Ridge subduction: kinematics and implications for the nature of mantle upwelling: Discussion
The authors, E. Farrar and J.M. Dixon, in a recent paper on "ridge subduction," discuss the magmatic and tectonic effects of overridden sea-floor spreading centres. Their paper provides a useful compilation of information on several ridge trench encounters, and elucidates a certain type of plate interaction called "eduction." However, much of their analysis is flawed because they (1) do not consider the geometry of subducted slabs and intervening slab windows; (2) misrepresent the importance of slab windows to magmatism and tectonism in the overriding plate, and to mantle currents; and (3) incorrectly link the concept of slab window formation to a model of ' 'passive" mantle upflow. Consequently, their paper overlooks the controlling influence that slab windows have on asthenosphere -lithosphere interactions when diverging plates are subducted. Since the authors present their material in the style of a comprehensive review, their omissions and misrepresentations warrant critical response. Numerous authors have established that ridge -trench encounters tend to affect the overriding plate in three main ways: (1) tectonic erosion of forearc, and related uplift of forearc and arc; (2) change in magmatic regime from arc-type volcanism to riftor plume-type volcanism, extending from forearc to retroarc; and (3) broad, thermally supported uplift, high heat flow, and extension (Stacey 1974; Fox et al. 1985; Merritts and Vincent 1989; Thorkelson and Taylor 1989; Hole 1990; Hole et al. 1991 ; Babcock et al. 1992; Hole and Larter 1993; references cited in the authors' paper). The authors concur, and appeal to "active" asthenospheric upwelling (e.g., Hess 1962; Gough 1984; Wilson 1988) beneath the overriding plate as a principal cause. Although the authors' depiction of active mantle dynamics is plausible, the geological context of their arguments is unacceptable. Specifically, they appeal to "ridge subduction" and mantle upwelling without providing a framework of subducted slab geometry. Without full appreciation of slab geometry and the constraints imposed by slab windows, their arguments are limited to a vague precept of mantle convection.