Mantle Flow at Subduction Zones
Tuesday, 22. January 2008, 16:22:27
I think that simple cartoons meant to get a better grasp on geological concepts unfortunately also can lead to misconceptions. A main problem is that they are 2-D drawings, where we ought to use 3-D. We may tend to forget that our Earth is a sphere and not flat. This is a fact that may hinder the fully understanding of plate tectonics.
Looking at cartoons of subduction zones you would think that the subducting slab would drag mantle material with it downwards and attract mantle material to the subduction zone from both sides in a more or less circular unidirectional flow on both sides of the subducted slab. Reading about three-dimensional numerical simulations of free subduction in a paper in Nature in March 2007 by Schellart et al. titled ‘Evolution and diversity of subduction zones controlled by slab width’ shows that the picture might be more complex than that. After all the character of the mantle flow field that accompanies subduction apparently remains poorly understood. An article in Science of 18 January 2008 is of further help. I am here referring to Long and Silver The Subduction Zone Flow Field from Seismic Anisotropy: A Global View.
The conventional model of mantle flow in the vicinity of subduction zones used to be that this flow is predominantly two-dimensional. In subduction zones, a wedge of mantle material is caught above the subducting plate and beneath the overlying crust. Flow within the mantle also produces flow in the wedge (“corner flow”) that can align mantle minerals. Such alignment should cause seismic waves to propagate faster in one direction, but systematic patterns have been difficult to discern. A global survey and analysis shows that the confusing patterns may reflect the migration of trenches forward (toward the direction of the incoming plate) and backward. This flow tends to induce flow parallel to the trench both in the mantle beneath the subducting slab (subslab mantle) and in the wedge (and around the slab edge) that scales with the rate of trench migration.
The model put forward by Long and Silver requires a thin decoupling zone between the downgoing slab and the subslab mantle. The existence of trench-parallel flow in the mantle wedge as a common phenomenon implies that along-strike transport of mantle material is important for many arcs, with consequences for magma formation, volatile transport, and thermal structure.
Trench-parallel flow was described at least as early as in 1994 by Russo and Silver in Trench-Parallel Flow Beneath the Nazca Plate from Seismic Anisotropy (Science of 25 February 1994).
Another couple of earlier articles in Science are also concerned with this issue, namely
• Smith et al.: A Complex Pattern of Mantle Flow in the Lau Backarc of 2001
• Behn et al : Trench-Parallel Anisotropy Produced by Foundering of Arc Lower Crust of 2007
PS: This post served as a contribution to the Accretionary Wedge #5: Geological Misconceptions and Pie published on 23 January 2008.
Looking at cartoons of subduction zones you would think that the subducting slab would drag mantle material with it downwards and attract mantle material to the subduction zone from both sides in a more or less circular unidirectional flow on both sides of the subducted slab. Reading about three-dimensional numerical simulations of free subduction in a paper in Nature in March 2007 by Schellart et al. titled ‘Evolution and diversity of subduction zones controlled by slab width’ shows that the picture might be more complex than that. After all the character of the mantle flow field that accompanies subduction apparently remains poorly understood. An article in Science of 18 January 2008 is of further help. I am here referring to Long and Silver The Subduction Zone Flow Field from Seismic Anisotropy: A Global View.
The conventional model of mantle flow in the vicinity of subduction zones used to be that this flow is predominantly two-dimensional. In subduction zones, a wedge of mantle material is caught above the subducting plate and beneath the overlying crust. Flow within the mantle also produces flow in the wedge (“corner flow”) that can align mantle minerals. Such alignment should cause seismic waves to propagate faster in one direction, but systematic patterns have been difficult to discern. A global survey and analysis shows that the confusing patterns may reflect the migration of trenches forward (toward the direction of the incoming plate) and backward. This flow tends to induce flow parallel to the trench both in the mantle beneath the subducting slab (subslab mantle) and in the wedge (and around the slab edge) that scales with the rate of trench migration.
The model put forward by Long and Silver requires a thin decoupling zone between the downgoing slab and the subslab mantle. The existence of trench-parallel flow in the mantle wedge as a common phenomenon implies that along-strike transport of mantle material is important for many arcs, with consequences for magma formation, volatile transport, and thermal structure.
Trench-parallel flow was described at least as early as in 1994 by Russo and Silver in Trench-Parallel Flow Beneath the Nazca Plate from Seismic Anisotropy (Science of 25 February 1994).
Another couple of earlier articles in Science are also concerned with this issue, namely
• Smith et al.: A Complex Pattern of Mantle Flow in the Lau Backarc of 2001
• Behn et al : Trench-Parallel Anisotropy Produced by Foundering of Arc Lower Crust of 2007
PS: This post served as a contribution to the Accretionary Wedge #5: Geological Misconceptions and Pie published on 23 January 2008.
WIDGETIZE