shear traction induced by the asthenospheric flow at the base of the overriding plate and the other is the gravitational collapse of the arc away from the structural high created by the vertical uprising branch of the convective flow (Fig. 2c). Attempts at quantifying the implications of the slab-induced corner flow by numerical and analogue modelling have provided information useful to understand the reliability of this interpretation.

The uprising branch of the convective flow is expected to be located at a distance of roughly 250-300 km from the volcanic arc (Toksoz and Hsui, 1978). This result is not much consistent with the observed distances between the volcanic arc and the centre of the extensional zone, which are mostly comprised between 100 and 150 km (e.g. Taylor and Karner, 1983).

The deviatoric tensional stress induced in the overriding plate by the combination of the spreading ridge (which may rise up to 1 km above the sea floor) and the bottom shear traction is predicted in the range 10-15 MPa (Toksoz and Hsui, 1978; Jurdy and Stefanik, 1983). However, as argued earlier, these values are considerably lower than the strength values estimated for both continental and oceanic lithosphere (100-260 MPa).

Numerical models of slab-induced corner flow (Davies and Stevenson, 1992) suggest that the velocities of asthenospheric flow below the arc may only be a fraction of the imposed subduction rate. This expected pattern cannot easily explain the relative kinematics at a number of subduction zones, as in the Mediterranean area, where geological and geophysical evidence indicate a trenchward motion of arcs higher than subduction rates (Dercourt et al., 1986; Mantovani et al., 1997, 2000a) and in the Tonga zone, where geological, seismological and space geodetic observations (Fig. 8) suggest a trench ward velocity (13-16 cm/y) of the Tonga arc considerably higher than the subduction rate (9-11 cm/y) of the Pacific lithosphere.

A major problem of the corner flow model is explaining why back arc extension did not occur in several consuming boundaries and why in a number of subduction zones back arc extension has ceased, while lithosphere consumption is still going on. One could expect, in fact, that the presumed asthenospheric flow should produce similar effects at subduction zones where slabs have comparable sizes and subduction rates.

Furthermore, this mechanism cannot easily explain the curved shape of arcs and the fact that back arc extension only occurs in limited sectors of consuming boundaries. This last difficulty is particularly evident in the Izu Bonin-proto Mariana arc-trench system (e.g. Taylor and Karner, 1983) where a continuos and laterally homogeneous subduction process has been reflected at the surface by a strongly heterogeneous behavior of the arcÕs deformation. Analogous difficulties would be encountered by any attempt to apply this model to the T-A-BA systems of the Mediterranean area, where the buckling of arcs is particularly evident (Mantovani et al., 1997, 2000a).

The dynamics of the corner flow model (Fig. 2c) implies that extension occurs in the central part of an upwelling zone, like an oceanic ridge, as also predicted by numerical modelling experiments (e.g., Keen, 1985). Thus, one could try to recognize the actual occurrence of this mechanism by the analysis of the morphological features of back arc zones, also keeping in mind that extension driven by horizontal forces (as presumably occurs in passive, i.e. kinematically induced processes) provides a generalized subsidence of the back arc zone. This discrimination could be hampered by the fact that tectonic troughs are generally bounded by uplifted shoulders. However, it has been demonstrated that this effect does not imply the presence of vertical additional forces, but simply represents a normal gravitational response to the trough's subsidence (Keen, 1985; Shemenda and Grocholsky, 1994). Once removed this possible ambiguity, the morphological features of most back arc basins seem to be more consistent with passive crustal stretching (see, e.g. Sibuet et al., 1987; Park et al., 1990; Wright et al., 1996).

The corner flow model may hardly be invoked to explain the occurrence of extension in some back arc basins, such as e.g. the Mariana and East Scotia, where the sites of active opening lie beyond the seismically defined deepest limit of the subducted lithosphere (e.g. Taylor and Karner, 1983).

Discussions as to unresolved problems with this model are also reported by other authors (e.g Uyeda and Kanamori, 1979;Taylor and Karner, 1983; Uyeda, 1986; Flower et al., 2001).

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