Spherical shell tectonics: Effects of sphericity and inextensibility on the geometry of the descending lithosphere

Publisher Website
Google Scholar
Abstract
The shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere. The lithosphere before subduction is a spherical shell, and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone. Upon bending a spherical shell often deforms in a very different way from a simple plate. We examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell, which shows little surface deformation under moderate stress. This examination is made region by region for most of the subduction zones in the world by means of an analogue method. The lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin. The Wadati‐Benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the International Seismological Centre (ISC) bulletins. The spherical shell is forced to fit the miniature of the Wadati‐Benioff zone. Fitting is first attempted only by bending. If a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region, the spherical shell is torn along this discontinuity or gap, and the goodness of fit is reexamined. The results of the analysis are summarized as follows: (1) The shape of the Wadati‐Benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation. (2) In almost all of the regions of poor fit with bending, a good fit can be achieved by tearing the spherical shell along the trace of low seismicity. The sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape. This means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it. Further, several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics.