Billen, Magali I. and Gurnis, Michael and Simons, Mark
Multiscale dynamics of the Tonga–Kermadec subduction zone.
Geophysical Journal International, 153
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Our understanding of mantle convection and the motion of plates depends intimately on our understanding of the viscosity structure of the mantle. While geoid and gravity observations have provided fundamental constraints on the radial viscosity structure of the mantle, the influence of short-wavelength variations in viscosity is still poorly understood. We present 2-D and 3-D finite-element models of mantle flow, including strong lateral viscosity variations and local sources of buoyancy, owing to both thermal and compositional effects. We first use generic 2-D models of a subduction zone to investigate how different observations depend on various aspects of the viscosity structure, in particular, the slab and lower-mantle viscosity and the presence of a low-viscosity region in the mantle wedge above the slab. We find that: (1) the strain rate provides a strong constraint on the absolute viscosity of the slab (10^(23) Pa s); (2) stress orientation within the slab is sensitive to the relative viscosity of the slab, lower mantle and the wedge; and (3) the stress state and topography of the overriding plate depend on the wedge viscosity and local sources of buoyancy. In particular, the state of stress in the overriding plate changes from compression to extension with the addition of a low-viscosity wedge. We then use observations of strain rate, stress orientation, dynamic topography and the geoid for the Tonga–Kermadec subduction zone as simultaneous constraints on the viscosity and buoyancy in a 3-D regional dynamic model. Together these observations are used to develop a self-consistent model of the viscosity and buoyancy by taking advantage of the sensitivity of each observation to different aspects of the dynamics, over a broad range of length-scales. The presence of a low-viscosity wedge makes it possible to match observations of shallow dynamic topography and horizontal extension within the backarc, and down-dip compression in the shallow portion of the slab. These results suggest that a low-viscosity wedge plays an important role in controlling the presence of backarc spreading. However, for a model with a low-viscosity and low-density region that provides a good fit to the observed topography, we find that a reduction of the slab density by a factor of 1.3 relative to the reference density model, is required to match the observed geoid. These results suggest that compensation of the slab by dynamic topography may be a much smaller effect at short to intermediate wavelengths than predicted by long-wavelength modelling of the geoid.
|Additional Information:||© 2003 Royal Astronomical Society.
Accepted 2002 October 9. Received 2002 September 17; in original form 2002 January 4.
We thank Don Anderson for discussions onwater in the mantle, Paul Asimow for his help in estimating the density anomalies within a wet subduction zonewedge and Brad Hager for his helpful review of the manuscript. Calculations were carried out on the parallel supercomputers of the Caltech Center for Advanced Computer Research (CACR). The software package GMT (Wessel & Smith 1995) was used for most of the figures. Supported byNSFgrants EAR-9725629 and EAR-9814577. Contribution no 8853, Division of Geological and Planetary Sciences, California Institute of Technology.|
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|Subject Keywords:||dynamics; lateral heterogeneity; mantle viscosity; subduction|
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|Caltech Division of Geological and Planetary Sciences||8853|
|Official Citation:||Billen, M. I., Gurnis, M. and Simons, M. (2003), Multiscale dynamics of the Tonga–Kermadec subduction zone. Geophysical Journal International, 153: 359–388. doi: 10.1046/j.1365-246X.2003.01915.x|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited On:||28 Nov 2012 19:40|
|Last Modified:||04 Oct 2016 22:56|
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