The dynamical origin of subduction zone topography

Abstract

Subduction zones are expressed topographically by long linear oceanic trenches flanked by a low outer rise on the seaward side and an island arc on the landward side. This topographic structure is reflected in free air gravity anomalies, suggesting that much of the topography originates from dynamical forces applied at the base of the crust. We have successfully reproduced the general topographic features of subduction zones by supposing that the stresses generated by the bending of the viscous lower lithosphere as it subducts are transmitted through the thin elastic upper portion of the lithosphere. The trench is due to a zone of extensional flow (associated with low pressure) in the upper part of the viscous lithosphere.The stresses in the subducting slab are computed using a finite element technique, assuming a Maxwell viscoelastic constitutive relation. Various dips (10 to 90") were investigated, as well as depth dependent and non-Newtonian (power law, n = 3) viscosities. Observed subduction zone dimensions are well reproduced by these models. The effective viscosity required at mid-depth in the lithosphere is about 6 x lo2* P. This low value is probably due to the stress dependence of the effective viscosity. However, these models also show that the topography of the subduction zone depends primarily upon the geometry of the subducting slab (dip, radius of curvature of the bend) rather than upon' its rheology. Shear stresses beneath the trench reach maxima of approximately 50 MPa. An interesting feature of some solutions is a dynamically supported bench or platform between the trench and island arc.

Flexure of the Indian plate and intraplate earthquakes

“…McKenzie and Fairhead (1997) conjecture that the longer observed wavelength is supported by mantle dynamics not considered in thin elastic plate models of the Indian collision. One such mechanism has been proposed by Melosh (1978) and Melosh and Raefsky (1908). In this model, a thin elastic plate is effectively supported by dynamic flow in the upper mantle, the flexural stress distribution in the elastic plate being similar though of lower amplitude than were the entire flexural bulge supported by bending of a thick plate.…”

Could Pantheon Fossae be the result of the Apollodorus crater-forming impact within the Caloris basin, Mercury?

“…2B), as well as those with non-symmetric exterior smooth plains loading. Kennedy et al (2008) used Tecton (Melosh and Raefsky, 1980) as a solver. Here we use the viscoelastic finite element program I-deas, which provides an ability to develop and solve spherical 3-D models and was applied in several previous tectonic studies (e.g., Freed et al, 2006Freed et al, , 2007.…”

Time‐dependent changes in failure stress following thrust earthquakes

“…Analyses were conducted using TECTON 2-D plane-strain, finite element models [Melosh and Raefsky, 1980] …”