Ridge subduction: kinematics and implications for the nature of mantle upwelling: Discussion

Abstract

The authors, E. Farrar and J.M. Dixon, in a recent paper on 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 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 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.